Tuesday, October 17, 2006

Windows Server #2 - arp Windows Server #2 - arp

Another interesting find that I stumbled into was the "arp" DOS command. It displays and modifies the IP to physical address translation tables.

Looking at low level network traces (using Ethereal), I was constantly amazed that every time one of my computers wanted to talk to another one, it would ask who had that computer's IP address. And the second computer would respond with its adaptor id. Then, the two would communicate. But then, a couple of minutes later, it would do the same thing. DNS searches were brutal - regardless of which DNS was being searched (Windows had to get the adaptor ID for the gateway for external DNS searches).

I couldn't help but think that since adaptors don't move around, and static IP addresses don't move that much more often, that Windows should be caching that information. And, it does. It just doesn't hold onto it very long.

And that is where the arp command comes in. With the -a switch, it displays the current ARP cache. But more imporantly, if you use the -s switch, you can set static mappings. It was a simple matter then to ping each of the permanently attached IP addresses, then, while all that info was in the ARP cache, dump the cache to a file (using arp -a redirected to the file), edit the resulting file to add arp -s to each line, and I now had a static arp mapping DOS batch file that could be run at boottime.

And, voila, no more of those "who has xx.xx.xx.xx"? query/ responses that were driving me crazy. Well, not exactly none - since the router still does them, but I have some ideas on that. But 90% are gone now.


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