Section: User Commands (1)
nmap - Network exploration tool and security scanner
[Scan Type(s)] [Options] <host or net #1 ... [#N]>
is designed to allow system administrators and curious
individuals to scan large networks to determine which hosts
are up and what services they are offering.
supports a large number of scanning techniques such as: UDP, TCP
connect(), TCP SYN (half open), ftp proxy (bounce attack),
Reverse-ident, ICMP (ping sweep), FIN, ACK sweep, Xmas Tree, SYN
sweep, IP Protocol, and Null scan. See the
section for more details. nmap also offers a number of
advanced features such as remote OS detection via TCP/IP
fingerprinting, stealth scanning, dynamic delay and
retransmission calculations, parallel scanning, detection of
down hosts via parallel pings, decoy scanning, port
filtering detection, direct (non-portmapper) RPC scanning,
fragmentation scanning, and flexible target and port
Significant effort has been put into decent nmap performance
for non-root users. Unfortunately, many critical kernel
interfaces (such as raw sockets) require root privileges.
nmap should be run as root whenever possible (not setuid root,
The result of running nmap is usually a list of interesting
ports on the machine(s) being scanned (if any). Nmap always
gives the port's "well known" service name (if any), number,
state, and protocol. The state is either 'open',
'filtered', or 'unfiltered'. Open means that the target
machine will accept() connections on that port. Filtered
means that a firewall, filter, or other network obstacle is
covering the port and preventing nmap from determining
whether the port is open. Unfiltered means that the port is
known by nmap to be closed and no firewall/filter seems to
be interfering with nmap's attempts to determine this.
Unfiltered ports are the common case and are only shown when
most of the scanned ports are in the filtered state.
Depending on options used, nmap may also report the
following characteristics of the remote host: OS in use, TCP
sequencability, usernames running the programs which have
bound to each port, the DNS name, whether the host is a
smurf address, and a few others.
Options that make sense together can generally be combined.
Some options are specific to certain scan modes.
tries to catch and warn the user about psychotic or
unsupported option combinations.
If you are impatient, you can skip to the
section at the end, which demonstrates common usage. You
can also run
for a quick reference page listing all the options.
- SCAN TYPES
TCP connect() scan: This is the most basic form of TCP
scanning. The connect() system call provided by your
operating system is used to open a connection to every
interesting port on the machine. If the port is listening,
connect() will succeed, otherwise the port isn't
reachable. One strong advantage to this technique is that
you don't need any special privileges. Any user on most UNIX
boxes is free to use this call.
This sort of scan is easily detectable as target host logs
will show a bunch of connection and error messages for the
services which accept() the connection just to have it
TCP SYN scan: 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 you wait for a response. A
SYN|ACK indicates the port is listening. A RST is indicative
of a non-listener. If a SYN|ACK is received, a RST is
immediately sent to tear down the connection (actually our
OS kernel does this for us). The primary advantage to this
scanning technique is that fewer sites will log it.
Unfortunately you need root privileges to build these custom
- -sF -sX -sN
Stealth FIN, Xmas Tree, or Null scan modes: There are times
when even SYN scanning isn't clandestine enough. Some
firewalls and packet filters watch for SYNs to restricted
ports, and programs like Synlogger and Courtney are
available to detect these scans. These advanced scans, on
the other hand, may be able to pass through unmolested.
The idea is that closed ports are required to reply to your
probe packet with an RST, while open ports must ignore the
packets in question (see RFC 793 pp 64). The FIN scan uses
a bare (surprise) FIN packet as the probe, while the Xmas
tree scan turns on the FIN, URG, and PUSH flags. The Null
scan turns off all flags. Unfortunately Microsoft (like
usual) decided to completely ignore the standard and do
things their own way. Thus this scan type will not work
against systems running Windows95/NT. On the positive side,
this is a good way to distinguish between the two platforms.
If the scan finds open ports, you know the machine is not a
Windows box. If a -sF,-sX,or -sN scan shows all ports
closed, yet a SYN (-sS) scan shows ports being opened, you
are probably looking at a Windows box. This is less useful
now that nmap has proper OS detection built in. There are
also a few other systems that are broken in the same way
Windows is. They include Cisco, BSDI, HP/UX, MVS, and IRIX.
All of the above send resets from the open ports when they
should just drop the packet.
Ping scanning: Sometimes you only want to know which hosts
on a network are up. Nmap can do this by sending ICMP echo
request packets to every IP address on the networks you
specify. Hosts that respond are up. Unfortunately, some
sites such as microsoft.com block echo request packets.
Thus nmap can also send a TCP ack packet to (by default)
port 80. If we get an RST back, that machine is up. A
third technique involves sending a SYN packet and waiting
for a RST or a SYN/ACK. For non-root users, a connect()
method is used.
By default (for root users), nmap uses both the ICMP and ACK
techniques in parallel. You can change the
option described later.
Note that pinging is done by default anyway, and only hosts
that respond are scanned. Only use this option if you wish
to ping sweep
doing any actual port scans.
UDP scans: This method is used to determine which UDP (User
Datagram Protocol, RFC 768) ports are open on a host. The
technique is to send 0 byte udp packets to each port on the
target machine. If we receive an ICMP port unreachable
message, then the port is closed. Otherwise we assume it is
Some people think UDP scanning is pointless. I usually
remind them of the recent Solaris rcpbind hole. Rpcbind can
be found hiding on an undocumented UDP port somewhere above
32770. So it doesn't matter that 111 is blocked by the
firewall. But can you find which of the more than 30,000
high ports it is listening on? With a UDP scanner you can!
There is also the cDc Back Orifice backdoor program which
hides on a configurable UDP port on Windows machines. Not
to mention the many commonly vulnerable services that
utilize UDP such as snmp, tftp, NFS, etc.
Unfortunately UDP scanning is sometimes painfully slow since
most hosts implement a suggestion in RFC 1812 (section
188.8.131.52) of limiting the ICMP error message rate. For
example, the Linux kernel (in net/ipv4/icmp.h) limits
destination unreachable message generation to 80 per 4
seconds, with a 1/4 second penalty if that is exceeded.
Solaris has much more strict limits (about 2 messages per
second) and thus takes even longer to scan.
detects this rate limiting and slows down accordingly,
rather than flood the network with useless packets that will
be ignored by the target machine.
As is typical, Microsoft ignored the suggestion of the RFC
and does not seem to do any rate limiting at all on Win95
and NT machines. Thus we can scan all 65K ports of a
IP protocol scans: This method is used to determine which IP protocols
are supported on a host. The technique is to send raw IP packets
without any further protocol header to each specified protocol on the
target machine. If we receive an ICMP protocol unreachable message,
then the protocol is not in use. Otherwise we assume it is open.
Note that some hosts (AIX, HP-UX, Digital UNIX) and firewalls may not
send protocol unreachable messages. This causes all of the protocols
to appear "open".
Because the implemented technique is very similar to UDP port scanning,
ICMP rate limit might apply too. But the IP
protocol field has only 8 bits, so at most 256 protocols can be
probed which should be possible in reasonable time anyway.
- -sI <zombie host[:probeport]>
Idlescan: 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). I am planning to put a
more detailed explanation up at
http://www.insecure.org/nmap/nmap_documentation.html in the near
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). Obviously this is
crucial information when prioritizing attack targets. Otherwise, you
penetration testers might have to expend considerable resources "owning" an
intermediate system, only to find out that its IP isn't even trusted
by the target host/network you are ultimately after.
You can add a colon followed by a port number if you wish to probe
a particular port on the zombie host for IPID changes. Otherwise Nmap
will use the port it uses by default for "tcp pings".
ACK scan: This advanced method is usually used to map out
firewall rulesets. In particular, it can help determine
whether a firewall is stateful or just a simple packet
filter that blocks incoming SYN packets.
This scan type sends an ACK packet (with random looking
acknowledgement/sequence numbers) to the ports specified.
If a RST comes back, the ports is classified as
"unfiltered". If nothing comes back (or if an ICMP
unreachable is returned), the port is classified as
"filtered". Note that
usually doesn't print "unfiltered"
ports, so getting
ports shown in the output is usually a sign that all the
probes got through (and returned RSTs). This scan will
obviously never show ports in the "open" state.
Window scan: This advanced scan is very similar to the ACK
scan, except that it can sometimes detect open ports as well
as filtered/nonfiltered due to an anomaly in the TCP window
size reporting by some operating systems. Systems
vulnerable to this include at least some versions of AIX,
Amiga, BeOS, BSDI, Cray, Tru64 UNIX, DG/UX, OpenVMS, Digital
UNIX, FreeBSD, HP-UX, OS/2, IRIX, MacOS, NetBSD, OpenBSD,
OpenStep, QNX, Rhapsody, SunOS 4.X, Ultrix, VAX, and
VxWorks. See the nmap-hackers mailing list archive for a
RPC scan. This method works in combination with the various
port scan methods of Nmap. It takes all the TCP/UDP ports
found open and then 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
firewall (or protected by TCP wrappers). Decoys do not
currently work with RPC scan, at some point I may add decoy
support for UDP RPC scans.
List scan. This method simply generates and prints a list of
IPs/Names without actually pinging or port scanning them. DNS name
resolution will be performed unless you use -n.
- -b <ftp relay host>
FTP bounce attack: An interesting "feature" of the ftp
protocol (RFC 959) is support for "proxy" ftp
connections. In other words, I should be able to connect
from evil.com to the FTP server of target.com and request
that the server send a file ANYWHERE on the internet! Now
this may have worked well in 1985 when the RFC was
written. But in today's Internet, we can't have people
hijacking ftp servers and requesting that data be spit out
to arbitrary points on the internet. As *Hobbit* wrote back
in 1995, this protocol flaw "can be used to post virtually
untraceable mail and news, hammer on servers at various
sites, fill up disks, try to hop firewalls, and generally be
annoying and hard to track down at the same time." What we
will exploit this for is to (surprise, surprise) scan TCP
ports from a "proxy" ftp server. Thus you could connect to
an ftp server behind a firewall, and then scan ports that
are more likely to be blocked (139 is a good one). If the
ftp server allows reading from and writing to some directory
(such as /incoming), you can send arbitrary data to ports
that you do find open (nmap doesn't do this for you though).
The argument passed to the 'b' option is the host you want
to use as a proxy, in standard URL notation. The format is:
is optional. To determine what servers are vulnerable to
this attack, you can see my article in
51. And updated version is available at the
- GENERAL OPTIONS
None of these are required but some can be quite useful.
Do not try and ping hosts at all before scanning them. This
allows the scanning of networks that don't allow ICMP echo
requests (or responses) through their firewall.
microsoft.com is an example of such a network, and thus you
should always use
when portscanning microsoft.com.
Use TCP "ping" to determine what hosts are up. Instead of
sending ICMP echo request packets and waiting for a
response, we spew out TCP ACK packets throughout the target
network (or to a single machine) and then wait for responses
to trickle back. Hosts that are up should respond with a
RST. This option preserves the efficiency of only scanning
hosts that are up while still allowing you to scan
networks/hosts that block ping packets. For non root users,
we use connect(). To set the destination port of the probe
packets use -PT<port number>. The default port is 80, since
this port is often not filtered out.
This option uses SYN (connection request) packets instead of
ACK packets for root users. Hosts that are up should
respond with a RST (or, rarely, a SYN|ACK). You can set the
destination port in the same manner as -PT above.
This option uses a true ping (ICMP echo request) packet. It
finds hosts that are up and also looks for subnet-directed
broadcast addresses on your network. These are IP addresses
which are externally reachable and translate to a broadcast
of incomming IP packets to a subnet of computers. These
should be eliminated if found as they allow for numerous
denial of service attacks (Smurf is the most common).
Uses an ICMP timestamp request (code 13) packet to find listening hosts.
except uses a netmask request (ICMP code 17).
This is the default ping type. It uses both the ACK (
) and ICMP echo request (
) sweeps in parallel. This way you can get firewalls that filter
either one (but not both). The TCP probe destination port can be set
in the same manner as with -PT above.
This option activates remote host identification via TCP/IP
fingerprinting. In other words, it uses a bunch of
techniques to detect subtleties in the underlying operating
system network stack of the computers you are scanning. It
uses this information to create a 'fingerprint' which it
compares with its database of known OS fingerprints (the
nmap-os-fingerprints file) to decide what type of system you
If Nmap is unable to guess the OS of a machine, and conditions are
good (eg at least one open port), 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.
Note that if you leave an IP address on the form, the machine may be
scanned when we add the fingerprint (to validate that it works).
The -O option also enables several other tests. One is the "Uptime"
measurement, which uses the TCP timestamp option (RFC 1323) to guess
when a machine was last rebooted. This is only reported for machines
which provide this information.
Another test enabled by -O is TCP Sequence Predictability
Classification. This is a measure that describes approximately how
hard it is to establish a forged TCP connection against the remote
host. This is useful for exploiting source-IP based trust
relationships (rlogin, firewall filters, etc) or for hiding the source
of an attack. 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 with -v.
When verbose mode (-v) is on with -O, IPID 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.
This turns on TCP reverse ident scanning. As noted by Dave
Goldsmith in a 1996 Bugtraq post, the ident protocol (rfc
1413) allows for the disclosure of the username that owns
any process connected via TCP, even if that process didn't
initiate the connection. So you can, for example, connect to
the http port and then use identd to find out whether the
server is running as root. This can only be done with a full
TCP connection to the target port (i.e. the -sT scanning
is used, the remote host's identd is queried for each open
port found. Obviously this won't work if the host is not
This option causes the requested SYN, FIN, XMAS, or NULL
scan 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. My favorite sniffer segmentation faulted
immediately upon receiving the first 36-byte fragment. After
that comes a 24 byte one! While this method won't get by
packet filters and firewalls that queue all IP fragments
(like 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.
Note that I do not yet have this option working on all
systems. It works fine for my Linux, FreeBSD, and OpenBSD
boxes and some people have reported success with other *NIX
Verbose mode. This is a highly recommended option and it
gives out more information about what is going on. You can
use it twice for greater effect. Use
a couple of times if you really want to get crazy with
scrolling the screen!
This handy option display a quick reference screen of nmap
usage options. As you may have noticed, this man page is
not exactly a 'quick reference' :)
- -oN <logfilename>
This logs the results of your scans in a normal
form into the file you specify as an argument.
- -oX <logfilename>
This logs the results of your scans in
form into the file you specify as an argument. This allows programs
to easily capture and interpret Nmap results. You can give the
argument '-' (without quotes) to shoot output into stdout (for shell
pipelines, etc). In this case normal output will be suppressed.
Watch out for error messages if you use this (they will still go to
stderr). Also note that '-v' may cause some extra information to be
printed. The Document Type Definition (DTD) defining the XML output
structure is available at http://www.insecure.org/nmap/nmap.dtd .
- -oG <logfilename>
This logs the results of your scans in a
form into the file you specify as an argument. This simple format
provides all the information on one line (so you can easily grep for
port or OS information and see all the IPs. This used to be the
preferred mechanism for programs to interact with Nmap, but now we
recommend XML output (-oX instead). This simple format may not
contain as much information as the other formats. You can give the
argument '-' (without quotes) to shoot output into stdout (for shell
pipelines, etc). In this case normal output will be suppressed.
Watch out for error messages if you use this (they will still go to
stderr). Also note that '-v' will cause some extra information to
- -oA <basefilename>
This tells Nmap to log in ALL the majore formats (normal, grepable,
and XML). You give a base for the filename, and the output files will
be base.nmap, base.gnmap, and base.xml.
- -oS <logfilename>
thIs l0gz th3 r3suLtS of YouR ScanZ iN a
f0rM iNto THe fiL3 U sPecfy 4s an arGuMEnT! U kAn gIv3
the 4rgument '-' (wItHOUt qUOteZ) to sh00t output iNT0
- --resume <logfilename>
A network scan that is cancelled due to control-C, network
outage, etc. can be resumed using this option. The
logfilename must be either a normal (-oN) or machine
parsable (-oM) log from the aborted scan. No other options
can be given (they will be the same as the aborted scan).
Nmap will start on the machine after the last one
successfully scanned in the log file.
Tells Nmap to append scan results to any output files you have specified
rather than overwriting those files.
- -iL <inputfilename>
Reads target specifications from the file specified RATHER
than from the command line. The file should contain a list
of host or network expressions seperated by spaces, tabs, or
newlines. Use a hyphen (-) as
if you want nmap to read host expressions from
stdin (like at the end of a pipe). See the section
for more information on the expressions you fill the file with.
This option tells Nmap to generate its own hosts to scan by
simply picking random numbers :). It will never end. This
can be useful for statistical sampling of the Internet to
estimate various things. If you are ever really bored, try
nmap -sS -iR -p 80
to find some web servers to look at.
- -p <port ranges>
This option specifies what ports you want to specify. For
example '-p 23' will only try port 23 of the target host(s).
'-p 20-30,139,60000-' scans ports between 20 and 30, port
139, and all ports greater than 60000. The default is to
scan all ports between 1 and 1024 as well as any ports
listed in the services file which comes with nmap. For IP protocol
scanning (-sO), this specifies the protocol number you wish to scan
When scanning both TCP and UDP ports, you can specify a particular
protocol by preceding the port numbers by "T:" or "U:". 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 &
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.
- -F Fast scan mode.
Specifies that you only wish to scan for ports listed in the services
file which comes with nmap (or the protocols file for -sO). This is
obviously much faster than scanning all 65535 ports on a host.
- -D <decoy1 [,decoy2][,ME],...>
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 extremely 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 you
want your IP address to be used. If your put 'ME' in the
6th position or later, some common port scan detectors (such
as Solar Designer's excellent scanlogd) are unlikeley to
show your IP address at all. If you don't use 'ME', nmap
will put you in a random position.
Note that the hosts you use as decoys should be up or you
might accidently 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).
Also note that some (stupid) "port scan detectors" will
firewall/deny routing to hosts that attempt port scans.
Thus you might inadvertantly cause the machine you scan to
lose connectivity with the decoy machines you are using.
This could cause the target machines major problems if the
decoy is, say, its internet gateway or even "localhost".
Thus you might want to be careful of this option. The real
moral of the story is that detectors of spoofable port scans
should not take action against the machine that seems like
it is port scanning them. It could just be a decoy!
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 (
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, although many
(currently most) do not restrict spoofed IP packets at all.
- -S <IP_Address>
In some circumstances,
may not be able to determine your source address (
will tell you if this is the case). In this situation, use
-S with your IP address (of the interface you wish to send
Another possible use of this flag is to spoof the scan to
make the targets think that
is scanning them. Imagine a company being repeatedly port
scanned by a competitor! This is not a supported usage (or
the main purpose) of this flag. I just think it raises an
interesting possibility that people should be aware of
before they go accusing others of port scanning them.
would generally be required for this sort of usage.
- -e <interface>
Tells nmap what interface to send and receive packets on.
Nmap should be able to detect this but it will tell you if
- -g <portnumber>
Sets the source port number used in scans. Many naive
firewall and packet filter installations make an exception
in their ruleset to allow DNS (53) or FTP-DATA (20) packets
to come through and establish a connection. Obviously this
completely subverts the security advantages of the firewall
since intruders can just masquerade as FTP or DNS by
modifying their source port. Obviously for a UDP scan you
should try 53 first and TCP scans should try 20 before 53.
Note that this is only a request -- nmap will honor it only
if and when it is able to. For example, you can't do TCP
ISN sampling all from one host:port to one host:port, so
nmap changes the source port even if you used -g.
Be aware that there is a small performance penalty on some
scans for using this option, because I sometimes store
useful information in the source port number.
- --data_length <number>
Normally Nmap sends minimalistic packets that only contain a header.
So its TCP packets are generally 40 bytes and ICMP echo requests are
just 28. This option tells Nmap to append the given number of
zero-filled bytes to most of the packets it sends. OS detection (-O)
packets are not affected, but most pinging and portscan packets are.
This slows things down, but can be slightly less conspicuous.
Tells Nmap to
do reverse DNS resolution on the active IP addresses it finds. Since DNS is often slow, this can help speed things up.
Tells Nmap to
do reverse DNS resolution on the target IP addresses. Normally
this is only done when a machine is found to be alive.
to randomize the order in which ports are scanned.
Tells Nmap to shuffle each group of up to 2048 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 (see below).
- -M <max sockets>
Sets the maximum number of sockets that will be used in
parallel for a TCP connect() scan (the default). This is
useful to slow down the scan a little bit and avoid crashing
remote machines. Another approach is to use -sS, which is
generally easier for machines to handle.
- TIMING OPTIONS
Generally Nmap does a good job at adjusting for Network
characteristics at runtime and scanning as fast as possible
while minimizing that chances of hosts/ports going
undetected. However, there are same cases where Nmap's
default timing policy may not meet your objectives. The
following options provide a fine level of control over the
- -T <Paranoid|Sneaky|Polite|Normal|Aggressive|Insane>
These are canned timing policies for conveniently expressing
your priorities to Nmap.
slowly in the hopes of avoiding detection by IDS systems.
It serializes all scans (no parallel scanning) and generally
waits at least 5 minutes between sending packets.
is similar, except it
only waits 15 seconds between sending packets.
is meant to ease load on the network and reduce the chances
of crashing machines. It serializes the probes and waits
0.4 seconds between them.
is the default Nmap behaviour, which tries to run as quickly
as possible without overloading the network or missing
mode adds a 5 minute timeout per host and it never waits
more than 1.25 seconds for probe responses.
is only suitable for very fast networks or where you don't
mind losing some information. It times out hosts in 75
seconds and only waits 0.3 seconds for individual probes.
It does allow for very quick network sweeps though :). You
can also reference these by number (0-5). For example, '-T
0' gives you Paranoid mode and '-T 5' is Insane mode.
These canned timing modes should NOT be used in combination
with the lower level controls given below.
- --host_timeout <milliseconds>
Specifies the amount of time Nmap is allowed to spend
scanning a single host before giving up on that IP. The
default timing mode has no host timeout.
- --max_rtt_timeout <milliseconds>
Specifies the maximum amount of time Nmap is allowed to wait
for a probe response before retransmitting or timing out
that particular probe. The default mode sets this to about
- --min_rtt_timeout <milliseconds>
When the target hosts start to establish a pattern of
responding very quickly, Nmap will shrink the amount of time
given per probe. This speeds up the scan, but can lead to
missed packets when a response takes longer than usual.
With this parameter you can guarantee that Nmap will wait at
least the given amount of time before giving up on a probe.
- --initial_rtt_timeout <milliseconds>
Specifies the initial probe timeout. This is generally only
useful when scanning firwalled hosts with -P0. Normally
Nmap can obtain good RTT estimates from the ping and the
first few probes. The default mode uses 6000.
- --max_parallelism <number>
Specifies the maximum number of scans Nmap is allowed to
perform in parallel. Setting this to one means Nmap will
never try to scan more than 1 port at a time. It also
effects other parallel scans such as ping sweep, RPC scan,
- --scan_delay <milliseconds>
amount of time Nmap must wait between probes. This is
mostly useful to reduce network load or to slow the scan way
down to sneak under IDS thresholds.
Everything that isn't an option (or option argument) in nmap
is treated as a target host specification. The simplest
case is listing single hostnames or IP addresses on the
command line. If you want to scan a subnet of IP addresses,
you can append
to the hostname
or IP address.
must be between 0 (scan the whole internet) and 32 (scan the
single host specified). Use /24 to scan a class 'C' address
and /16 for a class 'B'.
Nmap also has a more powerful notation which lets you
specify an IP address using lists/ranges for each element.
Thus you can scan the whole class 'B' network 192.168.*.* by
specifying '192.168.*.*' or '192.168.0-255.0-255' or even
'192.168.1-50,51-255.1,2,3,4,5-255'. And of course you can
use the mask notation: '192.168.0.0/16'. These are all
equivalent. If you use asterisks ('*'), remember that most
shells require you to escape them with back slashes or
protect them with quotes.
Another interesting thing to do is slice the Internet the
other way. Instead of scanning all the hosts in a class
specifying hosts to scan, see the
Here are some examples of using nmap, from simple and normal
to a little more complex/esoteric. Note that actual numbers
and some actual domain names are used to make things more
concrete. In their place you should substitute
your own network.
I do not think portscanning other networks is illegal; nor
should portscans be construed by others as an attack. I
have scanned hundreds of thousands of machines and have
received only one complaint. But I am not a lawyer and some
(anal) people may be annoyed by
probes. Get permission first or use at your own risk.
nmap -v target.example.com
This option scans all reserved TCP ports on the machine
target.example.com . The -v means turn on verbose mode.
nmap -sS -O target.example.com/24
Launches a stealth SYN scan against each machine that is up
out of the 255 machines on class 'C' where
target.example.com resides. It also tries to determine what
operating system is running on each host that is up and
running. This requires root privileges because of the SYN
scan and the OS detection.
nmap -sX -p 22,53,110,143,4564 198.116.*.1-127
Sends an Xmas tree scan to the first half of each of the 255
possible 8 bit subnets in the 198.116 class 'B' address
space. We are testing whether the systems run sshd, DNS,
pop3d, imapd, or port 4564. Note that Xmas scan doesn't
work on Microsoft boxes due to their deficient TCP stack.
Same goes with CISCO, IRIX, HP/UX, and BSDI boxes.
nmap -v --randomize_hosts -p 80 '*.*.2.3-5'
Rather than focus on a specific IP range, it is sometimes
interesting to slice up the entire Internet and scan a small
sample from each slice. This command finds all web servers
on machines with IP addresses ending in .2.3, .2.4, or .2.5
find more interesting machines starting at 127. so you might
want to use '127-222' instead of the first asterisks because
that section has a greater density of interesting machines
host -l company.com | cut '-d ' -f 4 | ./nmap -v -iL -
Do a DNS zone transfer to find the hosts in company.com and
then feed the IP addresses to
The above commands are for my GNU/Linux box. You may need
different commands/options on other operating systems.
Bugs? What bugs? Send me any that you find. Patches are
nice too :) Remember to also send in new OS fingerprints so
we can grow the database. Nmap will give you a submission
URL when an appropriate fingerprint is found.
The newest version of
can be obtained from
is (C) 1995-2001 by Insecure.Com LLC
is also distributed along with nmap. It is copyrighted by
Van Jacobson, Craig Leres and Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of
California, Berkeley, CA. The version distributed with nmap
may be modified, pristine sources are available from
This program is free software; you can redistribute it
and/or modify it under the terms of the GNU General Public
License as published by the Free Software Foundation;
Version 2. This guarantees your right to use, modify, and
redistribute Nmap under certain conditions. If this license
is unacceptable to you, Insecure.Org may be willing to sell
alternative licenses (contact firstname.lastname@example.org).
Source is provided to this software because we believe users
have a right to know exactly what a program is going to do
before they run it. This also allows you to audit the
software for security holes (none have been found so far).
Source code also allows you to port Nmap to new platforms, fix bugs,
and add new features. You are highly encouraged to send your changes
to email@example.com for possible incorporation into the main
distribution. By sending these changes to Fyodor or one the
insecure.org development mailing lists, it is assumed that you are
offering Fyodor the unlimited, non-exclusive right to reuse, modify,
and relicense the code. This is important because the inability to
relicense code has caused devastating problems for other Free Software
projects (such as KDE and NASM). Nmap will always be available Open
Source. If you wish to specify special license conditions of your
contributions, just say so when you send them.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY;
without even the implied warranty of
FITNESS FOR A PARTICULAR PURPOSE.
See the GNU
General Public License for more details (it is in the COPYING file of
It should also be noted that Nmap has been known to crash
certain poorly written applications, TCP/IP stacks, and even
Nmap should never be run against mission critical systems
unless you are prepared to suffer downtime. We acknowledge
here that Nmap may crash your systems or networks and we
disclaim all liability for any damage or problems Nmap could
Because of the slight risk of crashes and because a few black hats like
to use Nmap for reconnaissance prior to attacking systems, there are
administrators who become upset and may complain when their system is
scanned. Thus, it is often advisable to request permission before
doing even a light scan of a network.
Nmap should never be run with privileges (eg suid root) for security
All versions of Nmap equal to or greater than 2.0 are
believed to be Year 2000 (Y2K) compliant in all respects.
There is no reason to believe versions earlier than 2.0 are
susceptible to problems, but we have not tested them.
- TARGET SPECIFICATION
This document was created by
using the manual pages.