Work packages/Updating DNS for non-functioning mirrors

Alan Bawden has a setup for updating the DNS for sharedname.org with 10 second latency. We need to make that production, and then hook up the monitoring of shared names servers (currently via nagios) so that servers that are not working are removed from the DNS entry. See, e.g. Issue/Server_online_failure

The cases that need to be detected and acted on quickly are:

• Server connects but doesn't respond quickly enough
• Server responds with incorrect information
• Can't connect to server

We need to determine what the timeouts are for these (I'm guessing 1-2 seconds) and at what frequency we want to be checking (several times per minute?)