Domain Naming System

Names not Numbers

Names are given to computers connected to the Internet to make it easier on the humans. On the wire, the Internet uses IP addresses, not names.

Names are generally easier to remember than strings of numbers. Just imagine, that instead of having to carry around a little book of the telephone numbers of all your friends, you could just dial up their name on the telephone!

Also, Internet names are more stable than Internet addresses. The higher-order parts of Internet addresses are dictated by network topology and Internet Service Provider constraints. If a machine is moved between (sub-)networks its IP address will change, but its name need not.

A machine with a particular name can be connected anywhere on the Internet.

Host Naming

Originally, there were only a few hosts at Monash, and Monash wasn't connected to the Internet, hosts could have nice short names, and it was simple enough to keep track of them all in a little file:

vax1	130.194.1.1
vax2	130.194.1.2
vax3	130.194.1.3
...
vx15	130.194.1.15
monu1	130.194.1.101
monu3	130.194.1.103

Unfortunately, this model doesn't scale too well.

With the one centrally maintained Internet hosts file it became increasingly difficult to choose a new unique name, have the central authority update the hosts file, and then distribute the modified file out to every host.

There are currently about 13,000 machines registered on the Monash University network, and the number grows at about 40 per week.

There are some unknown number of millions of hosts on the Internet, being added at the rate of some tens of thousands per week.

A new solution was required.

Domain Name Concepts

The domain name space is a tree structure, with components of the domain name listed from least-significant to most-significant and separated by dots.

host.subdomain.domain.top

Subdomains are a means of subdividing the Domain Naming namespace into managable chunks, and also for delegating these subdomains to local administrators to look after.

You can't have two machines with the same host name (e.g. fred) in the same subdomain (e.g. dept.org.com), but you can have two machines with the same host name in different subdomains (e.g. dept1.org.com and dept2.org.com).

At Monash, there are subdomains for various departments (or faculties). This allows departments to choose their own domain names independently of other departments, and to apply local policies as to naming conventions, proceedures to be followed when adding/modifying/removing entries. Most subdomains of monash.edu.au are managed using the addhost database, but some are not, in particular

cs.monash.edu.au
vifp.monash.edu.au

There is no strong association between IP subnets and DNS subdomains. A machine with any Monash subdomain name can be connected to any Monash subnet.

Information Stored

The DNS system stores various types of information:

A records: Given a domain name, return it's IP addresses.
PTR records: Given an IP address, return the corresponding domain name.
MX records: Given a domain name, return an ordered list of where to attempt to deliver Email.
HINFO and TXT records: Given a domain name, return descriptive information.

DNS Servers

DNS server programs are run on a number of machines around the world.

Some DNS servers are the primary source of information for some subdomains and others have secondary copies of the information. If a DNS server doesn't know the answer to a particular query, it will ask another server "closer" to where the information is stored. The DNS server will also cache that answer for answering some subsequent query.

The DNS system forms an enourmous world-wide distributed database, with distributed management, replication and caching.

Given the wide variety of network links, O/S types, DNS server software, DNS administrator skills and workload out there in the Internet, it is amazing that the DNS system works as well as it does.

Further Information

See the DNS Requests For Comments (RFC), the "BIND Operations Guide", and "DNS and BIND" from O'Reilly and Associates.