[dns-operations] MX record definition?

Edward Lewis Ed.Lewis at neustar.biz
Thu Dec 18 14:28:46 UTC 2008

At 17:08 +1100 12/18/08, Mark Andrews wrote:

>	Which would only be legal if you are running in your own private
>	namespace.


>	Not on the big I Internet.  Maybe in your own private namespace.

The DNS is not only for the global public Internet.

Now, to get back to the original question - this thread is a reason 
why there is no document saying it is really poor practice to put an 
all numeric domain name in an MX record.  On the one hand, it is true 
that it won't work on the global public Internet because all numeric 
domain names are not defined and never will be.

(We can't have an all numeric TLD under the global public Internet's 
root zone as managed by IANA/ICANN because of IDN issues with the 
directionality of characters.  Punctuation ['.'] and numbers [0-9] 
have no directionality in the character hence you can't determine if 
a portion of a domain name is localized as ".0" or "0."  Yadda, 
yadda, yadda, IOW, numbers might 'jump' over labels.  See 
http://stupid.domain.name/node/683 for a start on that.)

So, numeric names in MX records is a bad idea (on the global public 
Internet).  But the reason it isn't documented (within DNS) is that 
it is legal to do so as far as the protocol is concerned and can be 
made to work in some environments.  Whenever someone tries to argue 
that "it's legal in the protocol" someone else responds with "but it 
won't work" and we wind up seeing about a half-dozen or so e-mails 
fighting it out, followed by a lull of a few months until the next 
question comes up on the list.

Merry festivus (http://en.wikipedia.org/wiki/Festivus) to all...
Edward Lewis
NeuStar                    You can leave a voice message at +1-571-434-5468

Never confuse activity with progress.  Activity pays more.

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