nmap
(1)
Name
nmap - Network exploration tool and security / port scanner
Synopsis
nmap [Scan Type...] [Options] {target specification}
Description
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.
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Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: 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 (74.207.244.221)
Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
The newest version of Nmap can be obtained from blue]-
http://nmap.org]. The newest version of this man page is
available at blue]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 blue]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
blue]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 6.25 ( 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, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
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-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:
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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 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -Pn -p 80
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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, 192.168.10.0/24 would scan the 256 hosts between
192.168.10.0 (binary: 11000000 10101000 00001010 00000000)
and 192.168.10.255 (binary: 11000000 10101000 00001010
11111111), inclusive. 192.168.10.40/24 would scan exactly
the same targets. Given that the host scanme.nmap.org. is
at the IP address 64.13.134.52, the specification
scanme.nmap.org/16 would scan the 65,536 IP addresses
between 64.13.0.0 and 64.13.255.255. 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 192.168.0.0/16 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 192.168.3.1, 192.168.4.1, 192.168.5.1, and
192.168.7.1. 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
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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 192.168.0.0/8 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
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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 10.0.0.0/8. 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
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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
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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
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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.
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-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
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likely to get a response.. The payload database is
described at blue]-
http://nmap.org/book/nmap-payloads.html].
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.
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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 blue]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 (blue]RFC
792][3]. and blue]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.
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-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 blue]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.
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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. 192.168.0.0/24)
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
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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.
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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.
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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
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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 blue]-
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
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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, blue]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,
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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) .
blue]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
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--scanflags option described in the next section)
exploit a subtle loophole in the blue]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.
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-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 blue]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
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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
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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
blue]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 nmap-hackers mailing
list.. 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
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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 (blue]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.
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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
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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
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(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 blue]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.
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--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-probes ports 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
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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 (blue]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 blue]-
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,
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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 blue]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 blue]-
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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 blue]-
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
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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 blue]-
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 blue]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
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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 blue]-
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
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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.
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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
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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
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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
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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,
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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.
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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.
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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.. -T4 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.
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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
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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
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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
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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 blue]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
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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 blue]-
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
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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 blue]-
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. blue]RFC 2960][14]
originally defined Adler32 as checksum algorithm for
SCTP; blue]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
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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..
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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 blue]-
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 blue]Nmap::Scanner][15]. and
blue]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
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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
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the Nmap grepable output format is available from blue]-
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
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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 states.
These three states are treated just as they normally
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are, which means that open|filtered and unfiltered may
be condensed into counts if there are an overwhelming
number of them.
--iflist (List interfaces and routes) .
Prints the interface list and system routes as detected
by Nmap. This is useful for debugging routing problems
or device mischaracterization (such as Nmap treating a
PPP connection as ethernet).
Miscellaneous output options
--append-output (Append to rather than clobber output files)
.
When you specify a filename to an output format flag
such as -oX or -oN, that file is overwritten by default.
If you prefer to keep the existing content of the file
and append the new results, specify the --append-output
option. All output filenames specified in that Nmap
execution will then be appended to rather than
clobbered. This doesn't work well for XML (-oX) scan
data as the resultant file generally won't parse
properly until you fix it up by hand.
--resume filename (Resume aborted scan) .
Some extensive Nmap runs take a very long time--on the
order of days. Such scans don't always run to
completion. Restrictions may prevent Nmap from being run
during working hours, the network could go down, the
machine Nmap is running on might suffer a planned or
unplanned reboot, or Nmap itself could crash. The
administrator running Nmap could cancel it for any other
reason as well, by pressing ctrl-C. Restarting the whole
scan from the beginning may be undesirable. Fortunately,
if normal (-oN) or grepable (-oG) logs were kept, the
user can ask Nmap to resume scanning with the target it
was working on when execution ceased. Simply specify the
--resume option and pass the normal/grepable output file
as its argument. No other arguments are permitted, as
Nmap parses the output file to use the same ones
specified previously. Simply call Nmap as nmap --resume
logfilename. Nmap will append new results to the data
files specified in the previous execution. Resumption
does not support the XML output format because combining
the two runs into one valid XML file would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform
XML output) .
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 initially installed by Nmap.
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Run the XML file through an XSLT processor such as
blue]xsltproc][17]. to produce an HTML file. Directly
opening the XML file in a browser no longer works well
because modern browsers limit the locations a stylesheet
may be loaded from. 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://nmap.org/svn/docs/nmap.xsl. This tells an XSLT
processor to load the latest version of the stylesheet
from Nmap.Org. The --webxml option does the same thing
with less typing and memorization. Loading the XSL from
Nmap.Org 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
location of nmap.xsl is used by default for privacy
reasons.
--webxml (Load stylesheet from Nmap.Org) .
This is a convenience option, nothing more than an alias
for --stylesheet http://nmap.org/svn/docs/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML) .
Specify this option to prevent Nmap from associating any
XSL stylesheet with its XML output. The xml-stylesheet
directive is omitted.
MISCELLANEOUS OPTIONS
This section describes some important (and not-so-important)
options that don't really fit anywhere else.
-6 (Enable IPv6 scanning) .
Nmap has IPv6 support for its most popular features.
Ping scanning, port scanning, version detection, and the
Nmap Scripting Engine all support IPv6. The command
syntax is the same as usual except that you also add the
-6 option. Of course, you must use IPv6 syntax if you
specify an address rather than a hostname. An address
might look like 3ffe:7501:4819:2000:210:f3ff:fe03:14d0,
so hostnames are recommended. The output looks the same
as usual, with the IPv6 address on the "interesting
ports" line being the only IPv6 giveaway.
While IPv6 hasn't exactly taken the world by storm, it
gets significant use in some (usually Asian) countries
and most modern operating systems support it. To use
Nmap with IPv6, both the source and target of your scan
must be configured for IPv6. If your ISP (like most of
them) does not allocate IPv6 addresses to you, free
tunnel brokers are widely available and work fine with
Nmap. I use the free IPv6 tunnel broker. service at
blue]http://www.tunnelbroker.net]. Other tunnel brokers
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are blue]listed at Wikipedia][18]. 6to4 tunnels are
another popular, free approach.
On Windows, raw-socket IPv6 scans are supported only on
ethernet devices (not tunnels), and only on Windows
Vista. and later. Use the --unprivileged. option in
other situations.
-A (Aggressive scan options) .
This option enables additional advanced and aggressive
options. I haven't decided exactly which it stands for
yet. Presently this enables OS detection (-O), version
scanning (-sV), script scanning (-sC) and traceroute
(--traceroute).. 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. However, because script scanning with the
default set is considered intrusive, you should not use
-A against target networks without permission. This
option only enables features, and not timing options
(such as -T4) or verbosity options (-v) that you might
want as well.
--datadir directoryname (Specify custom Nmap data file
location) .
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-db. If the
location of any of these files has been specified (using
the --servicedb or --versiondb options), that location
is used for that file. After that, Nmap searches these
files in the directory specified with the --datadir
option (if any). Any files not found there, are searched
for in the directory specified by the NMAPDIR.
environment variable. Next comes ~/.nmap. for real and
effective UIDs; or on Windows, HOME\AppData\Roaming\nmap
(where HOME is the user's home directory, like
C:\Users\user). This is followed by the location of the
nmap executable and the same location with ../share/nmap
appended. Then a compiled-in location such as
/usr/local/share/nmap or /usr/share/nmap.
--servicedb services file (Specify custom services file) .
Asks Nmap to use the specified services file rather than
the nmap-services data file that comes with Nmap. Using
this option also causes a fast scan (-F) to be used. See
the description for --datadir for more information on
Nmap's data files.
--versiondb service probes file (Specify custom service
probes file) .
Asks Nmap to use the specified service probes file
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rather than the nmap-service-probes data file that comes
with Nmap. See the description for --datadir for more
information on Nmap's data files.
--send-eth (Use raw ethernet sending) .
Asks Nmap to send packets at the raw ethernet (data
link) layer rather than the higher IP (network) layer.
By default, Nmap chooses the one which is generally best
for the platform it is running on. Raw sockets (IP
layer). are generally most efficient for Unix machines,
while ethernet frames are required for Windows operation
since Microsoft disabled raw socket support. Nmap still
uses raw IP packets on Unix despite this option when
there is no other choice (such as non-ethernet
connections).
--send-ip (Send at raw IP level) .
Asks Nmap to send packets via raw IP sockets rather than
sending lower level ethernet frames. It is the
complement to the --send-eth option discussed
previously.
--privileged (Assume that the user is fully privileged) .
Tells Nmap to simply assume that it is privileged enough
to perform raw socket sends, packet sniffing, and
similar operations that usually require root privileges.
on Unix systems. By default Nmap quits if such
operations are requested but geteuid is not zero.
--privileged is useful with Linux kernel capabilities
and similar systems that may be configured to allow
unprivileged users to perform raw-packet scans. Be sure
to provide this option flag before any flags for options
that require privileges (SYN scan, OS detection, etc.).
The NMAP_PRIVILEGED. environment variable may be set as
an equivalent alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket
privileges) .
This option is the opposite of --privileged. It tells
Nmap to treat the user as lacking network raw socket and
sniffing privileges. This is useful for testing,
debugging, or when the raw network functionality of your
operating system is somehow broken. The
NMAP_UNPRIVILEGED. environment variable may be set as
an equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting) .
This option is only useful for memory-leak debugging. It
causes Nmap to release allocated memory just before it
quits so that actual memory leaks are easier to spot.
Normally Nmap skips this as the OS does this anyway upon
process termination.
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-V; --version (Print version number) .
Prints the Nmap version number and exits.
-h; --help (Print help summary page) .
Prints a short help screen with the most common command
flags. Running Nmap without any arguments does the same
thing.
RUNTIME INTERACTION
During the execution of Nmap, all key presses are captured.
This allows you to interact with the program without
aborting and restarting it. Certain special keys will change
options, while any other keys will print out a status
message telling you about the scan. The convention is that
lowercase letters increase the amount of printing, and
uppercase letters decrease the printing. You may also press
`?' for help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
EXAMPLES
Here are some Nmap usage examples, from the simple and
routine to a little more complex and esoteric. Some actual
IP addresses and domain names are used to make things more
concrete. In their place you should substitute
addresses/names from your own network. While I don't think
port scanning other networks is or should be illegal, some
network administrators don't appreciate unsolicited scanning
of their networks and may complain. Getting permission first
is the best approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org.. This permission only includes scanning
via Nmap and not testing exploits or denial of service
attacks. To conserve bandwidth, please do not initiate more
than a dozen scans against that host per day. If this free
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scanning target service is abused, it will be taken down and
Nmap will report Failed to resolve given hostname/IP:
scanme.nmap.org. These permissions also apply to the hosts
scanme2.nmap.org, scanme3.nmap.org, and so on, though those
hosts do not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine
scanme.nmap.org . The -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up
out of the 256 IPs on the class C sized network where Scanme
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 OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half
of each of the 255 possible eight-bit subnets in the 198.116
class B address space. This tests whether the systems run
SSH, DNS, POP3, or IMAP on their standard ports, or anything
on port 4564. For any of these ports found open, version
detection is used to determine what application is running.
nmap -v -iR 100000 -Pn -p 80
Asks Nmap to choose 100,000 hosts at random and scan them
for web servers (port 80). Host enumeration is disabled with
-Pn since first sending a couple probes to determine whether
a host is up is wasteful when you are only probing one port
on each target host anyway.
nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG
logs/pb-port80scan.gnmap 216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging
them) and saves the output in grepable and XML formats.
NMAP BOOK
While this reference guide details all material Nmap
options, it can't fully demonstrate how to apply those
features to quickly solve real-world tasks. For that, we
released Nmap Network Scanning: The Official Nmap Project
Guide to Network Discovery and Security Scanning. Topics
include subverting firewalls and intrusion detection
systems, optimizing Nmap performance, and automating common
networking tasks with the Nmap Scripting Engine. Hints and
instructions are provided for common Nmap tasks such as
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taking network inventory, penetration testing, detecting
rogue wireless access points, and quashing network worm
outbreaks. Examples and diagrams show actual communication
on the wire. More than half of the book is available free
online. See blue]http://nmap.org/book] for more information.
BUGS
Like its author, Nmap isn't perfect. But you can help make
it better by sending bug reports or even writing patches. If
Nmap doesn't behave the way you expect, first upgrade to the
latest version available from blue]http://nmap.org]. If the
problem persists, do some research to determine whether it
has already been discovered and addressed. Try searching for
the error message on our search page at blue]-
http://insecure.org/search.html] or at Google. Also try
browsing the nmap-dev archives at blue]-
http://seclists.org/].. Read this full manual page as well.
If nothing comes of this, mail a bug report to
nmap-dev@insecure.org. Please include everything you have
learned about the problem, as well as what version of Nmap
you are running and what operating system version it is
running on. Problem reports and Nmap usage questions sent to
nmap-dev@insecure.org are far more likely to be answered
than those sent to Fyodor directly. If you subscribe to the
nmap-dev list before posting, your message will bypass
moderation and get through more quickly. Subscribe at blue]-
http://cgi.insecure.org/mailman/listinfo/nmap-dev].
Code patches to fix bugs are even better than bug reports.
Basic instructions for creating patch files with your
changes are available at blue]-
https://svn.nmap.org/nmap/HACKING]. Patches may be sent to
nmap-dev (recommended) or to Fyodor directly.
AUTHOR
Gordon "Fyodor" Lyon fyodor@insecure.org (blue]-
http://insecure.org])
Hundreds of people have made valuable contributions to Nmap
over the years. These are detailed in the CHANGELOG. file
which is distributed with Nmap and also available from
blue]http://nmap.org/changelog.html].
LEGAL NOTICES
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996-2012 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 Foundation; Version 2 with
the clarifications and exceptions described below. This
guarantees your right to use, modify, and redistribute this
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software under certain conditions. If you wish to embed Nmap
technology into proprietary software, we sell alternative
licenses (contact sales@insecure.com). Dozens of software
vendors already license Nmap technology such as host
discovery, port scanning, OS detection, and version
detection.
Note that the GPL places important restrictions on "derived
works", yet it does not provide a detailed definition of
that term. To avoid misunderstandings, 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-db 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
meant to clarify our interpretation of derived works with
some common examples. Our interpretation applies only to
Nmap--we don't speak for other people's GPL works.
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@insecure.com for further
information.
As a special exception to the GPL terms, Insecure.Com LLC
grants permission 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 combinations
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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 contract stating terms other than the terms above, then
that alternative license agreement takes precedence over
these comments.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005-2012 Insecure.Com LLC.
It is hereby placed under version 3.0 of the blue]Creative
Commons Attribution License][19]. This allows you
redistribute and modify the work as you desire, as long as
you credit the original source. Alternatively, you may
choose to treat this document as falling under the same
license as Nmap itself (discussed previously).
Source Code Availability and Community Contributions
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 nmap-dev@insecure.org for possible
incorporation into the main distribution. By sending these
changes to Fyodor or one of the Insecure.Org development
mailing lists, it is assumed that you are offering the Nmap
Project (Insecure.Com LLC) the unlimited, non-exclusive
right to reuse, modify, 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.
No Warranty.
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied
warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE. See the GNU General Public License v2.0 for more
details at blue]http://www.gnu.org/licenses/gpl-2.0.html],
or in the COPYING file included with Nmap.
It should also be noted that Nmap has occasionally been
known to crash poorly written applications, TCP/IP stacks,
and even operating systems.. While this is extremely rare,
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Nmap Reference Guide NMAP(1)
it is important to keep in mind. 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.
Inappropriate Usage
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 (e.g.
suid root).. That would open up a major security
vulnerability as other users on the system (or attackers)
could use it for privilege escalation.
Third-Party Software and Funding Notices
This product includes software developed by the blue]Apache
Software Foundation][20]. A modified version of the
blue]Libpcap portable packet capture library][21]. is
distributed along with Nmap. The Windows version of Nmap
utilized the Libpcap-derived blue]WinPcap library][22].
instead. Regular expression support is provided by the
blue]PCRE library][23],. which is open-source software,
written by Philip Hazel.. Certain raw networking functions
use the blue]Libdnet][24]. networking library, which was
written by Dug Song.. A modified version is distributed
with Nma.p Nmap can optionally link with the blue]OpenSSL
cryptography toolkit][25]. for SSL version detection
support. The Nmap Scripting Engine uses an embedded version
of the blue]Lua programming language][26].. The
blue]Liblinear linear classification library][27] is used
for our blue]IPv6 OS detection machine learning
techniques][28].
All of the third-party software described in this paragraph
is freely redistributable under BSD-style software licenses.
Binary packages for Windows and Mac OS X include support
libraries necessary to run Zenmap and Ndiff with Python and
PyGTK. (Unix platforms commonly make these libraries easy to
install, so they are not part of the packages.) A listing of
these support libraries and their licenses is included in
the LICENSES files.
This software was supported in part through the blue]Google
Summer of Code][29] and the blue]DARPA CINDER program][30]
(DARPA-BAA-10-84).
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United States Export Control.
Nmap only uses encryption when compiled with the optional
OpenSSL support and linked with OpenSSL. When compiled
without OpenSSL support, Insecure.Com LLC believes that Nmap
is not subject to U.S. blue]Export Administration
Regulations (EAR)][31] export control. As such, there is no
applicable ECCN (export control classification number) and
exportation does not require any special license, permit, or
other governmental authorization.
When compiled with OpenSSL support or distributed as source
code, Insecure.Com LLC believes that Nmap falls under U.S.
ECCN blue]5D002][32] ("Information Security Software"). We
distribute Nmap under the TSU exception for publicly
available encryption software defined in blue]EAR
740.13(e)][33].
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | diagnostic/nmap |
+---------------+------------------+
|Stability | Volatile |
+---------------+------------------+
NOTES
1. Nmap Network Scanning: The Official Nmap Project Guide
to Network Discovery and Security Scanning
http://nmap.org/book/
2. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
3. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
4. RFC 950
http://www.rfc-editor.org/rfc/rfc950.txt
5. RFC 1918
http://www.rfc-editor.org/rfc/rfc1918.txt
6. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
7. SCTP
http://www.rfc-editor.org/rfc/rfc4960.txt
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8. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
9. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
10. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
11. Lua programming language
http://lua.org
12. precedence
http://www.lua.org/manual/5.1/manual.html#2.5.3
13. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
14. RFC 2960
http://www.rfc-editor.org/rfc/rfc2960.txt
15. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
16. Nmap::Parser
http://nmapparser.wordpress.com/
17. xsltproc
http://xmlsoft.org/XSLT/
18. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
19. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
20. Apache Software Foundation
http://www.apache.org
21. Libpcap portable packet capture library
http://www.tcpdump.org
22. WinPcap library
http://www.winpcap.org
23. PCRE library
http://www.pcre.org
24. Libdnet
http://libdnet.sourceforge.net
25. OpenSSL cryptography toolkit
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http://www.openssl.org
26. Lua programming language
http://www.lua.org
27. Liblinear linear classification library
http://www.csie.ntu.edu.tw/~cjlin/liblinear/
28. IPv6 OS detection machine learning techniques
http://nmap.org/book/osdetect-guess.html#osdetect-guess-ipv6
29. Google Summer of Code
http://nmap.org/soc/
30. DARPA CINDER program
https://www.fbo.gov/index?s=opportunity&mode=form&id=585e02a51f77af5cb3c9e06b9cc82c48&tab=core&_cview=1
31. Export Administration Regulations (EAR)
http://www.access.gpo.gov/bis/ear/ear_data.html
32. 5D002
http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
33. EAR 740.13(e)
http://www.access.gpo.gov/bis/ear/pdf/740.pdf
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://nmap.org/dist/nmap-6.25.tgz
Further information about this software can be found on the
open source community website at http://nmap.org/.
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