Otdfbt

I am trying to use the TRACE target of IPtables but I can't seem to get any trace information logged. I want to use what is described here:
Debugger for Iptables.

From the iptables man for 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:chain-
 name: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.

I use the following rule: iptables -A PREROUTING -t raw -p tcp -j TRACE but nothing is appended either in /var/log/syslog or /var/log/kern.log!

Is there another step missing? Am I looking in the wrong place?

edit

Even though I can't find log entries, the TRACE target seems to be set up correctly since the packet counters get incremented:

# iptables -L -v -t raw
Chain PREROUTING (policy ACCEPT 193 packets, 63701 bytes)
 pkts bytes target prot opt in out source destination
 193 63701 TRACE tcp -- any any anywhere anywhere

Chain OUTPUT (policy ACCEPT 178 packets, 65277 bytes)
 pkts bytes target prot opt in out source destination

edit 2

The rule iptables -A PREROUTING -t raw -p tcp -j LOG does print packet information to /var/log/syslog... Why doesn't TRACE work?

Run:

modprobe ipt_LOG

That fixed it for me.

Seems like (i.e. works for me) with new kernel this is needed (for IPv4):

modprobe nf_log_ipv4
sysctl net.netfilter.nf_log.2=nf_log_ipv4

credits:

• https://www.centos.org/forums/viewtopic.php?f=47&t=54411


• upvoting other answers as they gave me important hints

I found that I needed to perform both of the previous answers, in this order:

sudo modprobe ipt_LOG
sudo sysctl net.netfilter.nf_log.2=ipt_LOG

Here are a couple of things that I discovered along the way.

You can get a list of valid loggers (along with the currently selected logger) with the following:

cat /proc/net/netfilter/nf_log

The numbers here represent the protocol family numbers, as defined in /usr/include/bits/socket.h. 2 is AF_INET (that's IPv4), and 10 is AF_INET6 (IPv6).

This worked for me sudo sysctl net.netfilter.nf_log.2=ipt_LOG