Important update (06/03/2017)
Important update (06/03/2017) to prevent this article misleading you to the assumption that you indded got to the right place.
Notice that the methods that are mentioned in the next article are not up-to-date(06/03/2017) and are expected to work only for specific setups. Setups which uses ssl-bump needs a much more complicated concept then the mentioned in the article to make it so skype clients will be able to run smooth with squid in the picture. Else then that skype in many cases will require direct access to the Internet and will not work in a very restricted networks with allow access only using a proxy. I belive that NTOP have some more details on how to somehow make skype work or be blocked in some cases. I recommend peeking at theri at: https://github.com/ntop/nDPI/search?utf8=✓&q=skype
Skype Access Controls
Warning: Any example presented here is provided "as-is" with no support or guarantee of suitability. If you have any further questions about these examples please email the squid-users mailing list.
Squid Configuration File
Configuration file to include.
Since FTP uses numeric IPs the Skype ACL must be exact including the port.
# Skype acl numeric_IPs dstdom_regex ^(([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+)|(\[([0-9a-f]+)?:([0-9a-f:]+)?:([0-9a-f]+|0-9\.]+)?\])):443 acl Skype_UA browser ^skype http_access deny numeric_IPS http_access deny Skype_UA
Recent releases of Skype have been evading the above restriction by not sending their User-Agent headers and using domain names. The following can be used to catch those installs, but be aware it will likely also catch other agents.
acl validUserAgent browser \S+ http_access deny !validUserAgent
This needs to be done before any restrictive CONNECT http_access controls.
acl numeric_IPs dstdom_regex ^(([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+)|(\[([0-9a-f]+)?:([0-9a-f:]+)?:([0-9a-f]+|0-9\.]+)?\])):443 acl Skype_UA browser ^skype http_access allow CONNECT localnet numeric_IPS Skype_UA
Note that Skype prefers the port 443 which is by default enabled in Squid anyway so this configuration is only needed when you block HTTPS access through the proxy.
If you limit HTTPS access to known sites only, then permitting Skype will break that policy.