Atlas has started to monitor squids with mrtg.
mrtg uses snmp. So to enable the monitoring you need your squid instance compiled with --enable-snmp. CERN binaries are already compiled with that option the default squid coming with SL5 OS might not, your site centralised squid service might not. You don't need snmpd or snmptracd (net-snmp rpm) running to make it work.
Once you are sure the binary is compiled with the right options and that port 3401 is not blocked by any firewall you need to add these lines to squid.conf
acl SNMPHOSTS src 184.108.40.206/24 localhost
acl SNMPMON snmp_community public
snmp_access allow SNMPMON SNMPHOSTS
snmp_access deny all
again if you are using the CERN rpms and the default frontier configuration you might not need to do that as there are already ACL lines for the monitoring.
Reload the configuration
service squid reload
snmpwalk -v2c -Cc -c public localhost:3401 .220.127.116.11.4.1.3495.1.1
you should get something similar to this:
SNMPv2-SMI::enterprises.3418.104.22.168.0 = INTEGER: 206648
SNMPv2-SMI::enterprises.3422.214.171.124.0 = INTEGER: 500136
SNMPv2-SMI::enterprises.34126.96.36.199.0 = Timeticks: (23672459) 2 days, 17:45:24.59
snmpwalk is part of net-snmp-utils rpm.
It takes a while for the monitoring to catch up. Don't expect an immediate response.
Additional information on squid/snmp can be found here http://wiki.squid-cache.org/Features/Snmp
NOTE: If you are also upgrading pay attention to the fact that in the latest CERN rpms the init script fn-local-squid.sh might try to regenerate your squid.conf.