[NBLUG/talk] System Message

[Mitch Patenaude]

<robertchow999 at yahoo.com> wrote:
> For wall to work (in most *nix systems, anyway), I
> believe that the user has to have mesg=y set.

Not when it's run by root.  

Historical footnote: It used to be mesg {y|n} was determined by
whether the terminal device was writable by the user doing the writing
(and therefore whether the user-space process "write" could open the
device to write to it), but having globally writable terminal devices
was considered a security hole (think spoofing attacks.)  So then the
executables were made setgid, and the tty devices were made 620 when
mesg=y, and 600 when mesg=n.  Still not perfect.

However, root doesn't care about permission, so it doesn't matter if
the user is "allowing" messages or not.. root can write there.

Another interesting footnote... back in the old days when large
machines had multiple users, setting the ticky-bit on your tty was a
sign you wanted to go in on pizza delivery, which is why it is
occasionally still called the "pizza bit".  Maybe this was only a UC
Berkeley thing, but I thought it was common practice.

  -- Mitch