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nc - arbitrary TCP and UDP connections and listens

 

NAME

      nc - arbitrary TCP and UDP connections and listens
 

SYNOPSIS

      nc [-46DEdhklnorStUuvz] [-e IPsec_policy] [-i interval]
         [-P proxy_username] [-p source_port] [-s source_ip_address] [-T ToS]
         [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]] [hostname]
         [port[s]]
 

DESCRIPTION

      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:
 
                  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.
 
      -D      Enable debugging on the socket.
 
      -d      Do not attempt to read from stdin.
 
      -h      Prints out nc help.
 
      -E      Shortcut for "-e ’in ipsec esp/transport//require’ -e ’out ipsec
              esp/transport//require’", which enables IPsec ESP transport mode
              in both directions.
 
      -e      If IPsec support is available, then one can specify the IPsec
              policies to be used using the syntax described in
              ipsec_set_policy(3).  This flag can be specified up to two times,
              as typically one policy for each direction is needed.
 
      -i interval
              Specifies a delay time interval between lines of text sent and
              received.  Also causes a delay time between connections to multi‐
              ple ports.
 
      -k      Forces nc to stay listening for another connection after its cur‐
              rent 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      “Once-only mode”.  By default, nc does not terminate on EOF con‐
              dition on input, but continues until the network side has been
              closed down.  Specifying -o will make it terminate on EOF as
              well.
 
      -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.  It is an error to use this option
              in conjunction with the -l option.
 
      -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_ip_address
              Specifies the IP of the interface which is used to send the pack‐
              ets.  It is an error to use this option in conjunction with the
              -l option.
 
      -T ToS  Specifies IP Type of Service (ToS) for the connection.  Valid
              values are the tokens “lowdelay”, “throughput”, “reliability”, or
              an 8-bit hexadecimal value preceded by “0x”.
 
      -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.
 
      -v      Have nc 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_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 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      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.
 
      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).
      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’).
      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 cap‐
      tured 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 automati‐
      cally.
      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 neces‐
      sary 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
      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 run‐
      ning, 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
 
      Open a TCP connection to port 42 of host.example.com using IPsec ESP for
      incoming and outgoing traffic.
 
            $ nc -E host.example.com 42
 
      Open a TCP connection to port 42 of host.example.com using IPsec ESP for
      outgoing traffic only.
 
            $ nc -e ’out ipsec esp/transport//require’ 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 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
      cat(1), ssh(1)
 

AUTHORS

      Original implementation by *Hobbit* 〈hobbit@avian.org〉.
      Rewritten with IPv6 support by Eric Jackson 〈ericj@monkey.org〉.
 

CAVEATS

      UDP port scans will always succeed (i.e. report the port as open), ren‐
      dering the -uz combination of flags relatively useless.
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.