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Debugger for Iptables

Hi,

How can I know how many packets were dropped by my iptables rules ??

Is there any debugging mechanism to see which rule is Dropping my packet or Accepting it ??

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marked as duplicate by Zoredache, splattne Mar 25 '10 at 8:16

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
I just answered a similar question here. serverfault.com/questions/122157/debugger-for-iptables/… –  Zoredache Mar 25 '10 at 5:55
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3 Answers

Run iptables -L -v (add -t nat for NAT rules), and you'll see packet and byte counters next to each of your rules. That'll show you which of your rules was the cause of a particular packet being accepted/rejected (whichever counter increased is the cause).

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Eventhough if I restart the machine I can still see packet and byte counters not set to 0. Chain PREROUTING (policy ACCEPT 1824 packets, 312K bytes) How can I reset these values to NULL at the startup ?? –  codingfreak Oct 26 '09 at 7:36
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From iptables(8): -Z, --zero [chain] Zero the packet and byte counters in all chains. –  womble Oct 26 '09 at 10:25
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Another method of debugging your rules is to add an identical rule to the one you're interested in, but set the action to being:

-j LOG --log-prefix "rule description"

Every time your rule matches, you'll get a line in syslog with lots of useful information about the packet.

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+1 agreed... e.g. it's a common (and often good) practice to log some or all dropped packets. –  David Z Oct 26 '09 at 6:49
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If you have a recent enough kernel and version of iptables you can use the TRACE target (Seems to be builtin on at least Debian 5.0). You should set the conditions of your trace to be as specific as possible and disable any TRACE rules when you are not debugging because it does spew a lot of information to the logs.

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 added rules like this

iptables -t raw -A PREROUTING -p tcp --destination 192.168.0.0/24 --dport 80 -j TRACE
iptables -t raw -A OUTPUT -p tcp --destination 192.168.0.0/24 --dport 80 -j TRACE

You will be supplied with output that looks like this.

# cat /var/log/kern.log | grep 'TRACE:'
Mar 24 22:41:52 enterprise kernel: [885386.325658] TRACE: raw:PREROUTING:policy:2 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.12.152 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=80 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325689] TRACE: mangle:PREROUTING:policy:1 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.12.152 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=80 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325713] TRACE: nat:PREROUTING:rule:1 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.12.152 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=80 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: nat:nat.1:rule:1 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.12.152 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=80 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: mangle:INPUT:policy:1 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: filter:INPUT:rule:2 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: filter:in_world:rule:1 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: filter:in_world_all_c1:return:2 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: filter:in_world:rule:2 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
Mar 24 22:41:52 enterprise kernel: [885386.325731] TRACE: filter:in_world_irc_c2:return:2 IN=eth0 OUT= MAC=00:1d:7d:aa:e3:4e:00:04:4b:05:b4:dc:08:00 SRC=192.168.32.18 DST=192.168.32.10 LEN=52 TOS=0x00 PREC=0x00 TTL=128 ID=30561 DF PROTO=TCP SPT=53054 DPT=3128 SEQ=3653700382 ACK=0 WINDOW=8192 RES=0x00 SYN URGP=0 OPT (020405B40103030201010402)
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Here you may find additional information about the theme: adminberlin.de/iptables-debugging –  zzeroo Jun 21 '13 at 10:39
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