Blocking Skype using Squid

Blocking

# Skype

acl numeric_IPs dstdom_regex 

^(([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+)|(\[([0-9af]+)?:([0-9af:]+)?:([0-9af]+)?\])):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

SPF Vs Sender ID

Are SPF and Sender ID the same?

SPF and Sender ID are not the same. Both validate e-mail sender addresses, and both use similar methods to do so. Both publish policy records in DNS. Both use the same syntax for their policy records. So you have some excuse to be confused. They differ in what they validate and what "layer" of the e-mail system they are concerned with.

What is SPF?

SPF (defined in RFC 4408) validates the HELO domain and the MAIL FROM address given as part of the SMTP protocol (RFC 2821 – the "envelope" layer). The MAIL FROM address is usually displayed as "Return-Path" if you select the "Show all headers" option in your e-mail client. Domain owners publish records via DNS that describe their policy for which machines are authorized to use their domain in the HELO and MAIL FROM addresses, which are part of the SMTP protocol.

What is Sender ID?

Sender ID (defined in RFC 4406) is a Microsoft protocol derived from SPF (hence the identical syntax), which validates one of the message's address header fields defined by RFC 2822. Which one it validates is selected according to an algorithm called PRA (Purported Responsible Address, RFC 4407). The algorithm aims to select the header field with the e-mail address "responsible" for sending the message.
Since it was derived from SPF, Sender ID can also validate the MAIL FROM. But it defines the new PRA identity to validate, and defines new sender policy record tags that specify whether a policy covers MAIL FROM (called MFROM by Sender ID), PRA, or both.