Discuss the absence of a syntactic distinction (such as use of a final ‘.’) between absolute and relative names in DNS.
What will be an ideal response?
DNS servers only accept complete domain names without a final ‘.’, such as dcs.qmul.ac.uk. Such names are referred to the DNS root, and in that sense are absolute. However, resolvers are configured with a list of domain names which they append to client-supplied names, called a domain suffix list. For example, when supplied with a name fred in the department of Computer Science at Queen Mary and Westfield College, a resolver appends.dcs.qmul.ac.uk to get fred.dcs.qmul.ac.uk, which it then submits to a server. If this should be unbound, the resolver tries fred.qmul.ac.uk. Eventually, if necessary, the resolver will submit the name fred to a server. Some resolvers accept a final after a domain name. This signifies to the resolver that the name is to be sent directly to the server as it is (but stripped of its final ‘.’); the final ‘.’ is not acceptable domain name syntax.
In practice the lack of syntactic distinction between relative names (fred) and absolute names (fred.dcs.qmul.ac.uk) is not a problem because of the conventions governing first-level domain names. No-one uses single-component names referred to the root (such as gov, edu, uk), so a single-component name is always relative to some subdomain. In principle, a multi-component name such as ac.uk uttered in the domain elvis.edu could refer to a (bound) domain ac.uk.elvis.edu, but normally organisations neither need to nor want to install such confusing names in their subdomains.
An advantage to the lack of syntactic distinction between absolute and relative names is that the DNS name space could, in principle, be reconfigured. We could, for example, transform edu, gov, com etc. into edu.us, gov.us, com.us etc. and still correctly resolve names such as purdue.edu in the USA by configuring all resolvers in the USA to include.us in their domain suffix list.
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