Changing euid (effective user ID)

This post has moved to eklausmeier.goip.de/blog/2014/01-13-changing-euid-effective-user-id.

Sometimes you have to create applications in an environment where you are not able to su to root, or use sudo. Nevertheless you are working with two or more user IDs to make your application work. For example, your business application is running under user ID 1555 (let’s call this user u1555), while your web-server is running under user ID 1000 (let’s call this user u1000). You want to switch from u1000 to u1555 without explicitly providing the password of u1555.