Novell Inc. is taking The SCO Group Inc. to task over the latter's legal claims over Unix and against Linux software.

Novell said it never transferred the copyrights and patents of Unix System V when it sold the software to SCO in 1995. SCO claims all Unix flavors in use today are based on Unix System V, and that it owns the software code and licensing rights to that software.

Novell, however, said SCO is apparently aware it lacks these copyrights and patents because over the past few months SCO has "repeatedly asked Novell to transfer the copyrights to SCO, requests Novell has rejected," Novell's Chief Executive Officer (CEO) and President Jack Messman wrote to SCO's President and CEO Darl McBride.

Novell included the entire text of that letter, along with additional comments, in a press release issued Wednesday.

SCO launched an initiative in January called SCOsource to more aggressively enforce the licensing of its Unix software. That initiative led to SCO's US$1 billion lawsuit against IBM Corp. and to later SCO allegations that Unix code it owns has been illegally copied into the Linux operating system kernel.

SCO responded to Novell on Wednesday saying it owns "contract rights" to Unix and that its lawsuit against IBM doesn't involve patents or copyrights but rather breach-of-contract allegations.

SCO said it has the contractual right to "prevent improper donations of UNIX code, methods or concepts into Linux by any UNIX vendor" and that it intends to "protect and enforce" all of the company's contracts with its over 6,000 licensees.

"Copyrights and patents are protection against strangers. Contracts are what you use against parties you have relationships with. From a legal standpoint, contracts end up being far stronger than anything you could do with copyrights," SCO said in its response.

SCO recently struck up a licensing deal with Microsoft Corp. for SCO's Unix software. That deal raised eyebrows, since it brought together two of the most disliked companies among Linux backers. Microsoft executives have said that Linux software has become a significant challenger to Microsoft products.

In its letter Wednesday, Novell's Messman also asks SCO to present specific evidence to back up its claims that proprietary Unix code from SCO has been illegally copied into Linux software, including the Linux kernel.

The Linux operating system can be obtained free of charge and its source code can be modified, copied and redistributed by anyone. Linux includes a kernel, which is developed by Linux Torvalds and volunteer programmers worldwide, and GNU operating system software from the Free Software Foundation Inc. SCO said recently it hasn't yet found infringements to its proprietary code in GNU software.

SCO announced recently it was suspending its own Linux business and sent letters to about 1,500 large companies warning them they could be held liable for intellectual property violations related to their use of Linux software.