This week I was in Grenoble for the Embedded Linux Conference Europe. On the seond day of the conference -- Friday -- I was one of the few people wandering around in a suit. Even the guys who normally wear suits had dressed down to deal with the nitty-gritty of kernel threads, time sources, and boot time optimization.
So I talked about licenses. And license obligations. And interesting bits of the GPL version 2. There's one clause of the GPL version 2 that I'd like to single out because it's one that is surprising to me -- and rarely mentioned. Clause 6.
6. Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License.
So, let's suppose you (Bob) receive a program under the GPLv2 from the original author (licensor, called Alice), and pass it on unmodified to a third party, Charles. Clause 1 of the GPL applies, because it is verbatim. You just need to give Charles a copy of the license, and Charles receives a license from Alice automatically. There is in my reading no license, no relation whatsoever between you and Charles. Now if Charles distributes the program, the only license he could violate is Alice's, and it is Alice who would need to enforce any violation.
This is actually how it works, too -- suppose Alice wrote a part of the Linux kernel, she grants a license to any recipient (Charles) even though there are second parties involved (Bob); Alice can enjoin Charles to satisfy the requirements of the license. (For Alice perhaps read 'Arald)
Now suppose you, Bob, modify the program before distributing it. In that case, clause 2b comes into play and the resulting work (based on the program) is also covered by the GPL version 2. When you distribute it, Charles receives a license from you to the modified work, and Charles receives a license from Alice to the original program. This is powerful, because Charles now has two licenses and two licensors that he must satisfy -- but also weak, it seems to me, because Charles could violate one of the licenses and still keep the other. So if Bob sues Charles over compliance, Charles could just switch to the original Alice version (unless Alice enforces compliance issues as well).
It's unlikely that would work in practice, because Charles wants the modifications done by Bob. Probably.
PS. Thanks to Saul Goode for some very careful and relevant comments to my previous writing on the GPL version 2; in particular pointing to the US Copyright Code and how it doesn't restrict running the program at all -- as indeed the GPLv2 itself writes "The act of running the program is now trestricted."