Netcat

Low level send and receive TCP/UDP data.

Versions

In Ubuntu 12.04, netcat and nc are both symlinks to nc.openbsd.

There is also the netcat-traditional package which offers another version (TODO is it the GNU netcat?)

http://superuser.com/questions/324812/versions-of-netcat

This tutorial considers the BSD version by default.

nc

Executable name.

Basic usage

Make a TCP HTTP get request and print the response:

printf 'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n' | nc example.com 80

Multiple requests

nc sends lines as you type them and over a single TCP connection if the server feels like taking it (and it should on HTTP 1.1):

(
  printf 'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n';
  sleep 2;
  printf 'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n';
) | nc example.com 80 | grep HTTP

returns 2 responses. The same could be done by manually typing the requests in.

u

UDP

UDP instead of TCP.

l

Listen for requests made on a port.

Send response from stdin.

Good way to test tools that send requests like curl.

Example:

printf 'HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\nHello curl!\n' \
  | nc -kl localhost 8000

On another terminal:

curl localhost:8000

The nc terminal prints its input:

GET / HTTP/1.1
User-Agent: curl/7.22.0 (i686-pc-linux-gnu) libcurl/7.22.0 OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
Host: localhost:8000
Accept: */*

And curl will print the reply it got: Hello curl!.

Same with another nc instead of curl:

echo 'abc' | nc localhost 8000

ECHO server

To do multiple tests of what is being sent, just wrap in a while and give an empty reply:

while true; do printf '' | nc -l localhost 8000; done

If -l is given, then the hostname is optional. If the hostname is not given, nc listens on all interfaces (TODO confirm).

More advanced one that does multiple connections: http://stackoverflow.com/questions/8375860/echo-server-with-bash

v

Give more verbose output.

E.g., on nc -l, prints an extra line:

Connection from 127.0.0.1 port 8000 [tcp/*] accepted

before the request.

k

Keep listing after the first connection instead of shutting down.

Requires the option -l.

Terminal 1:

nc -kl localhost 8000

Terminal 2:

echo 'abc' | nc localhost 8000
echo 'def' | nc localhost 8000

Terminal 1 has printed:

abc
def

C

If the last character is a newline \n, replace it with CRLF.

HTTPS

Not possible with nc:

printf 'GET / HTTP/1.0\r\n\r\n' | nc google.com 443

Returns empty.

Consider openssl or ncat http://superuser.com/questions/346958/can-the-telnet-netcat-client-communicate-over-ssl

ncat

nc version from nmap package.

c

Construct response with command.

HTTPS

ssl

printf 'GET / HTTP/1.1\r\nHost: github.com\r\n\r\n' | ncat --ssl github.com 443

As of Dec 2014, Facebook is annoying and requires a known user agent, or else you will get redirected to /unsupportedbrowser

printf 'GET / HTTP/1.1\r\nHost: www.facebook.com\r\nUser-Agent: Mozilla/5.0\r\n\r\n' \
| ncat --ssl www.facebook.com 443

The Host is mandatory or you get a redirect. TODO why