4.3. Domain Name Service (DNS)

Starting with the Lhotse release, Rocks clusters contain a fully-operational DNS server on the frontend. This name server coordinates the name->ip address mapping for each node in the cluster. In previous versions of Rocks, hostnames were resolved using a NIS map of the /etc/hosts file.

The switch to a full-fledged name service requires more discipline with our naming practices. We choose a domain name for the internal cluster, ".local" by default, and strictly separate internal names from external ones.

One problem with standard UNIX naming (and Linux in particular), is that a machine has only one name. This becomes an issue for machines like the frontend, which have two network interfaces: one on the internal private network, and one on the public network.

While external services such as Globus requires the frontend to be named by its public address, internal systems such as the queuing system (PBS, etc) prefers the frontend to carry the internal local name.

In Rocks, we have made the decision that all ".local" names resolve to an interface on the private cluster network. This includes all nodes and the eth0 interface of the frontend, and generally these names map to IP addresses in the 10.x.x.x range.

For Globus compatibility, the frontend node is named with its public name. This means a "hostname" command will return its public name, rather than one ending with ".local". Some internal systems are made more complicated by this choice, but those that correctly use the standard resolver library (in libc) have no problems.

New nodes added with "insert-ethers" will automatically be added to the local DNS domain. To see a complete list of node names, execute the following commands.

$ host -l local

4.3.1. Extending DNS

Rocks provides a mechanism to put external hostnames under the DNS control of your cluster. Generally, external hosts have names served by site-wide DNS servers. However if there is no external DNS server available, you may want to use your frontend's DNS server to handle the name->IP mappings for certain non-cluster nodes.

Since the DNS configuration file is automatically generated by a dbreport, you cannot add static configuration to the standard zone files in /var/named. Instead, put local name mappings in the file:

/var/named/rocks.domain.local

And reverse mappings (IP->name) in:

/var/named/reverse.rocks.domain.local

These files are automatically included by the Rocks dns dbreport, which can be refreshed with the command:

# insert-ethers --update

These files are in the BIND configuration format, just like the standard rocks.domain and reverse.rocks.domain files that are generated by the Rocks dbreport.

Your external hosts will have names in the .local cluster domain.

Errors in your local DNS files will cause the entire local cluster domain naming to fail. Proceed with caution.