Domain Name Service

The Domain Name System (DNS) is the method by which Internet addresses in mnemonic form such as sunc.scit.wlv.ac.uk. are converted into the equivalent numeric IP address such as 134.220.4.1. To the user and application process this translation is a service provided either by the local host or from a remote host via the Internet. The DNS server (or resolver) may communicate with other Internet DNS servers if it cannot translate the address itself. These notes will discuss

1. The form of mnemonic addresses understood by DNS
2. The hierarchichal structure of DNS servers and their databases
3. How DNS servers communicate with each other
4. The user and program interfaces to DNS

DNS name structure

DNS names are constructed hierarchichally. The highest level of the hierarchy being the last component or label of the DNS address. Labels can be up to 63 characters long and are case insensitive. A maximum length of 255 characters is allowed. Labels must start with a letter and can only consist of letters, digits and hyphens. [Unfortunately some administrators construct names that start with digits. This is wrong and can easily cause problems with software that simply inspects the first character of a host address to determine whether a DNS name or an IP address has been quoted.]

Note In the early days of the Internet users in at least one country (the United Kingdom) adopted a similar scheme with the highest hierarchical level appearing first rather than last. I.e. uk.ac.wlv.scit.sun rather than sunc.scit.wlv.ac.uk. This practice is, fortunately, obsolete.

DNS addresses can be relative or fully qualified. A fully qualified address includes all the labels and is globally unique. A relative address can be converted by appending the local domain information. For example sunc.scit.wlv.ac.uk is a fully qualified name for the host sunc in the domain scit.wlv.ac.uk. Strictly there should be a stop at the end of a fully qualified name but this is often overlooked.

The final most significant label of a fully qualified name can fall into one of three classes

• arpa

This is a special facility used for reverse translation, i.e. going from IP address to fully qualified domain address. If everything is properly configured a suitably framed query for 1.4.220.134.in-addr.arpa will return sunc.scit.wlv.ac.uk. Details of this will be described later.

• 3 letter codes

The DNS was orginally introduced in the United States of America and the final component of an address was intended to indicate the type of organisation hosting the computer. Some of the three letter final labels (edu, gov, mil) are still only used by organisations based in the USA, others can be used anywhere in the world.

The three letter codes are

• Two letter codes

The final two letter codes indicate the country of origin and are defined in ISO 3166 with the minor exception that uk is used for the United Kingdom rather than gb although there are some .gb sites. [This apparently happened because the ISO committee was unaware that Northern Ireland was part of the United Kingdom but not part of Great Britain.]

The two letter code us is used by some sites in the United States of America.

In some countries there are sub-domains indicating the type of organisation such as ac.uk, co.uk, sch.uk in the United Kingdom and edu.au and com.au in Australia. Most European countries have not adopted this useful practice. A fuller discussion of the United Kingdom DNS domains is provided by the UK Network Information Centre.

To obtain a domain address it is necessary to identify the administrator of the required domain and then all that is basically necessary is to send the administrator the required code and the associated IP address and they will, if they accept the request, include the details in their databases. Conditions for acceptance vary widely between administrators, the administrators for the com and org being, apparently, quite happy to accept anything from anywhere.

DNS servers and their databases

For any group of computers partaking of the DNS naming scheme there is likely to be a single definitive list of DNS names and associated IP addresses. The group of computers included in this list is called a zone. A zone could be a top level national domain or a university department. Within a zone DNS service for subsidiary zones may be delegated along with a subsidiary domain. The computer that maintains the master list for a zone is said to have authority for that zone and will be the primary name server for that zone, there will also be secondaries for that zone.

When any process needs to determine an IP address given a DNS address it calls upon the local host to resolve the address. This can be done in a variety of ways

• Table lookup. On Unix hosts the table is called /etc/hosts
• The process communicates with a local name server process. This is commonly called named on a Unix system. named initially obtains information from /etc/hosts but also maintains a cache of recent requests.
• It sends a message to a remote system that is identified from the information in the file /etc/resolv.conf.
• Finally if a network information system (NIS) is in use DNS service may be one of the facilities provided by the network information system. Most SUN systems work this way although the NIS master will use one or more of the techniques described above to build and maintain the master database.

If a named process cannot resolve an address locally it will call upon higher authority. Ultimately it will attempt to contact the system that is authoritative for the zone in question, however, unless the information is cached or in the hosts local files then it will not know the address of the authoritative server. This problem is resolved by recursive resolution of requests, i.e. any DNS server will pass requests it cannot handle to a higher level server and so on until either the request can be handled (either by sending a message to the identified authoritative host) or until the root of the DNS name space is reached.

There are eight servers that can serve requests at the root of the DNS name space, all servers should know their IP addresses so that DNS service can be offered even if there are no cached addresses and no local servers indicated by the /etc/resolv.conf file. The root servers will know the IP addresses of the servers for all the national DNS zones and the three letter zones.

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