nc.openbsd

NC(1)                        General Commands Manual                       NC(1)

NAME
     nc — arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc [-46bCDdhklnrStUuvZz] [-I length] [-i interval] [-O length]
        [-P proxy_username] [-p source_port] [-q seconds] [-s source]
        [-T toskeyword] [-V rtable] [-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      Forces nc to use IPv4 addresses only.

     -6      Forces nc to use IPv6 addresses only.

     -b      Allow broadcast.

     -C      Send CRLF as line-ending.

     -D      Enable debugging on the socket.

     -d      Do not attempt to read from stdin.

     -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.

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

     -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.

     -q seconds
             after EOF on stdin, wait the specified number of seconds and then
             quit. If seconds is negative, wait forever.

     -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, 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      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 to be used.  The default is 0.

     -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
             HTTPS).

     -Z      DCCP mode.

     -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.

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’).

     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 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.

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

           $ nc -zv host.example.com 80 20 22
           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!

     The ports are scanned by the order you given.

     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.

                                February 23, 2017