y0001006@ws.rz.tu-bs.de (Thomas Boroske) writes:

> 
> Thanks a lot, I'll try that. I already tried a single-user *startup*, 
> but that only mounts the filesystem read-only (hmm).

You can upgrade the file system to read-write by doing:

mount -u /

(if it's in /etc/fstab), or:

mount -u /dev/[sw]d0a /

> My Unix book (unfortunately about SVR4) says you can use shutdown 
> to switch security level, but the BSD one doesn't support this.

SVR4 doesn't have the same concept of `security level'.  You can in
fact use `shutdown' (with no `-r' or `-h' argument) to go to
single-user mode, which will also downgrade the `security level'.

> BTW: Is chflags schg MEANT to be irreversible in normal (root) mode ?

Yes, that's the whole point.  To prevent ordinary users from changing
the file, you would use the traditional Un*x-style permissions.