ncat usage

quota from "http://nmap.org/ncat/guide/ncat-usage.html":

Main usage:

Ncat always operates in one of two basic modes: connect mode and listen mode. In connect mode, Ncat initiates a connection (or sends UDP data) to a service that is listening somewhere. For those familiar with socket programming, connect mode is like using the connect function. In listen mode, Ncat waits for an incoming connection (or data receipt), like using the bind and listen functions. You can think of connect mode as “client” mode and listen mode as “server” mode.

Connect mode:
ncat <host> [<port>]

Listen mode:
ncat -l [<host>] [<port>]

In listen mode, <host> controls the address on which Ncat listens; if you omit it, Ncat will bind to all local interfaces (INADDR_ANY). If the port number is omitted, Ncat uses its default port 31337.Typically only privileged (root) users may bind to a port number lower than 1024. A listening TCP server normally accepts only one connection and will exit after the client disconnects. Combined with the --keep-open option, Ncat accepts multiple concurrent connections up to the connection limit. With --keep-open (or -k for short), the server receives everything sent by any of its clients, and anything the server sends is sent to all of them. A UDP server will communicate with only one client (the first one to send it data), because in UDP there is no list of “connected” clients.

Ncat can use TCP, UDP, SCTP, SSL, IPv4, IPv6, and various combinations of these. TCP over IPv4 is the default.

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File transfer using ncat:

input file on host1
output file on host2

Transfer a file, receiver listens
host2$ ncat -l > outputfile
host1$ ncat --send-only host2 < inputfile

Transfer a file, sender listens
host1$ ncat -l --send-only < inputfile
host2$ ncat host1 > outputfile


Transfer a bundle of files

host2$ ncat -l | tar xzv
host1$ tar czv <files> | ncat --send-only host2

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Example. Running a command with --sh-exec

ncat -l --sh-exec "echo `pwd`"

Example. Ncat as mail client

$ ncat -C mail.example.com 25
220 mail.example.com ESMTP
HELO client.example.com
250 mail.example.com Hello client.example.com
MAIL FROM:a@example.com
250 OK
RCPT TO:b@example.com
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
From: a@example.com
To: b@example.com
Subject: Greetings from Ncat

Hello. This short message is being sent by Ncat.
.
250 OK
QUIT
221 mail.example.com closing connection