Subject: raw sockets ignore IP_HDRINCL data

From: Mick McCreath <[email-masked]>
Date: Fri, 01 Jun 2007 15:36:27 +1000

Hi,

I have a multi-homed system like this:

eth1: 1.2.3.4 GW: 1.2.3.1
eth2: 2.3.4.5 GW: 2.3.4.1

route ouput snippet is:

default 2.3.4.1 0.0.0.0 UG 3 0 0 eth2
default 1.2.3.1 0.0.0.0 UG 4 0 0 eth1
default 2.3.4.1 0.0.0.0 UG 4 0 0 eth2

When I run traceroute -i eth1 5.4.3.2, the UDP packets are routed out
eth2.

Has anybody seen this behaviour with traceroute??

I have done some debugging, and believe this to be an ipv4 kernel bug.
My research came up with the following:

By the traceroute source code, it uses raw sockets and relies on
IP_HDRINCL (which is defined in the kernel source <netinet/in.h>) to
select the routing interface and does not bind the socket to the source
IP address.

I hacked the traceroute source code to do a bind if the -i option is used,
and the correct GW is used.

I am using a 2.4.29 kernel and have traced through the kernel and
in .../net/ipv4/raw.c, raw_sendmsg() does not read the data to get the
IP header and the source address, which then results in a 0.0.0.0 source
address and hence the routing, ip_route_output(), pulls the default
gateway with the lowest metric, in this case eth2.

I have also used a 2.6.21 kernel and similiar behaviour happens. The
2.6.21 kernel source code does not read the IP header from the data
either.

Am I reading/interpreting the source code correctly?

Any other suggestions/more information would be great.

Cheers,

Mick

-- 
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Mick McCreath
Sr Engineer

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