NMAP(1) Nmap Reference Guide NMAP(1)
NAME
nmap - Network exploration tool and security / port scanner
SYNOPSIS
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION
Nmap ("Network Mapper") is an open source tool for network exploration
and security auditing. It was designed to rapidly scan large networks,
although it works fine against single hosts. Nmap uses raw IP packets
in novel ways to determine what hosts are available on the network,
what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type of
packet filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits, many
systems and network administrators find it useful for routine tasks
such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the "interesting ports table".. That table lists the
port number and protocol, service name, and state. The state is either
open, filtered, closed, or unfiltered. Open. means that an
application on the target machine is listening for connections/packets
on that port. Filtered. means that a firewall, filter, or other
network obstacle is blocking the port so that Nmap cannot tell whether
it is open or closed. Closed. ports have no application listening on
them, though they could open up at any time. Ports are classified as
unfiltered. when they are responsive to Nmap's probes, but Nmap cannot
determine whether they are open or closed. Nmap reports the state
combinations open|filtered. and closed|filtered. when it cannot
determine which of the two states describe a port. The port table may
also include software version details when version detection has been
requested. When an IP protocol scan is requested (-sO), Nmap provides
information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap arguments used
in this example are -A, to enable OS and version detection, script
scanning, and traceroute; -T4 for faster execution; and then the two
target hostnames.
Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Nmap scan report for scanme.nmap.org (<IP_ADDRESS>)
Host is up (0.029s latency).
rDNS record for <IP_ADDRESS>: li86-221.members.linode.com
Not shown: 995 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp open http Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open nping-echo Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel
TRACEROUTE (using port 53/tcp)
HOP RTT ADDRESS
[Cut first 10 hops for brevity]
11 17.65 ms li86-221.members.linode.com (<IP_ADDRESS>)
Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
The newest version of Nmap can be obtained from http://nmap.org. The
newest version of this man page is available at
http://nmap.org/book/man.html. It is also included as a chapter of
Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning (see http://nmap.org/book/).
OPTIONS SUMMARY
This options summary is printed when Nmap is run with no arguments, and
the latest version is always available at
https://svn.nmap.org/nmap/docs/nmap.usage.txt. It helps people remember
the most common options, but is no substitute for the in-depth
documentation in the rest of this manual. Some obscure options aren't
even included here.
Nmap 6.40 ( http://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, <IP_ADDRESS>; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sn: Ping Scan - disable port scan
-Pn: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-args-file=filename: provide NSE script args in a file
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
--script-help=<Lua scripts>: Show help about scripts.
<Lua scripts> is a comma separted list of script-files or
script-categories.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in seconds, or append 'ms' (milliseconds),
's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <numprobes>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use -vv or more for greater effect)
-d: Increase debugging level (use -dd or more for greater effect)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--log-errors: Log errors/warnings to the normal-format output file
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enable OS detection, version detection, script scanning, and traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sn <IP_ADDRESS>/16 <IP_ADDRESS>/8
nmap -v -iR 10000 -Pn -p 80
SEE THE MAN PAGE (http://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
TARGET SPECIFICATION
Everything on the Nmap command-line that isn't an option (or option
argument) is treated as a target host specification. The simplest case
is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For this,
Nmap supports CIDR-style. addressing. You can append /numbits to an
IPv4 address or hostname and Nmap will scan every IP address for which
the first numbits are the same as for the reference IP or hostname
given. For example, <IP_ADDRESS>/24 would scan the 256 hosts between
<IP_ADDRESS> (binary: 11000000 10101000 00001010 00000000) and
<IP_ADDRESS> (binary: 11000000 10101000 00001010 11111111),
inclusive. <IP_ADDRESS>/24 would scan exactly the same targets. Given
that the host scanme.nmap.org. is at the IP address <IP_ADDRESS>, the
specification scanme.nmap.org/16 would scan the 65,536 IP addresses
between <IP_ADDRESS> and <IP_ADDRESS>. The smallest allowed value is /0,
which targets the whole Internet. The largest value is /32, which scans
just the named host or IP address because all address bits are fixed.
CIDR notation is short but not always flexible enough. For example, you
might want to scan <IP_ADDRESS>/16 but skip any IPs ending with .0 or
.255 because they may be used as subnet network and broadcast
addresses. Nmap supports this through octet range addressing. Rather
than specify a normal IP address, you can specify a comma-separated
list of numbers or ranges for each octet. For example,
192.168.0-255.1-254 will skip all addresses in the range that end in .0
or .255, and 192.168.3-5,7.1 will scan the four addresses <IP_ADDRESS>,
<IP_ADDRESS>, <IP_ADDRESS>, and <IP_ADDRESS>. Either side of a range may
be omitted; the default values are 0 on the left and 255 on the right.
Using - by itself is the same as 0-255, but remember to use 0- in the
first octet so the target specification doesn't look like a
command-line option. Ranges need not be limited to the final octets:
the specifier 0-255.0-255.13.37 will perform an Internet-wide scan for
all IP addresses ending in 13.37. This sort of broad sampling can be
useful for Internet surveys and research.
IPv6 addresses can only be specified by their fully qualified IPv6
address or hostname. CIDR and octet ranges aren't yet supported for
IPv6.
IPv6 addresses with non-global scope need to have a zone ID suffix. On
Unix systems, this is a percent sign followed by an interface name; a
complete address might be fe80::a8bb:ccff:fedd:eeff%eth0. On Windows,
use an interface index number in place of an interface name:
fe80::a8bb:ccff:fedd:eeff%1. You can see a list of interface indexes by
running the command netsh.exe interface ipv6 show interface.
Nmap accepts multiple host specifications on the command line, and they
don't need to be the same type. The command nmap scanme.nmap.org
<IP_ADDRESS>/8 10.0.0,1,3-7.- does what you would expect.
While targets are usually specified on the command lines, the following
options are also available to control target selection:
-iL inputfilename (Input from list) .
Reads target specifications from inputfilename. Passing a huge list
of hosts is often awkward on the command line, yet it is a common
desire. For example, your DHCP server might export a list of 10,000
current leases that you wish to scan. Or maybe you want to scan all
IP addresses except for those to locate hosts using unauthorized
static IP addresses. Simply generate the list of hosts to scan and
pass that filename to Nmap as an argument to the -iL option.
Entries can be in any of the formats accepted by Nmap on the
command line (IP address, hostname, CIDR, IPv6, or octet ranges).
Each entry must be separated by one or more spaces, tabs, or
newlines. You can specify a hyphen (-) as the filename if you want
Nmap to read hosts from standard input rather than an actual file.
The input file may contain comments that start with # and extend to
the end of the line.
-iR num hosts (Choose random targets) .
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain
private, multicast, or unallocated address ranges are automatically
skipped. The argument 0 can be specified for a never-ending scan.
Keep in mind that some network administrators bristle at
unauthorized scans of their networks and may complain. Use this
option at your own risk! If you find yourself really bored one
rainy afternoon, try the command nmap -Pn -sS -p 80 -iR 0 --open.
to locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks) .
Specifies a comma-separated list of targets to be excluded from the
scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can be
useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react adversely
to port scans, or subnets administered by other people.
--excludefile exclude_file (Exclude list from file) .
This offers the same functionality as the --exclude option, except
that the excluded targets are provided in a newline-, space-, or
tab-delimited exclude_file rather than on the command line.
The exclude file may contain comments that start with # and extend
to the end of the line.
HOST DISCOVERY
One of the very first steps in any network reconnaissance mission is to
reduce a (sometimes huge) set of IP ranges into a list of active or
interesting hosts. Scanning every port of every single IP address is
slow and usually unnecessary. Of course what makes a host interesting
depends greatly on the scan purposes. Network administrators may only
be interested in hosts running a certain service, while security
auditors may care about every single device with an IP address. An
administrator may be comfortable using just an ICMP ping to locate
hosts on his internal network, while an external penetration tester may
use a diverse set of dozens of probes in an attempt to evade firewall
restrictions.
Because host discovery needs are so diverse, Nmap offers a wide variety
of options for customizing the techniques used. Host discovery is
sometimes called ping scan, but it goes well beyond the simple ICMP
echo request packets associated with the ubiquitous ping tool. Users
can skip the ping step entirely with a list scan (-sL) or by disabling
ping (-Pn), or engage the network with arbitrary combinations of
multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP probes. The goal of
these probes is to solicit responses which demonstrate that an IP
address is actually active (is being used by a host or network device).
On many networks, only a small percentage of IP addresses are active at
any given time. This is particularly common with private address space
such as <IP_ADDRESS>/8. That network has 16 million IPs, but I have seen it
used by companies with less than a thousand machines. Host discovery
can find those machines in a sparsely allocated sea of IP addresses.
If no host discovery options are given, Nmap sends an ICMP echo
request, a TCP SYN packet to port 443, a TCP ACK packet to port 80, and
an ICMP timestamp request. (For IPv6, the ICMP timestamp request is
omitted because it is not part of ICMPv6.) These defaults are
equivalent to the -PE -PS443 -PA80 -PP options. The exceptions to this
are the ARP (for IPv4) and Neighbor Discovery. (for IPv6) scans which
are used for any targets on a local ethernet network. For unprivileged
Unix shell users, the default probes are a SYN packet to ports 80 and
443 using the connect system call.. This host discovery is often
sufficient when scanning local networks, but a more comprehensive set
of discovery probes is recommended for security auditing.
The -P* options (which select ping types) can be combined. You can
increase your odds of penetrating strict firewalls by sending many
probe types using different TCP ports/flags and ICMP codes. Also note
that ARP/Neighbor Discovery (-PR). is done by default against targets
on a local ethernet network even if you specify other -P* options,
because it is almost always faster and more effective.
By default, Nmap does host discovery and then performs a port scan
against each host it determines is online. This is true even if you
specify non-default host discovery types such as UDP probes (-PU). Read
about the -sn option to learn how to perform only host discovery, or
use -Pn to skip host discovery and port scan all target hosts. The
following options control host discovery:
-sL (List Scan) .
The list scan is a degenerate form of host discovery that simply
lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does
reverse-DNS resolution on the hosts to learn their names. It is
often surprising how much useful information simple hostnames give
out. For example, fw.chi is the name of one company's Chicago
firewall. Nmap also reports the total number of IP addresses at
the end. The list scan is a good sanity check to ensure that you
have proper IP addresses for your targets. If the hosts sport
domain names you do not recognize, it is worth investigating
further to prevent scanning the wrong company's network.
Since the idea is to simply print a list of target hosts, options
for higher level functionality such as port scanning, OS detection,
or ping scanning cannot be combined with this. If you wish to
disable ping scanning while still performing such higher level
functionality, read up on the -Pn (skip ping) option.
-sn (No port scan) .
This option tells Nmap not to do a port scan after host discovery,
and only print out the available hosts that responded to the scan.
This is often known as a "ping scan", but you can also request that
traceroute and NSE host scripts be run. This is by default one step
more intrusive than the list scan, and can often be used for the
same purposes. It allows light reconnaissance of a target network
without attracting much attention. Knowing how many hosts are up is
more valuable to attackers than the list provided by list scan of
every single IP and host name.
Systems administrators often find this option valuable as well. It
can easily be used to count available machines on a network or
monitor server availability. This is often called a ping sweep, and
is more reliable than pinging the broadcast address because many
hosts do not reply to broadcast queries.
The default host discovery done with -sn consists of an ICMP echo
request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP
timestamp request by default. When executed by an unprivileged
user, only SYN packets are sent (using a connect call) to ports 80
and 443 on the target. When a privileged user tries to scan targets
on a local ethernet network, ARP requests are used unless --send-ip
was specified. The -sn option can be combined with any of the
discovery probe types (the -P* options, excluding -Pn) for greater
flexibility. If any of those probe type and port number options are
used, the default probes are overridden. When strict firewalls are
in place between the source host running Nmap and the target
network, using those advanced techniques is recommended. Otherwise
hosts could be missed when the firewall drops probes or their
responses.
In previous releases of Nmap, -sn was known as -sP..
-Pn (No ping) .
This option skips the Nmap discovery stage altogether. Normally,
Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as port
scans, version detection, or OS detection against hosts that are
found to be up. Disabling host discovery with -Pn causes Nmap to
attempt the requested scanning functions against every target IP
address specified. So if a class B target address space (/16) is
specified on the command line, all 65,536 IP addresses are scanned.
Proper host discovery is skipped as with the list scan, but instead
of stopping and printing the target list, Nmap continues to perform
requested functions as if each target IP is active. To skip ping
scan and port scan, while still allowing NSE to run, use the two
options -Pn -sn together.
For machines on a local ethernet network, ARP scanning will still
be performed (unless --disable-arp-ping or --send-ip is specified)
because Nmap needs MAC addresses to further scan target hosts. In
previous versions of Nmap, -Pn was -P0. and -PN..
-PS port list (TCP SYN Ping) .
This option sends an empty TCP packet with the SYN flag set. The
default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC. in nmap.h).. Alternate
ports can be specified as a parameter. The syntax is the same as
for the -p except that port type specifiers like T: are not
allowed. Examples are -PS22 and -PS22-25,80,113,1050,35000. Note
that there can be no space between -PS and the port list. If
multiple probes are specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are attempting
to establish a connection. Normally the destination port will be
closed, and a RST (reset) packet sent back. If the port happens to
be open, the target will take the second step of a TCP
three-way-handshake. by responding with a SYN/ACK TCP packet. The
machine running Nmap then tears down the nascent connection by
responding with a RST rather than sending an ACK packet which would
complete the three-way-handshake and establish a full connection.
The RST packet is sent by the kernel of the machine running Nmap in
response to the unexpected SYN/ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
RST or SYN/ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw TCP packets.. For unprivileged users, a
workaround is automatically employed. whereby the connect system
call is initiated against each target port. This has the effect of
sending a SYN packet to the target host, in an attempt to establish
a connection. If connect returns with a quick success or an
ECONNREFUSED failure, the underlying TCP stack must have received a
SYN/ACK or RST and the host is marked available. If the connection
attempt is left hanging until a timeout is reached, the host is
marked as down.
-PA port list (TCP ACK Ping) .
The TCP ACK ping is quite similar to the just-discussed SYN ping.
The difference, as you could likely guess, is that the TCP ACK flag
is set instead of the SYN flag. Such an ACK packet purports to be
acknowledging data over an established TCP connection, but no such
connection exists. So remote hosts should always respond with a RST
packet, disclosing their existence in the process.
The -PA option uses the same default port as the SYN probe (80) and
can also take a list of destination ports in the same format. If an
unprivileged user tries this, the connect workaround discussed
previously is used. This workaround is imperfect because connect is
actually sending a SYN packet rather than an ACK.
The reason for offering both SYN and ACK ping probes is to maximize
the chances of bypassing firewalls. Many administrators configure
routers and other simple firewalls to block incoming SYN packets
except for those destined for public services like the company web
site or mail server. This prevents other incoming connections to
the organization, while allowing users to make unobstructed
outgoing connections to the Internet. This non-stateful approach
takes up few resources on the firewall/router and is widely
supported by hardware and software filters. The Linux
Netfilter/iptables. firewall software offers the --syn convenience
option to implement this stateless approach. When stateless
firewall rules such as this are in place, SYN ping probes (-PS) are
likely to be blocked when sent to closed target ports. In such
cases, the ACK probe shines as it cuts right through these rules.
Another common type of firewall uses stateful rules that drop
unexpected packets. This feature was initially found mostly on
high-end firewalls, though it has become much more common over the
years. The Linux Netfilter/iptables system supports this through
the --state option, which categorizes packets based on connection
state. A SYN probe is more likely to work against such a system, as
unexpected ACK packets are generally recognized as bogus and
dropped. A solution to this quandary is to send both SYN and ACK
probes by specifying -PS and -PA.
-PU port list (UDP Ping) .
Another host discovery option is the UDP ping, which sends a UDP
packet to the given ports. For most ports, the packet will be
empty, though for a few a protocol-specific payload will be sent
that is more likely to get a response.. The --data-length. option
can be used to send a fixed-length random payload to every port or
(if you specify a value of 0) to disable payloads. You can also
disable payloads by specifying --data-length 0.
The port list takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 40125.. This default can be configured at compile-time
by changing DEFAULT_UDP_PROBE_PORT_SPEC. in nmap.h.. A highly
uncommon port is used by default because sending to open ports is
often undesirable for this particular scan type.
Upon hitting a closed port on the target machine, the UDP probe
should elicit an ICMP port unreachable packet in return. This
signifies to Nmap that the machine is up and available. Many other
types of ICMP errors, such as host/network unreachables or TTL
exceeded are indicative of a down or unreachable host. A lack of
response is also interpreted this way. If an open port is reached,
most services simply ignore the empty packet and fail to return any
response. This is why the default probe port is 40125, which is
highly unlikely to be in use. A few services, such as the Character
Generator (chargen) protocol, will respond to an empty UDP packet,
and thus disclose to Nmap that the machine is available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I once
owned a Linksys BEFW11S4 wireless broadband router. The external
interface of this device filtered all TCP ports by default, but UDP
probes would still elicit port unreachable messages and thus give
away the device.
-PY port list (SCTP INIT Ping) .
This option sends an SCTP packet containing a minimal INIT chunk.
The default destination port is 80 (configurable at compile time by
changing DEFAULT_SCTP_PROBE_PORT_SPEC. in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for the
-p except that port type specifiers like S: are not allowed.
Examples are -PY22 and -PY22,80,179,5060. Note that there can be no
space between -PY and the port list. If multiple probes are
specified they will be sent in parallel.
The INIT chunk suggests to the remote system that you are
attempting to establish an association. Normally the destination
port will be closed, and an ABORT chunk will be sent back. If the
port happens to be open, the target will take the second step of an
SCTP four-way-handshake. by responding with an INIT-ACK chunk. If
the machine running Nmap has a functional SCTP stack, then it tears
down the nascent association by responding with an ABORT chunk
rather than sending a COOKIE-ECHO chunk which would be the next
step in the four-way-handshake. The ABORT packet is sent by the
kernel of the machine running Nmap in response to the unexpected
INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
ABORT or INIT-ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root. is generally able to
send and receive raw SCTP packets.. Using SCTP INIT Pings is
currently not possible for unprivileged users..
-PE; -PP; -PM (ICMP Ping Types) .
In addition to the unusual TCP, UDP and SCTP host discovery types
discussed previously, Nmap can send the standard packets sent by
the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
request) packet to the target IP addresses, expecting a type 0
(echo reply) in return from available hosts.. Unfortunately for
network explorers, many hosts and firewalls now block these
packets, rather than responding as required by RFC 1122[2].. For
this reason, ICMP-only scans are rarely reliable enough against
unknown targets over the Internet. But for system administrators
monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo request
behavior.
While echo request is the standard ICMP ping query, Nmap does not
stop there. The ICMP standards (RFC 792[3]. and RFC 950[4]. "a
host SHOULD NOT implement these messages". Timestamp and address
mask queries can be sent with the -PP and -PM options,
respectively. A timestamp reply (ICMP code 14) or address mask
reply (code 18) discloses that the host is available. These two
queries can be valuable when administrators specifically block echo
request packets while forgetting that other ICMP queries can be
used for the same purpose.
-PO protocol list (IP Protocol Ping) .
One of the newer host discovery options is the IP protocol ping,
which sends IP packets with the specified protocol number set in
their IP header. The protocol list takes the same format as do port
lists in the previously discussed TCP, UDP and SCTP host discovery
options. If no protocols are specified, the default is to send
multiple IP packets for ICMP (protocol 1), IGMP (protocol 2), and
IP-in-IP (protocol 4). The default protocols can be configured at
compile-time by changing DEFAULT_PROTO_PROBE_PORT_SPEC. in nmap.h.
Note that for the ICMP, IGMP, TCP (protocol 6), UDP (protocol 17)
and SCTP (protocol 132), the packets are sent with the proper
protocol headers. while other protocols are sent with no
additional data beyond the IP header (unless the --data-length.
option is specified).
This host discovery method looks for either responses using the
same protocol as a probe, or ICMP protocol unreachable messages
which signify that the given protocol isn't supported on the
destination host. Either type of response signifies that the target
host is alive.
-PR (ARP Ping) .
One of the most common Nmap usage scenarios is to scan an ethernet
LAN. On most LANs, especially those using private address ranges
specified by RFC 1918[5], the vast majority of IP addresses are
unused at any given time. When Nmap tries to send a raw IP packet
such as an ICMP echo request, the operating system must determine
the destination hardware (ARP) address corresponding to the target
IP so that it can properly address the ethernet frame. This is
often slow and problematic, since operating systems weren't written
with the expectation that they would need to do millions of ARP
requests against unavailable hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP
requests. And if it gets a response back, Nmap doesn't even need to
worry about the IP-based ping packets since it already knows the
host is up. This makes ARP scan much faster and more reliable than
IP-based scans. So it is done by default when scanning ethernet
hosts that Nmap detects are on a local ethernet network. Even if
different ping types (such as -PE or -PS) are specified, Nmap uses
ARP instead for any of the targets which are on the same LAN. If
you absolutely don't want to do an ARP scan, specify
--disable-arp-ping.
For IPv6 (-6 option), -PR uses ICMPv6 Neighbor Discovery instead of
ARP. Neighbor Discovery, defined in RFC 4861, can be seen as the
IPv6 equivalent of ARP.
--disable-arp-ping (No ARP or ND Ping) .
Nmap normally does ARP or IPv6 Neighbor Discovery (ND) discovery of
locally connected ethernet hosts, even if other host discovery
options such as -Pn or -PE are used. To disable this implicit
behavior, use the --disable-arp-ping option.
The default behavior is normally faster, but this option is useful
on networks using proxy ARP, in which a router speculatively
replies to all ARP requests, making every target appear to be up
according to ARP scan.
--traceroute (Trace path to host) .
Traceroutes are performed post-scan using information from the scan
results to determine the port and protocol most likely to reach the
target. It works with all scan types except connect scans (-sT) and
idle scans (-sI). All traces use Nmap's dynamic timing model and
are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live)
in an attempt to elicit ICMP Time Exceeded messages from
intermediate hops between the scanner and the target host. Standard
traceroute implementations start with a TTL of 1 and increment the
TTL until the destination host is reached. Nmap's traceroute starts
with a high TTL and then decrements the TTL until it reaches zero.
Doing it backwards lets Nmap employ clever caching algorithms to
speed up traces over multiple hosts. On average Nmap sends 5–10
fewer packets per host, depending on network conditions. If a
single subnet is being scanned (i.e. <IP_ADDRESS>/24) Nmap may only
have to send two packets to most hosts.
-n (No DNS resolution) .
Tells Nmap to never do reverse DNS resolution on the active IP
addresses it finds. Since DNS can be slow even with Nmap's built-in
parallel stub resolver, this option can slash scanning times.
-R (DNS resolution for all targets) .
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver) .
By default, Nmap resolves IP addresses by sending queries directly
to the name servers configured on your host and then listening for
responses. Many requests (often dozens) are performed in parallel
to improve performance. Specify this option to use your system
resolver instead (one IP at a time via the getnameinfo call). This
is slower and rarely useful unless you find a bug in the Nmap
parallel resolver (please let us know if you do). The system
resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]] (Servers to use for reverse DNS
queries) .
By default, Nmap determines your DNS servers (for rDNS resolution)
from your resolv.conf file (Unix) or the Registry (Win32).
Alternatively, you may use this option to specify alternate
servers. This option is not honored if you are using --system-dns
or an IPv6 scan. Using multiple DNS servers is often faster,
especially if you choose authoritative servers for your target IP
space. This option can also improve stealth, as your requests can
be bounced off just about any recursive DNS server on the Internet.
This option also comes in handy when scanning private networks.
Sometimes only a few name servers provide proper rDNS information,
and you may not even know where they are. You can scan the network
for port 53 (perhaps with version detection), then try Nmap list
scans (-sL) specifying each name server one at a time with
--dns-servers until you find one which works.
PORT SCANNING BASICS
While Nmap has grown in functionality over the years, it began as an
efficient port scanner, and that remains its core function. The simple
command nmap target scans 1,000 TCP ports on the host target. While
many port scanners have traditionally lumped all ports into the open or
closed states, Nmap is much more granular. It divides ports into six
states: open, closed, filtered, unfiltered, open|filtered, or
closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the same
network as the target may show port 135/tcp as open, while a scan at
the same time with the same options from across the Internet might show
that port as filtered.
The six port states recognized by Nmap
An application is actively accepting TCP connections, UDP datagrams
or SCTP associations on this port. Finding these is often the
primary goal of port scanning. Security-minded people know that
each open port is an avenue for attack. Attackers and pen-testers
want to exploit the open ports, while administrators try to close
or protect them with firewalls without thwarting legitimate users.
Open ports are also interesting for non-security scans because they
show services available for use on the network.
A closed port is accessible (it receives and responds to Nmap probe
packets), but there is no application listening on it. They can be
helpful in showing that a host is up on an IP address (host
discovery, or ping scanning), and as part of OS detection. Because
closed ports are reachable, it may be worth scanning later in case
some open up. Administrators may want to consider blocking such
ports with a firewall. Then they would appear in the filtered
state, discussed next.
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The filtering
could be from a dedicated firewall device, router rules, or
host-based firewall software. These ports frustrate attackers
because they provide so little information. Sometimes they respond
with ICMP error messages such as type 3 code 13 (destination
unreachable: communication administratively prohibited), but
filters that simply drop probes without responding are far more
common. This forces Nmap to retry several times just in case the
probe was dropped due to network congestion rather than filtering.
This slows down the scan dramatically.
The unfiltered state means that a port is accessible, but Nmap is
unable to determine whether it is open or closed. Only the ACK
scan, which is used to map firewall rulesets, classifies ports into
this state. Scanning unfiltered ports with other scan types such as
Window scan, SYN scan, or FIN scan, may help resolve whether the
port is open.
Nmap places ports in this state when it is unable to determine
whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could also
mean that a packet filter dropped the probe or any response it
elicited. So Nmap does not know for sure whether the port is open
or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas scans
classify ports this way.
This state is used when Nmap is unable to determine whether a port
is closed or filtered. It is only used for the IP ID idle scan.
PORT SCANNING TECHNIQUES
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to
the task at hand. When I fail miserably and tow my jalopy to a real
mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of scan
techniques and choose the appropriate one (or combination) for a given
task. Inexperienced users and script kiddies,. on the other hand, try
to solve every problem with the default SYN scan. Since Nmap is free,
the only barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to determine
that you need a strut spring compressor, then you still have to pay
thousands of dollars for it.
Most of the scan types are only available to privileged users.. This
is because they send and receive raw packets,. which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS. Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts. Now, the
world is different. Computers are cheaper, far more people have
always-on direct Internet access, and desktop Unix systems (including
Linux and Mac OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all these
reasons, users have less need to run Nmap from limited shared shell
accounts. This is fortunate, as the privileged options make Nmap far
more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all
of its insights are based on packets returned by the target machines
(or firewalls in front of them). Such hosts may be untrustworthy and
send responses intended to confuse or mislead Nmap. Much more common
are non-RFC-compliant hosts that do not respond as they should to Nmap
probes. FIN, NULL, and Xmas scans are particularly susceptible to this
problem. Such issues are specific to certain scan types and so are
discussed in the individual scan type entries.
This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined
with any one of the TCP scan types. As a memory aid, port scan type
options are of the form -sC, where C is a prominent character in the
scan name, usually the first. The one exception to this is the
deprecated FTP bounce scan (-b). By default, Nmap performs a SYN Scan,
though it substitutes a connect scan if the user does not have proper
privileges to send raw packets (requires root access on Unix). Of the
scans listed in this section, unprivileged users can only execute
connect and FTP bounce scans.
-sS (TCP SYN scan) .
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive firewalls.
It is also relatively unobtrusive and stealthy since it never
completes TCP connections. SYN scan works against any compliant TCP
stack rather than depending on idiosyncrasies of specific platforms
as Nmap's FIN/NULL/Xmas, Maimon and idle scans do. It also allows
clear, reliable differentiation between the open, closed, and
filtered states.
This technique is often referred to as half-open scanning, because
you don't open a full TCP connection. You send a SYN packet, as if
you are going to open a real connection and then wait for a
response. A SYN/ACK indicates the port is listening (open), while a
RST (reset) is indicative of a non-listener. If no response is
received after several retransmissions, the port is marked as
filtered. The port is also marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received. The port is
also considered open if a SYN packet (without the ACK flag) is
received in response. This can be due to an extremely rare TCP
feature known as a simultaneous open or split handshake connection
(see http://nmap.org/misc/split-handshake.pdf).
-sT (TCP connect scan) .
TCP connect scan is the default TCP scan type when SYN scan is not
an option. This is the case when a user does not have raw packet
privileges. Instead of writing raw packets as most other scan types
do, Nmap asks the underlying operating system to establish a
connection with the target machine and port by issuing the connect
system call. This is the same high-level system call that web
browsers, P2P clients, and most other network-enabled applications
use to establish a connection. It is part of a programming
interface known as the Berkeley Sockets API. Rather than read raw
packet responses off the wire, Nmap uses this API to obtain status
information on each connection attempt.
When SYN scan is available, it is usually a better choice. Nmap has
less control over the high level connect call than with raw
packets, making it less efficient. The system call completes
connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take longer
and require more packets to obtain the same information, but target
machines are more likely to log the connection. A decent IDS will
catch either, but most machines have no such alarm system. Many
services on your average Unix system will add a note to syslog, and
sometimes a cryptic error message, when Nmap connects and then
closes the connection without sending data. Truly pathetic services
crash when this happens, though that is uncommon. An administrator
who sees a bunch of connection attempts in her logs from a single
system should know that she has been connect scanned.
-sU (UDP scans) .
While most popular services on the Internet run over the TCP
protocol, UDP[6] services are widely deployed. DNS, SNMP, and DHCP
(registered ports 53, 161/162, and 67/68) are three of the most
common. Because UDP scanning is generally slower and more difficult
than TCP, some security auditors ignore these ports. This is a
mistake, as exploitable UDP services are quite common and attackers
certainly don't ignore the whole protocol. Fortunately, Nmap can
help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined with
a TCP scan type such as SYN scan (-sS) to check both protocols
during the same run.
UDP scan works by sending a UDP packet to every targeted port. For
some common ports such as 53 and 161, a protocol-specific payload
is sent, but for most ports the packet is empty.. The
--data-length option can be used to send a fixed-length random
payload to every port or (if you specify a value of 0) to disable
payloads. If an ICMP port unreachable error (type 3, code 3) is
returned, the port is closed. Other ICMP unreachable errors (type
3, codes 1, 2, 9, 10, or 13) mark the port as filtered.
Occasionally, a service will respond with a UDP packet, proving
that it is open. If no response is received after retransmissions,
the port is classified as open|filtered. This means that the port
could be open, or perhaps packet filters are blocking the
communication. Version detection (-sV) can be used to help
differentiate the truly open ports from the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time out
and then conduct retransmissions just in case the probe or response
were lost. Closed ports are often an even bigger problem. They
usually send back an ICMP port unreachable error. But unlike the
RST packets sent by closed TCP ports in response to a SYN or
connect scan, many hosts rate limit. ICMP port unreachable
messages by default. Linux and Solaris are particularly strict
about this. For example, the Linux 2.4.20 kernel limits destination
unreachable messages to one per second (in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine
will drop. Unfortunately, a Linux-style limit of one packet per
second makes a 65,536-port scan take more than 18 hours. Ideas for
speeding your UDP scans up include scanning more hosts in parallel,
doing a quick scan of just the popular ports first, scanning from
behind the firewall, and using --host-timeout to skip slow hosts.
-sY (SCTP INIT scan) .
SCTP[7] is a relatively new alternative to the TCP and UDP
protocols, combining most characteristics of TCP and UDP, and also
adding new features like multi-homing and multi-streaming. It is
mostly being used for SS7/SIGTRAN related services but has the
potential to be used for other applications as well. SCTP INIT scan
is the SCTP equivalent of a TCP SYN scan. It can be performed
quickly, scanning thousands of ports per second on a fast network
not hampered by restrictive firewalls. Like SYN scan, INIT scan is
relatively unobtrusive and stealthy, since it never completes SCTP
associations. It also allows clear, reliable differentiation
between the open, closed, and filtered states.
This technique is often referred to as half-open scanning, because
you don't open a full SCTP association. You send an INIT chunk, as
if you are going to open a real association and then wait for a
response. An INIT-ACK chunk indicates the port is listening (open),
while an ABORT chunk is indicative of a non-listener. If no
response is received after several retransmissions, the port is
marked as filtered. The port is also marked filtered if an ICMP
unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans) .
These three scan types (even more are possible with the --scanflags
option described in the next section) exploit a subtle loophole in
the TCP RFC[8] to differentiate between open and closed ports. Page
65 of RFC 793 says that "if the [destination] port state is CLOSED
.... an incoming segment not containing a RST causes a RST to be
sent in response." Then the next page discusses packets sent to
open ports without the SYN, RST, or ACK bits set, stating that:
"you are unlikely to get here, but if you do, drop the segment, and
return."
When scanning systems compliant with this RFC text, any packet not
containing SYN, RST, or ACK bits will result in a returned RST if
the port is closed and no response at all if the port is open. As
long as none of those three bits are included, any combination of
the other three (FIN, PSH, and URG) are OK. Nmap exploits this with
three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like a
Christmas tree.
These three scan types are exactly the same in behavior except for
the TCP flags set in probe packets. If a RST packet is received,
the port is considered closed, while no response means it is
open|filtered. The port is marked filtered if an ICMP unreachable
error (type 3, code 1, 2, 3, 9, 10, or 13) is received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don't count on this though—most
modern IDS products can be configured to detect them. The big
downside is that not all systems follow RFC 793 to the letter. A
number of systems send RST responses to the probes regardless of
whether the port is open or not. This causes all of the ports to be
labeled closed. Major operating systems that do this are Microsoft
Windows, many Cisco devices, BSDI, and IBM OS/400. This scan does
work against most Unix-based systems though. Another downside of
these scans is that they can't distinguish open ports from certain
filtered ones, leaving you with the response open|filtered.
-sA (TCP ACK scan) .
This scan is different than the others discussed so far in that it
never determines open (or even open|filtered) ports. It is used to
map out firewall rulesets, determining whether they are stateful or
not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you use
--scanflags). When scanning unfiltered systems, open and closed
ports will both return a RST packet. Nmap then labels them as
unfiltered, meaning that they are reachable by the ACK packet, but
whether they are open or closed is undetermined. Ports that don't
respond, or send certain ICMP error messages back (type 3, code 1,
2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan) .
Window scan is exactly the same as ACK scan except that it exploits
an implementation detail of certain systems to differentiate open
ports from closed ones, rather than always printing unfiltered when
a RST is returned. It does this by examining the TCP Window field
of the RST packets returned. On some systems, open ports use a
positive window size (even for RST packets) while closed ones have
a zero window. So instead of always listing a port as unfiltered
when it receives a RST back, Window scan lists the port as open or
closed if the TCP Window value in that reset is positive or zero,
respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it. Systems
that don't support it will usually return all ports closed. Of
course, it is possible that the machine really has no open ports.
If most scanned ports are closed but a few common port numbers
(such as 22, 25, 53) are filtered, the system is most likely
susceptible. Occasionally, systems will even show the exact
opposite behavior. If your scan shows 1,000 open ports and three
closed or filtered ports, then those three may very well be the
truly open ones.
-sM (TCP Maimon scan) .
The Maimon scan is named after its discoverer, Uriel Maimon.. He
described the technique in Phrack Magazine issue #49 (November
1996).. Nmap, which included this technique, was released two
issues later. This technique is exactly the same as NULL, FIN, and
Xmas scans, except that the probe is FIN/ACK. According to RFC
793[8] (TCP), a RST packet should be generated in response to such
a probe whether the port is open or closed. However, Uriel noticed
that many BSD-derived systems simply drop the packet if the port is
open.
--scanflags (Custom TCP scan) .
Truly advanced Nmap users need not limit themselves to the canned
scan types offered. The --scanflags option allows you to design
your own scan by specifying arbitrary TCP flags.. Let your
creative juices flow, while evading intrusion detection systems.
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN. For
example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it's not very useful for scanning. The order these are specified in
is irrelevant.
In addition to specifying the desired flags, you can specify a TCP
scan type (such as -sA or -sF). That base type tells Nmap how to
interpret responses. For example, a SYN scan considers no-response
to indicate a filtered port, while a FIN scan treats the same as
open|filtered. Nmap will behave the same way it does for the base
scan type, except that it will use the TCP flags you specify
instead. If you don't specify a base type, SYN scan is used.
-sZ (SCTP COOKIE ECHO scan) .
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but send
an ABORT if the port is closed. The advantage of this scan type is
that it is not as obvious a port scan than an INIT scan. Also,
there may be non-stateful firewall rulesets blocking INIT chunks,
but not COOKIE ECHO chunks. Don't be fooled into thinking that this
will make a port scan invisible; a good IDS will be able to detect
SCTP COOKIE ECHO scans too. The downside is that SCTP COOKIE ECHO
scans cannot differentiate between open and filtered ports, leaving
you with the state open|filtered in both cases.
-sI zombie host[:probeport] (idle scan) .
This advanced scan method allows for a truly blind TCP port scan of
the target (meaning no packets are sent to the target from your
real IP address). Instead, a unique side-channel attack exploits
predictable IP fragmentation ID sequence generation on the zombie
host to glean information about the open ports on the target. IDS
systems will display the scan as coming from the zombie machine you
specify (which must be up and meet certain criteria). This
fascinating scan type is too complex to fully describe in this
reference guide, so I wrote and posted an informal paper with full
details at http://nmap.org/book/idlescan.html.
Besides being extraordinarily stealthy (due to its blind nature),
this scan type permits mapping out IP-based trust relationships
between machines. The port listing shows open ports from the
perspective of the zombie host. So you can try scanning a target
using various zombies that you think might be trusted. (via
router/packet filter rules).
You can add a colon followed by a port number to the zombie host if
you wish to probe a particular port on the zombie for IP ID
changes. Otherwise Nmap will use the port it uses by default for
TCP pings (80).
-sO (IP protocol scan) .
IP protocol scan allows you to determine which IP protocols (TCP,
ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol
numbers rather than TCP or UDP port numbers. Yet it still uses the
-p option to select scanned protocol numbers, reports its results
within the normal port table format, and even uses the same
underlying scan engine as the true port scanning methods. So it is
close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates
the power of open-source software. While the fundamental idea is
pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger. conceived the idea, wrote an excellent patch
implementing it, and sent it to the announce mailing list. (then
called nmap-hackers).. I incorporated that patch into the Nmap
tree and released a new version the next day. Few pieces of
commercial software have users enthusiastic enough to design and
contribute their own improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of
iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol
field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The exceptions are
TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for those
is included since some systems won't send them otherwise and
because Nmap already has functions to create them. Instead of
watching for ICMP port unreachable messages, protocol scan is on
the lookout for ICMP protocol unreachable messages. If Nmap
receives any response in any protocol from the target host, Nmap
marks that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed Other
ICMP unreachable errors (type 3, code 1, 3, 9, 10, or 13) cause the
protocol to be marked filtered (though they prove that ICMP is open
at the same time). If no response is received after
retransmissions, the protocol is marked open|filtered
-b FTP relay host (FTP bounce scan) .
An interesting feature of the FTP protocol (RFC 959[9]) is support
for so-called proxy FTP connections. This allows a user to connect
to one FTP server, then ask that files be sent to a third-party
server. Such a feature is ripe for abuse on many levels, so most
servers have ceased supporting it. One of the abuses this feature
allows is causing the FTP server to port scan other hosts. Simply
ask the FTP server to send a file to each interesting port of a
target host in turn. The error message will describe whether the
port is open or not. This is a good way to bypass firewalls because
organizational FTP servers are often placed where they have more
access to other internal hosts than any old Internet host would.
Nmap supports FTP bounce scan with the -b option. It takes an
argument of the form username:password@server:port. Server is the
name or IP address of a vulnerable FTP server. As with a normal
URL, you may omit username:password, in which case anonymous login
credentials (user: anonymous password:-wwwuser@) are used. The port
number (and preceding colon) may be omitted as well, in which case
the default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released,
but has largely been fixed. Vulnerable servers are still around, so
it is worth trying when all else fails. If bypassing a firewall is
your goal, scan the target network for port 21 (or even for any FTP
services if you scan all ports with version detection) and use the
ftp-bounce. NSE script. Nmap will tell you whether the host is
vulnerable or not. If you are just trying to cover your tracks, you
don't need to (and, in fact, shouldn't) limit yourself to hosts on
the target network. Before you go scanning random Internet
addresses for vulnerable FTP servers, consider that sysadmins may
not appreciate you abusing their servers in this way.
PORT SPECIFICATION AND SCAN ORDER
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans the most
common 1,000 ports for each protocol.
-p port ranges (Only scan specified ports) .
This option specifies which ports you want to scan and overrides
the default. Individual port numbers are OK, as are ranges
separated by a hyphen (e.g. 1-1023). The beginning and/or end
values of a range may be omitted, causing Nmap to use 1 and 65535,
respectively. So you can specify -p- to scan ports from 1 through
65535. Scanning port zero. is allowed if you specify it
explicitly. For IP protocol scanning (-sO), this option specifies
the protocol numbers you wish to scan for (0–255).
When scanning a combination of protocols (e.g. TCP and UDP), you
can specify a particular protocol by preceding the port numbers by
T: for TCP, U: for UDP, S: for SCTP, or P: for IP Protocol. The
qualifier lasts until you specify another qualifier. For example,
the argument -p U:53,111,137,T:21-25,80,139,8080 would scan UDP
ports 53, 111,and 137, as well as the listed TCP ports. Note that
to scan both UDP and TCP, you have to specify -sU and at least one
TCP scan type (such as -sS, -sF, or -sT). If no protocol qualifier
is given, the port numbers are added to all protocol lists. Ports
can also be specified by name according to what the port is
referred to in the nmap-services. You can even use the wildcards *
and ? with the names. For example, to scan FTP and all ports whose
names begin with "http", use -p ftp,http*. Be careful about shell
expansions and quote the argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For example,
the following will scan all ports in nmap-services equal to or
below 1024: -p [-1024]. Be careful with shell expansions and quote
the argument to -p if unsure.
-F (Fast (limited port) scan) .
Specifies that you wish to scan fewer ports than the default.
Normally Nmap scans the most common 1,000 ports for each scanned
protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in
order to know which ports are the most common. If port frequency
information isn't available, perhaps because of the use of a custom
nmap-services file, Nmap scans all named ports plus ports 1-1024.
In that case, -F means to scan only ports that are named in the
services file.
-r (Don't randomize ports) .
By default, Nmap randomizes the scanned port order (except that
certain commonly accessible ports are moved near the beginning for
efficiency reasons). This randomization is normally desirable, but
you can specify -r for sequential (sorted from lowest to highest)
port scanning instead.
--port-ratio ratio<decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than the
one given. ratio must be between 0.0 and 1.1.
--top-ports n
Scans the n highest-ratio ports found in nmap-services file. n
must be 1 or greater.
SERVICE AND VERSION DETECTION
Point Nmap at a remote machine and it might tell you that ports 25/tcp,
80/tcp, and 53/udp are open. Using its nmap-services. database of
about 2,200 well-known services,. Nmap would report that those ports
probably correspond to a mail server (SMTP), web server (HTTP), and
name server (DNS) respectively. This lookup is usually accurate—the
vast majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can
and do run services on strange ports..
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories) of
your companies or clients, you really want to know which mail and DNS
servers and versions are running. Having an accurate version number
helps dramatically in determining which exploits a server is vulnerable
to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes. database
contains probes for querying various services and match expressions to
recognize and parse responses. Nmap tries to determine the service
protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g. ISC
BIND, Apache httpd, Solaris telnetd), the version number, hostname,
device type (e.g. printer, router), the OS family (e.g. Windows,
Linux). When possible, Nmap also gets the Common Platform Enumeration
(CPE). representation of this information. Sometimes miscellaneous
details like whether an X server is open to connections, the SSH
protocol version, or the KaZaA user name, are available. Of course,
most services don't provide all of this information. If Nmap was
compiled with OpenSSL support, it will connect to SSL servers to deduce
the service listening behind that encryption layer.. Some UDP ports
are left in the open|filtered state after a UDP port scan is unable to
determine whether the port is open or filtered. Version detection will
try to elicit a response from these ports (just as it does with open
ports), and change the state to open if it succeeds. open|filtered TCP
ports are treated the same way. Note that the Nmap -A option enables
version detection among other things. A paper documenting the
workings, usage, and customization of version detection is available at
http://nmap.org/book/vscan.html.
When RPC services are discovered, the Nmap RPC grinder. is
automatically used to determine the RPC program and version numbers. It
takes all the TCP/UDP ports detected as RPC and floods them with SunRPC
program NULL commands in an attempt to determine whether they are RPC
ports, and if so, what program and version number they serve up. Thus
you can effectively obtain the same info as rpcinfo -p even if the
target's portmapper is behind a firewall (or protected by TCP
wrappers). Decoys do not currently work with RPC scan..
When Nmap receives responses from a service but cannot match them to
its database, it prints out a special fingerprint and a URL for you to
submit if to if you know for sure what is running on the port. Please
take a couple minutes to make the submission so that your find can
benefit everyone. Thanks to these submissions, Nmap has about 6,500
pattern matches for more than 650 protocols such as SMTP, FTP, HTTP,
etc..
Version detection is enabled and controlled with the following options:
-sV (Version detection) .
Enables version detection, as discussed above. Alternatively, you
can use -A, which enables version detection among other things.
-sR. is an alias for -sV. Prior to March 2011, it was used to
active the RPC grinder separately from version detection, but now
these options are always combined.
--allports (Don't exclude any ports from version detection) .
By default, Nmap version detection skips TCP port 9100 because some
printers simply print anything sent to that port, leading to dozens
of pages of HTTP GET requests, binary SSL session requests, etc.
This behavior can be changed by modifying or removing the Exclude
directive in nmap-service-probes, or you can specify --allports to
scan all ports regardless of any Exclude directive.
--version-intensity intensity (Set version scan intensity) .
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between one and
nine. The lower-numbered probes are effective against a wide
variety of common services, while the higher-numbered ones are
rarely useful. The intensity level specifies which probes should be
applied. The higher the number, the more likely it is the service
will be correctly identified. However, high intensity scans take
longer. The intensity must be between 0 and 9.. The default is 7..
When a probe is registered to the target port via the
nmap-service-probesports directive, that probe is tried regardless
of intensity level. This ensures that the DNS probes will always be
attempted against any open port 53, the SSL probe will be done
against 443, etc.
--version-light (Enable light mode) .
This is a convenience alias for --version-intensity 2. This light
mode makes version scanning much faster, but it is slightly less
likely to identify services.
--version-all (Try every single probe) .
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
--version-trace (Trace version scan activity) .
This causes Nmap to print out extensive debugging info about what
version scanning is doing. It is a subset of what you get with
--packet-trace.
OS DETECTION
One of Nmap's best-known features is remote OS detection using TCP/IP
stack fingerprinting. Nmap sends a series of TCP and UDP packets to the
remote host and examines practically every bit in the responses. After
performing dozens of tests such as TCP ISN sampling, TCP options
support and ordering, IP ID sampling, and the initial window size
check, Nmap compares the results to its nmap-os-db. database of more
than 2,600 known OS fingerprints and prints out the OS details if there
is a match. Each fingerprint includes a freeform textual description of
the OS, and a classification which provides the vendor name (e.g. Sun),
underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type
(general purpose, router, switch, game console, etc). Most fingerprints
also have a Common Platform Enumeration (CPE). representation, like
cpe:/o:linux:linux_kernel:2.6.
If Nmap is unable to guess the OS of a machine, and conditions are good
(e.g. at least one open port and one closed port were found), Nmap will
provide a URL you can use to submit the fingerprint if you know (for
sure) the OS running on the machine. By doing this you contribute to
the pool of operating systems known to Nmap and thus it will be more
accurate for everyone.
OS detection enables some other tests which make use of information
that is gathered during the process anyway. One of these is TCP
Sequence Predictability Classification. This measures approximately how
hard it is to establish a forged TCP connection against the remote
host. It is useful for exploiting source-IP based trust relationships
(rlogin, firewall filters, etc) or for hiding the source of an attack.
This sort of spoofing is rarely performed any more, but many machines
are still vulnerable to it. The actual difficulty number is based on
statistical sampling and may fluctuate. It is generally better to use
the English classification such as "worthy challenge" or "trivial
joke". This is only reported in normal output in verbose (-v) mode.
When verbose mode is enabled along with -O, IP ID sequence generation
is also reported. Most machines are in the "incremental" class, which
means that they increment the ID field in the IP header for each packet
they send. This makes them vulnerable to several advanced information
gathering and spoofing attacks.
Another bit of extra information enabled by OS detection is a guess at
a target's uptime. This uses the TCP timestamp option (RFC 1323[10]) to
guess when a machine was last rebooted. The guess can be inaccurate due
to the timestamp counter not being initialized to zero or the counter
overflowing and wrapping around, so it is printed only in verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at http://nmap.org/book/osdetect.html.
OS detection is enabled and controlled with the following options:
-O (Enable OS detection) .
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets) .
OS detection is far more effective if at least one open and one
closed TCP port are found. Set this option and Nmap will not even
try OS detection against hosts that do not meet this criteria. This
can save substantial time, particularly on -Pn scans against many
hosts. It only matters when OS detection is requested with -O or
-A.
--osscan-guess; --fuzzy (Guess OS detection results) .
When Nmap is unable to detect a perfect OS match, it sometimes
offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these (equivalent)
options make Nmap guess more aggressively. Nmap will still tell you
when an imperfect match is printed and display its confidence level
(percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries against a
target) .
When Nmap performs OS detection against a target and fails to find
a perfect match, it usually repeats the attempt. By default, Nmap
tries five times if conditions are favorable for OS fingerprint
submission, and twice when conditions aren't so good. Specifying a
lower --max-os-tries value (such as 1) speeds Nmap up, though you
miss out on retries which could potentially identify the OS.
Alternatively, a high value may be set to allow even more retries
when conditions are favorable. This is rarely done, except to
generate better fingerprints for submission and integration into
the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE)
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and
flexible features. It allows users to write (and share) simple scripts
(using the Lua programming language[11],
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one or
more categories. Currently defined categories are auth, broadcast,
default. discovery, dos, exploit, external, fuzzer, intrusive,
malware, safe, version, and vuln. These are all described at
http://nmap.org/book/nse-usage.html#nse-categories.
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
The Nmap Scripting Engine is described in detail at
http://nmap.org/book/nse.html and is controlled by the following
options:
-sC .
Performs a script scan using the default set of scripts. It is
equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a
target network without permission.
--script filename|category|directory|expression[,...] .
Runs a script scan using the comma-separated list of filenames,
script categories, and directories. Each element in the list may
also be a Boolean expression describing a more complex set of
scripts. Each element is interpreted first as an expression, then
as a category, and finally as a file or directory name.
There are two special features for advanced users only. One is to
prefix script names and expressions with + to force them to run
even if they normally wouldn't (e.g. the relevant service wasn't
detected on the target port). The other is that the argument all
may be used to specify every script in Nmap's database. Be cautious
with this because NSE contains dangerous scripts such as exploits,
brute force authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute. Absolute
names are used directly. Relative paths are looked for in the
scripts of each of the following places until found: --datadir
$NMAPDIR.
~/.nmap (not searched on Windows).
HOME\AppData\Roaming\nmap (only on Windows).
the directory containing the nmap executable
the directory containing the nmap executable, followed by
../share/nmap
NMAPDATADIR.
the current directory.
When a directory name is given, Nmap loads every file in the directory
whose name ends with .nse. All other files are ignored and directories
are not searched recursively. When a filename is given, it does not
have to have the .nse extension; it will be added automatically if
necessary. Nmap scripts are stored in a scripts subdirectory of the
Nmap data directory by default (see
http://nmap.org/book/data-files.html). For efficiency, scripts are
indexed in a database stored in scripts/script.db,. which lists the
category or categories in which each script belongs. When referring to
scripts from script.db by name, you can use a shell-style ‘*’ wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as http-auth
and http-open-proxy. The argument to --script had to be in quotes
to protect the wildcard from the shell.
More complicated script selection can be done using the and, or, and
not operators to build Boolean expressions. The operators have the same
precedence[12] as in Lua: not is the highest, followed by and and then
or. You can alter precedence by using parentheses. Because expressions
contain space characters it is necessary to quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script "default,safe". It
loads all scripts that are in the default category or the safe
category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe
categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories, except
for those whose names start with http-.
--script-args n1=v1,n2={n3=v3},n4={v4,v5} .
Lets you provide arguments to NSE scripts. Arguments are a
comma-separated list of name=value pairs. Names and values may be
strings not containing whitespace or the characters ‘{’, ‘}’, ‘=’,
or ‘,’. To include one of these characters in a string, enclose the
string in single or double quotes. Within a quoted string, ‘\’
escapes a quote. A backslash is only used to escape quotation marks
in this special case; in all other cases a backslash is interpreted
literally. Values may also be tables enclosed in {}, just as in
Lua. A table may contain simple string values or more name-value
pairs, including nested tables. Many scripts qualify their
arguments with the script name, as in xmpp-info.server_name. You
may use that full qualified version to affect just the specified
script, or you may pass the unqualified version (server_name in
this case) to affect all scripts using that argument name. A script
will first check for its fully qualified argument name (the name
specified in its documentation) before it accepts an unqualified
argument name. A complex example of script arguments is
--script-args
'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'.
The online NSE Documentation Portal at http://nmap.org/nsedoc/
lists the arguments that each script accepts.
--script-args-file filename .
Lets you load arguments to NSE scripts from a file. Any arguments
on the command line supersede ones in the file. The file can be an
absolute path, or a path relative to Nmap's usual search path
(NMAPDIR, etc.) Arguments can be comma-separated or
newline-separated, but otherwise follow the same rules as for
--script-args, without requiring special quoting and escaping,
since they are not parsed by the shell.
--script-help filename|category|directory|expression|all[,...] .
Shows help about scripts. For each script matching the given
specification, Nmap prints the script name, its categories, and its
description. The specifications are the same as those accepted by
--script; so for example if you want help about the ftp-anon
script, you would run nmap --script-help ftp-anon. In addition to
getting help for individual scripts, you can use this as a preview
of what scripts will be run for a specification, for example with
nmap --script-help default.
--script-trace .
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source, the
target and the transmitted data. If more than 5% of all transmitted
data is not printable, then the trace output is in a hex dump
format. Specifying --packet-trace enables script tracing too.
--script-updatedb .
This option updates the script database found in scripts/script.db
which is used by Nmap to determine the available default scripts
and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts
directory or if you have changed the categories of any script. This
option is generally used by itself: nmap --script-updatedb.
TIMING AND PERFORMANCE
One of my highest Nmap development priorities has always been
performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to blink,
but adds up when you are scanning hundreds or thousands of hosts.
Moreover, certain scan options such as UDP scanning and version
detection can increase scan times substantially. So can certain
firewall configurations, particularly response rate limiting. While
Nmap utilizes parallelism and many advanced algorithms to accelerate
these scans, the user has ultimate control over how Nmap runs. Expert
users carefully craft Nmap commands to obtain only the information they
care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters can
also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in seconds by
default, though you can append ‘ms’, ‘s’, ‘m’, or ‘h’ to the value to
specify milliseconds, seconds, minutes, or hours. So the --host-timeout
arguments 900000ms, 900, 900s, and 15m all do the same thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel
scan group sizes) .
Nmap has the ability to port scan or version scan multiple hosts in
parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results can't
be provided until the whole group is finished. So if Nmap started
out with a group size of 50, the user would not receive any reports
(except for the updates offered in verbose mode) until the first 50
hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It
starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.
The exact default numbers depend on the options given. For
efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap
will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery phase
of a scan. This includes plain ping scans (-sn). Host discovery
always works in large groups of hosts to improve speed and
accuracy.
The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common choice
is 256 to scan a network in Class C sized chunks. For a scan with
many ports, exceeding that number is unlikely to help much. For
scans of just a few port numbers, host group sizes of 2048 or more
may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust probe
parallelization) .
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing ideal
parallelism based on network performance. If packets are being
dropped, Nmap slows down and allows fewer outstanding probes. The
ideal probe number slowly rises as the network proves itself
worthy. These options place minimum or maximum bounds on that
variable. By default, the ideal parallelism can drop to one if the
network proves unreliable and rise to several hundred in perfect
conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap's ability
to control parallelism dynamically based on network conditions. A
value of 10 might be reasonable, though I only adjust this value as
a last resort.
The --max-parallelism option is sometimes set to one to prevent
Nmap from sending more than one probe at a time to hosts. The
--scan-delay option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout
time (Adjust probe timeouts) .
Nmap maintains a running timeout value for determining how long it
will wait for a probe response before giving up or retransmitting
the probe. This is calculated based on the response times of
previous probes.
If the network latency shows itself to be significant and variable,
this timeout can grow to several seconds. It also starts at a
conservative (high) level and may stay that way for a while when
Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout than
the defaults can cut scan times significantly. This is particularly
true for pingless (-Pn) scans, and those against heavily filtered
networks. Don't get too aggressive though. The scan can end up
taking longer if you specify such a low value that many probes are
timing out and retransmitting while the response is in transit.
If all the hosts are on a local network, 100 milliseconds
(--max-rtt-timeout 100ms) is a reasonable aggressive value. If
routing is involved, ping a host on the network first with the ICMP
ping utility, or with a custom packet crafter such as Nping. that
is more likely to get through a firewall. Look at the maximum round
trip time out of ten packets or so. You might want to double that
for the --initial-rtt-timeout and triple or quadruple it for the
--max-rtt-timeout. I generally do not set the maximum RTT below
100 ms, no matter what the ping times are. Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be useful when
a network is so unreliable that even Nmap's default is too
aggressive. Since Nmap only reduces the timeout down to the minimum
when the network seems to be reliable, this need is unusual and
should be reported as a bug to the nmap-dev mailing list..
--max-retries numtries (Specify the maximum number of port scan probe
retransmissions) .
When Nmap receives no response to a port scan probe, it could mean
the port is filtered. Or maybe the probe or response was simply
lost on the network. It is also possible that the target host has
rate limiting enabled that temporarily blocked the response. So
Nmap tries again by retransmitting the initial probe. If Nmap
detects poor network reliability, it may try many more times before
giving up on a port. While this benefits accuracy, it also lengthen
scan times. When performance is critical, scans may be sped up by
limiting the number of retransmissions allowed. You can even
specify --max-retries 0 to prevent any retransmissions, though that
is only recommended for situations such as informal surveys where
occasional missed ports and hosts are acceptable.
The default (with no -T template) is to allow ten retransmissions.
If a network seems reliable and the target hosts aren't rate
limiting, Nmap usually only does one retransmission. So most target
scans aren't even affected by dropping --max-retries to a low value
such as three. Such values can substantially speed scans of slow
(rate limited) hosts. You usually lose some information when Nmap
gives up on ports early, though that may be preferable to letting
the --host-timeout expire and losing all information about the
target.
--host-timeout time (Give up on slow target hosts) .
Some hosts simply take a long time to scan. This may be due to
poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few
percent of the scanned hosts can eat up a majority of the scan
time. Sometimes it is best to cut your losses and skip those hosts
initially. Specify --host-timeout with the maximum amount of time
you are willing to wait. For example, specify 30m to ensure that
Nmap doesn't waste more than half an hour on a single host. Note
that Nmap may be scanning other hosts at the same time during that
half an hour, so it isn't a complete loss. A host that times out is
skipped. No port table, OS detection, or version detection results
are printed for that host.
--scan-delay time; --max-scan-delay time (Adjust delay between probes)
.
This option causes Nmap to wait at least the given amount of time
between each probe it sends to a given host. This is particularly
useful in the case of rate limiting.. Solaris machines (among many
others) will usually respond to UDP scan probe packets with only
one ICMP message per second. Any more than that sent by Nmap will
be wasteful. A --scan-delay of 1s will keep Nmap at that slow rate.
Nmap tries to detect rate limiting and adjust the scan delay
accordingly, but it doesn't hurt to specify it explicitly if you
already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate limiting,
the scan slows down dramatically. The --max-scan-delay option
specifies the largest delay that Nmap will allow. A low
--max-scan-delay can speed up Nmap, but it is risky. Setting this
value too low can lead to wasteful packet retransmissions and
possible missed ports when the target implements strict rate
limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS)..
--min-rate number; --max-rate number (Directly control the scanning
rate) .
Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to know
an appropriate scanning rate for a network, or you may have to
guarantee that a scan will be finished by a certain time. Or
perhaps you must keep Nmap from scanning too quickly. The
--min-rate and --max-rate options are designed for these
situations.
When the --min-rate option is given Nmap will do its best to send
packets as fast as or faster than the given rate. The argument is a
positive real number representing a packet rate in packets per
second. For example, specifying --min-rate 300 means that Nmap will
try to keep the sending rate at or above 300 packets per second.
Specifying a minimum rate does not keep Nmap from going faster if
conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given
maximum. Use --max-rate 100, for example, to limit sending to 100
packets per second on a fast network. Use --max-rate 0.1 for a slow
scan of one packet every ten seconds. Use --min-rate and --max-rate
together to keep the rate inside a certain range.
These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery
scans. Other features like OS detection implement their own timing.
There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is faster
than the fastest rate at which Nmap can send, which is dependent on
hardware. In this case Nmap will simply send packets as fast as
possible, but be aware that such high rates are likely to cause a
loss of accuracy. The second case is when Nmap has nothing to send,
for example at the end of a scan when the last probes have been
sent and Nmap is waiting for them to time out or be responded to.
It's normal to see the scanning rate drop at the end of a scan or
in between hostgroups. The sending rate may temporarily exceed the
maximum to make up for unpredictable delays, but on average the
rate will stay at or below the maximum.
Specifying a minimum rate should be done with care. Scanning faster
than a network can support may lead to a loss of accuracy. In some
cases, using a faster rate can make a scan take longer than it
would with a slower rate. This is because Nmap's adaptive
retransmission algorithms will detect the network congestion caused
by an excessive scanning rate and increase the number of
retransmissions in order to improve accuracy. So even though
packets are sent at a higher rate, more packets are sent overall.
Cap the number of retransmissions with the --max-retries option if
you need to set an upper limit on total scan time.
--defeat-rst-ratelimit .
Many hosts have long used rate limiting. to reduce the number of
ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell Nmap
to ignore those rate limits (for port scans such as SYN scan which
don't treat non-responsive ports as open) by specifying
--defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a
rate-limited RST response. With a SYN scan, the non-response
results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
closed and filtered ports isn't worth the extra time.
--nsock-engine epoll|kqueue|poll|select .
Enforce use of a given nsock IO multiplexing engine. Only the
select(2)-based fallback engine is guaranteed to be available on
your system. Engines are named after the name of the IO management
facility they leverage. Engines currenty implemented are epoll,
kqueue, poll, and select, but not all will be present on any
platform. Use nmap -V to see which engines are supported.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template) .
While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them
confusing. Moreover, choosing the appropriate values can sometimes
take more time than the scan you are trying to optimize. So Nmap
offers a simpler approach, with six timing templates. You can
specify them with the -T option and their number (0–5) or their
name. The template names are paranoid (0), sneaky (1), polite (2),
normal (3), aggressive (4), and insane (5). The first two are for
IDS evasion. Polite mode slows down the scan to use less bandwidth
and target machine resources. Normal mode is the default and so -T3
does nothing. Aggressive mode speeds scans up by making the
assumption that you are on a reasonably fast and reliable network.
Finally insane mode. assumes that you are on an extraordinarily
fast network or are willing to sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they wish
to be, while leaving Nmap to pick the exact timing values. The
templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
-T4. prohibits the dynamic scan delay from exceeding 10 ms for TCP
ports and -T5 caps that value at 5 ms. Templates can be used in
combination with fine-grained controls, and the fine-grained
controls will you specify will take precedence over the timing
template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that option
even when you add fine-grained controls so that you benefit from
those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because they
think it is less likely to crash hosts or because they consider
themselves to be polite in general. They often don't realize just
how slow -T polite. really is. Their scan may take ten times
longer than a default scan. Machine crashes and bandwidth problems
are rare with the default timing options (-T3) and so I normally
recommend that for cautious scanners. Omitting version detection is
far more effective than playing with timing values at reducing
these problems.
While -T0. and -T1. may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to set the
exact timing values you need rather than rely on the canned -T0 and
-T1 values.
The main effects of T0 are serializing the scan so only one port is
scanned at a time, and waiting five minutes between sending each
probe. T1 and T2 are similar but they only wait 15 seconds and 0.4
seconds, respectively, between probes. T3 is Nmap's default
behavior, which includes parallelization.. does the equivalent of
--max-rtt-timeout 1250ms --initial-rtt-timeout 500ms --max-retries
6 and sets the maximum TCP scan delay to 10 milliseconds. T5 does
the equivalent of --max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m as
well as setting the maximum TCP scan delay to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any two
nodes. This allows hosts to act as true peers, serving and retrieving
information from each other. People could access all of their home
systems from work, changing the climate control settings or unlocking
the doors for early guests. This vision of universal connectivity has
been stifled by address space shortages and security concerns. In the
early 1990s, organizations began deploying firewalls for the express
purpose of reducing connectivity. Huge networks were cordoned off from
the unfiltered Internet by application proxies, network address
translation, and packet filters. The unrestricted flow of information
gave way to tight regulation of approved communication channels and the
content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling casual
reconnaissance is often a key goal of implementing the devices.
Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented defenses.
One of the best methods of understanding your network security posture
is to try to defeat it. Place yourself in the mind-set of an attacker,
and deploy techniques from this section against your networks. Launch
an FTP bounce scan, idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are increasingly
monitoring traffic with intrusion detection systems (IDS). All of the
major IDSs ship with rules designed to detect Nmap scans because scans
are sometimes a precursor to attacks. Many of these products have
recently morphed into intrusion prevention systems (IPS). that
actively block traffic deemed malicious. Unfortunately for network
administrators and IDS vendors, reliably detecting bad intentions by
analyzing packet data is a tough problem. Attackers with patience,
skill, and the help of certain Nmap options can usually pass by IDSs
undetected. Meanwhile, administrators must cope with large numbers of
false positive results where innocent activity is misdiagnosed and
alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is that
these methods would still be used by attackers, who would just find
other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting
firewalls and IDS systems. It takes skill and experience. A tutorial is
beyond the scope of this reference guide, which only lists the relevant
options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU) .
The -f option causes the requested scan (including ping scans) to
use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters,
intrusion detection systems, and other annoyances to detect what
you are doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named Sniffit
segmentation faulted immediately upon receiving the first fragment.
Specify this option once, and Nmap splits the packets into eight
bytes or less after the IP header. So a 20-byte TCP header would be
split into three packets. Two with eight bytes of the TCP header,
and one with the final four. Of course each fragment also has an IP
header. Specify -f again to use 16 bytes per fragment (reducing the
number of fragments).. Or you can specify your own offset size
with the --mtu option. Don't also specify -f if you use --mtu. The
offset must be a multiple of eight. While fragmented packets won't
get by packet filters and firewalls that queue all IP fragments,
such as the CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel,
some networks can't afford the performance hit this causes and thus
leave it disabled. Others can't enable this because fragments may
take different routes into their networks. Some source systems
defragment outgoing packets in the kernel. Linux with the iptables.
connection tracking module is one such example. Do a scan while a
sniffer such as Wireshark. is running to ensure that sent packets
are fragmented. If your host OS is causing problems, try the
--send-eth. option to bypass the IP layer and send raw ethernet
frames.
Fragmentation is only supported for Nmap's raw packet features,
which includes TCP and UDP port scans (except connect scan and FTP
bounce scan) and OS detection. Features such as version detection
and the Nmap Scripting Engine generally don't support fragmentation
because they rely on your host's TCP stack to communicate with
target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys) .
Causes a decoy scan to be performed, which makes it appear to the
remote host that the host(s) you specify as decoys are scanning the
target network too. Thus their IDS might report 5–10 port scans
from unique IP addresses, but they won't know which IP was scanning
them and which were innocent decoys. While this can be defeated
through router path tracing, response-dropping, and other active
mechanisms, it is generally an effective technique for hiding your
IP address.
Separate each decoy host with commas, and you can optionally use
ME. as one of the decoys to represent the position for your real
IP address. If you put ME in the sixth position or later, some
common port scan detectors (such as Solar Designer's. excellent
Scanlogd). are unlikely to show your IP address at all. If you
don't use ME, Nmap will put you in a random position. You can also
use RND. to generate a random, non-reserved IP address, or
RND:number to generate number addresses.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy to
determine which host is scanning if only one is actually up on the
network. You might want to use IP addresses instead of names (so
the decoy networks don't see you in their nameserver logs).
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase. Decoys
are also used during remote OS detection (-O). Decoys do not work
with version detection or TCP connect scan. When a scan delay is in
effect, the delay is enforced between each batch of spoofed probes,
not between each individual probe. Because decoys are sent as a
batch all at once, they may temporarily violate congestion control
limits.
It is worth noting that using too many decoys may slow your scan
and potentially even make it less accurate. Also, some ISPs will
filter out your spoofed packets, but many do not restrict spoofed
IP packets at all.
-S IP_Address (Spoof source address) .
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish to
send packets through.
Another possible use of this flag is to spoof the scan to make the
targets think that someone else is scanning them. Imagine a company
being repeatedly port scanned by a competitor! The -e option and
-Pn are generally required for this sort of usage. Note that you
usually won't receive reply packets back (they will be addressed to
the IP you are spoofing), so Nmap won't produce useful reports.
-e interface (Use specified interface) .
Tells Nmap what interface to send and receive packets on. Nmap
should be able to detect this automatically, but it will tell you
if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number) .
One surprisingly common misconfiguration is to trust traffic based
only on the source port number. It is easy to understand how this
comes about. An administrator will set up a shiny new firewall,
only to be flooded with complaints from ungrateful users whose
applications stopped working. In particular, DNS may be broken
because the UDP DNS replies from external servers can no longer
enter the network. FTP is another common example. In active FTP
transfers, the remote server tries to establish a connection back
to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active FTP from port 20,
many administrators have fallen into the trap of simply allowing
incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other
cases, administrators consider this a short-term stop-gap measure
until they can implement a more secure solution. Then they forget
the security upgrade.
Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these insecure
rules. Even Microsoft has been guilty. The IPsec filters that
shipped with Windows 2000 and Windows XP contain an implicit rule
that allows all TCP or UDP traffic from port 88 (Kerberos). In
another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the
source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are equivalent)
to exploit these weaknesses. Simply provide a port number and Nmap
will send packets from that port where possible. Most scanning
operations that use raw sockets, including SYN and UDP scans,
support the option completely. The option notably doesn't have an
effect for any operations that use normal operating system sockets,
including DNS requests, TCP connect scan,. version detection, and
script scanning. Setting the source port also doesn't work for OS
detection, because Nmap must use different port numbers for certain
OS detection tests to work properly.
--data-length number (Append random data to sent packets) .
Normally Nmap sends minimalist packets containing only a header. So
its TCP packets are generally 40 bytes and ICMP echo requests are
just 28. Some UDP ports. and IP protocols. get a custom payload
by default. This option tells Nmap to append the given number of
random bytes to most of the packets it sends, and not to use any
protocol-specific payloads. (Use --data-length 0 for no random or
protocol-specific payloads.. OS detection (-O) packets are not
affected. because accuracy there requires probe consistency, but
most pinging and portscan packets support this. It slows things
down a little, but can make a scan slightly less conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string
(Send packets with specified ip options) .
The IP protocol[13] offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options are
rarely seen due to practicality and security concerns. In fact,
many Internet routers block the most dangerous options such as
source routing. Yet options can still be useful in some cases for
determining and manipulating the network route to target machines.
For example, you may be able to use the record route option to
determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being
dropped by a certain firewall, you may be able to specify a
different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with \x then the two digits. You may repeat certain characters by
following them with an asterisk and then the number of times you
wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a hex
string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,.
record-timestamp,. or both options together, respectively. Loose
or strict source routing. may be specified with an L or S followed
by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of using
IP options with Nmap, see http://seclists.org/nmap-dev/2006/q3/52.
--ttl value (Set IP time-to-live field) .
Sets the IPv4 time-to-live field in sent packets to the given
value.
--randomize-hosts (Randomize target host order) .
Tells Nmap to shuffle each group of up to 16384 hosts before it
scans them. This can make the scans less obvious to various network
monitoring systems, especially when you combine it with slow timing
options. If you want to randomize over larger group sizes, increase
PING_GROUP_SZ. in nmap.h. and recompile. An alternative solution
is to generate the target IP list with a list scan (-sL -n -oN
filename), randomize it with a Perl script, then provide the whole
list to Nmap with -iL..
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address) .
Asks Nmap to use the given MAC address for all of the raw ethernet
frames it sends. This option implies --send-eth. to ensure that
Nmap actually sends ethernet-level packets. The MAC given can take
several formats. If it is simply the number 0, Nmap chooses a
completely random MAC address for the session. If the given string
is an even number of hex digits (with the pairs optionally
separated by a colon), Nmap will use those as the MAC. If fewer
than 12 hex digits are provided, Nmap fills in the remainder of the
six bytes with random values. If the argument isn't a zero or hex
string, Nmap looks through nmap-mac-prefixes to find a vendor name
containing the given string (it is case insensitive). If a match is
found, Nmap uses the vendor's OUI (three-byte prefix). and fills
out the remaining three bytes randomly. Valid --spoof-mac argument
examples are Apple, 0, 01:02:03:04:05:06, deadbeefcafe, 0020F2, and
Cisco. This option only affects raw packet scans such as SYN scan
or OS detection, not connection-oriented features such as version
detection or the Nmap Scripting Engine.
--badsum (Send packets with bogus TCP/UDP checksums) .
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets
sent to target hosts. Since virtually all host IP stacks properly
drop these packets, any responses received are likely coming from a
firewall or IDS that didn't bother to verify the checksum. For more
details on this technique, see http://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP checksums)
.
Asks Nmap to use the deprecated Adler32 algorithm for calculating
the SCTP checksum. If --adler32 is not given, CRC-32C (Castagnoli)
is used. RFC 2960[14] originally defined Adler32 as checksum
algorithm for SCTP; RFC 4960[7] later redefined the SCTP checksums
to use CRC-32C. Current SCTP implementations should be using
CRC-32C, but in order to elicit responses from old, legacy SCTP
implementations, it may be preferable to use Adler32.
OUTPUT
Any security tool is only as useful as the output it generates. Complex
tests and algorithms are of little value if they aren't presented in an
organized and comprehensible fashion. Given the number of ways Nmap is
used by people and other software, no single format can please
everyone. So Nmap offers several formats, including the interactive
mode for humans to read directly and XML for easy parsing by software.
In addition to offering different output formats, Nmap provides options
for controlling the verbosity of output as well as debugging messages.
Output types may be sent to standard output or to named files, which
Nmap can append to or clobber. Output files may also be used to resume
aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output,. and it is sent to standard output
(stdout).. There is also normal output,. which is similar to
interactive except that it displays less runtime information and
warnings since it is expected to be analyzed after the scan completes
rather than interactively.
XML output. is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output. which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt. for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each format
may only be specified once. For example, you may wish to save normal
output for your own review while saving XML of the same scan for
programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or two
describing the scan, placed in a directory named after the company I'm
scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn't specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes Nmap
to deactivate interactive output, and instead print results in the
format you specified to the standard output stream. So the command nmap
-oX - target will send only XML output to stdout.. Serious errors may
still be printed to the normal error stream, stderr..
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit the
flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal format
output files named G- and Xscan.xml respectively.
All of these arguments support strftime-like. conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as
in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and %D
is the same as %m%d%y. A % followed by any other character just yields
that character (%% gives you a percent symbol). So -oX 'scan-%T-%D.xml'
will use an XML file with a name in the form of scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to
output files rather than clobbering them. All of these options are
described below.
Nmap Output Formats
-oN filespec (normal output) .
Requests that normal output be directed to the given filename. As
discussed above, this differs slightly from interactive output.
-oX filespec (XML output) .
Requests that XML output be directed to the given filename. Nmap
includes a document type definition (DTD) which allows XML parsers
to validate Nmap XML output. While it is primarily intended for
programmatic use, it can also help humans interpret Nmap XML
output. The DTD defines the legal elements of the format, and often
enumerates the attributes and values they can take on. The latest
version is always available from
https://svn.nmap.org/nmap/docs/nmap.dtd.
XML offers a stable format that is easily parsed by software. Free
XML parsers are available for all major computer languages,
including C/C++, Perl, Python, and Java. People have even written
bindings for most of these languages to handle Nmap output and
execution specifically. Examples are Nmap::Scanner[15]. and
Nmap::Parser[16]. in Perl CPAN. In almost all cases that a
non-trivial application interfaces with Nmap, XML is the preferred
format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is simply
to load the XML output in a web browser such as Firefox or IE. By
default, this will only work on the machine you ran Nmap on (or a
similarly configured one) due to the hard-coded nmap.xsl filesystem
path. Use the --webxml or --stylesheet options to create portable
XML files that render as HTML on any web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT) .
Script kiddie output is like interactive output, except that it is
post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and
spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly
"helping them".
-oG filespec (grepable output) .
This output format is covered last because it is deprecated. The
XML output format is far more powerful, and is nearly as convenient
for experienced users. XML is a standard for which dozens of
excellent parsers are available, while grepable output is my own
simple hack. XML is extensible to support new Nmap features as they
are released, while I often must omit those features from grepable
output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be trivially
searched and parsed with standard Unix tools such as grep, awk,
cut, sed, diff, and Perl. Even I usually use it for one-off tests
done at the command line. Finding all the hosts with the SSH port
open or that are running Solaris takes only a simple grep to
identify the hosts, piped to an awk or cut command to print the
desired fields.
Grepable output consists of comments (lines starting with a pound
(#)). and target lines. A target line includes a combination of
six labeled fields, separated by tabs and followed with a colon.
The fields are Host, Ports, Protocols, Ignored State, OS, Seq
Index, IP ID, and Status.
The most important of these fields is generally Ports, which gives
details on each interesting port. It is a comma separated list of
port entries. Each port entry represents one interesting port, and
takes the form of seven slash (/) separated subfields. Those
subfields are: Port number, State, Protocol, Owner, Service, SunRPC
info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable output
format is available from
http://nmap.org/book/output-formats-grepable-output.html.
-oA basename (Output to all formats) .
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They are
stored in basename.nmap, basename.xml, and basename.gnmap,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level) .
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as
they are found and completion time estimates are provided when Nmap
thinks a scan will take more than a few minutes. Use it twice or
more for even greater verbosity: -vv, or give a verbosity level
directly, for example -v3..
Most changes only affect interactive output, and some also affect
normal and script kiddie output. The other output types are meant
to be processed by machines, so Nmap can give substantial detail by
default in those formats without fatiguing a human user. However,
there are a few changes in other modes where output size can be
reduced substantially by omitting some detail. For example, a
comment line in the grepable output that provides a list of all
ports scanned is only printed in verbose mode because it can be
quite long.
-d (Increase debugging level) .
When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (-v), debugging is enabled with a command-line
flag (-d) and the debug level can be increased by specifying it
multiple times,. as in -dd, or by setting a level directly. For
example, -d9 sets level nine. That is the highest effective level
and will produce thousands of lines unless you run a very simple
scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if
you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren't
always self-explanatory. You may get something like: Timeout vals:
srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987 rttvar:
14987 to: 100000. If you don't understand a line, your only
recourses are to ignore it, look it up in the source code, or
request help from the development list (nmap-dev).. Some lines are
self explanatory, but the messages become more obscure as the debug
level is increased.
--reason (Host and port state reasons) .
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type of
the packet that determined a port or hosts state. For example, A
RST packet from a closed port or an echo reply from an alive host.
The information Nmap can provide is determined by the type of scan
or ping. The SYN scan and SYN ping (-sS and -PS) are very detailed,
but the TCP connect scan (-sT) is limited by the implementation of
the connect system call. This feature is automatically enabled by
the debug option (-d). and the results are stored in XML log files
even if this option is not specified.
--stats-every time (Print periodic timing stats) .
Periodically prints a timing status message after each interval of
time. The time is a specification of the kind described in the
section called "TIMING AND PERFORMANCE"; so for example, use
--stats-every 10s to get a status update every 10 seconds. Updates
are printed to interactive output (the screen) and XML output.
--packet-trace (Trace packets and data sent and received) .
Causes Nmap to print a summary of every packet sent or received.
This is often used for debugging, but is also a valuable way for
new users to understand exactly what Nmap is doing under the
covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If you
only care about the goings on of the version detection subsystem,
use --version-trace instead. If you only care about script tracing,
specify --script-trace. With --packet-trace, you get all of the
above.
--open (Show only open (or possibly open) ports) .
Sometimes you only care about ports you can actually connect to
(open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk, and
Perl, but this feature was added due to overwhelming requests.
Specify --open to only see hosts with at least one open,
open|filtered, or unfiltered port, and only see ports in those