On Sat, 27 Nov 2010, John R. Levine wrote:
I have been pondering the swamp which is IPv6 DNS blacklists and whitelists.
I think IPv6 blacklists are probably not ever going to be practical or
desirable. An MTA with only an IPv6 address will not have reachability
comparable to an IPv4 address for many decades, if ever. Certainly it will
be far less useful than an MTA on a pool address is today. So I don't see
actual mail servers sending desirable mail without IPv4. Since such a mail
server would have poor connectivity, it is likely only an "owned" machine
would attempt to send mail via IPv6, and one wouldn't want mail from such
a machine.
I know that many legitimate MTAs already have both IPv4 and IPv6
addresses, my argument is that they will always have at least IPv4.
There is the further point that while it is possible to maintain a
"badness" list in a 32 bit space, it isn't really possible to maintain
such a list in 128-bit (or even 64-bit) space, especially when the bad
guys don't have to keep track of which addresses they have already used.
Accepting mail only from IPv4 will be the norm and will not result in any
lost mail, even far into the future. Devices with only IPv6, such as smart
phones and desktops will send mail through smarthosts with IPv4
connectivity.
Daniel Feenberg
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