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iptables debugging

Has it ever happened to you that iptables was apparently not working as expected, and, in an effort to find out what's going on, you littered your ruleset with logging rules all over the place, or some other awkward kludge?
Now, it turns out that there's a much more convenient and cleaner way to find out which chains a packet traverses. This is based on using ip{,6}tables' raw table, with the TRACE target. For the following discussion, it helps if you keep an eye on this excellent iptables flow diagram.

In the diagram, we can see that the raw table has two built-in chains: PREROUTING and OUTPUT, which together cover both the input and output of packets. In other words, any packet in the system, be it for the local system, locally generated, or forwarded, traverses one or both these chains in the raw table. These chains support a special target, namely TRACE. Here's what the man page says about TRACE:

TRACE
This target marks packes so that the kernel will log every rule which match the packets as those traverse the tables, chains,
rules. (The ipt_LOG or ip6t_LOG module is required for the logging.) The packets are logged with the string prefix: "TRACE:
tablename:chainname:type:rulenum " where type can be "rule" for plain rule, "return" for implicit rule at the end of a user
defined chain and "policy" for the policy of the built in chains.
It can only be used in the raw table.

If you're building your own kernel, you need to enable or build as module at least CONFIG_NETFILTER_XT_TARGET_TRACE, one or both of CONFIG_IP_NF_RAW and CONFIG_IP6_NF_RAW to get the raw table, and again one or both of CONFIG_IP_NF_TARGET_LOG and CONFIG_IP6_NF_TARGET_LOG for logging (that's where the debug info will be written to). The TRACE target should be available from kernel 2.6.23 onwards.

Configuration

Configuration is really straightforward. Let's trace just ICMP echo packets for simplicity (of course, you'll trace whatever packets are relevant for your situation). First, we add rules to trace the relevant packets to the raw table's OUTPUT and PREROUTING chains, to catch all possible packet paths:

# for IPv4
# iptables -t raw -A OUTPUT -p icmp -j TRACE
# iptables -t raw -A PREROUTING -p icmp -j TRACE
# for IPv6
# ip6tables -t raw -A OUTPUT -p icmpv6 --icmpv6-type echo-request -j TRACE
# ip6tables -t raw -A OUTPUT -p icmpv6 --icmpv6-type echo-reply -j TRACE
# ip6tables -t raw -A PREROUTING -p icmpv6 --icmpv6-type echo-request -j TRACE
# ip6tables -t raw -A PREROUTING -p icmpv6 --icmpv6-type echo-reply -j TRACE

Yes, I should have specified the ICMP type for IPv4, too. But you get the idea, and for our purposes that works fine. For IPv6, where ICMP is used for many other things like neighbor discovery etc., tracing all ICMP types would pollute the traces with extra traffic.

Now, where is the debug information going to be logged? This info comes from the kernel, so it will use the LOG_KERN facility (kern.* in syslog speak). Check your logging daemon config to see where those messages are logged (they may end up in more than on file). On most machines, they end up in /var/log/kern.log.
Let's load the logging module (if it's not built-in)

# for IPv4
# modprobe ipt_LOG
# for IPv6
# modprobe ip6t_LOG

Some examples

Now we're ready to see tracing in action. Let's generate some ICMP traffic.

Local traffic, IPv4
# ping -c 1 www.example.com
PING www.example.com (192.0.32.10) 56(84) bytes of data.                                                       
64 bytes from www.example.com (192.0.32.10): icmp_seq=1 ttl=243 time=156 ms                                    

--- www.example.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 156.370/156.370/156.370/0.000 ms

Here's the trace in the log (line numbers and spacing added for readability):

 1 Jun 10 21:38:18 rowlf klogd: [39964.568632] TRACE: raw:OUTPUT:policy:2 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0
 2 Jun 10 21:38:18 rowlf klogd: [39964.568645] TRACE: mangle:OUTPUT:policy:1 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0
 3 Jun 10 21:38:18 rowlf klogd: [39964.568653] TRACE: nat:OUTPUT:policy:1 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0
 4 Jun 10 21:38:18 rowlf klogd: [39964.568662] TRACE: filter:OUTPUT:policy:1 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0
 5 Jun 10 21:38:18 rowlf klogd: [39964.568672] TRACE: mangle:POSTROUTING:policy:1 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0
 6 Jun 10 21:38:18 rowlf klogd: [39964.568679] TRACE: nat:POSTROUTING:policy:2 IN= OUT=br0 SRC=192.168.6.131 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=47431 SEQ=1 UID=0 GID=0

 7 Jun 10 21:38:18 rowlf klogd: [39964.725001] TRACE: raw:PREROUTING:policy:2 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=31737 PROTO=ICMP TYPE=0 CODE=0 ID=47431 SEQ=1
 8 Jun 10 21:38:18 rowlf klogd: [39964.725024] TRACE: mangle:PREROUTING:policy:1 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=31737 PROTO=ICMP TYPE=0 CODE=0 ID=47431 SEQ=1
 9 Jun 10 21:38:18 rowlf klogd: [39964.725051] TRACE: mangle:INPUT:policy:1 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=31737 PROTO=ICMP TYPE=0 CODE=0 ID=47431 SEQ=1
10 Jun 10 21:38:18 rowlf klogd: [39964.725066] TRACE: filter:INPUT:policy:5 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=31737 PROTO=ICMP TYPE=0 CODE=0 ID=47431 SEQ=1

There is a lot to see here. Looking at the iptables diagram mentioned at the beginning, it's possible to make sense of what we're seeing. Also note that after each table:CHAIN the matching rule(s) or policy in that chain are shown, to make debugging easier. The rules can be printed with iptables-save or iptables -S.
The first packet is a locally generated packet, and similarly the return packet is addressed to the local machine. It can clearly be seen that lines 1 to 6 trace the outgoing echo request packet, while lines from 7 to 10 trace the echo reply return packet. Note that the chains in the nat table are NOT traversed by the return packet. This is by design; only packets in the "NEW" state go through nat chains. Also note that the interface is a bridge, and iptables differentiates between IN (br0) and PHYSIN (eth0).

Transit traffic, IPv4

And now let's check out some transit traffic, IPv4:

 1 Jun 10 21:54:23 rowlf klogd: [40929.881158] TRACE: raw:PREROUTING:policy:2 IN=br2 OUT= PHYSIN=eth2 MAC=0e:19:c0:fe:15:cf:52:54:00:fa:9a:1d:08:00 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 2 Jun 10 21:54:23 rowlf klogd: [40929.881181] TRACE: mangle:PREROUTING:policy:1 IN=br2 OUT= PHYSIN=eth2 MAC=0e:19:c0:fe:15:cf:52:54:00:fa:9a:1d:08:00 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 3 Jun 10 21:54:23 rowlf klogd: [40929.881197] TRACE: nat:PREROUTING:policy:1 IN=br2 OUT= PHYSIN=eth2 MAC=0e:19:c0:fe:15:cf:52:54:00:fa:9a:1d:08:00 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 4 Jun 10 21:54:23 rowlf klogd: [40929.881224] TRACE: mangle:FORWARD:policy:1 IN=br2 OUT=br0 PHYSIN=eth2 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=63 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 5 Jun 10 21:54:23 rowlf klogd: [40929.881237] TRACE: filter:FORWARD:rule:2 IN=br2 OUT=br0 PHYSIN=eth2 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=63 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 6 Jun 10 21:54:23 rowlf klogd: [40929.881246] TRACE: mangle:POSTROUTING:policy:1 IN= OUT=br0 PHYSIN=eth2 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=63 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1
 7 Jun 10 21:54:23 rowlf klogd: [40929.881256] TRACE: nat:POSTROUTING:rule:1 IN= OUT=br0 PHYSIN=eth2 SRC=10.0.2.199 DST=192.0.32.10 LEN=84 TOS=0x00 PREC=0x00 TTL=63 ID=0 DF PROTO=ICMP TYPE=8 CODE=0 ID=54275 SEQ=1

 8 Jun 10 21:54:23 rowlf klogd: [40930.037578] TRACE: raw:PREROUTING:policy:2 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=40087 PROTO=ICMP TYPE=0 CODE=0 ID=54275 SEQ=1
 9 Jun 10 21:54:23 rowlf klogd: [40930.037598] TRACE: mangle:PREROUTING:policy:1 IN=br0 OUT= PHYSIN=eth0 MAC=00:1e:4f:f4:59:3e:00:00:5e:00:01:02:08:00 SRC=192.0.32.10 DST=192.168.6.131 LEN=84 TOS=0x00 PREC=0x00 TTL=243 ID=40087 PROTO=ICMP TYPE=0 CODE=0 ID=54275 SEQ=1
10 Jun 10 21:54:23 rowlf klogd: [40930.037623] TRACE: mangle:FORWARD:policy:1 IN=br0 OUT=br2 PHYSIN=eth0 SRC=192.0.32.10 DST=10.0.2.199 LEN=84 TOS=0x00 PREC=0x00 TTL=242 ID=40087 PROTO=ICMP TYPE=0 CODE=0 ID=54275 SEQ=1
11 Jun 10 21:54:23 rowlf klogd: [40930.037630] TRACE: filter:FORWARD:rule:1 IN=br0 OUT=br2 PHYSIN=eth0 SRC=192.0.32.10 DST=10.0.2.199 LEN=84 TOS=0x00 PREC=0x00 TTL=242 ID=40087 PROTO=ICMP TYPE=0 CODE=0 ID=54275 SEQ=1
12 Jun 10 21:54:23 rowlf klogd: [40930.037638] TRACE: mangle:POSTROUTING:policy:1 IN= OUT=br2 PHYSIN=eth0 SRC=192.0.32.10 DST=10.0.2.199 LEN=84 TOS=0x00 PREC=0x00 TTL=242 ID=40087 PROTO=ICMP TYPE=0 CODE=0 ID=54275 SEQ=1

This is again a echo request/echo reply exchange, sent by a machine in the network through this box. Incidentally, this computer also does NAT. Lines 1 to 7 trace the echo request packet, lines 8 to 12 trace the echo reply. Looking at the various IN=, OUT=, PHYSIN=, etc. fields it can be observed when decisions about which interfaces to use are made. NAT for outgoing packets happens in the nat:POSTROUTING chain, so the rewriting of the source address is not visible above (it could be seen using tcpdump, though); however, the reverse process is visible in the return packet: after line 9, the destination address changes from the local box (DST=192.168.6.131) to the original sender host (DST=10.0.2.199); also at the same time the output interface OUT=br2 is selected. Again note that, although the kernel does the reverse rewriting process as we just saw, no nat chain is traversed by the return packet, because the connection is no longer NEW.

Local traffic, IPv6

Now with IPv6 (2a00:1450:8002::63 is one of www.google.com's IPv6 addresses):

# ping6 -c1 2a00:1450:8002::63
PING 2a00:1450:8002::63(2a00:1450:8002::63) 56 data bytes
64 bytes from 2a00:1450:8002::63: icmp_seq=1 ttl=57 time=27.2 ms

--- 2a00:1450:8002::63 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 27.217/27.217/27.217/0.000 ms

The machine is using an IPv6-in-IPv4 tunnel, so the trace is particularly interesting. Here it is (make sure you scroll all the way to the right, there's a lot of info):

 1 Jun 11 09:57:34 scooter kernel: TRACE: raw:OUTPUT:policy:3 IN= OUT=ip6tun SRC=2001:0db8:f00d:0000:0000:0000:0000:0001 DST=2a00:1450:8006:0000:0000:0000:0000:0063 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=14867 SEQ=1 UID=0 GID=0
 2 Jun 11 09:57:34 scooter kernel: TRACE: mangle:OUTPUT:policy:1 IN= OUT=ip6tun SRC=2001:0db8:f00d:0000:0000:0000:0000:0001 DST=2a00:1450:8006:0000:0000:0000:0000:0063 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=14867 SEQ=1 UID=0 GID=0
 3 Jun 11 09:57:34 scooter kernel: TRACE: filter:OUTPUT:policy:1 IN= OUT=ip6tun SRC=2001:0db8:f00d:0000:0000:0000:0000:0001 DST=2a00:1450:8006:0000:0000:0000:0000:0063 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=14867 SEQ=1 UID=0 GID=0
 4 Jun 11 09:57:34 scooter kernel: TRACE: mangle:POSTROUTING:policy:1 IN= OUT=ip6tun SRC=2001:0db8:f00d:0000:0000:0000:0000:0001 DST=2a00:1450:8006:0000:0000:0000:0000:0063 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=14867 SEQ=1 UID=0 GID=0

 5 Jun 11 09:57:34 scooter kernel: TRACE: raw:OUTPUT:policy:1 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0
 6 Jun 11 09:57:34 scooter kernel: TRACE: mangle:OUTPUT:policy:1 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0
 7 Jun 11 09:57:34 scooter kernel: TRACE: nat:OUTPUT:policy:1 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0
 8 Jun 11 09:57:34 scooter kernel: TRACE: filter:OUTPUT:policy:1 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0
 9 Jun 11 09:57:34 scooter kernel: TRACE: mangle:POSTROUTING:policy:1 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0
10 Jun 11 09:57:34 scooter kernel: TRACE: nat:POSTROUTING:policy:4 IN= OUT=eth0 SRC=10.127.68.105 DST=172.21.17.6 LEN=124 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=41 UID=0 GID=0

11 Jun 11 09:57:34 scooter kernel: TRACE: raw:PREROUTING:policy:3 IN=ip6tun OUT= MAC=00:0a:e4:4e:9e:de:00:23:f8:9b:3d:3b:08:00:45:00:00:7c:f9:1f:00:00:fe:29:76:56:51:bb:51:06:51:bb:58:66 TUNNEL=172.21.17.6->10.127.68.105 SRC=2a00:1450:8006:0000:0000:0000:0000:0063 DST=2001:0db8:f00d:0000:0000:0000:0000:0001 LEN=104 TC=128 HOPLIMIT=57 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=14867 SEQ=1
12 Jun 11 09:57:34 scooter kernel: TRACE: mangle:PREROUTING:policy:1 IN=ip6tun OUT= MAC=00:0a:e4:4e:9e:de:00:23:f8:9b:3d:3b:08:00:45:00:00:7c:f9:1f:00:00:fe:29:76:56:51:bb:51:06:51:bb:58:66 TUNNEL=172.21.17.6->10.127.68.105 SRC=2a00:1450:8006:0000:0000:0000:0000:0063 DST=2001:0db8:f00d:0000:0000:0000:0000:0001 LEN=104 TC=128 HOPLIMIT=57 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=14867 SEQ=1
13 Jun 11 09:57:34 scooter kernel: TRACE: mangle:INPUT:policy:1 IN=ip6tun OUT= MAC=00:0a:e4:4e:9e:de:00:23:f8:9b:3d:3b:08:00:45:00:00:7c:f9:1f:00:00:fe:29:76:56:51:bb:51:06:51:bb:58:66 TUNNEL=172.21.17.6->10.127.68.105 SRC=2a00:1450:8006:0000:0000:0000:0000:0063 DST=2001:0db8:f00d:0000:0000:0000:0000:0001 LEN=104 TC=128 HOPLIMIT=57 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=14867 SEQ=1
14 Jun 11 09:57:34 scooter kernel: TRACE: filter:INPUT:policy:1 IN=ip6tun OUT= MAC=00:0a:e4:4e:9e:de:00:23:f8:9b:3d:3b:08:00:45:00:00:7c:f9:1f:00:00:fe:29:76:56:51:bb:51:06:51:bb:58:66 TUNNEL=172.21.17.6->10.127.68.105 SRC=2a00:1450:8006:0000:0000:0000:0000:0063 DST=2001:0db8:f00d:0000:0000:0000:0000:0001 LEN=104 TC=128 HOPLIMIT=57 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=14867 SEQ=1

The different phases of tunnelling can be observed (ip6tun is the tunnel interface): lines 1-4 are for the outgoing IPv6 packet, lines 5-10 for the outgoing encapsulating IPv4 packet (and they are printed even if IPv4 tracing is not enabled). Note that in the case of the return packet (lines 11-14), things are not symmetrical (or I'm missing something, which is also possible); however there is a TUNNEL=172.21.17.6->10.127.68.105 specification as part of the debugging information.
Also note that the IPv4 packet DOES traverse the nat chains, because this was explicitly set up to be the very first IPv4 packet in the tunnel connection (hence the connection was in state NEW); in practice, if the tunnel is in constant use, the connection will not be NEW and the nat chains will not usually be traversed. Lines 7 and 10 above will be missing from the traces.

Transit traffic, IPv6

Finally, here's a trace of transit IPv6 traffic (native, no IPv4 tunnelling):

 1 Jun 11 20:54:18 arch kernel: [ 1216.851243] TRACE: raw:PREROUTING:policy:3 IN=eth1 OUT= MAC=52:54:00:37:cc:22:52:54:00:ca:3b:5f:86:dd SRC=2001:0db8:0111:0000:0000:0000:0000:cafe DST=2001:0db8:0bad:f00d:0000:0000:0000:0001 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=39947 SEQ=1
 2 Jun 11 20:54:18 arch kernel: [ 1216.855156] TRACE: mangle:PREROUTING:policy:1 IN=eth1 OUT= MAC=52:54:00:37:cc:22:52:54:00:ca:3b:5f:86:dd SRC=2001:0db8:0111:0000:0000:0000:0000:cafe DST=2001:0db8:0bad:f00d:0000:0000:0000:0001 LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=39947 SEQ=1
 3 Jun 11 20:54:18 arch kernel: [ 1216.860565] TRACE: mangle:FORWARD:policy:1 IN=eth1 OUT=eth0 SRC=2001:0db8:0111:0000:0000:0000:0000:cafe DST=2001:0db8:0bad:f00d:0000:0000:0000:0001 LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=39947 SEQ=1
 4 Jun 11 20:54:18 arch kernel: [ 1216.863630] TRACE: filter:FORWARD:policy:1 IN=eth1 OUT=eth0 SRC=2001:0db8:0111:0000:0000:0000:0000:cafe DST=2001:0db8:0bad:f00d:0000:0000:0000:0001 LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=39947 SEQ=1
 5 Jun 11 20:54:18 arch kernel: [ 1216.866650] TRACE: mangle:POSTROUTING:policy:1 IN= OUT=eth0 SRC=2001:0db8:0111:0000:0000:0000:0000:cafe DST=2001:0db8:0bad:f00d:0000:0000:0000:0001 LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=128 CODE=0 ID=39947 SEQ=1

 6 Jun 11 20:54:18 arch kernel: [ 1216.872808] TRACE: raw:PREROUTING:policy:3 IN=eth0 OUT= MAC=52:54:00:51:e1:82:52:54:00:ee:63:0a:86:dd SRC=2001:0db8:0bad:f00d:0000:0000:0000:0001 DST=2001:0db8:0111:0000:0000:0000:0000:cafe LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=39947 SEQ=1
 7 Jun 11 20:54:18 arch kernel: [ 1216.878469] TRACE: mangle:PREROUTING:policy:1 IN=eth0 OUT= MAC=52:54:00:51:e1:82:52:54:00:ee:63:0a:86:dd SRC=2001:0db8:0bad:f00d:0000:0000:0000:0001 DST=2001:0db8:0111:0000:0000:0000:0000:cafe LEN=104 TC=0 HOPLIMIT=64 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=39947 SEQ=1
 8 Jun 11 20:54:18 arch kernel: [ 1216.884192] TRACE: mangle:FORWARD:policy:1 IN=eth0 OUT=eth1 SRC=2001:0db8:0bad:f00d:0000:0000:0000:0001 DST=2001:0db8:0111:0000:0000:0000:0000:cafe LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=39947 SEQ=1
 9 Jun 11 20:54:18 arch kernel: [ 1216.888587] TRACE: filter:FORWARD:policy:1 IN=eth0 OUT=eth1 SRC=2001:0db8:0bad:f00d:0000:0000:0000:0001 DST=2001:0db8:0111:0000:0000:0000:0000:cafe LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=39947 SEQ=1
10 Jun 11 20:54:18 arch kernel: [ 1216.892593] TRACE: mangle:POSTROUTING:policy:1 IN= OUT=eth1 SRC=2001:0db8:0bad:f00d:0000:0000:0000:0001 DST=2001:0db8:0111:0000:0000:0000:0000:cafe LEN=104 TC=0 HOPLIMIT=63 FLOWLBL=0 PROTO=ICMPv6 TYPE=129 CODE=0 ID=39947 SEQ=1

Conclusion

When we're done with our debugging, all we have to do is just delete the tracing rules:

# iptables -t raw -D OUTPUT -p icmp -j TRACE
# iptables -t raw -D PREROUTING -p icmp -j TRACE
# ip6tables -t raw -D OUTPUT -p icmpv6 --icmpv6-type echo-request -j TRACE
# ip6tables -t raw -D OUTPUT -p icmpv6 --icmpv6-type echo-reply -j TRACE
# ip6tables -t raw -D PREROUTING -p icmpv6 --icmpv6-type echo-request -j TRACE
# ip6tables -t raw -D PREROUTING -p icmpv6 --icmpv6-type echo-reply -j TRACE

Hopefully, using TRACE makes iptables debugging easier.

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3 Comments

  1. tudor says:

    Works after configure this:
    sudo sysctl net.netfilter.nf_log.2=ipt_LOG

    Thanks!

  2. Thank you for this post. Really informative and actually straigh to the point. Exactly what I needed

  3. Saibaba Telukunta says:

    You saved my day! Thank you.

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