Why does sgid shadow not work?
 

Hello all.

Short what we want to do: Install qmail with vpopmail and let vpopmail authenticate any non-virtual user against the system users for smtp-auth.

Therefore the auth-tool vchkpw must be able to read /etc/shadow to compare the passwords. This should be done by sgid shadow.

Problem ist: vchkpw won't execute with sgid shadow (or even wont execute with suid root!) when called from qmail-smtpd.

When it's called from commandline as a normal user sgid/suid works all fine, except in the case when we want to strace it: then sgid/suid refuses to work too.

What can be the problem?