ip_rt_bug weirdness

[Thomas Egerer <thomas.Egerer@secunet.com>]

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Hello *,

maybe it's my bad. Most probably! Even in that case I'd need someone to
point out the mistake to me!
I'm currently (trying) to write a transparent proxy application, using
libipq to capture packets + iptables' redirect mechanism.
The basic idea works as follows:
+---+      +---+      +---+
| S |<---->| P |<---->| D |
+---+ (1)  +---+  (2) +---+

(1) uses iptables' REDIRECT target; the received data is then forwarded,
    using another socket connection (2)
(2) uses libipq to do some kind of SNAT and change the local source
    address to S's address and vice versa for the incoming packets
    from D

So far the theory. The application works fine, as long, as I do not
remap the source port (destination port, respectively) from P to D. Once
I enable the port remapping I get syslog messages like the following:
[ 7742.939471] ip_rt_bug: [S' IP] -> [P's IP at (2)], ?
and a reset from P towards D, using exactly all the correct TCP
settings, except for the destination port, that is 1, sometimes 2, or 3.
I couldn't figure out, why.
By the way, the three-way-handshake works fine, the RSTs are generated
for the first packet to contain a TCP-payload. Also netstat tells me,
there is an established connection between P and D, but somehow (I
assume that this might be the trouble) looking for the corresponding
socket connection on P fails.
I'm totally puzzled why that happens. libipq reinjects the packets with
properly changed checksums and whatnot, yet the RSTs are generated.
I've also tried NF_REPEAT, instead of the NF_ACCEPT verdict. The
behavior remains identically.

Any ideas, anyone?

Thanks in advance

Thomas
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