journal

See a running log of all system messages:

1journalctl -f

Or just one user:

1journalctl --user -f

Or just one unit (sshd):

1journalctl -f -u sshd

Find errors since November

1journalctl --since=2018-11-01 --grep="EXT4-fs error"

Limit size to 2G.

1journalctl --vacuum-size=2G

Log the fact that you've installed your own dnsmasq on your system to journalctl, so that you can notice why your system's broken:

1logger "Installed new dnsmasq"
2sudo journalctl -f