On 17/08/2012 22:08, Daniel Feenberg wrote:
On Fri, 17 Aug 2012, Michael Thomas wrote:
On 08/17/2012 01:51 PM, Daniel Feenberg wrote:
Host operating systems -- all of them to my knowledge -- prefer v6
over v4 if you have a public v6 address. So the mere existence of a
AAAA associated with the MX will cause the sender to pick the v6
destination. I have a v6 mail system and got bitten because I had
forgot to put up the v6 reverse map. It will happen just as a natural
consequence of people enabling v6 on their infrastructure.
This sounds inconvenient. If I want to accept mail from one IPv6 host,
then all the IPv6 hosts will want to use IPv6, and unless I accept
mail from unknown IPv6 hosts, mail from hosts that would have been
accepted over IPv4 will be rejected?
This is a fair point. Given that IPv6 is rare at the moment and is
really an 'extension' to SMTP anyway, maybe we should be looking at a
further extension which allows an SMTP receiver to say 'retry on my IPv4
address'
However, it is up to the sending MTA whether it uses an IPv4 or IPv6
address. The 'OS may prefer IPv6', but the MTA could get all the
available IP addresses for the MX name, then chooses which to use. If
the OS has a 'connect to name' function then it may just connect to the
IPv6 address, but if you have to use 'gethostbyname' then 'connect to IP
address' functions then you can choose which results of the
'gethostbyname' function to use, and which to ignore, and which order to
try them in.
That's what ours does, and it can be configured to ignore the IPv6
addresses and just try to connect to the IPv4 ones, or to try IPv4
before IPv6.
This is especially true since more important hosts are more likely to
have access to IPv4 addresses. I actually wonder if the transition
could ever occur as long as IPv4 is supported at all by ISPs.
This is a fair point as well, and I do wonder that as well. We added
IPv6 support to our MTA as a 'selling point', but I'd generally
recommend users not to enable it.
I suspect that if IPv6 did become widely used for mail, then we'd have
to start moving towards a system of 'trusted MTAs' to try to limit the
spam issue. If we're going to do that, then there would be a limited
number of those, so why not have those 'trusted hosts' use IPv4? Then
any IPv6 SMTP would just be between mutually agreed upon MTAs, with the
'edge' MTA using IPv4 to talk to other 'unknown' MTAs. (hope that makes
sense)
-
Paul Smith Computer Services
Tel: 01484 855800
Vat No: GB 685 6987 53
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