iptables: log connection after SNAT/MASQUERADE command


 
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Old 07-19-2012
iptables: log connection after SNAT/MASQUERADE command

Hello!

I have the following problem with iptables in Debian 6:

My server works as a router and it needs to log server external IP+port for all outgoing connections.

But after command SNAT or MASQUERADE traffic is "lost".
I mean no following rules can catch those traffic.
Everything looks like SNAT/MASQUERADE is the "end" rule and it pushes traffic to network immediately.

Question:

How can I log routed connection data with server external IP and port?

---------- Post updated 19-07-12 at 12:20 PM ---------- Previous update was 18-07-12 at 10:27 PM ----------

These are rules from nat table:

-A POSTROUTING -p tcp -s 10.0.0.0/8 --syn -j MARK --set-mark 4
-A POSTROUTING -m mark --mark 4 -j LOG --log-prefix "TCPLO1 " --log-level debug
-A POSTROUTING -s 10.0.0.0/8 -o eth0 -j SNAT --to-source 12.23.34.56
-A POSTROUTING -m mark --mark 4 -j LOG --log-prefix "TCPLO2 " --log-level debug
-A POSTROUTING -p tcp --syn -j LOG --log-prefix "TCPLO3 " --log-level debug

First rule marks routed connection
second - logs marked connection but it don't contain IP and port selected by NAT.
third - executes SNAT
and next line don't log anything but should log marked connection
last line should log any connections and it logs server's own traffic and not routed connections
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iptables action in tc(8)                                               Linux                                              iptables action in tc(8)

NAME
xt - tc iptables action SYNOPSIS
tc ... action xt -j TARGET [ TARGET_OPTS ] DESCRIPTION
The xt action allows to call arbitrary iptables targets for packets matching the filter this action is attached to. OPTIONS
-j TARGET [ TARGET_OPTS ] Perform a jump to the given iptables target, optionally passing any target specific options in TARGET_OPTS. EXAMPLES
The following will attach a u32 filter to the ingress qdisc matching ICMP replies and using the xt action to make the kernel yell 'PONG' each time: tc qdisc add dev eth0 ingress tc filter add dev eth0 parent ffff: proto ip u32 match ip protocol 1 0xff match ip icmp_type 0 0xff action xt -j LOG --log-prefix PONG SEE ALSO
tc(8), tc-u32(8), iptables-extensions(8) iproute2 3 Mar 2016 iptables action in tc(8)