Subject: Re: [tproxy,regression] tproxy broken in 2.6.32

From: KOVACS Krisztian <[email-masked]>
Date: Sat, 28 Nov 2009 16:15:15 +0100

Hi,

On p, nov 27, 2009 at 11:05:32 -0500, jamal wrote:
> On Fri, 2009-11-27 at 09:26 +0100, KOVACS Krisztian wrote:
> > Hi,
> > 
> > On Thu, 2009-11-26 at 18:19 +0100, Andreas Schultz wrote:
> > > Hi,
> > > 
> > > git bisect shows that TPROXY has been broken by commit
> > > f7c6fd2465d8e6f4f89c5d1262da10b4a6d499d0, [PATCH] net: Fix RPF to work
> > > with policy routing
> > > 
> > > I had a look at the patch, and it seems logical that this would break TPROXY.
> > 
> > Indeed, that's a good catch. If this is indeed the problem you should be
> > able to work it around by disabling rpfilter on the ingress interface.
> > Does it work that way?
> 
> Not familiar with tproxy, but I suspect the system doesnt see the mark
> before policy routing happens. So probably the wrong route cache gets
> created. Easy to validate by dumping the route cache.
> If thats so, you have to set the mark in pre-route hook if it uses
> iptables.

It's already on prerouting, so that's not the problem.

The problem is that for tproxy to work we've used to have a rule like
this:

# ip rule add fwmark 1 lookup 100

plus a few iptables rules setting mark values.

The issue is that previously fib_validate_source ignored the mark set on
the skb, and thus when fib_validate_source() did a FIB lookup, it all went
fine, because it found a result of type RTN_UNICAST. However, with your
change, and because of the ip rule above not being specific enough now
it's returning with type RTN_LOCAL, and that's considered invalid and thus
the skb is dropped.

The workaround is using more specific ip rules that include the ingress
interface name:

# ip rule add dev eth0 fwmark 1 lookup 100

(repeat the above for each interface except lo.)

Andreas, would you mind giving it a try on your system?

-- 
KOVACS Krisztian