noble (1) nc_openbsd.1.gz

Provided by: netcat-openbsd_1.226-1ubuntu2_amd64 bug

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 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      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 already 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 datagrams 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 arguments, 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 temporary 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.

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.