How to fake the source-port of a TCP packet?

Question

Is there an easy way (or is it possible at all) to rewrite a TCP packet as it leaves the machine?
For example if I run

telnet binfalse.de 22

it will search for an unused port to leave the machine. In this case it's 46576:

root@srv % lsof -i -P -n | grep telnet
telnet    10150   user    3u  IPv4 1159425      0t0  TCP 1.2.3.4:46576->87.118.88.39:22 (ESTABLISHED)

But now I want to rewrite these packets to let the server think the requests came from port 1337, or somewhat like this. Of course I know that I don't have to expect an answer in my telnet session.

EDIT:
Of course on 1337 another program is listening, so telling telnet to speak through 1337 is no option...

Answer 1

Assuming you are speaking about Linux, iptables has a mangle table that can do all sorts of crazy things to outgoing TCP traffic. iptables NAT features might help as well, because it really sounds like you want to do "port address translation" or "manual NAT."

Answer 2

If you're really using the telnet protocol (or raw tcp streams), you can use nc -p <source_port>... -t negotiates telnet options. I have a terminal server below that accepts telnet connections on tcp/2015 and I will source from tcp/31415

# Window 1
[mpenning@hotcoffee ~]$ nc -p 31415 -t 192.168.12.117 2015
????????????????

8024-1(config-if-Po2)#

To verify...

[mpenning@hotcoffee tshark_wd]$ netstat -an | grep 31415
tcp        0      0 192.168.12.236:31415    192.168.12.117:2015     ESTABLISHED
[mpenning@hotcoffee tshark_wd]$

This assumes you do not already have a local socket listening on tcp/31415... if you already have a local tcp socket bound on the desired port, iptables is your only option.