[mrtg] Re: SNMP Errors

[Dan Ratzlaff]

It turns out in my case there is a bug in the SNMP agent of the device I'm trying to monitor.

---
Dan Ratzlaff
Network Administrator
Mental Health Center of East Central Kansas
(620) 343-2111 x271
dratzlaff at mhceck.org

-----Original Message-----
From: Dan Ratzlaff 
Sent: Tuesday, December 17, 2002 5:04 PM
To: mrtg at list.ee.ethz.ch
Subject: [mrtg] Re: SNMP Errors

I spent some time looking at the packets from tcpdump.

Looking at the packets in ethereal, I see no difference between the
Get-Requests that work and those that don't.  But I do see one difference in
the Get-Responses.  In the Request Id field, the Responses that MRTG doesn't
see is 5 bytes long, rather than 4 bytes.  (At least if I'm assuming
correctly)

02 04 1b de 29 8b decodes as Request Id: 0x1bde298b -- works!
02 05 00 fd 13 cb 70 decodes as Request Id: 0xfd13cb70 -- doesn't work...

Is the 04 telling me the number of bytes comprising the Request Id?  Is MRTG
confused since it's sending a 4 byte request ID and receiving a 5 byte
request ID?  Why would my switch sometimes send a 4 byte ID and sometimes a
5 byte ID?  Am I totally off base here?

-----Original Message-----
From: Dan Ratzlaff 
Sent: Tuesday, December 17, 2002 12:21 PM
To: mrtg at list.ee.ethz.ch
Subject: [mrtg] Re: SNMP Errors

I am having a similar problem, but only with certain devices and only part of the time.

I have been monitoring a Cisco router and several Linux firewalls via MRTG 2.9.22 for some time with no problems.  Now I'm trying to set up monitoring for our new managed switches.

snmpwalk works fine on these switches no matter what.  But mrtg and cfgmaker will only work approx 33% of the time.  The other 66% it reports timeouts talking to the switch.

But tcpdump running on the MRTG machine clearly shows the SNMP request going out, and the response coming in from the switch.  But for some reason mrtg or cfgmaker isn't getting the packets.  And I also see arp requests being sent out quite frequently while mrtg/cfgmaker is attempting the SNMP gets.

And the other three boxes I'm monitoring don't have any problems at all!

This is on RedHat Linux 7.1, kernel 2.4.9-34, mrtg-2.9.22

Anyone have a clue??
---
Dan Ratzlaff
Network Administrator
Mental Health Center of East Central Kansas
(620) 343-2111 x271
dratzlaff at mhceck.org

On Mon, Dec 16, 2002, Alex van den Bogaerdt wrote:
>
>On Mon, Dec 16, 2002 at 09:59:39AM -0500, Alan Rader wrote:
>> 
>> If anyone has any insight on the below, I would appreciate it.  I have no
>> clue and can't figure out why it just stopped.  I get the same error when
>> trying cfgmaker on ver 2.9.7 and 2.9.22, so I don't think it is mrtg itself.
>> But I am able to get info from my router via getif?
>
>Perhaps MRTG and getif use another source port number and you
>have a firewall somewhere inbetween (maybe on the mrtg host) that
>does allow udp161<->udp161 and doesn't allow udp<random> <->udp161?
>
>You could try tcpdump to see what's going on.

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