Polarian via Postfix-users:
> What technology do you use to pick between the protocols?
As documented at the link you mentioned, the Postfix SMTP CLIENT
can sort IP addresses, with the same MX preference, by their protocol.
RFC 5321 requires that the Postfix SMTP CLIENT connects to hosts
with primary MX preference, before connecting to hosts with a
secondary MX preference.
For example, given the following DNS records:
example.com. IN MX 10 primary.example.com
example.com. IN MX 20 secondary.example.com
primary.example.com. IN AAAA fc00::1
primary.example.com. IN A 10.0.0.1
secondary.example.com. IN A 10.0.0.2
secondary.example.com. IN AAAA fc00::2
With "smtp_address_preference = ipv4", Postfix will connect to:
- First, MX preference 10 address 10.0.0.1, then fc00::1,
- Then, MX preference 20 addresses 10.0.0.2, then fc00::2.
Note: Postfix may not try all IP addresses. This depends on on
the smtp_mx_address_limit and smtp_mx_session_limit settings.
> Is there any way I can make IPv6 the priority safely without damaging
> my ability to deliver mail (I do want IPv4 to be used if IPv6 fails, as
> a lot of mail servers are yet to support IPv6).
Use the Postfix smtp_address_preference default: random selection.
If an MX host has IPv4 and IPv6 addresses, this ensures that mail
won't get stuck in the queue when one of the protocols is not
working for that destination.
If a destination's MX host has no AAAA record, then Postfix will
not try to connect to it with IPv6. On the other hand, if a
destination's MX host has an AAAA record, but the Postfix host has
no IPv6 routes, then you should turn off IPv6 support in Postfix.
Wietse
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