Viktor Dukhovni via Postfix-users:
> On Tue, Mar 26, 2024 at 02:20:55PM -0400, Wietse Venema via Postfix-users
> wrote:
> > Viktor Dukhovni via Postfix-users:
> > > That's fine, the SRV records can be keyed by destination domain.
> >
> > Locally-managed SRV records, keyed by the final destination domain
> > name, to select a local relay host?
>
> Yes. The only tricky part is getting the queries to go to the right
> zone, which requires appending a suffix to the nexthop domain.
This is a very clever approach based on configuration data in DNS
and Postfix. Would a routing policy help here? I'm thinking of a
declarative interface (like postfwd for access policies).
This may require a non-blocking client that supports multiple
outstanding requests.
Wietse
> So a socketmap transport lookup that maps:
>
> foo.example -> dnslb:foo.example.dnslb.local:smtp
>
> and a custom DNS authoritative server handling "dnslb.local" (or other
> suitable suffix).
>
> To have separate load-balanced transport per-sender, one could have
>
> sender_dependent_transport:
> example.com dnslb1
> example.net dnslb2
> ...
>
> And the socketmap would then leave the transport unchanged, and tweak
> just the nexthop:
>
> foo.example -> :foo.example.dnslb.local:smtp
>
> Encoding both the sender domain and the recipient domain into a single
> lookup, as with the policy service, is otherwise difficult, so if
> all mail is single-recipient, one might use FILTER, and still SRV
> lookups on the backend:
>
> sender domain: belongs to client1
> recipient = ...@example.net
>
> action = FILTER dnslb:example.net.client1.dnsbl.local
>
> Then the DNS server can parse out the destination and client id and do
> as it pleases.
>
> --
> Viktor.
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