Access control is one of the main reason to use a proxy. The ACL system of Squid has options to control almost every aspect of an HTTP request. Access control can be restricted by time of day, by domain/URI, by user (logging into proxy), and by content. Configuration files are processed sequentially.

To enable the ACL named hourlyworkers to only use the proxy during business hours, do:

acl workinghours time MTWHF 08:00-18:00
http_access allow hourlyworkers workinghours
http_access deny hourlyworkers

To restrict by a part of the URI, do:

acl banned_reddit url_regex ˆhttp://.*reddit.com/.*$
http_access deny banned_reddit

To allow only authenticated users to use the following configuration, do:

acl valid_users proxy_auth REQUIRED
http_access allow valid_users
http_access deny all

When building ACLs or configuration files for Squid, remember that the first match wins. Therefore, start your ACLs with the most specific options in the beginning.