from rdp.advisory:

  The ICMP Router Discovery Protocol (IRDP) comes enabled by default on
DHCP clients that are running Microsoft Windows95 (w/winsock2),
Windows95b, Windows98, Windows98se, and Windows2000 machines.  By
spoofing IRDP Router Advertisements, an attacker can remotely add default
route entries on a remote system.  The default route entry added by the
attacker will be preferred over the default route obtained from the DHCP
server.

  SunOS systems will also intentionally use IRDP under specific
conditions. For Solaris2.6, the IRDP daemon, in.rdisc, will be started
if the following conditions are met:

		. The system is a host, not a router.
		. The system did not learn a default gateway from a 
		  DHCP server.
		. The system does not have any static routes.
		. The system does not have a valid /etc/defaultrouter
		  file.

-d.