If you are having problems with SNMP, you should first confirm that SNMP is running on pfSense. You can do this by navigating to Status | Services and looking for bsnmpd (the SNMP daemon) in the table. If it is not running even though you enabled it, there may be a resource-allocation issue.

If not, the most common problem with SNMP is that the agents are unable to communicate with bsnmpd. This could happen for a variety of reasons. There may be no connectivity between the agent and bsnmpd; it is unlikely, but it is still a possibility that should be considered before moving on to other possible causes. It is possible that access to the SNMP ports is being blocked by pfSense or another network device; check your firewall settings to see whether this is the case.

Once you have determined that a node has connectivity with the pfSense node running bsnmpd, you should make sure the agent software is running on the node. If the agent software is running, make sure the agent is sending the correct community string; without it, bsnmpd will reject any attempts to connect to it. Keep in mind that community strings are case-sensitive.

Once you have determined that a node has connectivity with the pfSense node running bsnmpd, you should make sure the agent software is running on the node. If the agent software is running, make sure the agent is sending the correct community string; without it, bsnmpd will reject any attempts to connect to it. Keep in mind that community strings are case-sensitive.

If none of these works, you may consider other possibilities, such as the following:

• Wrong version of SNMP: The SNMP daemon on pfSense runs version 3; the agent may be using an earlier version.
• Incorrect Object Identifier: In a custom probe, the Object Identifier (OID) used by the agent may be wrong.
• Software bug in the agent: This is not likely if the software is relatively stable, but is still a possibility.

In many cases, once the obvious possibilities have been exhausted, the process of troubleshooting an SNMP node is dependent on the platform you are using (Windows, Linux, macOS, and so on) and the software you are using. As a result, consulting any troubleshooting resources the software provider has made available is an important step in resolving the problem.