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18. SNMP

Contributors: Glenn Chisholm.

18.1 Does Squid support SNMP?

True SNMP support is available in squid 2 and above. A significant change in the implimentation occured starting with the development 2.2 code. Therefore there are two sets of instructions on how to configure SNMP in squid, please make sure that you follow the correct one.

18.2 Enabling SNMP in Squid

To use SNMP, it must first be enabled with the configure script, and squid rebuilt. To enable is first run the script:

        ./configure --enable-snmp  [ ... other configure options ]
Next, recompile after cleaning the source tree :
        make clean
        make all
        make install
Once the compile is completed and the new binary is installed the squid.conf file needs to be configured to allow access; the default is to deny all requests. The instructions on how to do this have been broken into two parts, the first for all versions of Squid from 2.2 onwards and the second for 2.1 and below.

18.3 Configuring Squid 2.2

To configure SNMP first specify a list of communities that you would like to allow access by using a standard acl of the form:

        acl aclname snmp_community string
For example:
        acl snmppublic snmp_community public
        acl snmpjoebloggs snmp_community joebloggs
This creates two acl's, with two different communities, public and joebloggs. You can name the acl's and the community strings anything that you like.

To specify the port that the agent will listen on modify the "snmp_port" parameter, it is defaulted to 3401. The port that the agent will forward requests that can not be furfilled by this agent to is set by "forward_snmpd_port" it is defaulted to off. It must be configured for this to work. Remember that as the requests will be originating from this agent you will need to make sure that you configure your access accordingly.

To allow access to Squid's SNMP agent, define an snmp_access ACL with the community strings that you previously defined. For example:

        snmp_access allow snmppublic localhost
        snmp_access deny all
The above will allow anyone on the localhost who uses the community public to access the agent. It will deny all others access.

If you do not define any snmp_access ACL's, then SNMP access is denied by default.

Finally squid allows to you to configure the address that the agent will bind to for incomming and outgoing traffic. These are defaulted to 0.0.0.0, changing these will cause the agent to bind to a specific address on the host, rather than the default which is all.

        snmp_incoming_address 0.0.0.0
        snmp_outgoing_address 0.0.0.0

18.4 Configuring Squid 2.1

Prior to Squid 2.1 the SNMP code had a number of issues with the ACL's. If you are a frequent user of SNMP with Squid, please upgrade to 2.2 or higher.

A sort of default, working configuration is:

        snmp_port 3401
        snmp_mib_path /local/squid/etc/mib.txt

        snmp_agent_conf view all .1.3.6 included
        snmp_agent_conf view squid .1.3.6 included
        snmp_agent_conf user squid - all all public
        snmp_agent_conf user all all all all  squid
        snmp_agent_conf community public squid squid
        snmp_agent_conf community readwrite all all

Note that for security you are advised to restrict SNMP access to your caches. You can do this easily as follows:

        acl snmpmanagementhosts 1.2.3.4/255.255.255.255 1.2.3.0/255.255.255.0
        snmp_acl public deny all !snmpmanagementhosts
        snmp_acl readwrite deny all
You must follow these instructions for 2.1 and below exactly or you are likely to have problems. The parser has some issues which have been corrected in 2.2.

18.5 How can I query the Squid SNMP Agent

You can test if your Squid supports SNMP with the snmpwalk program (snmpwalk is a part of the NET-SNMP project). Note that you have to specify the SNMP port, which in Squid defaults to 3401.

        snmpwalk -p 3401 hostname communitystring .1.3.6.1.4.1.3495.1.1
If it gives output like:
        enterprises.nlanr.squid.cacheSystem.cacheSysVMsize = 7970816
        enterprises.nlanr.squid.cacheSystem.cacheSysStorage = 2796142
        enterprises.nlanr.squid.cacheSystem.cacheUptime = Timeticks: (766299) 2:07:42.99
then it is working ok, and you should be able to make nice statistics out of it.

For an explanation of what every string (OID) does, you should refer to the Squid SNMP web pages.

18.6 What can I use SNMP and Squid for?

There are a lot of things you can do with SNMP and Squid. It can be useful in some extent for a longer term overview of how your proxy is doing. It can also be used as a problem solver. For example: how is it going with your filedescriptor usage? or how much does your LRU vary along a day. Things you can't monitor very well normally, aside from clicking at the cachemgr frequently. Why not let MRTG do it for you?

18.7 How can I use SNMP with Squid?

There are a number of tools that you can use to monitor Squid via SNMP. Many people use MRTG. Another good combination is NET-SNMP plus RRDTool. You might be able to find more information at the Squid SNMP web pages or ircache rrdtool scipts

18.8 Where can I get more information/discussion about Squid and SNMP?

General Discussion: cache-snmp@ircache.net These messages are archived.

Subscriptions should be sent to: cache-snmp-request@ircache.net.

18.9 Monitoring Squid with MRTG

Some people use MRTG to query Squid through its SNMP interface.

To get instruction on using MRTG with Squid please visit these pages:

  1. Cache Monitoring - How to set up your own monitoring by DFN-Cache
  2. Using MRTG to monitor Squid by Guido Serassio
  3. Squid Configuration Manual - Monitoring Squid by Visolve
  4. Using MRTG for Squid monitoring Desire II caching workshop session by Matija Grabnar
  5. How do I monitor my Squid 2 cache using MRT by The National Janet Web Cache Service

Further examples of Squid MRTG configurations can be found here:

  1. MRTG HOWTO Collection / Squid from MRTG
  2. using mrtg to monitor Squid from MRTG
  3. Chris' MRTG Resources
  4. MRTG & Squid by Glenn Chisholm
  5. Braindump by Joakim Recht


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