NC(1)                      OpenBSD Reference Manual                      NC(1)

NAME
     nc - arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc [-46dhklnrStUuvz] [-i interval] [-p source_port]
        [-s source_ip_address] [-w timeout] [-X socks_version]
        [-x proxy_address[:port]] [hostname] [port[s]]

DESCRIPTION
     The nc (or netcat) utility is used for just about anything under the sun
     involving TCP or UDP.  It can open TCP connections, send UDP packets,
     listen on arbitrary TCP and UDP ports, do port scanning, and deal with
     both IPv4 and IPv6.  Unlike telnet(1), nc scripts nicely, and separates
     error messages onto standard error instead of sending them to standard
     output, as telnet(1) does with some.

     Destination ports can be single integers or ranges.  Ranges are in the
     form nn-mm.

     Common uses include:

           o   simple TCP proxies
           o   shell-script based HTTP clients and servers
           o   network daemon testing
           o   and much, much more

     The options are as follows:

     -4      Forces nc to use IPv4 addresses only.

     -6      Forces nc to use IPv6 addresses only.

     -d      Do not attempt to read from stdin.

     -h      Prints out nc help.

     -i interval
             Specifies a delay time interval between lines of text sent and
             received.  Also causes a delay time between connections to multi-
             ple ports.

     -k      Forces nc to stay listening for another connection after its cur-
             rent connection is completed.  It is an error to use this option
             without the -l option.

     -l      Used to specify that nc should listen for an incoming connection
             rather than initiate a connection to a remote host.  It is an er-
             ror to use this option in conjunction with the -p, -s, or -z op-
             tions.

     -n      Do not do any DNS or service lookups on any specified addresses,
             hostnames or ports.

     -p source_port
             Specifies the source port nc should use, subject to privilege re-
             strictions and availability.  It is an error to use this option
             in conjunction with the -l option.

     -r      Specifies that source and/or destination ports should be chosen
             randomly instead of sequentially within a range or in the order
             that the system assigns them.

     -S      Enables the RFC 2385 TCP MD5 signature option.

     -s source_ip_address
             Specifies the IP of the interface which is used to send the pack-
             ets.  It is an error to use this option in conjunction with the
             -l option.

     -t      Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO
             and WILL requests.  This makes it possible to use nc to script
             telnet sessions.

     -U      Specifies to use Unix Domain Sockets.

     -u      Use UDP instead of the default option of TCP.

     -v      Have nc give more verbose output.

     -w timeout
             If a connection and stdin are idle for more than timeout seconds,
             then the connection is silently closed.  The -w flag has no ef-
             fect on the -l option, i.e. nc will listen forever for a connec-
             tion, with or without the -w flag.  The default is no timeout.

     -X socks_version
             Requests that nc should use the specified version of the SOCKS
             protocol when talking to a SOCKS proxy.  SOCKS versions 4 and 5
             are currently supported.  If the version is not specified, SOCKS
             version 5 is used.

     -x proxy_address[:port]
             Requests that nc should connect to hostname using a SOCKS proxy
             at proxy_address and port.  If port is not specified, port 1080
             is used.

     -z      Specifies that nc should just scan for listening daemons, without
             sending any data to them.  It is an error to use this option in
             conjunction with the -l option.

CLIENT/SERVER MODEL
     It is quite simple to build a very basic client/server model using nc.
     On one console, start nc listening on a specific port for a connection.
     For example:

           $ nc -l 1234

     nc is now listening on port 1234 for a connection.  On a second console
     (or a second machine), connect to the machine and port being listened on:

           $ nc 127.0.0.1 1234

     There should now be a connection between the ports.  Anything typed at
     the second console will be concatenated to the first, and vice-versa.
     After the connection has been set up, nc does not really care which side
     is being used as a `server' and which side is being used as a `client'.
     The connection may be terminated using an EOF (`^D').

DATA TRANSFER
     The example in the previous section can be expanded to build a basic data
     transfer model.  Any information input into one end of the connection
     will be output to the other end, and input and output can be easily cap-
     tured in order to emulate file transfer.

     Start by using nc to listen on a specific port, with output captured into
     a file:

           $ nc -l 1234 > filename.out

     Using a second machine, connect to the listening nc process, feeding it
     the file which is to be transferred:

           $ nc host.example.com 1234 < filename.in

     After the file has been transferred, the connection will close automati-
     cally.

TALKING TO SERVERS
     It is sometimes useful to talk to servers ``by hand'' rather than through
     a user interface.  It can aid in troubleshooting, when it might be neces-
     sary to verify what data a server is sending in response to commands is-
     sued by the client.  For example, to retrieve the home page of a web
     site:

           $ echo "GET" | nc host.example.com 80

     Note that this also displays the headers sent by the web server.  They
     can be filtered, using a tool such as sed(1), if necessary.

     More complicated examples can be built up when the user knows the format
     of requests required by the server.  As another example, an email may be
     submitted to an SMTP server using:

           $ nc localhost 25 << EOF
           HELO host.example.com
           MAIL FROM: <user@host.example.com>
           RCPT TO: <user2@host.example.com>
           DATA
           Body of email.
           .
           QUIT
           EOF

PORT SCANNING
     It may be useful to know which ports are open and running services on a
     target machine.  The -z flag can be used to tell nc not to initiate a
     connection, together with the -v (verbose) flag, to report open ports.
     For example:

           $ nc -vz host.example.com 20-30
           Connection to host.example.com 22 port [tcp/ssh] succeeded!
           Connection to host.example.com 25 port [tcp/smtp] succeeded!

     The port range was specified to limit the search to ports 20 - 30.

     Alternatively, it might be useful to know which server software is run-
     ning, and which versions.  This information is often contained within the
     greeting banners.  In order to retrieve these, it is necessary to first
     make a connection, and then break the connection when the banner has been
     retrieved.  This can be accomplished by specifying a small timeout with
     the -w flag, or perhaps by issuing a "QUIT" command to the server:

           $ echo "QUIT" | nc host.example.com 20-30
           SSH-1.99-OpenSSH_3.6.1p2
           Protocol mismatch.
           220 host.example.com IMS SMTP Receiver Version 0.84 Ready

EXAMPLES
     Open a TCP connection to port 42 of hostname, using port 31337 as the
     source port, with a timeout of 5 seconds:

           $ nc -p 31337 -w 5 hostname 42

     Open a UDP connection to port 53 of hostname:

           $ nc -u hostname 53

     Open a TCP connection to port 42 of example.host using 10.1.2.3 as the IP
     for the local end of the connection:

           $ nc -s 10.1.2.3 example.host 42

     Send UDP packets to ports 20-30 of example.host, and report which ones
     responded with an ICMP packet after three seconds:

           $ nc -uvz -w 3 hostname 20-30

     Create and listen on a Unix Domain Socket:

           $ nc -lU /var/tmp/dsocket

SEE ALSO
     cat(1)

AUTHORS
     Original implementation by *Hobbit* <hobbit@avian.org>.
     Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.

OpenBSD 3.5                      June 25, 2001                               4
