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Tuesday, 23 February 2010

Disable Skype Supernoding

Skype is one of the most popular pieces of VoIP and IM software available.  It is fast, free, good quality, fairly reliable and seems to work almost anywhere. It is also one of the few more secure pieces of communication software, using basic PGP Encryption for calls and instant messaging.

Skype operates on a peer to peer model, rather than a server to client model using is own Skype Protocol and proprietary VoIP Network. This means that the user directory is scattered between "nodes" in the network, rather than using an expensive, centralised infrastructure.

Each client has a list of "nodes" called the "host-cache." This is a list of IP Addresses and Port Numbers of "supernodes."
"Supernodes" relay communications to other clients that are behind firewalls. Any Skype client that is version 3.x or higher can become a supernode if it has good bandwidth, no firewall, and adequate processing power. Usually only a small percentage of skype users become "supernodes" and they hold a record containing the online presence of other skype users.

"Supernodes" are not pre-defined or configured, but are a dynamic feature of the Skype client. Any client that discovers it is well connected to the Internet is likely to offer itself as a super-node by advertising its connectivity to other Skype users.

Usually Network administrators and Internet Service Providers are both against having Skype on their network due to bandwidth usage by the software.

When a client becomes a "supernode" the incoming and outgoing bandwidth usage can increase considerably. The local Skype client may result in handling voice communications to and from clients all over the world, not just those originating or destined for the local user of the PC. Any user who installs Skype with the default configuration permits his computer and his organisation’s bandwidth to be used by any other Skype user.
It is possible that this could be considered a violation of an "acceptable usage policy," particularly on educational, government funded ISP's such as JA.NET.

Skype can be very hard to block on a network without disrupting other services. However according to skype, the ability for a client to become a "supernode" can be disabled in several ways.
  • Clients on a network behind a restrictive firewall and/or NAT (Network Address Translation) device tend to have "supernoding" disabled.
  • Clients behind a HTTP or SOCKS5 Proxy will not become "supernodes."
  • Changing a registry setting on the clients to disable the ability to become a "supernode."
The most typical and effective method is the registry key option, as this can be easily distributed via a Group Policy. Keeping in mind this will only hit the clients that are on your domain. Guest computers/laptops will NOT pick up the policy.
  • To disable the "supernoding" via the registry copy the following in to notepad and save as a .reg file.
Windows Registry Editor Version 5.00
  • Run the registry key on all of the clients, either manually or via a Group Policy.
If you choose to use a Group Policy to distribute the key, you may have to script it in to a batch file.

Use the command: regedit /s "[filename].reg"

There is an option in Group Policy on Server 2008 and Server 2008 R2 domains to distribute registry keys without using a script.

Another more solid solution is to build a SOCKS5 proxy and configure Skype clients to use that. Skype can be configured to use a SOCKS5 proxy, regardless of whether the client finds itself on a network with a public IP address or on one with a private IP address.

Remember as well, that if skype is going to be used on a corporate network, there is a "business" version that is easier to administer.

NOTE: Skype now has millions of users connected to it's network. To support these millions, Skype needs thousands, even hundreds of thousands of "supernodes." Beware, if all skype users disable the ability to become supernodes, then skype will eventually cease to function.

Further information can be found here.

A Skype Network Administrators Guide (PDF) can be found here.


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