Squid has some very detailed logging options which can be activated in this area.
/var/log/squid. If needed, you can find even more information about the logs in Squids Logs . In the wiki section Logs there are also statements regarding to the proxy logging which can be found in Proxy Logs and Proxy Reports.
/var/log/squid/useragent.log. This logging option collects data about the browser and the operating system from the clients.
Example with query term logging:
1326376573.979 172 192.168.205.2 TCP_MISS/200 3605 GET http://wiki.ipfire.org/de/configuration/network/proxy? do=login§ok=898d87a4cdeed875a0fa3e3254fgh0bf user DIRECT/126.96.36.199 text/html
Example without query term logging:
1326376486.158 122 192.168.205.2 TCP_MISS/200 3605 GET http://wiki.ipfire.org/de/configuration/network/proxy? user DIRECT/188.8.131.52 text/html
An example for useragent logging:
192.168.23.5 [08/Jan/2012:15:50:17 +0100] "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:8.0.1) Gecko/20100101 Firefox/8.0.1"