16 Oct 2010
Some quick Squid notes, from O’Reilly’s “Squid The Definitive Guide“
# check config file changes: % squid -k parse # load conf file changes: % squid -k reconfigure # start in foreground, display debug info: % squid -N -d1 -> look for "Ready to serve requests" message
Edit your squid.conf file and set the debug_options line to this:
debug_options ALL,1 33,2
Now, Squid writes a message to cache.log for each client request and another for each reply. The messages contain the request method, URI, whether the request/reply is allowed or denied, and the name of the last ACL that matched it.
Rule order – AND vs OR
- ACL elements are OR’d together ie *any* success causes success eg
acl FOO src 18.104.22.168 22.214.171.124
- access rules are AND’d together ie *any* failure causes failure eg
http_access allow FOO BAR BAZ
Or, as O’Reilly explains:
Recall that Squid uses OR logic when searching ACL elements. Any single value in an acl can cause a match. It’s the opposite for access rules, however. For http_access and the other rule sets, Squid uses AND logic. Consider this generic example:
access_list allow ACL1 ACL2 ACL3
For this rule to be a match, the request must match each of ACL1, ACL2, and ACL3. If any of those ACLs don’t match the request, Squid stops searching this rule and proceeds to the next.
dstdomain vs dst
It’s generally better to use dstdomain rather than dst in ACL’s:
- normally users request by domain name not ip address, and unless there is an entry in Squid’s ip cache, specifying dst will cause delays from dns lookups.
- target domains can change their ip address, requiring updating the config file
- nota bene: an entry of dst foo.com will actually get cached as an ip address
- however if the intention is to block access to URLs, users can bypass by requesting the ip directly (http://126.96.36.199/pr0n), unless TODO