NMAP(1) Nmap Reference Guide NMAP(1)
NAME
nmap - Network exploration tool and security / port scanner
SYNOPSIS
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION
Nmap (“Network Mapper”) is an open source tool for network exploration
and security auditing. It was designed to rapidly scan large networks,
although it works fine against single hosts. Nmap uses raw IP packets
in novel ways to determine what hosts are available on the network,
what services (application name and version) those hosts are offering,
what operating systems (and OS versions) they are running, what type of
packet filters/firewalls are in use, and dozens of other
characteristics. While Nmap is commonly used for security audits, many
systems and network administrators find it useful for routine tasks
such as network inventory, managing service upgrade schedules, and
monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the “interesting ports table”. That table lists the port
number and protocol, service name, and state. The state is either open,
filtered, closed, or unfiltered. Open means that an application on the
target machine is listening for connections/packets on that port.
Filtered means that a firewall, filter, or other network obstacle is
blocking the port so that Nmap cannot tell whether it is open or
closed. Closed ports have no application listening on them, though
they could open up at any time. Ports are classified as unfiltered when
they are responsive to Nmap’s probes, but Nmap cannot determine whether
they are open or closed. Nmap reports the state combinations
open|filtered and closed|filtered when it cannot determine which of the
two states describe a port. The port table may also include software
version details when version detection has been requested. When an IP
protocol scan is requested (-sO), Nmap provides information on
supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 14.1, “A representative Nmap
scan”. The only Nmap arguments used in this example are -A, to enable
OS and version detection, -T4 for faster execution, and then the two
target hostnames. Example 14.1. A representative Nmap scan.sp
# nmap -A -T4 scanme.nmap.org playground
Starting nmap ( http://insecure.org/nmap/ )
Interesting ports on scanme.nmap.org (205.217.153.62):
(The 1663 ports scanned but not shown below are in state: filtered)
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 3.9p1 (protocol 1.99)
53/tcp open domain
70/tcp closed gopher
80/tcp open http Apache httpd 2.0.52 ((Fedora))
113/tcp closed auth
Device type: general purpose
Running: Linux 2.4.X|2.5.X|2.6.X
OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)
Interesting ports on playground.nmap.org (192.168.0.40):
(The 1659 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE VERSION
135/tcp open msrpc Microsoft Windows RPC
139/tcp open netbios-ssn
389/tcp open ldap?
445/tcp open microsoft-ds Microsoft Windows XP microsoft-ds
1002/tcp open windows-icfw?
1025/tcp open msrpc Microsoft Windows RPC
1720/tcp open H.323/Q.931 CompTek AquaGateKeeper
5800/tcp open vnc-http RealVNC 4.0 (Resolution 400x250; VNC TCP port: 5900)
5900/tcp open vnc VNC (protocol 3.8)
MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
Device type: general purpose
Running: Microsoft Windows NT/2K/XP
OS details: Microsoft Windows XP Pro RC1+ through final release
Service Info: OSs: Windows, Windows XP
Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds
The newest version of Nmap can be obtained from
http://insecure.org/nmap/. The newest version of the man page is
available from http://insecure.org/nmap/man/.
OPTIONS SUMMARY
This options summary is printed when Nmap is run with no arguments, and
the latest version is always available at
http://insecure.org/nmap/data/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 4.20RC1 ( http://insecure.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 : Input from list of hosts/networks
-iR : Choose random targets
--exclude : Exclude hosts/networks
--excludefile : Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sP: Ping Scan - go no further than determining if host is online
-P0: Treat all hosts as online -- skip host discovery
-PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers : Specify custom DNS servers
--system-dns: Use OS’s DNS resolver
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 : Customize TCP scan flags
-sI : Idlescan
-sO: IP protocol scan
-b : FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p : Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
-F: Fast - Scan only the ports listed in the nmap-services file)
-r: Scan ports consecutively - don’t randomize
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity : 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)
OS DETECTION:
-O: Enable OS detection (try 2nd generation w/fallback to 1st)
-O2: Only use the new OS detection system (no fallback)
-O1: Only use the old (1st generation) OS detection system
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take are in milliseconds, unless you append ’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 : Parallel host scan group sizes
--min-parallelism/max-parallelism : Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout : Specifies
probe round trip time.
--max-retries : Caps number of port scan probe retransmissions.
--host-timeout : Give up on target after this long
--scan-delay/--max-scan-delay : Adjust delay between probes
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu : fragment packets (optionally w/given MTU)
-D : Cloak a scan with decoys
-S : Spoof source address
-e : Use specified interface
-g/--source-port : Use given port number
--data-length : Append random data to sent packets
--ip-options : Send packets with specified ip options
--ttl : Set IP time-to-live field
--spoof-mac : Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP checksum
OUTPUT:
-oN/-oX/-oS/-oG : Output scan in normal, XML, s|: Output in the three major formats at once
-v: Increase verbosity level (use twice for more effect)
-d[level]: Set or increase debugging level (Up to 9 is meaningful)
--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 : Resume an aborted scan
--stylesheet : XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Insecure.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enables OS detection and Version detection
--datadir : 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 -sP 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -P0 -p 80
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 IP 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 do exactly the same thing.
Given that the host scanme.nmap.org is at the IP address
205.217.153.62, the specification scanme.nmap.org/16 would scan the
65,536 IP addresses between 205.217.0.0 and 205.217.255.255. The
smallest allowed value is /1, which scans half the 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 are commonly 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 and or .255. 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 supported for IPv6
because they are rarely useful.
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
192.168.0.0/16 10.0.0,1,3-7.0-255 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 (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.
-iR (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 -sS -PS80
-iR 0 -p 80 to locate random web servers for browsing.
--exclude (Exclude hosts/networks)
Specifies a comma-separated list of targets to be excluded from
the scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can
be useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react
adversely to port scans, or subnetworks administered by other
people.
--excludefile (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.
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 (-P0), or engage the network with arbitrary combinations of
multi-port TCP SYN/ACK, UDP, 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 RFC1918-blessed 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 a TCP ACK packet
destined for port 80 and an ICMP Echo Request query to each target
machine. An exception to this is that an ARP scan is used for any
targets which are on a local ethernet network. For unprivileged UNIX
shell users, a SYN packet is sent instead of the ack using the
connect() system call. These defaults are equivalent to the -PA -PE
options. 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 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 -sP option to learn how to perform only host discovery, or
use -P0 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 -P0 option.
-sP (Ping Scan)
This option tells Nmap to only perform a ping scan (host
discovery), then print out the available hosts that responded to
the scan. No further testing (such as port scanning or OS
detection) is performed. This is 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 -sP option sends an ICMP echo request and a TCP packet to
port 80 by default. When executed by an unprivileged user, a SYN
packet is sent (using a connect() call) to port 80 on the
target. When a privileged user tries to scan targets on a local
ethernet network, ARP requests (-PR) are used unless --send-ip
was specified. The -sP option can be combined with any of the
discovery probe types (the -P* options, excluding -P0) for
greater flexibility. If any of those probe type and port number
options are used, the default probes (ACK and echo request) 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.
-P0 (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 -P0
causes Nmap to attempt the requested scanning functions against
every target IP address specified. So if a class B sized target
address space (/16) is specified on the command line, all 65,536
IP addresses are scanned. That second option character in -P0 is
a zero and not the letter O. 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.
-PS [portlist] (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 in nmap.h), but an alternate
port can be specified as a parameter. A comma separated list of
ports can even be specified (e.g.
-PS22,23,25,80,113,1050,35000), in which case probes will be
attempted against each port 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 3-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 3-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. This workaround
is also used for IPv6 connections, as raw IPv6 packet building
support is not yet available in Nmap.
-PA [portlist] (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, or an IPv6 target is
specified, 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 [portlist] (UDP Ping)
Another host discovery option is the UDP ping, which sends an
empty (unless --data-length is specified) UDP packet to the
given ports. The portlist takes the same format as with the
previously discussed -PS and -PA options. If no ports are
specified, the default is 31338. This default can be configured
at compile-time by changing DEFAULT_UDP_PROBE_PORT 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 31338, which is highly unlikely to be in use. A few services,
such as chargen, 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.
-PE; -PP; -PM (ICMP Ping Types)
In addition to the unusual TCP and UDP 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 [1]RFC 1122. 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 standard ([2]RFC 792) also specifies
timestamp request, information request, and address mask request
packets as codes 13, 15, and 17, respectively. While the
ostensible purpose for these queries is to learn information
such as address masks and current times, they can easily be used
for host discovery. A system that replies is up and available.
Nmap does not currently implement information request packets,
as they are not widely supported. RFC 1122 insists that “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 admins specifically block echo request packets
while forgetting that other ICMP queries can be used for the
same purpose.
-PR (ARP Ping)
One of the most common Nmap usage scenarios is to scan an
ethernet LAN. On most LANs, especially those using
RFC1918-blessed private address ranges, 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 --send-ip.
-n (No DNS resolution)
Tells Nmap to never do reverse DNS resolution on the active IP
addresses it finds. Since DNS can be slow even with Nmap’s
built-in parallel stub resolver, this option can slash scanning
times.
-R (DNS resolution for all targets)
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver)
By default, Nmap resolves IP addresses by sending queries
directly to the name servers configured on your host and then
listening for responses. Many requests (often dozens) are
performed in parallel to improve performance. Specify this
option to use your system resolver instead (one IP at a time via
the getnameinfo() call). This is slower and rarely useful unless
you find a bug in the Nmap parallel resolver (please let us know
if you do). The system resolver is always used for IPv6 scans.
--dns-servers (Servers to use for reverse DNS
queries)
By default Nmap will try to determine 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 more than 1660 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
open An application is actively accepting TCP connections or UDP
packets on this port. Finding these is often the primary goal of
port scanning. Security-minded people know that each open port
is an avenue for attack. Attackers and pen-testers want to
exploit the open ports, while administrators try to close or
protect them with firewalls without thwarting legitimate users.
Open ports are also interesting for non-security scans because
they show services available for use on the network.
closed 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.
filtered
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.
unfiltered
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.
open|filtered
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.
closed|filtered
This state is used when Nmap is unable to determine whether a
port is closed or filtered. It is only used for the IPID Idle
scan.
PORT SCANNING TECHNIQUES
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to
the task at hand. When I fail miserably and tow my jalopy to a real
mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless. The
art of port scanning is similar. Experts understand the dozens of scan
techniques and choose the appropriate one (or combination) for a given
task. Inexperienced users and script kiddies, on the other hand, try to
solve every problem with the default SYN scan. Since Nmap is free, the
only barrier to port scanning mastery is knowledge. That certainly
beats the automotive world, where it may take great skill to determine
that you need a strut spring compressor, then you still have to pay
thousands of dollars for it.
Most of the scan types are only available to privileged users. This is
because they send and receive raw packets, which requires root access
on UNIX systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on that
platform when WinPcap has already been loaded into the OS. Requiring
root privileges was a serious limitation when Nmap was released in
1997, as many users only had access to shared shell accounts. Now, the
world is different. Computers are cheaper, far more people have
always-on direct Internet access, and desktop UNIX systems (including
Linux and MAC OS X) are prevalent. A Windows version of Nmap is now
available, allowing it to run on even more desktops. For all these
reasons, users have less need to run Nmap from limited shared shell
accounts. This is fortunate, as the privileged options make Nmap far
more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that all
of its insights are based on packets returned by the target machines
(or firewalls in front of them). Such hosts may be untrustworthy and
send responses intended to confuse or mislead Nmap. Much more common
are non-RFC-compliant hosts that do not respond as they should to Nmap
probes. FIN, Null, and Xmas scans are particularly susceptible to this
problem. Such issues are specific to certain scan types and so are
discussed in the individual scan type entries.
This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) may be combined with any one of the TCP scan types. As a memory
aid, port scan type options are of the form -sC, where C is a prominent
character in the scan name, usually the first. The one exception to
this is the deprecated FTP bounce scan (-b). By default, Nmap performs
a SYN Scan, though it substitutes a connect scan if the user does not
have proper privileges to send raw packets (requires root access on
UNIX) or if IPv6 targets were specified. 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 intrusive
firewalls. SYN scan is relatively unobtrusive and stealthy,
since it never completes TCP connections. It also 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.
-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 or is scanning IPv6 networks. Instead of
writing raw packets as most other scan types do, Nmap asks the
underlying operating system to establish a connection with the
target machine and port by issuing the connect() system call.
This is the same high-level system call that web browsers, P2P
clients, and most other network-enabled applications use to
establish a connection. It is part of a programming interface
known as the Berkeley Sockets API. Rather than read raw packet
responses off the wire, Nmap uses this API to obtain status
information on each connection attempt.
When SYN scan is available, it is usually a better choice. Nmap
has less control over the high level connect() call than with
raw packets, making it less efficient. The system call completes
connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take
longer and require more packets to obtain the same information,
but target machines are more likely to log the connection. A
decent IDS will catch either, but most machines have no such
alarm system. Many services on your average UNIX system will add
a note to syslog, and sometimes a cryptic error message, when
Nmap connects and then closes the connection without sending
data. Truly pathetic services crash when this happens, though
that is uncommon. An administrator who sees a bunch of
connection attempts in her logs from a single system should know
that she has been connect scanned.
-sU (UDP scans)
While most popular services on the Internet run over the TCP
protocol, [3]UDP 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 an empty (no data) UDP header to every
targeted port. 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. Versions scan (-sV) can
be used to help differentiate the truly open ports from the
filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time
out and then conduct retransmissions just in case the probe or
response were lost. Closed ports are often an even bigger
problem. They usually send back an ICMP port unreachable error.
But unlike the RST packets sent by closed TCP ports in response
to a SYN or connect scan, many hosts rate limit ICMP port
unreachable messages by default. Linux and Solaris are
particularly strict about this. For example, the Linux 2.4.20
kernel limits destination unreachable messages to one per second
(in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target
machine will drop. Unfortunately, a Linux-style limit of one
packet per second makes a 65,536-port scan take more than 18
hours. Ideas for speeding your UDP scans up include scanning
more hosts in parallel, doing a quick scan of just the popular
ports first, scanning from behind the firewall, and using
--host-timeout to skip slow hosts.
-sN; -sF; -sX (TCP Null, FIN, and Xmas scans)
These three scan types (even more are possible with the
--scanflags option described in the next section) exploit a
subtle loophole in the [4]TCP RFC to differentiate between open
and closed ports. Page 65 says that “if the [destination] port
state is CLOSED .... an incoming segment not containing a RST
causes a RST to be sent in response.” Then the next page
discusses packets sent to open ports without the SYN, RST, or
ACK bits set, stating that: “you are unlikely to get here, but
if you do, drop the segment, and return.”
When scanning systems compliant with this RFC text, any packet
not containing SYN, RST, or ACK bits will result in a returned
RST if the port is closed and no response at all if the port is
open. As long as none of those three bits are included, any
combination of the other three (FIN, PSH, and URG) are OK. Nmap
exploits this with three scan types:
Null scan (-sN)
Does not set any bits (tcp flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up
like a Christmas tree.
These three scan types are exactly the same in behavior except
for the TCP flags set in probe packets. If a RST packet is
received, the port is considered closed, while no response means
it is open|filtered. The port is marked filtered if an ICMP
unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is
received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don’t count on this though
-- most modern IDS products can be configured to detect them.
The big downside is that not all systems follow RFC 793 to the
letter. A number of systems send RST responses to the probes
regardless of whether the port is open or not. This causes all
of the ports to be labeled closed. Major operating systems that
do this are Microsoft Windows, many Cisco devices, BSDI, and IBM
OS/400. This scan does work against most UNIX-based systems
though. Another downside of these scans is that they can’t
distinguish open ports from certain filtered ones, leaving you
with the response open|filtered.
-sA (TCP ACK scan)
This scan is different than the others discussed so far in that
it never determines open (or even open|filtered) ports. It is
used to map out firewall rulesets, determining whether they are
stateful or not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you
use --scanflags). When scanning unfiltered systems, open and
closed ports will both return a RST packet. Nmap then labels
them as unfiltered, meaning that they are reachable by the ACK
packet, but whether they are open or closed is undetermined.
Ports that don’t respond, or send certain ICMP error messages
back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan)
Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to
differentiate open ports from closed ones, rather than always
printing unfiltered when a RST is returned. It does this by
examining the TCP Window field of the RST packets returned. On
some systems, open ports use a positive window size (even for
RST packets) while closed ones have a zero window. So instead of
always listing a port as unfiltered when it receives a RST back,
Window scan lists the port as open or closed if the TCP Window
value in that reset is positive or zero, respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can’t always trust it.
Systems that don’t support it will usually return all ports
closed. Of course, it is possible that the machine really has no
open ports. If most scanned ports are closed but a few common
port numbers (such as 22, 25, 53) are filtered, the system is
most likely susceptible. Occasionally, systems will even show
the exact opposite behavior. If your scan shows 1000 open ports
and 3 closed or filtered ports, then those three may very well
be the truly open ones.
-sM (TCP Maimon scan)
The Maimon scan is named after its discoverer, Uriel Maimon. He
described the technique in Phrack Magazine issue #49 (November
1996). Nmap, which included this technique, was released two
issues later. This technique is exactly the same as Null, FIN,
and Xmas scans, except that the probe is FIN/ACK. According to
RFC 793 (TCP), a RST packet should be generated in response to
such a probe whether the port is open or closed. However, Uriel
noticed that many BSD-derived systems simply drop the packet if
the port is open.
--scanflags (Custom TCP scan)
Truly advanced Nmap users need not limit themselves to the
canned scan types offered. The --scanflags option allows you to
design your own scan by specifying arbitrary TCP flags. Let your
creative juices flow, while evading intrusion detection systems
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN.
For example, --scanflags URGACKPSHRSTSYNFIN sets everything,
though it’s not very useful for scanning. The order these are
specified in is irrelevant.
In addition to specifying the desired flags, you can specify a
TCP scan type (such as -sA or -sF). That base type tells Nmap
how to interpret responses. For example, a SYN scan considers
no-response to indicate a filtered port, while a FIN scan treats
the same as open|filtered. Nmap will behave the same way it does
for the base scan type, except that it will use the TCP flags
you specify instead. If you don’t specify a base type, SYN scan
is used.
-sI (Idlescan)
This advanced scan method allows for a truly blind TCP port scan
of the target (meaning no packets are sent to the target from
your real IP address). Instead, a unique side-channel attack
exploits predictable IP fragmentation ID sequence generation on
the zombie host to glean information about the open ports on the
target. IDS systems will display the scan as coming from the
zombie machine you specify (which must be up and meet certain
criteria). This fascinating scan type is too complex to fully
describe in this reference guide, so I wrote and posted an
informal paper with full details at
http://insecure.org/nmap/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 IPID
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 8-bit IP
protocol field. The headers are usually empty, containing no
data and not even the proper header for the claimed protocol.
The three exceptions are TCP, UDP, and ICMP. A proper protocol
header for those is included since some systems won’t send them
otherwise and because Nmap already has functions to create them.
Instead of watching for ICMP port unreachable messages, protocol
scan is on the lookout for ICMP protocol unreachable messages.
If Nmap receives any response in any protocol from the target
host, Nmap marks that protocol as open. An ICMP protocol
unreachable error (type 3, code 2) causes the protocol to be
marked as closed Other ICMP unreachable errors (type 3, code 1,
3, 9, 10, or 13) cause the protocol to be marked filtered
(though they prove that ICMP is open at the same time). If no
response is received after retransmissions, the protocol is
marked open|filtered
-b (FTP bounce scan)
An interesting feature of the FTP protocol ([5]RFC 959) 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
open port 21 (or even for any ftp services if you scan all ports
with version detection), then try a bounce scan using each. Nmap
will tell you whether the host is vulnerable or not. If you are
just trying to cover your tracks, you don’t need to (and, in
fact, shouldn’t) limit yourself to hosts on the target network.
Before you go scanning random Internet addresses for vulnerable
FTP servers, consider that sysadmins may not appreciate you
abusing their servers in this way.
PORT SPECIFICATION AND SCAN ORDER
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans all
ports up to and including 1024 as well as higher numbered ports listed
in the nmap-services file for the protocol(s) being scanned.
-p (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 both TCP and UDP ports, you can specify a
particular protocol by preceding the port numbers by T: or U:.
The qualifier lasts until you specify another qualifier. For
example, the argument -p U:53,111,137,T:21-25,80,139,8080 would
scan UDP ports 53,111,and 137, as well as the listed TCP ports.
Note that to scan both UDP & TCP, you have to specify -sU and at
least one TCP scan type (such as -sS, -sF, or -sT). If no
protocol qualifier is given, the port numbers are added to all
protocol lists.
-F (Fast (limited port) scan)
Specifies that you only wish to scan for ports listed in the
nmap-services file which comes with nmap (or the protocols file
for -sO). This is much faster than scanning all 65535 ports on a
host. Because this list contains so many TCP ports (more than
1200), the speed difference from a default TCP scan (about 1650
ports) isn’t dramatic. The difference can be enormous if you
specify your own tiny nmap-services file using the --datadir
option.
-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 port scanning
instead.
SERVICE AND VERSION DETECTION
Point Nmap at a remote machine and it might tell you that ports 25/tcp,
80/tcp, and 53/udp are open. Using its nmap-services database of about
2,200 well-known services, Nmap would report that those ports probably
correspond to a mail server (SMTP), web server (HTTP), and name server
(DNS) respectively. This lookup is usually accurate -- the vast
majority of daemons listening on TCP port 25 are, in fact, mail
servers. However, you should not bet your security on this! People can
and do run services on strange ports.
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories) of
your companies or clients, you really want to know which mail and DNS
servers and versions are running. Having an accurate version number
helps dramatically in determining which exploits a server is vulnerable
to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes database
contains probes for querying various services and match expressions to
recognize and parse responses. Nmap tries to determine the service
protocol (e.g. ftp, ssh, telnet, http), the application name (e.g. ISC
Bind, Apache httpd, Solaris telnetd), the version number, hostname,
device type (e.g. printer, router), the OS family (e.g. Windows, Linux)
and sometimes miscellaneous details like whether an X server is open to
connections, the SSH protocol version, or the KaZaA user name). 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. When RPC
services are discovered, the Nmap RPC grinder (-sR) is automatically
used to determine the RPC program and version numbers. Some UDP ports
are left in the open|filtered state after a UDP port scan is unable to
determine whether the port is open or filtered. Version detection will
try to elicit a response from these ports (just as it does with open
ports), and change the state to open if it succeeds. open|filtered TCP
ports are treated the same way. Note that the Nmap -A option enables
version detection among other things. A paper documenting the workings,
usage, and customization of version detection is available at
http://insecure.org/nmap/vscan/.
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 3,000
pattern matches for more than 350 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 to enable both OS detection and version
detection.
--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 (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 1 and
9. The lower-numbered probes are effective against a wide
variety of common services, while the higher numbered ones are
rarely useful. The intensity level specifies which probes should
be applied. The higher the number, the more likely it is the
service will be correctly identified. However, high intensity
scans take longer. The intensity must be between 0 and 9. The
default is 7. When a probe is registered to the target port via
the nmap-service-probesports directive, that probe is tried
regardless of intensity level. This ensures that the DNS probes
will always be attempted against any open port 53, the SSL probe
will be done against 443, etc.
--version-light (Enable light mode)
This is a convenience alias for --version-intensity 2. This
light mode makes version scanning much faster, but it is
slightly less likely to identify services.
--version-all (Try every single probe)
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
--version-trace (Trace version scan activity)
This causes Nmap to print out extensive debugging info about
what version scanning is doing. It is a subset of what you get
with --packet-trace.
-sR (RPC scan)
This method works in conjunction with the various port scan
methods of Nmap. It takes all the TCP/UDP ports found open 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. This is
automatically enabled as part of version scan (-sV) if you
request that. As version detection includes this and is much
more comprehensive, -sR is rarely needed.
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, IPID sampling, and the initial window size check,
Nmap compares the results to its nmap-os-fingerprints database of more
than 1500 known OS fingerprints and prints out the OS details if there
is a match. Each fingerprint includes a freeform textual description of
the OS, and a classification which provides the vendor name (e.g. Sun),
underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type
(general purpose, router, switch, game console, etc).
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 several other tests which make use of information
that is gathered during the process anyway. One of these is uptime
measurement, which uses the TCP timestamp option (RFC 1323) to guess
when a machine was last rebooted. This is only reported for machines
which provide this information. Another 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, IPID Sequence Generation is also reported.
Most machines are in the “incremental” class, which means that they
increment the ID field in the IP header for each packet they send. This
makes them vulnerable to several advanced information gathering and
spoofing attacks.
A paper documenting the workings, usage, and customization of OS
detection is available at http://insecure.org/nmap/osdetect/.
OS detection is enabled and controlled with the following options:
-O (Enable OS detection)
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable both OS detection and version detection. 2nd
generation OS detection is tried first. If that fails, Nmap will
either print out the host fingerprint and ask you to submit it
(if you are certain about what the target host is running), or
Nmap will fall back to the 1st generation OS detection system in
case its larger database has a match.
-O2 (2nd Generation OS Detection Only)
Enables 2nd generation OS detection, but never falls back to the
old (1st generation) system, even if it fails to find any match.
This saves time and can reduce the number of packets sent to
each target.
-O1 (1nd Generation OS Detection Only)
Tells Nmap to only use the old OS detection system. If -O2 just
gives you a fingerprint to submit, but you don’t know what OS
the target is running, try -O1. But in that case, don’t submit
the fingerprint as you don’t know for sure whether -O1 guess
correctly. If it was perfect, we wouldn’t have bothered to
create -O2.
This option, and all other vestiges of the old OS detection
system, will likely be removed in late 2006 or in 2007.
--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 -P0
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. This option only affects
second generation OS detection (-O2, the default) and not the
old system (-O1).
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 tens or hundreds of 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 milliseconds
by default, though you can append ‘s’, ‘m’, or ‘h’ to the value to
specify seconds, minutes, or hours. So the --host-timeout arguments
900000, 900s, and 15m all do the same thing.
--min-hostgroup ; --max-hostgroup (Adjust parallel
scan group sizes)
Nmap has the ability to port scan or version scan multiple hosts
in parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results
can’t be provided until the whole group is finished. So if Nmap
started out with a group size of 50, the user would not receive
any reports (except for the updates offered in verbose mode)
until the first 50 hosts are completed.
By default, Nmap takes a compromise approach to this conflict.
It starts out with a group size as low as five so the first
results come quickly and then increases the groupsize to as high
as 1024. The exact default numbers depend on the options given.
For efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup,
Nmap will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
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 ; --max-parallelism (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 1 if the network proves unreliable and rise to several
hundred in perfect conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap’s
ability to control parallelism dynamically based on network
conditions. A value of ten 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. This
can be useful in combination with --scan-delay (discussed
later), although the latter usually serves the purpose well
enough by itself.
--min-rtt-timeout , --max-rtt-timeout ,
--initial-rtt-timeout (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 (-P0) 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 is a
reasonable aggressive --max-rtt-timeout 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 hping2 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 100ms, no matter what the ping times are. Nor do I
exceed 1000ms.
--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 (Specify the maximum number of port scan probe
retransmissions)
When Nmap receives no response to a port scan probe, it could
mean the port is filtered. Or maybe the probe or response was
simply lost on the network. It is also possible that the target
host has rate limiting enabled that temporarily blocked the
response. So Nmap tries again by retransmitting the initial
probe. If Nmap detects poor network reliability, it may try many
more times before giving up on a port. While this benefits
accuracy, it also lengthen scan times. When performance is
critical, scans may be sped up by limiting the number of
retransmissions allowed. You can even specify --max-retries 0 to
prevent any retransmissions, though that is rarely recommended.
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 (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. I often 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 as well, 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 ; --max-scan-delay (Adjust delay between
probes)
This option causes Nmap to wait at least the given amount of
time between each probe it sends to a given host. This is
particularly useful in the case of rate limiting. Solaris
machines (among many others) will usually respond to UDP scan
probe packets with only one ICMP message per second. Any more
than that sent by Nmap will be wasteful. A --scan-delay of 1s
will keep Nmap at that slow rate. Nmap tries to detect rate
limiting and adjust the scan delay accordingly, but it doesn’t
hurt to specify it explicitly if you already know what rate
works best.
When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically. The --max-scan-delay
option specifies the largest delay that Nmap will allow. 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).
--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 nonresponsive ports as open) by
specifying --defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear
nonresponse 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 optional
is useful when you only care about open ports, and
distinguishing between closed and filtered ports isn’t worth the
extra time.
-T (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
10ms for TCP ports and -T5 caps that value at 5 milliseconds.
Templates can be used in combination with fine grained controls,
and the fine-grained controls will you specify will take
precedence over the timing template default for that parameter.
I recommend using -T4 when scanning reasonably modern and
reliable networks. Keep that option even when you add fine
grained controls so that you benefit from those extra minor
optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is
too aggressive for my taste. People sometimes specify -T2
because they think it is less likely to crash hosts or because
they consider themselves to be polite in general. They often
don’t realize just how slow -T Polite really is. Their scan may
take ten times longer than a default scan. Machine crashes and
bandwidth problems are rare with the default timing options
(-T3) and so I normally recommend that for cautious scanners.
Omitting version detection is far more effective than playing
with timing values at reducing these problems.
While -T0 and -T1 may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to set
the exact timing values you need rather than rely on the canned
-T0 and -T1 values.
The main effects of T0 are serializing the scan so only one port
is scanned at a time, and waiting five minutes between sending
each probe. T1 and T2 are similar but they only wait 15 seconds
and 0.4 seconds, respectively, between probes. T3 is Nmap’s
default behavior, which includes parallelization. T4 does the
equivalent of --max-rtt-timeout 1250 --initial-rtt-timeout 500
--max-retries 6 and sets the maximum TCP scan delay to 10
milliseconds. T5 does the equivalent of --max-rtt-timeout 300
--min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2
--host-timeout 15m as well as setting the maximum TCP scan delay
to 5ms.
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 mindset of an attacker,
and deploy techniques from this section against your networks. Launch
an FTP bounce scan, Idle scan, fragmentation attack, or try to tunnel
through one of your own proxies.
In addition to restricting network activity, companies are increasingly
monitoring traffic with intrusion detection systems (IDS). All of the
major IDSs ship with rules designed to detect Nmap scans because scans
are sometimes a precursor to attacks. Many of these products have
recently morphed into intrusion prevention systems (IPS) that actively
block traffic deemed malicious. Unfortunately for network
administrators and IDS vendors, reliably detecting bad intentions by
analyzing packet data is a tough problem. Attackers with patience,
skill, and the help of certain Nmap options can usually pass by IDSs
undetected. Meanwhile, administrators must cope with large numbers of
false positive results where innocent activity is misdiagnosed and
alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is that
these methods would still be used by attackers, who would just find
other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and subverting
firewalls and IDS systems. It takes skill and experience. A tutorial is
beyond the scope of this reference guide, which only lists the relevant
options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU)
The -f option causes the requested scan (including ping scans)
to use tiny fragmented IP packets. The idea is to split up the
TCP header over several packets to make it harder for packet
filters, intrusion detection systems, and other annoyances to
detect what you are doing. Be careful with this! Some programs
have trouble handling these tiny packets. The old-school sniffer
named Sniffit segmentation faulted immediately upon receiving
the first fragment. Specify this option once, and Nmap splits
the packets into 8 bytes or less after the IP header. So a
20-byte TCP header would be split into 3 packets. Two with eight
bytes of the TCP header, and one with the final four. Of course
each fragment also has an IP header. Specify -f again to use 16
bytes per fragment (reducing the number of fragments). Or you
can specify your own offset size with the --mtu option. Don’t
also specify -f if you use --mtu. The offset must be a multiple
of 8. While fragmented packets won’t get by packet filters and
firewalls that queue all IP fragments, such as the
CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some
networks can’t afford the performance hit this causes and thus
leave it disabled. 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 Ethereal 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.
-D (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 6th position or later, some
common port scan detectors (such as Solar Designer’s excellent
scanlogd) are unlikely to show your IP address at all. If you
don’t use ME, nmap will put you in a random position.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy
to determine which host is scanning if only one is actually up
on the network. You might want to use IP addresses instead of
names (so the decoy networks don’t see you in their nameserver
logs).
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase.
Decoys are also used during remote OS detection (-O). Decoys do
not work with version detection or TCP connect scan.
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 (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 -P0 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 (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 ; -g (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 complains from ungrateful
users whose applications stopped working. In particular, DNS may
be broken because the UDP DNS replies from external servers can
no longer enter the network. FTP is another common example. In
active FTP transfers, the remote server tries to establish a
connection back to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active ftp from port 20,
many admins 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, admins 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.
Nmap must use different port numbers for certain OS detection
tests to work properly, and DNS requests ignore the
--source-port flag because Nmap relies on system libraries to
handle those. Most TCP scans, including SYN scan, support the
option completely, as does UDP scan.
--data-length (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. This option tells Nmap to append the given number
of random bytes to most of the packets it sends. OS detection
(-O) packets are not affected 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 ; --ip-options (Send packets with specified ip options)
The [6]IP protocol offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options
are rarely seen due to practicality and security concerns. In
fact, many Internet routers block the most dangerous options
such as source routing. Yet options can still be useful in some
cases for determining and manipulating the network route to
target machines. For example, you may be able to use the record
route option to determine a path to a target even when more
traditional traceroute-style approaches fail. Or if your packets
are being dropped by a certain firewall, you may be able to
specify a different route with the strict or loose source
routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with \x then the two digits. You may repeat certain characters
by following them with an asterisk and then the number of times
you wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is
a hex string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,
record-timestamp, or both options together, respectively. Loose
or strict source routing may be specified with an L or S
followed by a space and then a space-separated list of IP
addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of
using IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/0052.html.
--ttl (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 8096 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 (Spoof MAC address)
Asks Nmap to use the given MAC address for all of the raw
ethernet frames it sends. This option implies --send-eth to
ensure that Nmap actually sends ethernet-level packets. The MAC
given can take several formats. If it is simply the string “0”,
Nmap chooses a completely random MAC 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 less than 12 hex digits are provided, Nmap fills in the
remainder of the 6 bytes with random values. If the argument
isn’t a 0 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
(3-byte prefix) and fills out the remaining 3 bytes randomly.
Valid --spoof-mac argument examples are Apple, 0,
01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco.
--badsum (Send packets with bogus TCP/UDP checksums)
Asks Nmap to use an invalid TCP or UDP checksum for packets sent
to target hosts. Since virtually all host IP stacks properly
drop these packets, any responses received are likely coming
from a firewall or IDS that didn’t bother to verify the
checksum. For more details on this technique, see
http://www.phrack.org/phrack/60/p60-0x0c.txt
OUTPUT
Any security tools is only as useful as the output it generates.
Complex tests and algorithms are of little value if they aren’t
presented in an organized and comprehensible fashion. Given the number
of ways Nmap is used by people and other software, no single format can
please everyone. So Nmap offers several formats, including the
interactive mode for humans to read directly and XML for easy parsing
by software.
In addition to offering different output formats, Nmap provides options
for controlling the verbosity of output as well as debugging messages.
Output types may be sent to standard output or to named files, which
Nmap can append to or clobber. Output files may also be used to resume
aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output, and it is sent to standard output (stdout).
There is also normal output, which is similar to interactive except
that it displays less runtime information and warnings since it is
expected to be analyzed after the scan completes rather than
interactively.
XML output is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each format
may only be specified once. For example, you may wish to save normal
output for your own review while saving XML of the same scan for
programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or two
describing the scan, placed in a directory named after the company I’m
scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn’t specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes Nmap
to deactivate interactive output, and instead print results in the
format you specified to the standard output stream. So the command nmap
-oX - target will send only XML output to stdout. Serious errors may
still be printed to the normal error stream, stderr.
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit the
flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal format
output files named G- and Xscan.xml respectively.
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 (Normal output)
Requests that normal output be directed to the given filename.
As discussed above, this differs slightly from interactive
output.
-oX (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
http://insecure.org/nmap/data/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 [7]Nmap::Scanner
and [8]Nmap::Parser in Perl CPAN. In almost all cases that a
non-trivial application interfaces with Nmap, XML is the
preferred format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is
simply to load the XML output in a web browser such as Firefox
or IE. By default, this will only work on the machine you ran
Nmap on (or a similarly configured one) due to the hard-coded
nmap.xsl filesystem path. Use the --webxml or --stylesheet
options to create portable XML files that render as HTML on any
web-connected machine.
-oS (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 (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 6 labeled fields, separated by tabs and followed
with a colon. The fields are Host, Ports, Protocols, Ignored
State, OS, Seq Index, IPID, and Status.
The most important of these fields is generally Ports, which
gives details on each interesting port. It is a comma separated
list of port entries. Each port entry represents one interesting
port, and takes the form of seven slash (/) separated subfields.
Those subfields are: Port number, State, Protocol, Owner,
Service, SunRPC info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable
output format is available from
http://www.unspecific.com/nmap-oG-output.
-oA (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 for even greater verbosity. Using it more than twice has
no effect.
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 [level] (Increase or set 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. Alternatively, you can set a debug level by
giving an argument to -d. For example, -d9 sets level nine. That
is the highest effective level and will produce thousands of
lines unless you run a very simple scan with very few ports and
targets.
Debugging output is useful when a bug is suspected in Nmap, or
if you are simply confused as to what Nmap is doing and why. As
this feature is mostly intended for developers, debug lines
aren’t always self-explanatory. You may get something like:
Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==>
srtt: 14987 rttvar: 14987 to: 100000. If you don’t understand a
line, your only recourses are to ignore it, look it up in the
source code, or request help from the development list
(nmap-dev). Some lines are self explanatory, but the messages
become more obscure as the debug level is increased.
--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.
--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 open, open|filtered, and unfiltered
ports. These three ports are treated just as they normally 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).
--log-errors (Log errors/warnings to normal mode output file)
Warnings and errors printed by Nmap usually go only to the
screen (interactive output), leaving any specified normal-fomat
output files uncluttered. But when you do want to see those
messages in the normal output file you specified, add this
option. It is useful when you aren’t watching the interactive
output or are trying to debug a problem. The messages will also
still appear in interactive mode. This will not work for most
errors related to bad command-line arguments, as Nmap may not
have initialized its output files yet. In addition, some Nmap
error/warning messages use a different system that does not yet
support this option. An alternative to using this option is
redirecting interactive output (including the standard error
stream) to a file. While most UNIX shells make that approach
easy, it can be difficult on Windows.
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 (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 admin 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 (Set XSL stylesheet to transform XML output)
Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
transla