Can I assume that nobody is 65534?

Question

I'm writing a setuid root program. This program needs to open a file for writing and then write some content. It needs euid 0 only for opening the file, then it can drop privileges.

To drop privileges, I could seteuid to the current uid. But I was thinking at switching to nobody:nogroup.

Now, I was wondering: can I assume that nobody is 65534 on every system (and nogroup is 65534 too)? Is it defined by some standard (POSIX, maybe)?

Answer 1

You can't. nobody has had at least a few different IDs across distros and time:

Historically, the user “nobody” was assigned UID -2 by several operating systems, although other values such as 2^(15)−1 = 32,767 are also in use, such as by OpenBSD. For compatibility between 16-bit and 32-bit UIDs, many Linux distributions now set it to be 2^(16)−2 = 65,534; the Linux kernel defaults to returning this value when a 32-bit UID does not fit into the return value of the 16-bit system calls. An alternative convention assigns the last UID of the range statically allocated for system use (0-99) to nobody: 99.