Add the following line in /etc/squid/squid.conf:

 acl NOCACHEDOMAIN dstdomain www.redhat.com
 no_cache deny NOCACHEDOMAIN

It will not cache any content come from the domain www.redhat.com. In /var/log/squid/access.log, it will show the page from that domain will always get “TCP_MISS” on consecutive visits.

 1197363963.721    892 127.0.0.1 TCP_MISS/200 11813 GET http://www.redhat.com/ - DIRECT/209.132.177.50 text/html
 1197364100.832    906 127.0.0.1 TCP_MISS/200 11813 GET http://www.redhat.com/ - DIRECT/209.132.177.50 text/html

Refresh patterns determine what is saved and served from the cache. Ideally, you would want your squid to follow the directions of the Web servers serving the content to determine what is cacheable and for how long. These directions are set as HTTP headers that are processed and understood by Squid. Unfortunately, the directions given by most servers are the Web servers' defaults, and do not produce significant bandwidth savings.

Refresh patterns are of the format:

 refresh_pattern [-i] regex min percent max [options]

where min and max are time values in minutes and percent is a percentage figure. The options are:

• override-expire – ignores the expire header from the Web server.
• override-lastmod – ignores the last modified date header from the Web server.
• reload-into-ims – a reload request from a client is converted into an If-Modified-Since request.
• ignore-reload – a client's no-cache or “reload from origin server” directive is ignored. The request can therefore be satisfied from the cache if available.
• ignore-no-cache – a no-cache directive from the Web server which makes an object non-cacheable is ignored.
• ignore-no-store – a no-store directive from the Web server which makes an object non-cacheable is ignored.
• ignore-private – a private directive from the Web server which makes an object non-cacheable is ignored.
• ignore-auth – objects requiring authorisation are non-cacheable. This option overrides this limitation.
• refresh-ims – a refresh request from a client is converted into an If-Modified-Since request.

Consult your configuration file to see which of these options are available in your version of Squid.

Refresh patterns are effective if there is no expire header from the origin server, or your refresh pattern has an ignore-expire option. Example:

 refresh_pattern -i \.gif$ 1440 20% 10080.

This says:

• If there is no expire header for all objects whose names end in .gif or .GIF (that is, image files) then:
• if the age (that is how long the object has been on your cache server) is less than 1,440 minutes, then consider it fresh and serve it and stop
• else if the age is greater than 10,080 minutes, consider it stale and go to the origin server for a fresh copy and stop
• else if the age is in between the min and max values, use the lm-factor to determine freshness. lm-factor is the ratio of the age on your cache server to the period since creation or modification of the object on the origin server as a percentage. So if the object was created 10,000 minutes ago on the origin server and it has been on my cache server for 1,800 minutes (that is the age) the lm-factor is 1,800/10,000 = 18%.
• If the lm factor is less than the percent in our refresh pattern (20%) then the object is considered fresh; serve it and stop
• else the object is stale, go for a fresh copy from the origin server.

For objects that scarcely change under the same file name, such as video, images, sound, executables, and archives, you can modify the refresh pattern to consider them fresh on your Squid for a longer time, increasing the probability of having hits. For example, you could modify our refresh pattern above to:

 refresh_pattern ^ftp: 1440 20% 10080 
 refresh_pattern ^gopher: 1440 0% 1440 
 refresh_pattern -i \.(gif|png|jpg|jpeg|ico)$ 10080 90% 43200 ignore-expire ignore-no-cache ignore-no-store ignore-private 
 refresh_pattern -i \.(iso|avi|wav|mp3|mp4|mpeg|swf|flv|x-flv)$ 43200 90% 432000 ignore-expire ignore-no-cache ignore-no-store ignore-private 
 refresh_pattern -i \.(deb|rpm|exe|zip|tar|tgz|ram|rar|bin|ppt|doc|tiff)$ 10080 90% 43200 ignore-expire ignore-no-cache ignore-no-store ignore-private 
 refresh_pattern -i \.index.(html|htm)$ 0 40% 10080 refresh_pattern -i \.(html|htm|css|js)$ 1440 40% 40320 
 refresh_pattern . 0 40% 40320

By default, Squid will not cache dynamic content. Dynamic content is determined by matching against either “cgi-bin” or “?”. This feature used to be activated via the “hierarchy_stoplist” and “cache deny” settings in older versions of Squid. In recent versions, starting with 3.1, this feature is activated via a refresh pattern such as refresh_pattern (/cgi-bin/|\?) 0 0% 0. This enables you to specify sites that serve dynamic content that could be made cacheable in bypass rules. For example, you could set up a refresh pattern such as:

 refresh_pattern -i movies.com/.* 10080 90% 43200 refresh_pattern (/cgi-bin/|\?) 0 0% 0

For the older versions of Squid, you will have to define an access control list (ACL) for the content providers you wish to make exceptions for, and use cache accept to exempt it before the cache deny rule. The following example is from the Squid wiki:

 # Let the client's favourite video site through 
 acl youtube dstdomain .youtube.com cache allow youtube 
 # Now stop other dynamic stuff being cached 
 hierarchy_stoplist cgi-bin ? 
 acl QUERY urlpath_regex cgi-bin \? 
 cache deny QUERY

Below, we configure one global delay pool at 64Kbps (8KBps). Traffic for which the ACL of destination domain is windowsupdate.com during the peak period of 10:00-16:00 will be limited to 64Kbps.

 acl winupdate dstdomain .windowsupdate.com 
 acl peakperiod time 10:00-16:00 
 delay_pools 1 
 delay_class 1 1 
 # 64 Kbit/s 
 delay_parameters 1 8000/8000 
 delay_access 1 allow winupdate peakperiod

After making changes like the ones above, my Squid's byte hit rate increased from about 8% to between 26-37%. If you are doing 33%, it means a third of all traffic is coming from your cache, and not from slower links across the Internet.