How can I configure my Linode to respond to SNMP?

Hi,

You'll need to first ensure that you've installed snmpd with one of the following commands, depending on your distribution:

yum install snmpd
apt install snmpd

You can verify what port/IP combination that SNMP is responding on by examining the output of netstat or ss.

# netstat -plntu
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      11070/sshd
tcp6       0      0 :::22                   :::*                    LISTEN      11070/sshd
udp    16896      0 127.0.0.1:161           0.0.0.0:*                           18670/snmpd
udp        0      0 0.0.0.0:52067           0.0.0.0:*                           18670/snmpd
udp6   15360      0 :::44191                :::*                                3018/systemd-timesy

From the output above, we can see that snmpd is only listening on the localhost. If you'd like your server to respond remotely, you'll need to configure snmpd to listen on your public IP or all IPs with the agentAddress directive in etc/snmp/snmpd.conf and restart snmpd:

#  AGENT BEHAVIOUR
#

#  Listen for connections from the local system only
agentAddress  udp:127.0.0.1:161
#  Listen for connections on all interfaces (both IPv4 *and* IPv6)
agentAddress udp:161,udp6:[::1]:161

You should also ensure that your firewall rules are configured properly to allow UDP traffic on port 161. I would suggest whitelisting IPs that you use in your monitoring for snmp traffic or editing snmpd.conf to disable the default public community setting as leaving this service open to the public leaves you vulnerable to participating in amplification attacks.