nmap - Network exploration tool and security scanner
nmap [Scan Type(s)] [Options] <host or net #1 ... [#N]>
Nmap is designed to allow system administrators and curious individuals
to scan large networks to determine which hosts are up and what ser-
vices they are offering. nmap supports a large number of scanning
techniques such as: UDP, TCP connect(), TCP SYN (half open), ftp proxy
(bounce attack), ICMP (ping sweep), FIN, ACK sweep, Xmas Tree, SYN
sweep, IP Protocol, and Null scan. See the Scan Types 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 flex-
ible target and port specification.
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 when-
ever possible (not setuid root, of course).
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 deter-
mine 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 charac-
teristics of the remote host: OS in use, TCP sequentiality, 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. nmap tries to catch and
warn the user about psychotic or unsupported option combinations.
If you are impatient, you can skip to the examples section at the end,
which demonstrates common usage. You can also run nmap -h for a quick
reference page listing all the options.
-sS 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 listen-
ing. 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 SYN
packets. This is the default scan type for privileged users.
-sT 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, oth-
erwise 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 immediately shut-
down. This is the default scan type for unprivileged users.
-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
The idea is that closed ports are required to reply to your
probe packet with an RST, while open ports must ignore the pack-
ets in question (see RFC 793 pp 64). Filered ports also tend to
drop probes without a response, so Nmap considers ports
"open|filtered" when it fails to elicit any response. If you
add version detection (-sV), it will try to verify whether the
ports are actually open and change the state as appropriate.
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.
-sP 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 -P option described
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 without doing any actual port scans.
-sV Version detection: After TCP and/or UDP ports are discovered
using one of the other scan methods, version detection
communicates with those ports to try and determine more about
what is actually running. A file called nmap-service-probes is
used to determine the best probes for detecting various services
and the match strings to expect. 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, and sometimes miscellaneous details like whether an X
server is open to connections or the SSH protocol version). If
Nmap was compiled with OpenSSL support, it will connect to SSL
servers to deduce the service listening behind the encryption.
When RPC services are discovered, the Nmap RPC grinder is used
to determine the RPC program and version numbers. Some UDP
ports are left in the "open|filtered" state after a UDP scan is
unable to determine whether the port is open or filtered. Ver-
sion 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. Note that the Nmap -A option also enables this
feature. For a much more detailed description of Nmap service
detection, read our paper at http://www.insecure.org/nmap/ver-
sionscan.html . There is a related --version_trace option which
causes Nmap to print out extensive debugging info about what
version scanning is doing (this is a subset of what you would
get with --packet_trace).
-sU UDP scans: This method is used to determine which UDP (User
Datagram Protocol, RFC 768) ports are open on a host. The tech-
nique 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. If a UDP response is received to the probe
(unusual), the port is open. If we get no response at all, the
state is "open|filtered", meaning that the port is either open
or packet filters are blocking the communication. Versions scan
(-sV) can be used to help differentiate the truly open ports
from the filtered ones.
Some people think UDP scanning is pointless. I usually remind
them of the Solaris rpcbind 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 Win-
dows machines. Not to mention the many commonly vulnerable ser-
vices 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 22.214.171.124)
of limiting the ICMP error message rate. For example, the Linux
kernel (in net/ipv4/icmp.h) limits destination unreachable mes-
sage 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. nmap
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 Windows machine
very quickly. Whoop!
-sO 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 tar-
get from your real IP address). Instead, a unique side-channel
attack exploits predictable "IP fragmentation ID" sequence gen-
eration on the zombie host to glean information about the open
ports on the target. IDS systems will display the scan as com-
ing from the zombie machine you specify (which must be up and
meet certain criteria). I wrote an informal paper about this
technique at http://www.insecure.org/nmap/idlescan.html .
Besides being extraordinarily stealthy (due to its blind
nature), this scan type permits mapping out IP-based trust rela-
tionships between machines. The port listing shows open ports
from the perspective of the zombie host. So you can try scan-
ning 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. Other-
wise, 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
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
-sA ACK scan: This advanced method is usually used to map out fire-
wall 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 acknowl-
edgment/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 nmap usually doesn’t print
"unfiltered" ports, so getting no 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"
-sW Window scan: This advanced scan is very similar to the ACK scan,
except that it can sometimes detect open ports as well as fil-
tered/unfiltered 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 full list.
-sR 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 "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, at some
point I may add decoy support for UDP RPC scans. This is auto-
matically enabled as part of version scan (-sV) if you request
-sL List scan. This method simply generates and prints a list of IP
addresses or hostnames 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: user-
name:password@server:port. Everything but server is optional.
To determine what servers are vulnerable to this attack, you can
see my article in Phrack 51. An updated version is available at
the nmap URL (http://www.insecure.org/nmap).
None of these are required but some can be quite useful. Note
that the -P options can now be combined -- you can increase your
odds of penetrating strict firewalls by sending many probe types
using different TCP ports/flags and ICMP codes.
-P0 Do not try to 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
-P0 or -PS80 when portscanning microsoft.com. Note that "ping"
in this context may involve more than the traditional ICMP echo
request packet. Nmap supports many such probes, including arbi-
trary combinations of TCP, UDP, and ICMP probes. By default,
Nmap sends an ICMP echo request and a TCP ACK packet to port 80.
Use TCP ACK "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 pre-
serves the efficiency of only scanning hosts that are up while
still allowing you to scan networks/hosts that block ping pack-
ets. For non root UNIX users, we use connect() and thus a SYN
is actually being sent. To set the destination ports of the
probe packets use -PA<port1>[,port2][...]. The default port is
80, since this port is often not filtered out. Note that this
option now accepts multiple, comma-separated port numbers.
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 ports
in the same manner as -PA above.
This option sends UDP probes to the specified hosts, expecting
an ICMP port unreachable packet (or possibly a UDP response if
the port is open) if the host is up. Since many UDP services
won’t reply to an empty packet, your best bet might be to send
this to expected-closed ports rather than open ones.
-PE 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
incoming 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).
-PP Uses an ICMP timestamp request (type 13) packet to find listen-
-PM Same as -PE and -PP except uses a netmask request (ICMP type
-PB This is the default ping type. It uses both the ACK ( -PA ) and
ICMP echo request ( -PE ) 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 -PA
above. Note that this flag is now deprecated as pingtype flags
can now be used in combination. So you should use both "PE" and
"PA" (or rely on the default behavior) to achieve this same
-O This option activates remote host identification via TCP/IP fin-
gerprinting. 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 informa-
tion 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 are scanning.
If Nmap is unable to guess the OS of a machine, and conditions
are good (e.g. 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 Clas-
sification. 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 hid-
ing the source of an attack. The actual difficulty number is
based on statistical sampling and may fluctuate. It is gener-
ally 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 sev-
eral advanced information gathering and spoofing attacks.
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 crite-
ria. This can save substantial time, particularly on -P0 scans
against many hosts. It only matters when OS detection is
requested (-O or -A options).
-A This option enables _a_dditional _a_dvanced and _a_ggressive
options. I haven’t decided exactly which it stands for yet :).
Presently this enables OS Detection (-O) and version scanning
(-sV). More features may be added in the future. The point is
to enable a comprehensive set of scan options without people
having to remember a large set of flags. This option only
enables features, and not timing options (such as -T4) or ver-
bosity options (-v) that you might wan’t as well.
-6 This options enables IPv6 support. All targets must be IPv6 if
this option is used, and they can be specified via normal DNS
name (AAAA record) or as a literal IP address such as
3ffe:501:4819:2000:210:f3ff:fe03:4d0 . Currently, connect() TCP
scan and TCP connect() Ping scan are supported. If you need UDP
or other scan types, have a look at http://nmap6.source-
-f This 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 fil-
ters, 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 8 bytes or less after the IP header. So a
20-byte TCP header would be split into 3 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 off-
set must be a multiple of 8. 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. Some source systems defragment outgoing
packets in the kernel. Linux with the ip tables connection
tracking module is one such example. Do a scan with a sniffer
such as ethereal running to ensure that sent packets are frag-
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 variants.
-v 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. You can also use -d a few times if
you really want to get crazy with scrolling the screen!
-h 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" :)
This logs the results of your scans in a normal human readable
form into the file you specify as an argument.
This logs the results of your scans in XML form into the file
you specify as an argument. This allows programs to easily cap-
ture 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 informa-
tion to be printed. The Document Type Definition (DTD) defining
the XML output structure is available at http://www.inse-
Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
translating XML output to HTML. The XML output includes an xml-
stylesheet directive which points to nmap.xml where it was ini-
tially installed by Nmap (or in the current working directory on
Windows). Simply load Nmap’s XML output in a modern web browser
and it should retrieve nmap.xsl from the filesystem and use it
to render results. If you wish to use a different stylesheet,
specify it as the argument to --stylesheet. You must pass the
full pathname or URL. One common invocation is --stylesheet
http://www.insecure.org/nmap/data/nmap.xsl . This tells a
browser to load the latest version of the stylesheet from Inse-
cure.Org. This makes it easier to view results on a machine
that doesn’t have Nmap (and thus nmap.xsl) installed. So the
URL is often more useful, but the local filesystem locaton of
nmap.xsl is used by default for privacy reasons.
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive is
This logs the results of your scans in a grepable 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 pre-
ferred 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 be printed.
This tells Nmap to log in ALL the major 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.
thIs l0gz th3 r3suLtS of YouR ScanZ iN a s|<ipT kiDd|3 f0rM iNto
THe fiL3 U sPecfy 4s an arGuMEnT! U kAn gIv3 the 4rgument "-"
(wItHOUt qUOteZ) to sh00t output iNT0 stDouT!@!!
A network scan that is canceled due to control-C, network out-
age, etc. can be resumed using this option. The logfilename
must be either a normal (-oN) or grepable (-oG) 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.
--exclude <host1 [,host2][,host3],...">
Specifies a list of targets (hosts, ranges, netblocks) that
should be excluded from a scan. Useful to keep from scanning
yourself, your ISP, particularly sensitive hosts, etc.
Same functionality as the --exclude option, only the excluded
targets are provided in an newline-delimited exclude_file rather
than on the command line.
Tells Nmap to append scan results to any output files you have
specified rather than overwriting those files.
Reads target specifications from the file specified RATHER than
from the command line. The file should contain a list of host
or network expressions separated by spaces, tabs, or newlines.
Use a hyphen (-) as inputfilename if you want nmap to read host
expressions from stdin (like at the end of a pipe). See the
section target specification for more information on the expres-
sions you fill the file with.
-iR <num hosts>
This option tells Nmap to generate its own hosts to scan by sim-
ply picking random numbers :). It will never end after the
given number of IPs has been scanned -- use 0 for a never-ending
scan. This option can be useful for statistical sampling of the
Internet to estimate various things. If you are ever really
bored, try nmap -sS -PS80 -iR 0 -p 80 to find some web servers
to look at.
-p <port ranges>
This option specifies what ports you want to specify. For exam-
ple "-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 for (0-255).
When scanning both TCP and UDP ports, you can specify a particu-
lar protocol by preceding the port numbers by "T:" or "U:". The
qualifier lasts until you specify another qualifier. For exam-
ple, 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 pro-
tocol qualifier is given, the port numbers are added to all pro-
-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 effec-
tive 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 you put "ME" in the 6th 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
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 name-
Also note that some "port scan detectors" will firewall/deny
routing to hosts that attempt port scans. The problem is that
many scan types can be forged (as this option demonstrates). So
attackers can cause such a machine to sever connectivity with
important hosts such as its internet gateway, DNS TLD servers,
sites like Windows Update, etc. Most such software offers
whitelist capabilities, but you are unlikely to enumerate all of
the critical machines. For this reason we never recommend tak-
ing action against port scans that can be forged, including SYN
scans, UDP scans, etc. The machine you block could just be a
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 ).
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.
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 your 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! 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 scan-
ning them. -e would generally be required for this sort of
Tells nmap what interface to send and receive packets on. Nmap
should be able to detect this but it will tell you if it cannot.
Sets the source port number used in scans. Many naive firewall
and packet filter installations make an exception in their rule-
set 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. Obvi-
ously 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 this
option. This is an alias for the shorter, but harder to remem-
ber, -g option.
Be aware that there is a small performance penalty on some scans
for using this option, because I sometimes store useful informa-
tion in the source port 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 random 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.
-n Tells Nmap to NEVER do reverse DNS resolution on the active IP
addresses it finds. Since DNS is often slow, this can help
speed things up.
-R Tells Nmap to ALWAYS do reverse DNS resolution on the target IP
addresses. Normally this is only done when a machine is found
to be alive.
-r Tells Nmap NOT to randomize the order in which ports are
Sets the IPv4 time to live field in sent packets to the given
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.
Tells Nmap to show all the packets it sends and receives in a
tcpdump-like format. This can be tremendously useful for debug-
ging, and is also a good learning tool.
Nmap obtains some special data at runtime in files named nmap-
service-probes, nmap-services, nmap-protocols, nmap-rpc, nmap-
mac-prefixes, and nmap-os-fingerprints. Nmap first searches
these files in the directory option to --datadir. Any files not
found there, are searched for in the directory specified by the
NMAPDIR environmental variable. Next comes ~/.nmap for real and
effective UIDs (POSIX systems only) or location of the Nmap exe-
cutable (Win32 only), and then a compiled-in location such as
/usr/local/share/nmap or /usr/share/nmap . As a last resort,
Nmap will look in the current directory.
Generally Nmap does a good job at adjusting for Network charac-
teristics at runtime and scanning as fast as possible while min-
imizing 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 scan timing:
These are canned timing policies for conveniently expressing
your priorities to Nmap. Paranoid mode scans very slowly in the
hopes of avoiding detection by IDS systems. It serializes all
scans (no parallel scanning) and generally waits at least 5 min-
utes between sending packets. Sneaky is similar, except it only
waits 15 seconds between sending packets. Polite is meant to
ease load on the network and reduce the chances of crashing
machines. It serializes the probes and waits at least 0.4 sec-
onds between them. Note that this is generally at least an
order of magnitude slower than default scans, so only use it
when you need to. Normal is the default Nmap behavior, which
tries to run as quickly as possible without overloading the net-
work or missing hosts/ports. Aggressive This option can make
certain scans (especially SYN scans against heavily filtered
hosts) much faster. It is recommended for impatient folks with
a fast net connection. Insane is only suitable for very fast
networks or where you don’t mind losing some information. It
times out hosts in 15 minutes and won’t wait more than 0.3 sec-
onds for individual probes. It does allow for very quick net-
work sweeps though :).
You can also reference these by number (0-5). For example,
"-T0" gives you Paranoid mode and "-T5" is Insane mode. If you
wish to set specific timing values such as --max_rtt_timeout or
--host_timeout, place them after any -T option on the command
line. Otherwise the defaults for the selected timing mode will
override your choices.
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.
Specifies the maximum amount of time Nmap is allowed to wait for
a probe response before retransmitting or timing out that par-
ticular probe. The default mode sets this to about 9000.
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.
Specifies the initial probe timeout. This is generally only
useful when scanning firewalled hosts with -P0. Normally Nmap
can obtain good RTT estimates from the ping and the first few
probes. The default mode uses 6000.
Specifies the maximum number of hosts that Nmap is allowed to
scan in parallel. Most of the port scan techniques support
multi-host operation, which makes them much quicker. Spreading
the load among multiple target hosts makes the scans gentler.
The downside is increased results latency. You need to wait for
all hosts in a group to finish, rather than having them pop up
one by one. Specify an argument of one for old-style (one host
at a time) Nmap behavior. Note that the ping scanner handles
its own grouping, and ignores this value.
Specifies the minimum host group size (see previous entry).
Large values (such as 50) are often beneficial for unattended
scans, though they do take up more memory. Nmap may override
this preference when it needs to, because a group must all use
the same network interface, and some scan types can only handle
one host at a time.
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, etc.
Tells Nmap to scan at least the given number of ports in paral-
lel. This can speed up scans against certain firewalled hosts
by an order of magnitude. But be careful -- results will become
unreliable if you push it too far.
Specifies the minimum 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. Nmap will
sometimes increase the delay itself when it detects many dropped
packets. For example, Solaris systems tend to respond with only
one ICMP port unreachable packet per second during a UDP scan.
So Nmap will try to detect this and lower its rate of UDP probes
to one per second.
As noted above, Nmap will sometimes enforce a special delay
between sending packets. This can provide more accurate results
while reducing network congestion, but it can slow the scans
down substantially. By default (with no -T options specified),
Nmap allows this delay to grow to one second per probe. This
option allows you to set a lower or higher maximum. Even if you
set it to zero, Nmap will have some delay between packet sends
so that it can wait for responses and avoid having too many out-
standing probes in parallel.
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 /mask to the hostname or IP
address. mask 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
Another interesting thing to do is slice the Internet the other way.
Instead of scanning all the hosts in a class "B", scan "*.*.5.6-7" to
scan every IP address that ends in .5.6 or .5.7 Pick your own numbers.
For more information on specifying hosts to scan, see the examples sec-
Here are some examples of using nmap, from simple and normal to a lit-
tle 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 addresses/names from 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 nmap probes. Get per-
mission first or use at your own risk.
nmap -v target.example.com
This option scans all reserved TCP ports on the machine target.exam-
ple.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. If you are root you might as
well add -sS. Also you will 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 nmap. 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 fin-
gerprint is found.
The newest version of nmap can be obtained from http://www.inse-
The Nmap Security Scanner is (C) 1996-2004 Insecure.Com LLC. Nmap is
also a registered trademark of Insecure.Com LLC. This program is free
software; you may redistribute and/or modify it under the terms of the
GNU General Public License as published by the Free Software Founda-
tion; Version 2. This guarantees your right to use, modify, and redis-
tribute this software under certain conditions. If you wish to embed
Nmap technology into proprietary software, we may be willing to sell
alternative licenses (contact email@example.com). Many security scan-
ner vendors already license Nmap technology such as our remote OS fin-
gerprinting database and code, service/version detection system, and
port scanning code.
Note that the GPL places important restrictions on "derived works", yet
it does not provide a detailed definition of that term. To avoid mis-
understandings, we consider an application to constitute a "derivative
work" for the purpose of this license if it does any of the following:
o Integrates source code from Nmap
o Reads or includes Nmap copyrighted data files, such as nmap-os-fin-
gerprints or nmap-service-probes.
o Executes Nmap and parses the results (as opposed to typical shell or
execution-menu apps, which simply display raw Nmap output and so are
not derivative works.)
o Integrates/includes/aggregates Nmap into a proprietary executable
installer, such as those produced by InstallShield.
o Links to a library or executes a program that does any of the above
The term "Nmap" should be taken to also include any portions or derived
works of Nmap. This list is not exclusive, but is just meant to clar-
ify our interpretation of derived works with some common examples.
These restrictions only apply when you actually redistribute Nmap. For
example, nothing stops you from writing and selling a proprietary
front-end to Nmap. Just distribute it by itself, and point people to
http://www.insecure.org/nmap/ to download Nmap.
We don’t consider these to be added restrictions on top of the GPL, but
just a clarification of how we interpret "derived works" as it applies
to our GPL-licensed Nmap product. This is similar to the way Linus
Torvalds has announced his interpretation of how "derived works"
applies to Linux kernel modules. Our interpretation refers only to
Nmap - we don’t speak for any other GPL products.
If you have any questions about the GPL licensing restrictions on using
Nmap in non-GPL works, we would be happy to help. As mentioned above,
we also offer alternative license to integrate Nmap into proprietary
applications and appliances. These contracts have been sold to many
security vendors, and generally include a perpetual license as well as
providing for priority support and updates as well as helping to fund
the continued development of Nmap technology. Please email sales@inse-
cure.com for further information.
As a special exception to the GPL terms, Insecure.Com LLC grants per-
mission to link the code of this program with any version of the
OpenSSL library which is distributed under a license identical to that
listed in the included Copying.OpenSSL file, and distribute linked com-
binations including the two. You must obey the GNU GPL in all respects
for all of the code used other than OpenSSL. If you modify this file,
you may extend this exception to your version of the file, but you are
not obligated to do so.
If you received these files with a written license agreement or con-
tract stating terms other than the terms above, then that alternative
license agreement takes precedence over these comments.
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 firstname.lastname@example.org for possible incorporation into the main distri-
bution. By sending these changes to Fyodor or one the Insecure.Org
development mailing lists, it is assumed that you are offering Fyodor
and Insecure.Com LLC the unlimited, non-exclusive right to reuse, mod-
ify, and relicense the code. Nmap will always be available Open
Source, but this is important because the inability to relicense code
has caused devastating problems for other Free Software projects (such
as KDE and NASM). We also occasionally relicense the code to third
parties as discussed above. 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 MER-
CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details at http://www.gnu.org/copyleft/gpl.html
, or in the COPYING file included with Nmap.
It should also be noted that Nmap has been known to crash certain
poorly written applications, TCP/IP stacks, and even operating systems.
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 cause.
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 installed with special privileges (eg suid root)
for security reasons.
This product includes software developed by the Apache Software Founda-
tion (http://www.apache.org/). The Libpcap portable packet capture
library is distributed along with nmap. Libpcap was originally copy-
righted by Van Jacobson, Craig Leres and Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berke-
ley, CA. It is now maintained by http://www.tcpdump.org .
Regular expression support is provided by the PCRE library package,
which is open source software, written by Philip Hazel, and copyright
by the University of Cambridge, England. See http://www.pcre.org/ .
Nmap can optionally link to the OpenSSL cryptography toolkit, which is
available from http://www.openssl.org/ .
US Export Control: Insecure.Com LLC believes that Nmap falls under US
ECCN (export control classification number) 5D992. This category is
called ’"Information Security" "software" not controlled by 5D002’.
The only restriction of this classification is AT (anti-terrorism),
which applies to almost all goods and denies export to a handful of
rogue nations such as Iran and North Korea. Thus exporting Nmap does
not require any special license, permit, or other governmental autho-
Man(1) output converted with