mjhjmtu

In my iptables, I have a rule which logs dropped packets:

-A INPUT -i eth0 -j LOG --log-prefix "FW: " --log-level 7
-A INPUT -i eth0 -j DROP

And in /etc/rsyslog.conf, I have another rule which sends these logs to a dedicated file /var/log/firewall.log.

:msg, contains, "FW: " -/var/log/firewall.log
& ~

The & ~ deletes the logs immediately, so that they don't flood syslog or other log files.

This works well, except that it floods dmesg with those firewall logs (not /var/log/dmesg but the output of command dmesg).

Is there a way to prevent these logs being shown in dmesg ?

You could use the NFLOG target instead of LOG:

NFLOG
 This target provides logging of matching packets. When this target is set for a
 rule, the Linux kernel will pass the packet to the loaded logging backend to log the
 packet. This is usually used in combination with nfnetlink_log as logging backend,
 which will multicast the packet through a netlink socket to the specified multicast
 group. One or more userspace processes may subscribe to the group to receive the
 packets. Like LOG, this is a non-terminating target, i.e. rule traversal continues
 at the next rule.

All you'd need is a nfnetlink_log capable logging program. Messages would go there and the userspace process would decide whether to log the packet or not.

Another thing you could try would be limiting the LOG rule to a specific threshold:

-A INPUT -i eth0 -m limit --limit 10/minutes -j LOG --log-prefix "FW: " --log-level 7
-A INPUT -i eth0 -j DROP

This would on average log 10 packets per minute. You could of course adjust this at your needs.

It may be related to the log-level you are using in iptables. As I understand it from the rsyslog documentation log levels are: "The priority is one of the following keywords, in ascending order: debug, info, notice, warning, warn (same as warning), err, error (same as err), crit, alert, emerg, panic (same as emerg)." What about trying to specify the log level in iptables by using its name, i.e. 'notice'.
Well serves me right for posting without checking as I now think that is not the issue at all. I implemented a similar scheme to that outlined above and I get the same issue. My centos 7 kernel is v3.10.0 and apparently since v3.5 it seems that kernel logging is done using /dev/kmsg and I presume dmesg somehow gets its input from there.

When you have set the log level to 7 with the command:

-A INPUT -i eth0 -j LOG --log-prefix "FW: " --log-level 7

Then you can simply filter out these messages by passing the level threshold to the dmesg:

dmesg --level=err,warn

Why do you care? dmesg is a low-level tool to print recent kernel messages, and you did ask for the kernel to log dropped packets.

Configure your system's syslog system to log iptables messages in a separate log file from other kernel messages, and use the log files that it writes instead of dmesg.