Linux netfilter / iptables : How to enable iptables TRACE chain handling with nf_log_syslog on RHEL8+?

Good day -

On a Linux RHEL8 system, I have enabled these iptables rules, which I am led to believe should enable ICMP packet syslog logging on interface ingress & egress :

    # iptables -L -t raw
    Chain PREROUTING (policy ACCEPT)
    target     prot opt source               destination         
    TRACE      icmp --  anywhere             anywhere            

Chain OUTPUT (policy ACCEPT)
target prot opt source destination
TRACE icmp -- anywhere anywhere

As described at : https://access.redhat.com/solutions/2313671 I have done :

    # modprobe  nf_log_ipv4
    # sysctl -w net.netfilter.nf_log.2=nf_log_ipv4

I also did:

    # modprobe nf_log_syslog

which I am led to believe replaces all previous nf_log* or ipt_LOG modules in modern (RHEL8 4.18.x+) kernels.

But, when I 'ping' a NAT'd (with iptables) IP address, no TRACE log messages appear in 'dmesg -c' output or in syslog (systemd.journald in use).

What am I missing ?

The most comprehensive discussion I have found on this issue so far on the web is at :

https://backreference.org/2010/06/11/iptables-debugging/ (thanks waldner!)

But this is getting rather old (2010-06-11) , and evidently does not apply to kernel 4.18+(RHEL) .

I have duplicated precisely the steps above on Fedora-36
(kernel v6.2.16) system , and it DOES work, TRACE log messages ARE generated :

  # iptables -t raw -A PREROUTING -p icmp -j TRACE
  # iptables -t raw -A OUTPUT -p icmp -j TRACE
  # modprobe nf_log_ipv4
  # echo nf_log_ipv4 > /proc/sys/net/netfilter/nf_log/2

But, these steps, when repeated on a RHEL8 kernel 4.18.0-477.13.1 host, do not work or produce any packet TRACE output in logs - this is what I am tearing what remains of my hair out trying to resolve.

Thanks in advance for any informative replies .

Best Regards,
Jason Vas Dias (SW+SYS+NET)-Engineer, West Cork, Eire.

Run "xtables-monitor --trace".

Good day Florian -

Thanks for the info about xtables-monitor - yes, that does give alot of extra information about rule chain processing.

But I'd just like to understand :
Why does this work under kernel v6.2.16 and not under v4.18.0-477 ? :
# iptables -t raw -A PREROUTING -p icmp -j TRACE
# iptables -t raw -A OUTPUT -p icmp -j TRACE
# modprobe nf_log_ipv4
# echo nf_log_ipv4 > /proc/sys/net/netfilter/nf_log/2

How can I enable the 'nf_log_syslog' module, so that it does in fact emit TRACE kernel messages to syslog, as it purports to be able to do, under v4.18.0-477 ?

xtables-monitor is great, it provides ALOT of information, but really I'd like to just trace packet ingress / egress to from interfaces, with messages written to syslog . Has this functionality been disabled somehow from the 'nf_log_syslog' module in v4.18.0-477 ? If so, how can I enable it ?

There is very little documentation about nf_log_syslog, besides that it is meant to take over everything done by ipt_LOG .

ipt_LOG WAS meant to log packets that meet the rules on the TRACE chain to syslog, no ? So how can I enable that functionality only with nf_log_syslog under v4.18.0-477 ?

I am reading its source code, it SEEMS like it should be getting triggered when 'sysctl netfilter.nf_log.2' is not NONE, no ? But the v4.18.0-477 version of it it is not doing so. Why ?

Best Regards,
Jason

You need to install iptables-legacy, not shipped in RHEL8.