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Re: synproxy mysteriously stopped working???

On Sun, Jan 25, 2004 at 03:50:22PM -0800, Scott L. Burson wrote:
> I repeat, this ruleset had been working fine with `synproxy state' for a
> good 3 weeks, and I don't think this was even the first time I had rebooted
> the firewall.  Could I have changed something that made synproxy stop
> working?  Conceivably, but I have no idea what and don't recall changing
> anything.
Something must have changed, try to find out what (even if it seems
impossible that it is related). Either the system was updated (when,
from/to what version?) or the ruleset has changed (what rules?).
If you changed nat rules, that might be a reason. synproxy doesn't work
with some nat translations (never did), for instance
  nat on $ext_if from to any ->
  pass in on $int_if from to any synproxy state
When connects to, synproxy will first handshake
with the client (which works), but then replay the handshake with through $ext_if, without applying the nat translation.
If this is what happens, it should be easy to spot with tcpdump -nvvvS
running on all interfaces, capturing one failing TCP connection attempt.
The handshake packets generated by synproxy are always passed
unconditionally (without matching/creating state or getting translated
further on subsequent interfaces).
Without a tcpdump it's hard to tell.