FAQ:Applications 03

Why can't I see anything from the agent?

Fundamentally, there are two basic reasons why a request may go unanswered. Either the management application does not like the request (so never sends it), or the agent does not like the request (so never responds). The simplest way to distinguish between the two is to run the command with the command-line option '-d'.

If this doesn't display a hex dump of the raw outgoing packet, then it's the client side which is dropping the request. Hopefully you should also see an error message, to help identify what's wrong.

If this displays one or more outgoing dumps (but nothing coming back), then the request is failing at the agent end. See the next entry for more details.

There are three further possibilities to consider:

One is that the agent may return a response to the original query, but the management application may not like this response, and refuse to display it. This is relatively unusual, and typically indicates a flaw with the remote agent. (I hope you're not contemplating the suggestion that the Net-SNMP command-line tools might contain bugs!)

The typical symptoms of this would be that the '-d' option would display a sequence of sending and received packet dumps, with the same contents each time. Ask on the mailing list for advice.

Alternatively, the agent may simply not support the MIB objects being requested. This is most commonly seen when using the snmpwalk tool (particularly with SNMPv1).

The symptoms here would be that '-d' would show two pairs of raw packet dumps - one a GETNEXT request (A1 in the sending packet), followed by a GET request (A0). Repeating the same request with the snmpgetnext command-line tool should show the information (if any) that the agent returned, which was then discarded by snmpwalk as irrelevant.

Note that this is how snmpwalk was designed to work. It is not an error.

Finally, it may be that the agent is simply taking too long to respond. The easiest way to test for this is to add the command-line options "-t 60 -r 0", which will send a single request (with no repetitions) and wait for a minute before giving up. This ought to be long enough for all but the most-overloaded agent, or inefficient MIB module!

If this turns out to be the cause, then ask on the mailing list for advice on options for improving the performance.

   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 query a string-indexed table value - why?
17. How should I specify string-index table values?
18. How do I send traps and notifications?
19. How do I receive traps and notifications?
20. How do I receive SNNMPv1 traps?
21. Why don't I receive incoming traps?
22. My traphandler script doesn't work when run like this - why not?
23. How can the agent receive traps and notifications?
24. How big can an SNMP request (or reply) be?
25. How can I monitor my systems (disk, memory, etc)?
26. Applications complain about entries in your example 'snmp.conf' file. Why?
27. OK, what should I put in snmp.conf?
28. How do I specify IPv6 addresses in tools command line arguments?