Does our implementation of kerberos support determining the default
realm from the default domain?  I have a need for that.  I use my laptop
in three different places, each with its own realm.  The laptop gets its
domain name via DHCP.  

I can see two ways to do this.  One is to define a new DHCP option (as
in dhcp-options(5)).  I could put somthing into /etc/dhclient-exit-hooks
to change default_realm in /etc/krb5.conf to the value of this new
option.  Another way is to cook up a shell script for /etc/rc.d that
selects the default_realm.