raw sockets ignore IP_HDRINCL data

raw sockets ignore IP_HDRINCL data

[Mick McCreath]

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

-- 
--
Mick McCreath
Sr Engineer

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Re: raw sockets ignore IP_HDRINCL data

[Herbert Xu]

Mick McCreath <mick_mccreath@au.securecomputing.com> wrote:
> 
> When I run traceroute -i eth1 5.4.3.2, the UDP packets are routed out
> eth2.
> 
> Has anybody seen this behaviour with traceroute??

Well the version of traceroute here actually does a BINDTODEVICE so it
will go out of the right interface.  It also binds the socket so that
it gets the right source address.

I can see your point of view that it would be nice if the kernel read
the IP headers in making its routing decision.  However, the current
behaviour has been there forever and it's unclear what we might break
by changing it.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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Re: raw sockets ignore IP_HDRINCL data

[Mick McCreath]

Hi Herbet,

Thanks for the feedback.

On Sat, 2007-06-02 at 21:07 +1000, Herbert Xu wrote:
> Mick McCreath <mick_mccreath@au.securecomputing.com> wrote:
> > 
> > When I run traceroute -i eth1 5.4.3.2, the UDP packets are routed out
> > eth2.
> > 
> > Has anybody seen this behaviour with traceroute??
> 
> Well the version of traceroute here actually does a BINDTODEVICE so it
> will go out of the right interface.  It also binds the socket so that
> it gets the right source address.
> 

Which traceroute are you using??

I am using version 1.4a12 by Van Jacobson downloaded from Lawrence
Berkeley Labs.

In this one, it only binds if IP_HDRINCL is not defined (in a #ifndef).

> I can see your point of view that it would be nice if the kernel read
> the IP headers in making its routing decision.  However, the current
> behaviour has been there forever and it's unclear what we might break
> by changing it.

I sort of thought that might be the case :-)

Cheers,

-- 
--
Mick McCreath
Sr Engineer

Secure Computing®
your trusted source for enterprise security™
www.securecomputing.com
NASDAQ: SCUR

61 7 3435 2803 (Direct Phone)
mick_mccreath@securecomputing.com

Secure Computing Corporation
825 Stanley Street
Woolloongabba
Queensland 4102
Australia

The information contained in this email message may be privileged,
confidential and protected from disclosure. If you are not the intended
recipient, any review, dissemination, distribution or copying is
strictly prohibited. If you have received this email message in error,
please notify the sender by reply email and delete the message and any
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Re: raw sockets ignore IP_HDRINCL data

[Herbert Xu]

On Mon, Jun 04, 2007 at 08:52:07AM +1000, Mick McCreath wrote:
> 
> Which traceroute are you using??
> 
> I am using version 1.4a12 by Van Jacobson downloaded from Lawrence
> Berkeley Labs.

The one from Debian which is based on the same source.  Hmm, it
looks like I came to the same conclusion that you did and added
a patch to it five years ago to get it to bind the interface :)

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt