The Art of Port Scanning

by Fyodor <>

(Last significant update: Sat Sep 6 03:24:53 GMT 1997)


This paper details many of the techniques used to determine what ports (or similar protocol abstraction) of a host are listening for connections. These ports represent potential communication channels. Mapping their existence facilitates the exchange of information with the host, and thus it is quite useful for anyone wishing to explore their networked environment, including hackers. Despite what you have heard from the media, the Internet is NOT all about TCP port 80. Anyone who relies exclusively on the WWW for information gathering is likely to gain the same level of proficiency as your average AOLer, who does the same. This paper is also meant to serve as an introduction to and ancillary documentation for a coding project I have been working on. It is a full featured, robust port scanner which (I hope) solves some of the problems I have encountered when dealing with other scanners and when working to scan massive networks. The tool, nmap, supports the following:

The freely distributable source code is appended to this paper.


Scanning, as a method for discovering exploitable communication channels, has been around for ages. The idea is to probe as many listeners as possible, and keep track of the ones that are receptive or useful to your particular need. Much of the field of advertising is based on this paradigm, and the "to current resident" brute force style of bulk mail is an almost perfect parallel to what we will discuss. Just stick a message in every mailbox and wait for the responses to trickle back.

Scanning entered the h/p world along with the phone systems. Here we have this tremendous global telecommunications network, all reachable through codes on our telephone. Millions of numbers are reachable locally, yet we may only be interested in 0.5% of these numbers, perhaps those that answer with a carrier.

The logical solution to finding those numbers that interest us is to try them all. Thus the field of "wardialing" arose. Excellent programs like Toneloc were developed to facilitate the probing of entire exchanges and more. The basic idea is simple. If you dial a number and your modem gives you a CONNECT, you record it. Otherwise the computer hangs up and tirelessly dials the next one.

While wardialing is still useful, we are now finding that many of the computers we wish to communicate with are connected through networks such as the Internet rather than analog phone dialups. Scanning these machines involves the same brute force technique. We send a blizzard of packets for various protocols, and we deduce which services are listening from the responses we receive (or don't receive).


Over time, a number of techniques have been developed for surveying the protocols and ports on which a target machine is listening. They all offer different benefits and problems. Here is a line up of the most common:


Prior to writing nmap, I spent a lot of time with other scanners exploring the Internet and various private networks (note the avoidance of the "intranet" buzzword). I have used many of the top scanners available today, including strobe by Julian Assange, netcat by *Hobbit*, stcp by Uriel Maimon, pscan by Pluvius, ident-scan by Dave Goldsmith, and the SATAN tcp/udp scanners by Wietse Venema. These are all excellent scanners! In fact, I ended up hacking most of them to support the best features of the others. Finally I decided to write a whole new scanner, rather than rely on hacked versions of a dozen different scanners in my /usr/local/sbin. While I wrote all the code, nmap uses a lot of good ideas from its predecessors. I also incorporated some new stuff like fragmentation scanning and options that were on my "wish list" for other scanners. Here are some of the (IMHO) useful features of nmap:

Some other, more minor options:
 -v (verbose): This is highly recommended for interactive use.  Among other
useful messages, you will see ports come up as they are found, rather than
having to wait for the sorted summary list.

 -r (randomize): This will randomize the order in which the target host's
ports are scanned.

 -q (quash argv): This changes argv[0] to FAKE_ARGV ("pine" by default).
It also eliminates all other arguments, so you won't look too suspicious in
'w' or 'ps' listings.

 -h for an options summary.

 -R show and resolve all hosts, even down ones.
Also look for, which is the web site I plan to put future versions and more information on. In fact, you would be well advised to check there right now. (If that isn't where you are reading this).
Example Usage

To launch a stealth scan of the entire class 'B' networks and for the popularly exploitable imapd daemon:
# nmap -Up 143
To do a standard tcp scan on the reserved ports of host <target>:
> nmap target
To check the class 'C' network on which sits for popular services (via fragmented SIN scan):
# nmap -fsp 21,22,23,25,80,110
To scan the same network for all the services in your /etc/services via (very fast) tcp scan:
> nmap -F
To scan using the ftp bounce attack off of
> nmap -Db
To find hosts that are up in the the adjacent class C's 193.14.12, .13, .14, .15, ... , .30:
> nmap -P '193.14.[12-30].*'
If you don't want to have to quote it to avoid shell interpretation, this does the same thing:
> nmap -P 193.14.12-30.0-255