On 2026-03-23 at 10:36:42 UTC-0400 (Mon, 23 Mar 2026 15:36:42 +0100)
Matus UHLAR - fantomas via Postfix-users <[email protected]>
is rumored to have said:
With DNS round-robin your users will complain whene either one
of IP addresses is unreachable.
On 23.03.26 11:01, Bill Cole via Postfix-users wrote:
This is not such a huge issue when you do the "round-robin" with
very short (non-zero!) TTLs on the A records. It was worse some
years ago when some resolvers would sort answers, but I don't
think anything still does that.
On 2026-03-23 at 11:07:23 UTC-0400 (Mon, 23 Mar 2026 16:07:23 +0100)
Matus UHLAR - fantomas via Postfix-users <[email protected]>
is rumored to have said:
Do you mean providing different DNS record when one of servers is
down?
On 23.03.26 11:32, Bill Cole via Postfix-users wrote:
No. I mean that the TTLs should be short enough that on attempts after
the first, the client will recheck DNS and likely get a different IP.
What if they get the same IP address, or the same set of IP addresses in the
same order? Because you can NOT guarantee this.
Thus, I don't think it makes sense to play with DNS here.
Some users may notice that a connection starts slow when the first
attempt hits a bad IP but as the connection will be almost certainly
be retried either automatically or manually, it should work well
enough to forestall complaints, as long as outages are infrequent and
short.
I mean, providing multiple IP addresses: clients that first try to
connect to address that is down will complain that the connection
takes too long.
Properly-designed clients will retry additional IPs of an A record. If
a MUA doesn't do that automatically (as web browsers will) a user will
almost always try again after an initial timeout.
This requires providing multiple IP addresses in DNS all the times.
DNS servers should provide addresses in random order (BIND does)
So there's only one case I see where short TTL makes sense - you have server
that can provide different DNS results if one of servers is down.
I still see load balancer as a better solution.
--
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