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Re: MX to CNAME and (mis)interptretation of 2821

2007-12-14 06:45:21

Paul Smith wrote:

IMHO, 2821bis should state that "putting CNAMES as the target of MX records is not allowed, as stated in RFC 2181"

Paul, I generally agree with your other points in your message.

But this still won't change the BCP requirement for "Industry Ready" software to recognize it. So even if it was stated strongly, what you will end up doing is someone new who will implement it as stated and then get all bent out of shape when he comes across those CNAME situations in the real world.

Guess who has to change?

While he might get on mission and initially succeed in getting DNS administrators to change their setups, eventually he will get tired of it and begin to recognized and add support for it - bitching and moaning at the same time. If he doesn't - he simply isn't normal. <grin> The fact is, sometimes it is not feasible for DNS setup to be changed.

So IMO, -1 on changing RFC 29821bis.

Consider the fact it is already general practice for DNS software to recognize CNAME issues, including but not exclusive to:

    - Watching and Limiting CNAME loops [1]
    - Resolving CNAMES from A records lookups [4] [5]

Yes, it is expected MX resolves to A records [2], the main concern being that it will cause extra queries/network burdens and early resolvers did not expect it.

But due to the nature of the world, those A records can easily end up being CNAME RRs [3]. It is prudent that Industry Ready software be prepare for that. This SMTP Operational Experience is officially recognized in RFC 3974 [4]

Mandating it in RFC 2821bis isn't going to change that. Even if you are looking at future generation DNS admins doing things correctly for MX resolution, I still say the DNS client software will still need to take into account the MX resolution will point to CNAMES, even if the practice is deprecated.


Hector Santos, CTO


[2] RFC 2181.

[3] See RFC 974 for historical perspective.

[4] RFC 3974 SMTP Operational Experience in Mixed IPv4/v6 Environments