Routing loop + raw IP packets : looking for clues ...

Aziz Kezzou french.linuxian at
Tue May 31 04:07:49 PDT 2005

Hi all,
For the purpose of my project I am simpulating a 3-hop network with
QEMU on my workstation, as follows :

|Daemon1(user process)|---tun0---| Daemon 2 on QEMU |---tun1---|
Daemon 3 (user porcess) |

My workstation  is running FC3 and QEMU is running FreeBSD5.3
I am tryning to exchange raw IP packets between daemon 1 and 3 through
daemon 2. I've succeded to by pass  the system routing and get packets
sent from daemon 1 to daemon 2 and from daemon 2 to daemon 3. BUT, on
daemon 3 I can not receive them ??!!

However, when I listen with tcpdump on tun0 and tun1 I see the packets
travelling as expected :

What could prevent me from receiving packets intended for daemon 3? I
am suspecting some hard coded rule to prevent routing loops (which is
my case) in the Linux kernel, am I on the right track ?

Note : I disabled completely my firewalls, so this could not be the reason.

Any help is highly appreciated.


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