Netcat (NC) Syntax

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Description

This article gives information on the Linux/UNIX command Netcat (nc).


Install

RHEL

yum install nc.x86_64


Ubuntu

sudo apt-get install netcat


Manual

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 stan‐
     dard 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 connec‐
             tions 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 speci‐
             fied 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 deci‐
             mal.

     -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 destina‐
     tion 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 redi‐
     recting 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:

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 exam‐
     ple, 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 nec‐
     essary.

     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.


Examples

Listen on a Specific Port

nc -l <port>


Sources

http://www.computerhope.com/unix/nc.htm