Virtualization software maker VMware is facing a lawsuit alleging that it has been violating the GPLv2 free software license for years with its use of Linux and other source code in ESXi.

Linux kernel developer Christoph Hellwig filed the suit in the district court of Hamburg, Germany with funding from the nonprofit Software Freedom Conservancy, which works to "promote, improve, develop, and defend" free and open source software. The case centers on "a combined work that VMware allegedly created by combining their own code ('vmkernel') with portions of Linux's code, which was licensed only under GPLv2," the group said in an FAQ describing the lawsuit.

VMware denies the lawsuit's accusations, calling them "without merit," but it did not address them specifically in its public statement on the matter. The ESXi hypervisor is a key part of VMware's leading position in the enterprise virtualization market. VMware, which is owned by EMC, made $1.7 billion in revenue and $326 million in net income in the most recent quarter.

Hellwig is one of the most active developers of the Linux kernel. He "has publicly denounced VMware's misuse of GPL-licensed code since 2007," the Software Freedom Conservancy said in its announcement yesterday. "In 2011, Conservancy discovered that VMware had failed to provide nor offer any source code for the version of BusyBox included in VMware's ESXi products (as required by BusyBox's license, GPLv2)."

The Conservancy began negotiations with VMware "to seek compliance on all GPL'd components in the ESXi project," but progress was slow through 2013, the group said. Hellwig had begun working more closely with the Conservancy in late 2012 to analyze "the non-compliant releases of ESXi that VMware provided," the group wrote. "After studying these materials over a long period, it became apparent that VMware's current ESXi products infringed many of Hellwig's own copyrights, due to VMware's failure to comply with Linux's license, GPLv2."

"During Hellwig's investigations, Conservancy continued to negotiate with VMware," it said. "Sadly, VMware's legal counsel finally informed Conservancy in 2014 that VMware had no intention of ceasing their distribution of proprietary-licensed works derived from Hellwig's and other kernel developers' copyrights, despite the terms of GPLv2. Conservancy therefore had no recourse but to support Hellwig's court action."

Court documents have not been released because "Court proceedings are not public by default in Germany," the Conservancy's FAQ said.

"In addition to other ways VMware has not complied with the requirements of the GPL, Conservancy and Hellwig specifically assert that VMware has combined copyrighted Linux code, licensed under GPLv2, with their own proprietary code called 'vmkernel' and distributed the entire combined work without providing nor offering complete, corresponding source code for that combined work under terms of the GPLv2," the Conservancy wrote. "Hellwig is an extensive copyright holder in the portions of Linux that VMware misappropriated and used together in a single, new work without permission."

VMware told Ars, "We believe the lawsuit is without merit. VMware embraces, participates in, and is committed to the open source community. We believe we will prevail on all issues through the judicial process in Germany.” The company said it would not provide any further comment.

The Free Software Foundation (FSF), which wrote the General Public License (GPL), voiced support for Hellwig's lawsuit.

"VMware, like everyone, is free to use, modify, and distribute such software under the GPL, so long as they make available the human-readable source code corresponding to their version of the software when they distribute it," the FSF wrote. "Unfortunately, VMware has broken this promise by not releasing the source code for the version of the operating system kernel they distribute with their ESXi software."

The Conservancy is seeking donations to help pay for the lawsuit, which it says could take years to resolve. The group decided to pursue litigation only after exploring "every other possible alternative" over the past few years, it said.