It's widely known that two processes running on same machine can use the loopback network pseudo-interface with address 127.0.0.1 for communication even when there's no real network interface present. The loopback interface neatly solves the problem of providing a usable destination address when there are no other interfaces or when those interfaces are unusable because they are not active. An example of an inactive interface might be an Ethernet interface with no link integrity (or carrier detect) signal. Hence an unplugged laptop may be used to develop unicast applications simply by configuring those applications to use the loopback interface's address of 127.0.0.1.
Unfortunately, the kernel's usual loopback interface isn't used for multicast and isn't capable of forwarding multicast from application to application on the same machine. Ethernet interfaces are capable of forwarding multicast, but only when they are connected to a network. This is seldom an issue on desktops that are generally connected, but it does come up with laptops that tend to spend some of their time unplugged.
The 29West LBM messaging product may be configured to use only unicast, thereby allowing development even on an unplugged laptop.
A simpler solution may be to loopback an Ethernet interface so that the kernel sees it once again as being capable of forwarding multicast between applications. A $5.99 commercial product will do the trick. Or, you can build your own loopback plug.
Note that there may be a time delay between when Ethernet loopback is established and when the interface becomes usable for multicast loopback. The interface won't be usable till it has an IP address. It may take a minute or so for a DHCP client to give up on trying to find a DHCP server. At least on Windows, after DHCP times out, the interface should get a 169.254.x.y "link local" address if the machine doesn't have a cached DHCP lease that is still valid.
Copyright 2004 - 2008 29West, Inc.