[AG-TECH] Rat and Cisco VPN

Robert Olson olson at mcs.anl.gov
Wed Aug 4 06:59:45 CDT 2004

[Background for those who are coming in in the middle: we use the Cisco VPN 
for offsite access at ANL; it has always caused problems with using rat].

In looking at debug output from another libcommon app, I realized what the 
problem has been with rat and the VPN.

The libcommon code I added to make it deal properly with multiple network 
interfaces on Windows works by querying the system for the best route to 
use for a multicast address, and then binds the UDP sockets to that address.

If you look at the routing table when the VPN is up you'll find something 
like this:
       10       10      30

(The first row is the VPN route, the second is the local network, and the 
third is "MS Loopback" interface that Justin pointed me at that you can 
install via the device mgr - gives you a live interface in the absence of 
actual network. Required for doing development of this sort offline).

The last column is the interface metric; note that the VPN and the local 
network have the same metric; this means that the route chosen is chosen 
arbitrarily between the two. And in fact, windows chooses the VPN route, so 
rat will use that route as well.

If you bring up the properties panel for the VPN interface, and click 
through to the advanced settings for TCP/IP, you find a checkbox for 
"Automatic metric". Uncheck that, and set a metric > 10 (I used 15). Now, 
the routing table looks like this:
       15       10      30

and rat and windows will choose the local network interface for routing 
multicast, and things should be happier. I've not yet tried this on a 
network with live multicast, but I suspect it should work.


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