NAME

     nc — arbitrary TCP and UDP connections and listens

SYNOPSIS

     nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl] [-m minttl] [-O length]
        [-P proxy_username] [-p source_port] [-q seconds] [-s sourceaddr] [-T keyword]
        [-V rtable] [-W recvlimit] [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
        [destination] [port]

DESCRIPTION

     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      Use IPv4 addresses only.

     -6      Use IPv6 addresses only.

     -b      Allow broadcast.

     -C      Send CRLF as line-ending.  Each line feed (LF) character from the input data is
             translated into CR+LF before being written to the socket.  Line feed characters that
             are already preceded with a carriage return (CR) are not translated.  Received data
             is not affected.

     -D      Enable debugging on the socket.

     -d      Do not attempt to read from stdin.

     -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).  Cannot be used with -U.

     -h      Print out the nc help text and exit.

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

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

     -k      When a connection is completed, listen for another one.  Requires -l.  When used
             together with the -u option, the server socket is not connected and it can receive
             UDP datagrams from multiple hosts.

     -l      Listen for an incoming connection rather than initiating a connection to a remote
             host.  The destination and port to listen on can be specified either as non-optional
             arguments, or with options -s and -p respectively.  Cannot be used together with -x
             or -z.  Additionally, any timeouts specified with the -w option are ignored.

     -M ttl  Set the TTL / hop limit of outgoing packets.

     -m minttl
             Ask the kernel to drop incoming packets whose TTL / hop limit is under minttl.

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

     -n      Do not perform domain name resolution.  If a name cannot be resolved without DNS, an
             error will be reported.

     -O length
             Specify 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
             Specify the source port nc should use, subject to privilege restrictions and
             availability.

     -q seconds
             after EOF on stdin, wait the specified number of seconds and then quit. If seconds
             is negative, wait forever (default).  Specifying a non-negative seconds implies -N.

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

     -S      Enable the RFC 2385 TCP MD5 signature option.

     -s sourceaddr
             Set the source address to send packets from, which is useful on machines with
             multiple interfaces.  For UNIX-domain datagram sockets, specifies the local
             temporary socket file to create and use so that datagrams can be received.  Cannot
             be used together with -x.

     -T keyword
             Change the IPv4 TOS/IPv6 traffic class value.  keyword may be one of critical,
             inetcontrol, lowcost, 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      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      Use UNIX-domain sockets.  Cannot be used together with -F or -x.  On Linux, if the
             name starts with an at symbol (`@') it is read as an abstract namespace socket: the
             leading `@' is replaced with a NUL byte before binding or connecting.  For details,
             see unix(7).

     -u      Use UDP instead of TCP.  Cannot be used together with -x.  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 to be used.

     -v      Produce more verbose output.

     -W recvlimit
             Terminate after receiving recvlimit packets from the network.

     -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
             Use proxy_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]
             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 HTTPS).  An IPv6 address can be specified unambiguously by enclosing
             proxy_address in square brackets.  A proxy cannot be used with any of the options
             -lsuU.

     -Z      DCCP mode.

     -z      Only scan for listening daemons, without sending any data to them.  Cannot be used
             together with -l.

     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 specified as a numeric port number or as a service name.  Port ranges may be
     specified as numeric port numbers of the form nn-mm.  In general, a destination port must be
     specified, unless the -U option is given.

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 -N 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’), as the -N flag was given.

     There is no -c or -e option in this netcat, but you still can execute a command after
     connection being established by redirecting file descriptors. Be cautious here because
     opening a port and let anyone connected execute arbitrary command on your site is DANGEROUS.
     If you really need to do this, here is an example:

     On ‘server’ side:

           $ rm -f /tmp/f; mkfifo /tmp/f
           $ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

     On ‘client’ side:

           $ nc host.example.com 1234
           $ (shell prompt from host.example.com)

     By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address
     127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that
     port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.

     When connection is terminated, nc quits as well. Use -k if you want it keep listening, but
     if the command quits this option won't restart it or keep nc running. Also don't forget to
     remove the file descriptor once you don't need it anymore:

           $ rm -f /tmp/f

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 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 host.example.com 1234 < filename.in

     After the file has been transferred, the connection will close automatically.

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 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 site:

           $ printf "GET / HTTP/1.0\r\n\r\n" | 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 [-C] 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 to report open ports, rather than initiate a connection.
     Usually it's useful to turn on verbose output to stderr by use this option in conjunction
     with -v option.

     For example:

           $ nc -zv 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, and is scanned by
     increasing order (unless the -r flag is set).

     You can also specify a list of ports to scan, for example:

           $ nc -zv host.example.com http 20 22-23
           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!
           nc: connect to host.example.com 23 (tcp) failed: Connection refused

     The ports are scanned by the order you given (unless the -r flag is set).

     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 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 host.example.com, using port 31337 as the source port,
     with a timeout of 5 seconds:

           $ nc -p 31337 -w 5 host.example.com 42

     Open a UDP connection to port 53 of host.example.com:

           $ nc -u host.example.com 53

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

           $ nc -s 10.1.2.3 host.example.com 42

     Create and listen on a UNIX-domain stream socket:

           $ nc -lU /var/tmp/dsocket

     Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, 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 host.example.com 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 host.example.com 42

SEE ALSO

     cat(1), ssh(1)

AUTHORS

     Original implementation by *Hobbit* <hobbit@avian.org>.
     Rewritten with IPv6 support by
     Eric Jackson <ericj@monkey.org>.
     Modified for Debian port by Aron Xu ⟨aron@debian.org⟩.

CAVEATS

     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.