Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single hosts. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many systems and network administrators find it useful for routine tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental information on each depending on the options used. Key among that information is the “interesting ports table”. That table lists the port number and protocol, service name, and state. The state is either open, filtered, closed, or unfiltered. Open means that an application on the target machine is listening for connections/packets on that port. Filtered means that a firewall, filter, or other network obstacle is blocking the port so that Nmap cannot tell whether it is open or closed. Closed ports have no application listening on them, though they could open up at any time. Ports are classified as unfiltered when they are responsive to Nmap’s probes, but Nmap cannot determine whether they are open or closed. Nmap reports the state combinations open|filtered and closed|filtered when it cannot determine which of the two states describe a port. The port table may also include software version details when version detection has been requested. When an IP protocol scan is requested (-sO), Nmap provides information on supported IP protocols rather than listening ports.
In addition to the interesting ports table, Nmap can provide further information on targets, including reverse DNS names, operating system guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap arguments used in this example are -A, to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution; and then the two target hostnames.
Example 1. A representative Nmap scan
nmap -A -T4 221.194.128.175
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2009-01-12 17:32 CST
Interesting ports on 221.194.128.175:
Not shown: 1670 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 3.9p1 (protocol 1.99)
53/tcp open domain ISC Bind 9.X
80/tcp open http Apache httpd 2.2.9 ((Unix) PHP/5.2.6)
135/tcp filtered msrpc
137/tcp filtered netbios-ns
139/tcp filtered netbios-ssn
445/tcp filtered microsoft-ds
593/tcp filtered http-rpc-epmap
3306/tcp open mysql MySQL (unauthorized)
4444/tcp filtered krb524
Device type: general purpose
Running: Linux 2.4.X|2.5.X
OS details: Linux 2.4.0 – 2.5.20
Options Summary
Nmap 4.11 ( http://www.insecure.org/nmap/ )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
–exclude <host1[,host2][,host3],…>: Exclude hosts/networks
–excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan – simply list targets to scan
-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 <serv1[,serv2],…>: 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
-sN/sF/sX: TCP Null, FIN, and Xmas scans
–scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idlescan
-sO: IP protocol scan
-b <ftp relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
-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 <level>: Set from 0 (light) to 9 (try all probes)
–version-light: Limit to most likely probes (intensity 2)
–version-all: Try every single probe (intensity 9)
–version-trace: Show detailed version scan activity (for debugging)
OS DETECTION:
-O: Enable OS detection
–osscan-limit: Limit OS detection to promising targets
–osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in 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 <size>: Parallel host scan group sizes
–min-parallelism/max-parallelism <time>: Probe parallelization
–min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
–max-retries <tries>: Caps number of port scan probe retransmissions.
–host-timeout <time>: Give up on target after this long
–scan-delay/–max-scan-delay <time>: Adjust delay between probes
FIREWALL/IDS EVASION AND SPOOFING:
-f; –mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],…>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/–source-port <portnum>: Use given port number
–data-length <num>: Append random data to sent packets
–ttl <val>: Set IP time-to-live field
–spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
–badsum: Send packets with a bogus TCP/UDP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use twice for more effect)
-d[level]: Set or increase debugging level (Up to 9 is meaningful)
–packet-trace: Show all packets sent and received
–iflist: Print host interfaces and routes (for debugging)
–log-errors: Log errors/warnings to the normal-format output file
–append-output: Append to rather than clobber specified output files
–resume <filename>: Resume an aborted scan
–stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
–webxml: Reference stylesheet from 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 <dirname>: Specify custom Nmap data file location
–send-eth/–send-ip: Send using raw ethernet frames or IP packets
–privileged: Assume that the user is fully privileged
-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
SEE THE MAN PAGE FOR MANY MORE OPTIONS, DESCRIPTIONS, AND EXAMPLES
Nmap Examples
Here are some Nmap usage examples, from the simple and routine to a little more complex and esoteric. Some actual IP addresses and domain names are used to make things more concrete. In their place you should substitute addresses/names from your own network.. While I don’t think port scanning other networks is or should be illegal, some network administrators don’t appreciate unsolicited scanning of their networks and may complain. Getting permission first is the best approach.
For testing purposes, you have permission to scan the host scanme.nmap.org. This permission only includes scanning via Nmap and not testing exploits or denial of service attacks. To conserve bandwidth, please do not initiate more than a dozen scans against that host per day. If this free scanning target service is abused, it will be taken down and Nmap will report Failed to resolve given hostname/IP: scanme.nmap.org. These permissions also apply to the hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do not currently exist.
This option scans all reserved TCP ports on the machine scanme.nmap.org . The -v option enables verbose mode.
Launches a stealth SYN scan against each machine that is up out of the 255 machines on “class C” network where Scanme resides. It also tries to determine what operating system is running on each host that is up and running. This requires root privileges because of the SYN scan and OS detection.
nmap -sV -p 21,53,110,123,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of each of the 255 possible eight-bit subnets in the 198.116 class B address space. This tests whether the systems run SSH, DNS, POP3, or IMAP on their standard ports, or anything on port 4564. For any of these ports found open, version detection is used to determine what application is running.
Asks Nmap to choose 100,000 hosts at random and scan them for web servers (port 80). Host enumeration is disabled with -PN since first sending a couple probes to determine whether a host is up is wasteful when you are only probing one port on each target host anyway.
nmap -PN -p80 -oX logs/port80scan.xml -oG logs/port80scan.gnmap 221.194.128.0/20
This scans 4096 IPs for any web servers (without pinging them) and saves the output in grepable and XML formats.
