Command Section
NC(1)                   FreeBSD General Commands Manual                  NC(1)

     nc - arbitrary TCP and UDP connections and listens

     nc [-46DdEFhklNnrStUuvz] [-e IPsec_policy] [-I length] [-i interval]
        [--no-tcpopt] [-O length] [-P proxy_username] [-p source_port]
        [-s source] [-T toskeyword] [-V rtable] [-w timeout]
        [-X proxy_protocol] [-x proxy_address[:port]] [destination] [port]

     The nc (or netcat) utility is used for just about anything under the sun
     involving TCP, UDP, or UNIX-domain sockets.  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.

     Common uses include:

              simple TCP proxies
              shell-script based HTTP clients and servers
              network daemon testing
              a SOCKS or HTTP ProxyCommand for ssh(1)
              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.

     -E      Shortcut for "-e 'in ipsec esp/transport//require' -e 'out ipsec
             esp/transport//require'", which enables IPsec ESP transport mode
             in both directions.

     -e      If IPsec support is available, then one can specify the IPsec
             policies to be used using the syntax described in
             ipsec_set_policy(3).  This flag can be specified up to two times,
             as typically one policy for each direction is needed.

     -F      Pass the first connected socket using sendmsg(2) to stdout and
             exit.  This is useful in conjunction with -X to have nc perform
             connection setup with a proxy but then leave the rest of the
             connection to another program (e.g. ssh(1) using the
             ssh_config(5) ProxyUseFdpass option).

     -h      Prints out nc help.

     -I length
             Specifies the size of the TCP receive buffer.

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

     -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.  When used together with the -u
             option, the server socket is not connected and it can receive UDP
             datagrams from multiple hosts.

     -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.  Additionally, any timeouts specified with the -w option
             are ignored.

     -N      shutdown(2) the network socket after EOF on the input.  Some
             servers require this to finish their work.

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

             Disables the use of TCP options on the socket, by setting the
             boolean TCP_NOOPT socket option.

     -O length
             Specifies the size of the TCP send buffer.

     -P proxy_username
             Specifies a username to present to a proxy server that requires
             authentication.  If no username is specified then authentication
             will not be attempted.  Proxy authentication is only supported
             for HTTP CONNECT proxies at present.

     -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
             Specifies the IP of the interface which is used to send the
             packets.  For UNIX-domain datagram sockets, specifies the local
             temporary socket file to create and use so that datagrams can be
             received.  It is an error to use this option in conjunction with
             the -l option.

     -T toskeyword
             Change IPv4 TOS value.  toskeyword may be one of critical,
             inetcontrol, lowdelay, netcontrol, throughput, reliability, or
             one of the DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7;
             or a number in either hex or decimal.

     -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.  For UNIX-domain
             sockets, use a datagram socket instead of a stream socket.  If a
             UNIX-domain socket is used, a temporary receiving socket is
             created in /tmp unless the -s flag is given.

     -V rtable
             Set the routing table (``FIB'') to be used.

     -v      Have nc give more verbose output.

     -w timeout
             Connections which cannot be established or are idle timeout after
             timeout seconds.  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 proxy_protocol
             Requests that nc should use the specified protocol when talking
             to the proxy server.  Supported protocols are ``4'' (SOCKS v.4),
             ``5'' (SOCKS v.5) and ``connect'' (HTTPS proxy).  If the protocol
             is not specified, SOCKS version 5 is used.

     -x proxy_address[:port]
             Requests that nc should connect to destination using a proxy at
             proxy_address and port.  If port is not specified, the well-known
             port for the proxy protocol is used (1080 for SOCKS, 3128 for

     -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.

     destination can be a numerical IP address or a symbolic hostname (unless
     the -n option is given).  In general, a destination must be specified,
     unless the -l option is given (in which case the local host is used).
     For UNIX-domain sockets, a destination is required and is the socket path
     to connect to (or listen on if the -l option is given).

     port can be a single integer or a range of ports.  Ranges are in the form
     nn-mm.  In general, a destination port must be specified, unless the -U
     option is given.

     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 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').

     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
     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 -N 1234 <

     After the file has been transferred, the connection will close

     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

           $ printf "GET / HTTP/1.0\r\n\r\n" | nc 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
           MAIL FROM:<[email protected]>
           RCPT TO:<[email protected]>
           Body of email.

     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 to report open ports,
     rather than initiate a connection.  For example:

           $ nc -z 20-30
           Connection to 22 port [tcp/ssh] succeeded!
           Connection to 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 20-30
           Protocol mismatch.
           220 IMS SMTP Receiver Version 0.84 Ready

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

           $ nc -p 31337 -w 5 42

     Open a UDP connection to port 53 of

           $ nc -u 53

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

           $ nc -s 42

     Open a TCP connection to port 42 of using IPsec ESP for
     incoming and outgoing traffic.

           $ nc -E 42

     Open a TCP connection to port 42 of using IPsec ESP for
     outgoing traffic only.

           $ nc -e 'out ipsec esp/transport//require' 42

     Create and listen on a UNIX-domain stream socket:

           $ nc -lU /var/tmp/dsocket

     Connect to port 42 of via an HTTP proxy at,
     port 8080.  This example could also be used by ssh(1); see the
     ProxyCommand directive in ssh_config(5) for more information.

           $ nc -x10.2.3.4:8080 -Xconnect 42

     The same example again, this time enabling proxy authentication with
     username ``ruser'' if the proxy requires it:

           $ nc -x10.2.3.4:8080 -Xconnect -Pruser 42

     The nc utility exits 0 on success, and >0 if an error occurs.

     cat(1), setfib(1), ssh(1), tcp(4)

     Original implementation by *Hobbit* <[email protected]>.
     Rewritten with IPv6 support by Eric Jackson <[email protected]>.

     UDP port scans using the -uz combination of flags will always report
     success irrespective of the target machine's state.  However, in
     conjunction with a traffic sniffer either on the target machine or an
     intermediary device, the -uz combination could be useful for
     communications diagnostics.  Note that the amount of UDP traffic
     generated may be limited either due to hardware resources and/or
     configuration settings.

FreeBSD 11.1-RELEASE-p4       September 26, 2015       FreeBSD 11.1-RELEASE-p4
Command Section