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NAME    [Toc]    [Back]

     nc - arbitrary TCP and UDP connections and listens

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

     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
     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      Enable debugging on the socket.

     -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 multiple

     -k      Forces nc to stay listening for  another  connection
after its current
 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 error
 to use this option in conjunction with  the  -p,
-s, or -z options.

     -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 restrictions
  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 packets.
   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 effect
 on the -l option, i.e. nc will  listen  forever
for a connection,
  with  or without the -w flag.  The default is
no timeout.

     -X socks_version
             Requests that nc should use the specified version of
             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.

     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
     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 1234

     There  should  now  be a connection between the ports.  Anything typed at
     the second console will be concatenated to  the  first,  and
     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    [Toc]    [Back]

     The example in the previous section can be expanded to build
a basic data
     transfer model.  Any information input into one end  of  the
     will be output to the other end, and input and output can be
easily captured
 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 automatically.

TALKING TO SERVERS    [Toc]    [Back]

     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 necessary
  to verify what data a server is sending in response to
commands issued
 by the client.  For example, to retrieve the home  page
of a web

           $ 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>
           Body of email.

PORT SCANNING    [Toc]    [Back]

     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 running,
 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

           $ echo "QUIT" | nc host.example.com 20-30
           Protocol mismatch.
           220  host.example.com  IMS  SMTP Receiver Version 0.84

EXAMPLES    [Toc]    [Back]

     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 as the IP
     for the local end of the connection:

           $ nc -s 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    [Toc]    [Back]


AUTHORS    [Toc]    [Back]

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

OpenBSD     3.6                           June      25,      2001
[ Back ]
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