FAQ:Applications 05

Requests always seem to timeout, and don't give me anything back. Why?

There are a number of possible causes of this.

The most likely are the agent access control settings (who is allowed access by the agent itself), or firewall/packet filtering settings (who is allowed access by the underlying operating system).

A fuller list of possible causes (with indications of how to check for each) is as follows:

• is the machine you are querying up and running?

(Does it respond to ping or similar requests?)

• is there an SNMP agent running on it?

(Run ps -ef | grep snmp or netstat -an | grep 161)

• are the requests arriving, or being blocked (e.g. by a firewall)?

(Restart the agent using snmpd -f -Le -d

and see if it shows the incoming packet dumps)

• is the agent simply taking a long time to respond?

(The snmpd -f -Le -d command should show a series of incoming PDUs, followed eventually by the outgoing PDUs.

Try the request again with a long timeout value, e.g. snmpcmd -t 120 ....)

• do the agent's control settings allow this request?

(The snmpd -f -Le -d command will show a series of incoming PDUs with no corresponding outgoing PDUs)

If the agent is not configured to allow access for a particular community, then no error response will be returned. The Net-SNMP tools will retry the request a number of times, before reporting a timeout error.

If the agent is configured to allow partial access for a given community, then requests that fall outside this authorised access will result in an error response.
(SNMP agents can be very fussy over who they talk to!)

See the entries on access control in the AGENT section for how to configure the Net-SNMP agent to allow suitable access. For other vendors' agents, you will need to consult the relevant documentation.

   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?