Re: iptables MARK + ip rule fwmark on locally generated packets

From: Fredrik Ax
Date: Fri Jan 22 2010 - 05:16:34 EST


On Fri, Jan 22, 2010 at 11:09:02AM +0100, Fredrik Ax wrote:

> Hi guys,
>
> I'm a pretty experienced Linux / network developer and administrator,
> but I can't get my head around this one.
>
> The long story is that I have a box used as router/fw/proxy running
> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> interfaces is used for doing masquerading of other
> subnets. Masquerading (not snat) is chosen because the interfaces are
> on dhcp, and I don't want to have to rewrite the fw rules each time I
> get a new addr ... already have enough with dhclient-hooks for fixing
> the routing tables dns-updates, etc ;-) What I basically want to do is
> make the proxy's request to go out the same ifc as the masqueraded
> packets getting a src addr of s41.s42.s43.s44. Other locally generated
> packets should get a src addr s21.s22.s23.s24.
>
> To accomplish this I'm using iptables to mark all, to port 80, locally
> generated tcp packets:
>
> % iptables -t mangle -vnL OUTPUT
> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
> pkts bytes target prot opt in out source destination
> 1114 181K MARK tcp -- * * 0.0.0.0/0 0.0.0.0/0 tcp dpt:80 MARK set 0x4
>
> I have verified that the iptables rule marks them fine enough.
>
> Then the ip rule with prio 99 below should then catch them and route
> according to table eth4 below. That rule however does, for some reason
> not match those packets, instead they are routed according to table
> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24. If I
> disable that rule they are routed according the the prio 300 rule
> (getting src addr s31.s32.s33.s34).
>
> prompt% ip rule
> 0: from all lookup local
> 1: from all lookup main
> 99: from all fwmark 0x4 lookup eth4
> 100: from 10.116.254.0/26 lookup eth4
> 100: from 10.116.255.34 lookup eth3
> 100: from 10.116.255.64/26 lookup eth4
> 200: from all lookup eth2
> 300: from all lookup eth3
> 400: from all lookup eth4
> 32767: from all lookup default
>
> prompt% ip route show table eth2
> broadcast b1.b2.b3.b4 dev eth2 scope link src s21.s22.s23.s24
> broadcast n1.n2.n3.n4 dev eth2 scope link src s21.s22.s23.s24
> n1.n2.n3.n4/m dev eth2 scope link src s21.s22.s23.s24
> default via g1.g2.g3.g4 dev eth2 src s21.s22.s23.s24
>
> prompt% ip route show table eth3
> broadcast b1.b2.b3.b4 dev eth3 scope link src s31.s32.s33.s34
> broadcast n1.n2.n3.n4 dev eth3 scope link src s31.s32.s33.s34
> n1.n2.n3.n4/m dev eth3 scope link src s31.s32.s33.s34
> default via g1.g2.g3.g4 dev eth3 src s31.s32.s33.s34
>
> prompt% ip route show table eth4
> broadcast b1.b2.b3.b4 dev eth4 scope link src s41.s42.s43.s44
> broadcast n1.n2.n3.n4 dev eth4 scope link src s41.s42.s43.s44
> n1.n2.n3.n4/m dev eth4 scope link src s41.s42.s43.s44
> default via g1.g2.g3.g4 dev eth4 src s41.s42.s43.s44

You might also want to know that the local routes for eth2-4 are
removed in the local table, and that the main table holds no default
routes.

>
> What am I doing wrong here?
>
> TIA
> /frax

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