Troubleshooting squid issues

Website not opening via squid, however it opens directly

If a website does not opens via squid, but it opens directly than look at squid access log. The main reasons are:

Site is also using non-standard ports (other than 80, 443) via JavaScript / Flash and squid is configured to deny such requests. This should be visible in squid logs as port number would appear in log line with DENIED message.
Some ACL such as port ACL described before is blocking such access. Try adding "http_access allow all" for given IP or for entire proxy (if practical), to see if the problem is due to ACL. If "http_access allow all" makes things work than it is definitely an ACL related issue. Now comment / reversing various ACLs one by one till the problem is solved can help in pin-pointing the ACL which is causing the problem.
DNS server is reply includes IPv6 address but only IPv4 is supported. This should ideally be solved on DNS using steps similar to Disabling IPv6 lookups in bind. However for fixing it on squid side use 'dns_v4_first on'. Using 'tcp_outgoing_address <IP_of_isp_interface>' might also help. This problem can be detected using logs. Typical error message shown to client on browser in such cases in "Network is unreachable".

Some of this learned from: