nc / netcat

Read and write data across networks - arbitrary TCP and UDP connections and listens.

Syntax
      nc [-46AcDCdhklnrtUuvz] [-b boundif] [-i interval] [-p source_port]
            [-s source_ip_address] [-w timeout] [-X proxy_protocol]
               [-x proxy_address[:port]] [hostname] [port[s]]

Options
   -4      Use IPv4 addresses only.

   -6      Use IPv6 addresses only.

   -A      Set SO_RECV_ANYIF on socket.

   -b boundif
           The interface to bind the socket to.

   -c      Send CRLF as line-ending.

   -C      Force nc not to use cellular data context.

   -d      Do not attempt to read from stdin.
           This will prevent any text being sent from a client to the server.

   -D      Enable debugging on the socket.

   -h      Print help.

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

   -G conntimeout
           TCP connection timeout in seconds.

   -H keepidle
           Initial TCP keep alive timeout in seconds.

   -I keepintvl
           Interval for repeating TCP keep alive timeouts in seconds.

   -J keepcnt
           Number of times to repeat TCP keep alive packets.

   -k      Force 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      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.

   -L num_probes
           Number of probes to send to the peer before declaring that the
           peer is not reachable and generating an adpative timeout
           read/write event.

   -n      Skip all DNS or service lookups on addresses, hostnames or ports.

   -p source_port
           Specify 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      Select the source and/or destination ports randomly instead of
           sequentially within a range or in the order that the system assigns them.

   -s source_ip_address
           The IP of the interface which is used to send the packets.
           It is an error to use this option in conjunction with the -l option.

   -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.

   -u      Use UDP instead of the default option of TCP.

   -v      Give more verbose output.

   -w timeout
           If a connection and stdin are idle for more than timeout seconds,
           then the connection is silently closed.  The -w flag has no
           effect on the -l option, i.e. nc will listen forever for a con-
           nection, with or without the -w flag.  The default is no timeout.

   -X proxy_version
           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]
           Connect to hostname 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      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.

hostname can be a numerical IP address or a symbolic hostname (unless the -n option is given). In general, a hostname must be specified, unless the -l option is given (in which case the local host is used).

port[s] can be single integers or ranges. Ranges are in the form nn-mm. In general, a destination port must be specified, unless the -U option is given (in which case a socket must be specified).

The nc (or netcat) utility is used for just about anything under the sun involving TCP or UDP. 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:

Client / Server Model

Netcat can perform basic client/server functions that allow 'chat' between machines.
On one console, start nc listening on a specific port for a connection:
# On a computer A with IP 10.10.10.10 we listen on port 64
$ nc -l -p 64

nc is now listening on port 64 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 64

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).

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 64 > 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 64 < filename.in
or
$ cat filename.in | nc host.example.com -p 64

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: $ echo -n "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 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. For example:
$ nc -z 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.


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

UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.
For more advanced port scanning, use nmap.

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 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 more connections you and your lover make, not just between your bodies, but between your minds, your hearts, and your souls, the more you will strengthen the fabric of your relationship, and the more real moments you will experience together” ~ Barbara De Angelis

Related

cat - Concatenate and print (display) the content of files
ncat - Read and write data across networks (a rewrite of netcat as part of the larger nmap toolset)
ssh(1) - OpenSSH SSH client (remote login program)


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