Cached CNAMEs

Simon Waters Simon at wretched.demon.co.uk
Fri Sep 13 20:45:51 UTC 2002


DJ wrote:
> 
> Now does the cache

It caches records, so it'll cache both.

You could have checked this by typing "dig www.microsoft.com"
twice two seconds apart, and note how the CNAME TTL has
decreased between them.

> The second question is that I want to change noddy.com to be and A record
> and fred.co.uk to be a CNAME for noddy.com
> My question is that if I do is is there ANYdanger on a cache that it will
> have cached as in (a) above BUT the TTL of fred.co.uk has timed-out so it
> now holds fred.co.uk as a CNAME for noddy.com (which it still holds itself
> as a CNAME as its TTL has not expired) i.e. a CNAME loop on a cache.
> 
> OR will the above never happen ???

It could happen, it wouldn't cause the end of DNS as we know it
otherwise every script kiddie on the planet would have done it
by now.

If you reduce the TTL values a short while before the change you
can avoid problems.

In this case it is sufficient to change both to A records for
the duration of the relevant TTL, as the A record will always
work, and never loop.


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