nc

NC(1) General Commands Manual NC(1)

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  arbi
   trary  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  al
           ready 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 data
           grams 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  argu
           ments, 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 tem
           porary 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.  For some options, the value 0 requests that the system choose a port number.

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.

Debian April 1, 2024 NC(1)