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Blocking Skype Using Squid and OpenBSD

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the proxy on is subjective (I chose OpenBSD as my network OS of choice for its ... The setup is basically Squidproxy running over OpenBSD. PF (packet ...

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Blocking Skype Using Squid and OpenBSD Page 1November 13th 2005 Written by rootn0deBlocking Skype Using Squid and OpenBSD Abstract :After much digging online for an effective way to stop this pesky app that ishighly decentralised and a big pain to blocked I finally found a way to do quite nicely. It has been working perfectly fine on our corporate network and we have had no complaints of users being denied access to legitimate web destinations(that are in compliance with our security policy of course). I used Squidproxyrunning on an OpenBSD server to carry out the below. The choice of OS to run the proxy on is subjective (I chose OpenBSD as my network OS of choice for itsproven security record and excellent reliability) and has no effect over the actual blocking mechanism. The same can be accomplished on any other BSD or Linux flavour. Background :This basic writeup will not delve deeply into the operation of Skype but will quickly highlight the main challenges of blocking this application. As mentionedthe below is not an accurate study of how Skype operates and is not be a comprehensive analysis of its behaviour : 1) Skype will initially attempt to contact supernodes the IPs of which are in a file stored along with the other files that Skype installs. The first method of contact is direct. The source ports that Skype attempts to connect from are nondefaultports. From my observations I could see that the UDP source port 1247 is the initial control channel. Once the connection is established the rest of the Page 2communications is done in TCP over nondefault source ports with ranges sweeping from 29403000. In general any company that is serious about its security policy would have strict egress filtering rules which makes identifying the nondefault source/destination ports that Skype uses irrelevant since they would be blocked anyway. 2) If the above fails Skype will use the proxy server specified i ...

Source: www.net-security.org

 

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