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

[Fredrik Ax <frax@axnet.nu>]

On Fri, Jan 22, 2010 at 11:53:45AM +0100, Patrick McHardy wrote:

> Fredrik Ax wrote:
> > On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:
> > 
> >> 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).
> >>>
> >>> ...
> >>>
> >>>
> >>> What am I doing wrong here?
> >> Source address selection happens before the packet is even generated,
> >> so iptables marking in OUTPUT can't affect it.
> > 
> > So, to accomplish this I would have to oute it through a dummy
> > interface to make iptables able to mark it before it goes out?
> 
> You need some criteria for your routing rules that is available
> when the socket is routed. That's everything but the packet mark.
> Using a seperate device will work.
> 
> For ethernet, the macvlan device might be a good choice if you
> don't mind using different MAC addresses for each IP.

Thanks, I'll have a look at it ... 

Just one more question, the host is actually run as a domU on XEN and
all of the eth2-4 interfaces are on a in dom0 created bridge, bridging
in a vlan where the tagged traffic is on a blanace-rr bond-device.

Would it create any problems creating a macvlan device on top of this?

Thanks AGAIN,
/frax



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