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Identifying DNS servers at the University of Bristol

From December 7 2004 JANET name servers will no longer operate as "recursive" servers. This means that they will no longer respond to queries for domains for which they are not authoritative. Consequently, you should no longer include references to them in your host "resolver" files. In the local context, this means that you should not include references to our off-site secondary server (address 193.63.106.103). More information is available at JANET Off-site Resolver Service page.

One of the things you all need to do in advising on the configuration of IP networked systems is to identify "suitable" DNS name servers. As you know, we have 2 official local servers plus an official off-site secondary. In order of preference these are:

  • 137.222.10.36
  • 137.222.10.39
  • 193.63.106.103
lookup

Increasingly these days, however, we are configuring departmental systems as unofficial secondary DNS servers (and should encourage all departments to set these up, where possible). Where these exist, they should be used in preference to any other name server. What this means is that where a local name server is available and the software to be configured supports multiple DNS servers, then the 137.222.10.36 address should be replaced with the IP address of the local server. For example, In the Computer Centre, sun.cse.bris.ac.uk (IP address 137.222.12.1) is configured as a DNS server, so systems in the Computer Centre, on the 137.222.12.0 subnet, should be configured to use the name servers 137.222.12.1, 137.222.10.39 and 193.63.106.103 in order of preference.

The problem you'll have is in identifying whether a local server is available in the first place. To do this, you'll need to know the domain name of the system you're going to configure (as a user of DNS services) and the subnet to which the system is (to be) connected. With this information, you can then consult the DNS and look for registered servers that will serve the domain on the subnet. The convention I have is that all departmental servers are registered with names (usually as aliases) of the form dnsN.domain.bris.ac.uk, where N starts at 1 and increments (where more than 1 server is available in a domain). An example will probably help:
sun% nslookup
Default Server: localhost
Address: 127.0.0.1

Firstly, say you were going to set up a system in the phy.bris.ac.uk domain on subnet 137.222.30.0:
> dns1.phy.bris.ac.uk.
Server: localhost
Address: 127.0.0.1

Name: vishnu.phy.bris.ac.uk
Address: 137.222.30.2
Aliases: dns1.phy.bris.ac.uk

This shows that a local server for the phy.bris.ac.uk domain is available on the subnet 137.222.30.0

Another example - doing the same for the cse.bris.ac.uk domain in the 137.222.12.0 subnet....
> dns1.cse.bris.ac.uk.
Server: localhost
Address: 127.0.0.1

Name: sun.cse.bris.ac.uk
Address: 137.222.12.1
Aliases: dns1.cse.bris.ac.uk

Again - got one.

How about the cse.bris.ac.uk domain on the 137.222.14.0 subnet? We shouldn't (but could) use sun.cse.bris.ac.uk because it's not on the same subnet. If there is a second server registered for the cse.bris.ac.uk domain, it will have the name dns2.cse.bris.ac.uk.
> dns2.cse.bris.ac.uk.
Server: localhost
Address: 127.0.0.1

Name: puff.cse.bris.ac.uk
Address: 137.222.14.100
Aliases: dns2.cse.bris.ac.uk

So, once again we've identified a suitable DNS server for the cse.bris.ac.uk, this time on the 137.222.14.0 subnet.