Snow Leopard and DNS/AD problem

Question

Here's a good one. We have an equal number of Macs & PCs on a small (c. 20 seats) office network, with a W2008 server acting as Domain Controller, AD, DNS and DHCP server, as well as fileserver. Most, but not all, of the Macs are bound to the AD, all of them need to connect to it to access files. All the Macs are running 10.6.6.
Last week, 2 Macs stopped being able to connect to the server, both by ethernet and wirelessly. They therefore created self-assigned IP addresses. By manually assigning IP addresses, they can access other network shares but lost Internet connectivity since they aren't connecting to the DNS server. In desparation, I reinstalled the OS from scratch on one machine. It worked for a day, then stopped being able to connect to the AD server again.
This week, my MacBook Pro started to do the same, so now it's serious.
All 3 affected machines were bound to the AD server. But so are others which aren't affected. When you plug the ethernet cable in there's a momentary connection which then drops. When I ping the server, I get 'Permission denied', so it seems to be a permissions issue.
AppleCare say the Macs can't be at fault. My external W2008 server support team say the same about the server.
Anyone any idea what on earth is going on?

Answer 1

You might want to use something like Wireshark to see what's actually happening on the network. There's all sorts of things that can cause this. The most common one is that a network switch's routing tables have become fubar'ed and the switch needs to be reset. The symptoms that you see when this happens are that certain packets are simply dropped by the switch for particular systems or particular ports.
Another possibility is Windows DHCP service has locked up. You wouldn't notice on a computer until the DHCP lease has expired (and you may be using leases that are a month long, for example). Computers would, after their lease is up, negotiate renewal of the lease with the DHCP service, but if the service fails to renew, the computer is supposed to give up it's IP. Windows systems, by default, will not follow the rules and if a lease expires and negotiation fails, they'll just carrying on using the expired info. Without DHCP info, they won't be able to connect to AD, the DC, or make DNS queries.
If you assign them static IP addresses, why not assign them static DNS address too? System Preferences > Network. Click on the "Advanced..." button, select the DNS tab, and add your DNS servers (and outside ones, if you like) to the list.