resolving differently depending on location?

[Barry Margolin]

In article <dj17cv$2hks$1 at sf1.isc.org>,
 Kevin Darcy <kcd at daimlerchrysler.com> wrote:

> BIND *does* however, have support for "sortlist". One can have the name 
> resolve to all of the location-specific IPs, and then sort them 
> according to the source IP of the DNS client. This only works 
> *reliably*, however, when the sortlist configuration of all resolvers is 
> tightly controlled.

Right, so it's useless for a public web site.

> Note that the sortlist approach also assumes that the DNS client address 
> is also the same as, or close to, the client for whatever service one is 
> trying to provide (HTTP, SMTP, whatever). Due to proxying and numerous 
> other factors, this isn't always a good assumption. But Akamai and 
> others seem to do fairly well making exactly the same assumption, so YMMV...

If you're just trying to select a server on the same continent or ISP 
backbone as the client, the assumption will be pretty good.  Also, many 
ISPs make use of anycast for their resolvers, so the resolver will be 
relatively close to the client on the backbone; therefore, choosing a 
server close to the resolver should be OK.

While there may be occasional cases where it doesn't choose the optimal 
server, on average it seems like it can be expected to be better than 
random choice.  But having the server do it at the application level 
should generally be even better.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***