limiting a user to login only via serial link

[James Herbert <jamesherbert@gmx.net>]

Hi,

I'm using TeraTerm to talk to a 3.4-stable vanilla i386 box via a null 
modem cable on com0.

I've created a user 'serial' to use on this link for monitoring 
purposes, and so that I can talk people through fixing stuff when i'm 
not there, without having to give other passwords out, etc.

What I'd like to do is limit this user to only be able to log in via the 
serial port. I've had a look at login.conf(5) and other bits and bobs 
but can't really see how to do this.

Enlightenment much appreciated!

TIA,

James