I'm working on a server program and I'm trying to decide which open
source license to use for it.

Normally I'd just go with GPL, but there's one concern I have about
GPL: If I'm not mistaken, it allows people to modify their software,
and they don't have to release their modifications if they don't
distribute the software to anyone.

Since my program is a *server* program, I'm concerned that if I release
it under GPL, other people can make improvements to their servers, and
run their improved servers without sharing their improvements with
everyone else.

Is there an existing well-known open-source license that says "you must
submit any changes you make to the author, even if you aren't
distributing the application to anyone"?