From Net-SNMP Wiki

Why doesn't the agent respond?

Assuming that the client application is actually sending the request (see the previous entry), there are two main likely causes for the agent not to respond. Either it doesn't receive the request (e.g. it's being blocked by a firewall or packet filtering), or it receives the request, but is unwilling (or unable) to process it. Note that if an agent receives an SNMPv1 or SNMPv2c request with an unknown community string, then it will not return an error response - the request simply times out.

If the remote system is running the Net-SNMP agent, then the easiest way to check what's going wrong is to shut down the agent, and re-start it using the options:

             -f -Le -d

This will display raw dumps of packets seen (or sent) by the agent, just as the '-d' flag did for the client side in the previous entry.

Restart the agent with these flags, and send the same query as before. If the agent doesn't display anything in response to this request, then it's probably some form of firewall settings, which are preventing the agent from ever seeing the request.

If the agent displays a dump of the incoming request, but nothing going out, then the most likely cause is access control settings. See the relevant entries in the AGENT section for details.

A third possibility is that the agent is responding to the request, but only after a long delay. This would be indicated by a series of incoming packet dumps (showing various retries from the client side), followed by several outgoing dumps - possibly long after the client tool has given up in disgust. See the entry The agent worked for a while, then stopped responding. Why? later in this section.

The same basic causes could also affect other vendors' SNMP agents. Please consult the relevant documentation for how to investigate and address such problems.

   FAQ:Applications
   

1. How do I add a MIB?
2. How do I add a MIB to the tools?
3. Why can't I see anything from the agent?
4. Why doesn't the agent respond?
5. I can see the system group, but nothing else. Why?
6. Why can't I see values in the <ENTERPRISE> tree?
7. The agent worked for a while, then stopped responding. Why?
8. Requesting an object fails with "Unknown Object Identifier" Why?
9. Why do I get "noSuchName" when asking for "sysUpTime" (or similar)?
10. Why do I sometimes get "End of MIB" when walking a tree, and sometimes not?
11. How do I use SNMPv3?
12. Why can't I set any variables in the MIB?
13. Variables seem to disappear when I try to set them. Why?
14. Why can't I change sysLocation (or sysContact)?
15. I get an error when trying to set a negative value - why?
16. I get an error when trying to get a string-indexed table value - why?
17. What quotes should I use for string index values?
18. How do I send traps and notifications?
19. How do I handle traps and notifications?
20. My traphandler script doesn't work when run like this - why not?
21. How big can an SNMP request (or reply) be?
22. How can I monitor my systems (disk, memory, etc)?
23. Applications complain about entries in your example 'snmp.conf' file. Why?
24. OK, what should I put in snmp.conf?
25. How do I specify IPv6 addresses in tools command line arguments?