Anycast provider for SMTP?

Mon Jun 15 18:13:02 UTC 2015

> On Jun 15, 2015, at 10:50 AM, Joe Hamelin <joe at nethead.com> wrote:
> 
> I have a mail system where there are two MX hosts, one in the US and one in
> Europe.  Both have a DNS MX record metric of 10 so a bastardized
> round-robin takes place.  This does not work so well when one site goes
> down.   My solution will be to place a load balancer in a hosting site
> (virtual, of course) and have it provide HA.  But what about HA for the
> LB?

It seems like you may be over-thinking this.

You could, in fact, use anycast, in one of two ways:

You could anycast the DNS, with servers in the US and Europe, and different MX metrics between the two, so anyone who’s nearby the European DNS server will see the European MX host as the first-choice, and anyone nearer the US DNS server will see the US MX host as first-choice.

Or you could skip the MX records, and just put both US and European SMTP servers on the same IP address, which would save a lot of steps and simplify the system, but leave you with the _very_ occasional corner-case of someone equal-path-length load-balancing traffic to you such that half of one TCP session goes to Europe, and half the the US.  That’s a bogeyman that scares a lot of people into not using anycast for TCP services, particularly long-lived ones, but it’s a theoretical problem rather than an actually-observed-in-the-wild problem.  But since it scares people, it’s probably safer just doing the DNS anycast, rather than SMTP anycast, to avoid startling the easily-upset out there.  :-)

Either of these is vastly simpler and more reliable than trying to throw a load balancer into the mix.  As you note, load balancers aren’t particularly HA.  Always replace load balancers with crossconnects.  Much more HA.

                                -Bill

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