journalctl


Ubuntu how clear journal logs and free up some disk space

On a machine that has Ubuntu 20.04LTS was recently running out of space, while using the Disk Usage Analysis tool we noticed that /var/log/journal was taking a bit more than 4 GB.

We knew that the machine was not hosting any kind of public service nor did it have any hardware problems, so we decided to clear up old logs. To do so, we used the following command that removed all logs that were older than two days.

sudo journalctl --vacuum-time=2d;

The result was great as it saved 3.9 GB of space:

Vacuuming done, freed 3.9G of archived journals from /var/log/journal/ee4a566eacf347dbb47e03b3f33821a1.

More information on journalctl can be found here. You can find more options on removing old logs, for example limiting the total size of logs that you want to keep, using this variation which will keep only 50 MB of data:

sudo journalctl --vacuum-size=50M;

A simple way to find which DHCP server gave you an IP

Recently, we were trying to find which DHCP server was responding to the messages on the network. Using a DHCP-enabled client on a Fedora 26 GNU/Linux we grepped the contents of journalctl to find the DHCP acknowledgment messages (DHCPACK) and figure out the IP of the DHCP server.

The command we used was the following:

sudo journalctl | grep DHCPACK;

And it gave us results such as the ones below:

[[email protected] ~]$ sudo journalctl | grep DHCPACK
Nov 12 13:08:28 sys-net dhclient[578]: DHCPACK from 10.1.101.252 (xid=0x80ec760c)
Nov 12 13:08:34 sys-net dhclient[720]: DHCPACK from 10.1.101.252 (xid=0x2ed6486f)
Nov 12 11:51:19 sys-net dhclient[1248]: DHCPACK from 10.1.101.252 (xid=0xe3dd491c)
Nov 12 12:02:09 sys-net dhclient[1407]: DHCPACK from 10.1.101.252 (xid=0x1fa42c2d)
Nov 12 12:11:03 sys-net dhclient[1508]: DHCPACK from 10.1.101.252 (xid=0x91c3990a)
Nov 12 12:14:06 sys-net dhclient[1607]: DHCPACK from 10.1.101.252 (xid=0x57ebb515)
Nov 12 12:19:27 sys-net dhclient[1710]: DHCPACK from 10.1.101.252 (xid=0x5450c250)
Nov 12 12:19:39 sys-net dhclient[1776]: DHCPACK from 10.1.101.252 (xid=0x2c38d517)
Nov 12 12:39:53 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:40:51 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:41:51 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:42:44 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:43:33 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:44:31 sys-net dhclient[1837]: DHCPACK from 192.168.1.1 (xid=0xe7a1182d)
Nov 12 12:46:20 sys-net dhclient[2053]: DHCPACK from 192.168.1.1 (xid=0xbb006001)

It is important to use sudo or else you will not be seeing messages from other users and the system. As, only users in groups ‘adm’, ‘systemd-journal’, ‘wheel’ can see all messages.