[Coco] domain names

[William Astle]

James Dessart wrote:
> 
> On Thu, 16 Sep 2004, Roger Taylor wrote:
> 
> 
>>What about the delay it takes for DNS info to get updated at all the ISPs 
>>around the globe?  Somebody out there is bound to be pointing to your old 
>>IP address(es).
> 
> 
> The way DNS works is that the IPs for a given domain are only stored on
> one server. Sometimes servers implement caching, to speed up queries, but
> for the most part those caches don't stay around for very long.

I work in the internet industry. In fact, I work for a hosting company. 
I would like to provide a more correct description of the way DNS works.

The DNS system is hierarchical. There are 13 root name servers which 
delegate the top level domains (.com, .net, .ca, .au, .tv, and so on). 
Each of these top level domains has a group of name servers that are 
authoritative for them. These servers (plural!) contain authoritative 
delegations for second level domains (like coco3.com) and so on. Note 
that there are multiple authoritative servers for most domains, usually 
at least two. How they obtain their information is unspecified although 
it is usually a form of master/slave relationship where one has the 
authoritative copy and the others simply grab the information off it at 
regular intervals. Thus, the information for a given domain is probably 
stored in at least two places although there is likely only one 
authoritative place.

Now, to help spread the load so the authoritative location doesn't fall 
over dead, other servers cache responses. Each record (like an IP 
address) has a "time to live" (TTL) associated with it. This TTL 
determines (normally) how long the response is cached. Many providers 
are using TTLs of a few hours although TTLs in excess of a day are 
common. This caching behaviour is actually desirable in most cases. When 
you change an IP address, however, this causes some issues as you have 
to wait for all the caches to time out before everyone in the world sees 
the change. [1]

This works wonderfully most of the time. The wrinkle comes in when you 
change hosting for the DNS for a domain. This means that the delegation 
pointers in the parent domain (like .com) need to be changed. Now while 
the information for the domain in question may have a TTL of a few 
hours, the delegation information probably has a TTL of one or two days. 
This is what causes the biggest headache when changing providers because 
when you change web hosts, you are usually changing the DNS service 
provider as well. (Most web hosting includes DNS hosting.)

There are reasonably good ways of transferring a web site. This usually 
involves moving the DNS hosting while maintaining the dns records for 
the old host, but with a low TTL. Then, after a few days, update the dns 
information. Theoretically, then, you end up with only a small glitch in 
web presense. Handling email during a transition can be a bit of a 
problem, though. [2]

So really, it requires some level of planning to make a completely 
smooth transition. This can be significantly improved if the domain 
owner is using a third party DNS host with a clue or reasonable self 
management facilities. It can also be siginficantly hampered by 
incompetence at all levels and stages of the process. [3]

Hopefully I have fully confused everyone now. I'll crawl back under my 
rock now. :)

William Astle



[1] Sometimes there are broken name servers that never expire their 
caches. Also, anyone whose name server has not recently looked up a 
domain (and thus has not got it cached) will see the new information 
immediately. Regular visitors, however, have the highest chance of being 
affected by the cache affects.

[2] The email side of things often requires a clue from both the old and 
new providers.

[3] Not to mention the ubiquitous "fat finger" mode of operation.