In a somewhat ironic move, Novell has announced it is prepared to use its existing software patents to protect the open-source software it ships.

In statements posted on its website and in an e-mail sent to customers, the networking giant voewed to fight fire with fire if open-source products are targeted using patent infringement claims in the US courts.

The announcement is aimed at reassuring purchasers of the company's open source software, such as the Suse Linux operating system and Ximian desktop software, that these products are as safe from legal attacks. The company said it would use the "same measures generally used to defend proprietary software products," in the event that any of the open source projects included in Novell's be accused of patent infringement.

"If somebody comes after an open source technology that we ship with a patent claim, then we're going to go after that company," said Bruce Lowry, a Novell spokesman.

Though software patent disputes are fairly common in the high technology industry, open source software's vulnerability to a patent suit has received particular attention recently, spurred in part by SCO's claims that the Linux kernel violates its intellectual property.

Since open-source code tends to be created by diverse groups of developers who write code that can be easily scrutinised, some critics have argued it is easier for companies to make IP claims against it.

One study, funded by insurance provider Open Source Risk Management, found that 283 registered software patents, including 27 held by Microsoft, could conceivably be used as the basis of lawsuits against the Linux kernel. "There is a non-trivial risk of patents being asserted against Linux," said the study's author, Dan Ravicher, at the time. Ravicher is executive director of the Public Patent Foundation, a not-for-profit organization that advocates for changes to US patent policy.

Recently, the City of Munich halted a 14,000 PC Linux roll-out to investigate the patent liabilities associated with such a large deployment of open source software. The city eventually decided to go ahead with Linux after concluding that the risk of software patent infringement was "very small".

Novell's statement follows similar declarations from open source providers such as IBM and Red Hat. In August, IBM's technology and marketing VP, Nick Donofrio, said his company would not use its patents against the Linux kernel. And Red Hat has recently begun amassing a patent portfolio, to be used only for "defensive purposes".

Novell is in a unique position to defend open source software, Lowry said, because with 411 patents in its portfolio, it has far more patents than other open-source providers, such as Red Hat.

Linux advocate Bruce Perens, who has long warned of the patent threat to open source software, welcomed Novell's new patent policy, but said that a patent defence might have no effect against a company that had cross-licensed patents with the company. "The only problem that can potentially come up is when one of Novell's own partners is the one who is bringing actions against open source," Perens said.

Lowry declined to comment on which, if any, companies had cross-licensing deals with Novell. "We don't make our cross-licensing arrangements public," he said. "However, cross-licensing issues were certainly considered in the formulation of the patent policy statement we just put out."

Linux vendors, including Novell, could do more to mitigate the patent threat, Perens said. "I would like to see statements like this from other companies that would both explicitly promise not to use their patents against open source and promise to stand beside us in defending open source from patents. I would like such documents to be legally binding. The ones I've seen so far fall short of that."

Though Novell's patent policy doesn't say whether or not the company will ever use its patents against Linux, Novell has no intention of using its patents against open source, Lowry said. "It's nice that they say they won't use their patents against open source," Perens said. "They should write it down explicitly in the next revision of this policy."