IPv6 isn't SMTP

Franck Martin fmartin at linkedin.com
Thu Mar 27 02:16:40 UTC 2014

On Mar 26, 2014, at 5:47 PM, Fred Baker (fred) <fred at cisco.com> wrote:

> On Mar 25, 2014, at 8:31 PM, Cutler James R <james.cutler at consultant.com> wrote:
>> 3.  Arguing about IPv6 in the context of requirements upon SMTP connections is playing that uncomfortable game with one’s own combat boots.  And not particularly productive.
> That is one of my two big take-aways from this conversation. The other is that operators of SMTP MTAs should implement RDNS for them, which I thought we already knew.

It is in several industry recommendations cf for instance BCP at www.m3aawg.org 

> To my knowledge, there are three impacts that IPv6 implementation makes on an SMTP implementation. One is that the OS interface to get the address of the next MUA or MTA needs to use getaddrinfo() instead of gethostbyname() (and would do well to observe RFC 6555’s considerations). Another is that, whether on an incoming or an outbound connection, when the application gets its own address from the OS (binary or as a character string), it needs to allocate more storage for the data structure. The third is that it needs to be able to interpret user at 2001:db8::1 as well as user at dns-name and user at 

and user at 2001:db8::1.25 with user at Who had the good idea to use : for IPv6 addresses while this is the separator for the port in IPv4? A few MTA are confused by it.

> All things considered, that’s a pretty narrow change set.
> Everyone here, no doubt, is clueful enough to implement RDNS for their MTAs. We know that there are people in the world that don’t implement it for IPv4. Yet, here we are, using SMTP/IPv4 to discuss this, and I don’t hear anyone saying that IPv4 isn’t ready for prime time as a result of the fact of some operators not implementing RDNS.
There is some confusion between MX selection and address selection, I tried to document it, and resolve the ambiguities in http://datatracker.ietf.org/doc/draft-martin-smtp-target-host-selection-ipv4-IPv6/ (comments at apps-discuss at ietf.org)

Remember 70 to 90% of email is spam, blacklists can drop as much as 75% of spam at connection time (an IPv6 blacklist has problems due to size and impact on DNS). If we mess up the transition of SMTP to IPv6, less than 1 out of 10 emails in your mailbox will be remotely interesting….

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