netcat

[http://dummy.us.eu.org/robert (Robert)]

NAME
       nc - TCP/IP swiss army knife

SYNOPSIS
       nc [-options] hostname port[s] [ports] ...
       nc -l -p port [-options] [hostname] [port]

DESCRIPTION
       netcat  is  a  simple  unix utility which reads and writes
       data across network connections, using TCP or  UDP  protocol.
       It is designed to be a reliable "back-end" tool that
       can be used directly or easily driven  by  other  programs
       and  scripts.  At the same time, it is a feature-rich net
       work debugging and exploration tool, since it  can  create
       almost  any kind of connection you would need and has sev
       eral interesting built-in capabilities.  Netcat,  or  "nc"
       as  the actual program is named, should have been supplied
       long ago as another one of those cryptic but standard Unix
       tools.

       In  the  simplest usage, "nc host port" creates a TCP
       connection to the given port on the given target host.   Your
       standard input is then sent to the host, and anything that
       comes back across the connection is sent to your  standard
       output.   This  continues  indefinitely, until the network
       side of the connection shuts down.  Note that this  behav
       ior  is  different from most other applications which shut
       everything down and exit after an end-of-file on the stan
       dard input.

       Netcat  can  also  function  as a server, by listening for
       inbound connections on arbitrary ports and then doing  the
       same  reading and writing.  With minor limitations, netcat
       doesn't really care if it runs  in  "client"  or  "server"
       mode  --  it still shovels data back and forth until there
       isn't any more left.  In  either  mode,  shutdown  can  be
       forced after a configurable time of inactivity on the 
       network side.

       And it can do this via UDP too, so netcat is possibly  the
       "udp  telnet-like" application you always wanted for testing
       your UDP-mode servers.  UDP, as the "U" implies, gives
       less  reliable  data transmission than TCP connections and
       some systems may have trouble  sending  large  amounts  of
       data that way, but it's still a useful capability to have.

       You may be asking "why not just use telnet to  connect  to
       arbitrary  ports?"  Valid question, and here are some 
       reasons.  Telnet has the "standard input EOF" problem, so one
       must  introduce  calculated  delays  in driving scripts to
       allow network output to finish.  This is the  main  reason
       netcat  stays  running  until  the  *network* side closes.
       Telnet also  will  not  transfer  arbitrary  binary  data,
       because  certain  characters  are  interpreted  as  telnet
       options and are thus removed from the data stream.  Telnet
       also  emits  some  of  its diagnostic messages to standard
       output, where netcat keeps such things  religiously  
       separated  from  its *output* and will never modify any of the
       real data in transit unless you *really* want it to.   And
       of  course  telnet  is  incapable of listening for inbound
       connections, or using UDP instead.   Netcat  doesn't  have
       any  of these limitations, is much smaller and faster than
       telnet, and has many other advantages.

OPTIONS
       -g gateway   source-routing hop point[s], up to 8

       -G num       source-routing pointer: 4, 8, 12, ...

       -h           display help

       -i secs      delay interval for lines sent, ports scanned

       -l           listen mode, for inbound connects

       -n           numeric-only IP addresses, no DNS

       -o file      hex dump of traffic

       -p port      local port number (port numbers can be  indi­
                    vidual or ranges: lo-hi [inclusive])

       -q seconds   after  EOF  is  detected,  wait the specified
                    number of seconds and then quit.

       -b           allow UDP broadcasts

       -r           randomize local and remote ports

       -s addr      local source address

       -t           enable telnet negotiation

       -e prog      specify program to exec  after  connect  (use
                    with caution)

       -u           UDP mode

       -v           verbose [use twice to be more verbose]

       -w secs      timeout for connects and final net reads

       -z           zero-I/O mode [used for scanning]

BUGS
       Some port names in /etc/services contain hyphens -- netcat
       currently will  not  correctly  parse  those,  so  specify
       ranges using numbers if you can.