DNS in plain English

The name and the destination are separate

A domain name is a label. The machines that actually hold a website, or accept email, are somewhere else entirely, identified by numbers rather than words. Something has to sit between the two and translate, and that something is the domain name system, universally shortened to DNS. When a browser is given the address of a choir's website, it asks the system where that name points, receives an answer, and then goes to the place named in the answer.

This separation is the reason a website can move to an entirely new provider without the address changing, and the reason an address can stop working while the website itself sits perfectly intact on a server that nobody is being sent to any more. Almost every mysterious outage in a small organisation is a pointing problem rather than a content problem.

Nameservers: where the answers come from

The register entry for a name contains very little: the holder, the dates, and the addresses of the nameservers responsible for it. Nameservers are the machines that hold the actual answers about where the name points. Setting them is the one DNS decision made at the registrar rather than anywhere else, and it determines which service is in charge of everything below.

That is why a website provider so often asks for the nameservers to be changed. It is asking to be put in charge of the answers. It is also why a group that has forgotten which company runs its DNS can find out by looking at the nameservers on the record, which usually carry the name of the responsible company in plain sight.

The records that do the work

Underneath the nameservers sit the individual records, each answering a different question about the name. The ones that matter to an ordinary organisation are few:

• A record: sends visitors to the server holding the website.
• CNAME: says that one name should be answered the same way as another, commonly used for the www version of an address.
• MX: says which mail servers accept email for the name, and is the reason email and website can live at completely different companies.
• TXT: holds short pieces of text used to prove control of the name to another service, and to publish email authentication policy.

The most important practical fact about these records is that they are independent. Changing where a website lives means changing the A record and leaves the MX untouched, so email keeps flowing. Changing everything at once, which is what happens when nameservers are switched without copying the existing records across, takes the email down with the website. That failure is common enough to have its own page, keeping email working.

Caching, and the illusion of propagation

Answers are not fetched from the source every time. Each record carries a lifetime, and every machine that asks for it is entitled to remember the answer for that long before asking again. Multiply that across internet providers, office networks, phones and browsers, and a change made at one moment reaches different people at different times over the following hours.

This is what is meant by propagation, and the word is slightly misleading, because nothing is being pushed outwards. Old answers are simply expiring at different moments. The practical effect is familiar: the treasurer sees the new site immediately, the secretary still sees the old one, and both are looking at a correctly configured domain.

A change that looks half applied for a few hours is usually cached answers expiring at different times. A change that looks identical after a full day is usually a mistake.

Planning a change so it does not look broken

Two habits remove most of the drama. The first is to lower the lifetime on the affected records a day or two before a planned move, so that the world forgets the old answers quickly when the switch is made. The second is to have the new destination ready and tested before anything is repointed, so that the gap between the old answer expiring and the new one working is as close to zero as possible.

Where a name is also changing hands or moving to another provider at the same time, do not combine the operations. Move the name first, verify that the records survived intact, and change the pointing afterwards. The sequence is set out in transferring to another person, and following it turns a nerve-wracking evening into an unremarkable one.