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enricoschiattarella 78b931b935 Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect (#38 ) * Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect * Fixed style.		2017-01-10 13:26:08 -08:00
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Makefile	Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect (#38 )	2017-01-10 13:26:08 -08:00
README.md	Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect (#38 )	2017-01-10 13:26:08 -08:00
server.c	Simple TCP server to show how to retrieve original dest IP:port after an iptables redirect (#38 )	2017-01-10 13:26:08 -08:00

README.md

Sample TCP Server

This is a simple TCP server that listens on the specified port (3490 by default) and replies to incoming connections by providing info about src/destination ip/port.

It demonstrates the use of the getsockopt() system call with the ORIGINAL_DST option to retrieve the original destination ip/port after an iptables redirect.

So, for example, if you have the server listening on port 3490 on the local machine and an iptables rule like:

iptables -t nat -I OUTPUT 1 -p tcp --dport 4000:5000 -j REDIRECT --to-port 3490

your will see:

$ telnet localhost 3490
FROM 127.0.0.1:44978, TO 127.0.0.1:3490, ORIG DEST 127.0.0.1:3490

$ telnet localhost 4100
FROM 127.0.0.1:35476, TO 127.0.0.1:3490, ORIG DEST 127.0.0.1:4100

$ telnet 1.1.1.1 5000
FROM 100.100.100.100:60275, TO 127.0.0.1:3490, ORIG DEST 1.1.1.1:5000