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Old 01-29-2014, 03:41 PM   #16
Ser Olmy
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Here's what I think. The outbound packets from the PC with the address are being routed properly, and replies are coming in. The tcpdump session proved that. I'd say that proves the policy routing is actually working; the outbound packet is being processed by the right routing table.

The question is, why isn't the reply packet being routed back to It can't be policy routing, as the packet doesn't match the criteria in the iptables fwmark rule, and shouldn't be subject to policy routing anyway.

The way I see it, there are just three mechanisms that could prevent the packet from reaching
  1. Routing
  2. Reverse path verification
  3. The iptables firewall
You've ruled out (1) and (2), which leaves (3).

I think you may be onto something when you suspect a kernel/iproute2/firewall bug. If the policy routing somehow conflicts with the iptables state engine, the reply packet may not match the state rules for ESTABLISHED or RELATED traffic.

I believe it's possible to troubleshoot this further. How about adding a temporary rule to the top of the FORWARD chain in the filter table, like this:
iptables -I FORWARD 1 -i eth0.666 -s -d -p icmp -j ACCEPT
That rule would match ANY icmp packet from to arriving at eth0.666, regardless of whether it's a reply or not. The packet would have to reach the routing engine, no matter how erroneous the entries in the firewall state engine might be.

Could you try the ping test again with that rule in place? (There should be no risk of it disturbing network traffic, and it can be easily removed afterwards with iptables -D FORWARD 1.)
Old 01-30-2014, 06:44 AM   #17
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I solved it ......finally it works


Finally after so much mess.....I was able to get solution after reading this

Actually in rhel6 value of /proc/sys/net/ipv4/conf/<interface>/rp_filter is an integer not a boolean

and particularly in rhel 6 even if you do it "0" it defaults to strict filteration so....

In our implementation we have to use these two lines in syscyl.conf ::

net.ipv4.conf.default.rp_filter = 2
net.ipv4.conf.all.rp_filter = 2

I shall be thankfull to the people out there who took so serious interest in my issue over here

Best Regards

Rahul Grover

Last edited by rahulgrover; 01-30-2014 at 06:55 AM. Reason: mis typed
Old 02-03-2017, 05:39 PM   #18
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Registered: Nov 2015
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@Ser olmy
cat /proc/sys/net/ipv4/conf/eth0.666/rp_filter returns 0
why do we need to disable rp_filter here? i don't see asymmetric routing here


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