Re-routing packets via netfilter (ip_rt_bug)

From: Yair Itzhaki
Date: Mon Apr 25 2005 - 03:48:07 EST

* Summary:
While traversing packets through Netfilter, changing dest address from a foreign to a local address causes the packet to drop (and show up at ip_rt_bug(), along a syslog entry).

* Description:
I'm using libipq/ip_tables and ip_queue to trap packets to a userspace VPN product, using nothing but standard kernel modules (and my own VPN proxy app).

The packets flowing into or out of the machine get diverted to a userspace application, src/dest addresses are modified, and injected back into the IP stack.

For example, an outgoing packet (that has a foreign dest addr) is overridden with a local dest address, hoping it would end up at the local VPN listener.

Under kernel 2.4 this works fine.
In 2.6 it breaks.

* Details:
An outgoing packet (has a non-local dest addr) is queued and recognized at the ip_queue userspace app. Its dest addr+port are set to that of the local machine (to get to my userspace VPN app).
The modified packet is marked NF_ACCEPT and sent back into the kernel, but ends up at the ip_rt_bug function (with a syslog entry).

* Assumed bug analysis:
Due to the destination address change, the packet needed to go through routing once again, since it's no longer an outgoing packet.
This does happen in the ip_route_me_harder function, which sets the dst->output to point at ip_rt_bug.
Since this was an outgoing packet (in the NF_IP_LOCAL_OUT chain), the final operation done on the packet is calling the *okfn function, which points to dst->output which is ip_rt_bug.

I would have expected the routing function to realize it needs to re-evaluate the route, and set the *okfn to dst->input instead.

* Kernel version:
2.6.9-prep, (Red Hat 3.4.2-6.fc3) compiled locally with no modifications.

Please advise (and please CC "YAIR at ARX.COM")

A similar problem has been reported a while back but never replied (

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