Keeping firewall logs out of Linux’s kernel log with ulogd2

A few words about iptables vs nft ^

nftables is the new thing and iptables is deprecated, but I haven’t found time to convert everything to nft rules syntax yet.

I’m still using iptables rules but it’s the iptables frontend to nftables. All of this works both with legacy iptables and with nft but with different syntax.

Logging with iptables ^

As a contrived example let’s log inbound ICMP packets at a maximum rate of 1 per second:

-A INPUT -m limit --limit 1/s -p icmp -j LOG --log-level 7 --log-prefix "ICMP: "

The Problem ^

If you have logging rules in your firewall then they’ll log to your kernel log, which is available at /dev/kmsg. The dmesg command displays the contents of /dev/kmsg but /dev/kmsg is a fixed size circular buffer, so after a while your firewall logs will crowd out every other thing.

On a modern systemd system this stuff does get copied to the journal, so if you set that to be persistent then you can keep the kernel logs forever. Or you can additionally run a syslog daemon like rsyslogd, and have that keep things forever.

Either way though your dmesg or journalctl -k commands are only going to display the contents of the kernel’s ring buffer which will be a limited amount.

I’m not that interested in firewall logs. They’re nice to have and very occasionally valuable when debugging something, but most of the time I’d rather they weren’t in my kernel log.

An answer: ulogd2 ^

One answer to this problem is ulogd2. ulogd2 is a userspace logging daemon into which you can feed netfilter data and have it log it in a flexible way, to multiple different formats and destinations.

I actually already use it to log certain firewall things to a MariaDB database for monitoring purposes, but you can also emit plain text, JSON, netflow and all manner of things. Since I’m already running it I decided to switch my general firewall logging to it.

Configuring ulogd2 ^

I added the following to /etc/ulogd.conf:

# This one for logging to local file in emulated syslog format.
stack=log2:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,print1:PRINTPKT,emu1:LOGEMU
 
[log2]
group=2
 
[emu1]
file="/var/log/iptables_ulogd2.log"
sync=1

I already had a stack called log1 for logging to MariaDB, so I called the new one log2 with its output being emu1.

The log2 section can then be told to expect messages from netfilter group 2. Don’t worry about this, just know that this is what you refer to in your firewall rules, and you can’t use group 0 because that’s used for something else.

The emu1 section then says which file to write this stuff to.

That’s it. Restart the daemon.

Configuring iptables ^

Now it’s time to make iptables log to netfilter group 2 instead of its normal LOG target. As a reminder, here’s what the rule was like before:

-A INPUT -m limit --limit 1/s -p icmp -j LOG --log-level 7 --log-prefix "ICMP: "

And here’s what you’d change it to:

-A INPUT -m limit --limit 1/s -p icmp -j NFLOG --nflog-group 2 --nflog-prefix "ICMP:"

The --nflog-group 2 needs to match what you put in /etc/ulogd.conf.

You’re now logging with ulogd2 and none of this will be going to the kernel log buffer. Don’t forget to rotate the new log file! Or maybe you’d like to play with logging this as JSON or into a SQLite DB?

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