Re: Let's try to be productive...

On Mon, 6 Jan 2014, John Levine wrote:

>> 2. The net is moving to IPv6.
>>
>> One example is this vastly expands the IP blacklisting and IP mobility
>> landscape.
>
> Yeah, that's the interesting one.  We all seem to agree that the
> minimum granularity for blacklisting will be /64, although there are
> hosting companies who made poor decisions and put all the customers in
> a rack in the same /64.  My inclination is to say tough luck.
>
Agreed that IPv6 DNSBL isn't straightforward, but it may not be necessary.

There is some logic to not accepting mail on IPv6 at all. While there are 
currently some MTAs with ipv6 capability, there are apparently no MTAs 
that lack IPv4 fallback. So if you simply fail to support IPv6 for SMTP
then the senders will fall back to IPv4, and your DNSBL is as effective as 
ever. How long will this work? Certainly for a long time and possibly 
forever. Consider that most mail admins would resist using an IPv4 address
that was balcklisted on a fraction of 1% of receivers. So anyone 
establishing an IPv6 only MTA needs to wait till 99+% of MTAs support 
IPv6. That isn't likely for many years.

Daniel Feenberg
NBER
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