Decrease snmpd logging level on Ubuntu 18.04

I recently updated some servers from Ubuntu 16.04 to 18.04, and found that the snmpd daemon was generating way too many log entries in /var/log/syslog - one for every SNMP query coming from our monitoring system.

In older Ubuntus I had edited /etc/defaults/snmpd to change the SNMPDOPTS line to have a different -Ls parameter, but it seems that on the new Ubuntu, the systemd service for snmp doesn't use that defaults file at all. A comment on this serverfault question gave me a clue on how to fix it in systemd - I thought I'd elaborate here.

If you run 

systemctl cat snmpd.service

You see the current service file:

# /lib/systemd/system/snmpd.service
[Unit]
Description=Simple Network Management Protocol (SNMP) Daemon.
After=network.target
ConditionPathExists=/etc/snmp/snmpd.conf

[Service]
Environment="MIBSDIR=/usr/share/snmp/mibs:/usr/share/snmp/mibs/iana:/usr/share/snmp/mibs/ietf:/usr/share/mibs/site:/usr/share/snmp/mibs:/usr/share/mibs/iana:/usr/share/mibs/ietf:/usr/share/mibs/netsnmp"
Environment="MIBS="
Type=simple
ExecStartPre=/bin/mkdir -p /var/run/agentx
ExecStart=/usr/sbin/snmpd -Lsd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux,mteTrigger,mteTriggerConf -f
ExecReload=/bin/kill -HUP $MAINPID

[Install]
WantedBy=multi-user.target

I wanted to override the ExecStart line with something different. To do that, run

systemctl edit snmpd.service

This brings up your default editor with a blank file. I entered these new lines:

# Override default "-Lsd" paramter to "-LSwd" to decrease logging level
[Service]
ExecStart=
ExecStart=/usr/sbin/snmpd -LSwd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux,mteTrigger,mteTriggerConf -f

The first ExecStart= line is a bit odd, without it you get an error:

snmpd.service: Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.

so the first line 'clears' the setting before processing your own version.

Save your file (systemctl edit stores it as /etc/systemd/system/snmpd.service.d/override.conf), and then run service snmpd restart to have take effect. If you re-run systemctl cat snmpd.service you should now see:

# /lib/systemd/system/snmpd.service
[Unit]
Description=Simple Network Management Protocol (SNMP) Daemon.
After=network.target
ConditionPathExists=/etc/snmp/snmpd.conf

[Service]
Environment="MIBSDIR=/usr/share/snmp/mibs:/usr/share/snmp/mibs/iana:/usr/share/snmp/mibs/ietf:/usr/share/mibs/site:/usr/share/snmp/mibs:/usr/share/mibs/iana:/usr/share/mibs/ietf:/usr/share/mibs/netsnmp"
Environment="MIBS="
Type=simple
ExecStartPre=/bin/mkdir -p /var/run/agentx
ExecStart=/usr/sbin/snmpd -Lsd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux,mteTrigger,mteTriggerConf -f
ExecReload=/bin/kill -HUP $MAINPID

[Install]
WantedBy=multi-user.target

# /etc/systemd/system/snmpd.service.d/override.conf
# Override default "-Lsd" paramter to "-LSwd" to decrease logging level
[Service]
ExecStart=
ExecStart=/usr/sbin/snmpd -LSwd -Lf /dev/null -u Debian-snmp -g Debian-snmp -I -smux,mteTrigger,mteTriggerConf -f

Which is a combination of the default service along with your override.

If you have other servers you want to copy your /etc/systemd/system/snmpd.service.d/override.conf file to, you need to run

systemctl daemon-reload
service snmpd restart

to have it take effect.