John Levine criticized the first version of RMX, saying that major
domains like Yahoo would never be able to fit all their networks into
512 bytes. Indeed, pobox.com is hardly a huge ISP, and our MX servers
span seven or eight distinct networks.
The issue is one of complexity, do we put that complexity in every client
that looks up the information or do we put it into the tools to publish and
distribute this information.
Not of complexity -- one of function.
Many DNS implementations are broken and won't fall back to TCP
properly. Therefore any response over 512 bytes may fail.
Better from a complexity point of view to have a two or three part
query:
--> TXT domain.com
<-- v=spf1 mx
--> MX domain.com
<-- MX 10 foo.domain.com
<-- MX 20 bar.otherdomain.com
<-- A foo.domain.com 127.0.0.1
--> A bar.otherdomain.com
<-- A bar.otherdomain.com 0.0.0.0
If all of your MXes are in your domain, the addresses can be returned as
additional data. If they're not, or there's not enough room (ie: yahoo),
then there's well-defined steps to query it, already implemented in
resolver libraries.
No such code exists to work around broken UDP-only DNS.
Ari
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