resolving differently depending on location?

[Kevin Darcy]

Barry Margolin wrote:

>In article <dj0nh1$13c$1 at sf1.isc.org>,
> Sid Shapiro <sid_shapiro at bio-rad.com> wrote:
>
>  
>
>>Hello,
>>In a global intranet, some of my people would like to have connections 
>>made to "close" servers. That is you ask for a connection to, say, 
>>local.company.com and you get connected to a server which is closer to you 
>>depending on where you are. 
>>
>>When I was asked this, my first reaction was that this was a misuse of 
>>DNS, but then I remembered that other places - ebay, or google have 
>>geographically placed servers and connections are made to the "closest" 
>>server to you.
>>
>>Is this done in DNS or is it handled by the web (or whatever) 
>>applications?
>>    
>>
>
>Both ways.  It can be done in DNS using custom DNS servers (e.g. Cisco 
>Distributed Director) -- BIND doesn't have any support for this.
>  
>
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. Otherwise, intermediate resolvers might end up 
re-sorting the address list when answering from cache and thus messing 
up the sort order. (Lowering the TTL on the records is one way to defeat 
caching, of course, but rather anti-social).

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...

                                                                         
                                                   - Kevin