Google is following IBM and Oracle in circling the wagons around open source Linux. The company is expected to announce Tuesday that it has become a member of the Open Invention Network, a group that pools Linux patents as a means of turning back any patent infringement challenge to Linux.

The addition of Google means seven companies have formed a bulwark of protection around Linux.

"This is a way for everyone to get together and look after Linux development," said Chris DiBona, open source programs manager for Google in an interview. Every user of the Google search engine is being supported by Linux, and Google manages its internal Web indexing and analysis of Web pages with systems running on clusters of Linux servers. "Linux is very important to Google," DiBona said.

"We are pleased to have Google become our first end-user licensee," said Jerry Rosenthal, CEO of OIN on the group's Web site. Other members are vendors that run Linux on their products or distribute it as part of their products.

Patent holders typically mount a defense against a possible infringement challenge by threatening to mount their own case against the would-be challenger. The more patents in an organization's portfolio, the more likely it will be able to mount a counter challenge and the better its potential defense, according to Todd McClelland in the Atlanta office of Alston & Bird LLP, a patent law firm.

The Open Invention Network's set of Linux patents numbered over 100 before Google joined its ranks. DiBona couldn't say exactly how many patents Google will add to the portfolio, but he noted Google staffers include many Linux contributors. The OIN was founded in 2005 and in addition to IBM, Oracle and Google, includes Red Hat, Novell, Philips, NEC and Sony. Members share their Linux patents with each other and offer the prospect of a joint defense if Linux is confronted with a legal challenge.

No patent holder has ever sued Linux developers or a Linux distributor, and none is likely to, asserted Jerry Rosenthal, CEO of Open Invention Network, when he welcomed Oracle into the group March 27.

IBM and Oracle are examples of vendors that feature Linux as a key part of their product strategy. Google is one of the largest users of Linux, with multiple data centers in Mountain View, New York and other locations around the world powering its search engine.

As the OIN stores up ammunition, Microsoft has been signing agreements with small Linux distributors, such as LG Electronics, Linspire and Xandros that say Microsoft will not sue their customers for patent infringement because they've signed a patent agreement. It signed a similar deal with Novell last November, but Novell spokesmen later disavowed supporting any assertion that Microsoft IP had found its way into Linux. Dell has also signed an agreement with Microsoft.

After signing the Novell deal in November, Microsoft CEO Steve Ballmer asserted Linux was an IT and Microsoft IP was found in Linux. Microsoft Chief Counsel Brad Smith said in a May 13 Fortune magazine article that his firm believed that Linux violated 235 of its patents. But neither party has named any violated patents to give Linux defenders a chance to respond.