Proxy Advertising on Multihomed Hosts

If a host with multiple interfaces advertises services by using slpd and proxy registration, the service URLs that are advertised by slpd must contain reachable host names or addresses. If unicast routing is enabled between the interfaces, hosts on all subnets can reach hosts on other subnets. Proxy registrations can also be made for a service on any subnet. If, however, unicast routing is disabled, service clients on one subnet cannot reach services on another subnet through the multihomed host. However, those clients might be able to reach the services through another router.

For example, suppose the host with default host name bigguy has three interface cards on three different unrouted subnets. The host names on these subnets are bigguy, with IP address 192.147.142.42, bigguy1, with IP address 192.147.143.42, and bigguy2, with IP address 192.147.144.42. Now suppose that a legacy printer, oldprinter, is connected to the 143 subnet and that the URL service:printing:lpr://oldprinter/queue1 is configured with the net.slp.interfaces to listen on all interfaces. The oldprinter URL is proxy-advertised on all interfaces. The machines on the 142 and 144 subnets receive the URL in response to service requests, but are unable to access the oldprinter service.

The solution to this problem is to perform the proxy advertisement with slpd running on a machine that is connected to the 143 subnet only, rather than on the multihomed host. Only hosts on the 143 subnet can obtain the advertisement in response to a service request.