Anycast provider for SMTP?

Wed Jun 17 19:13:38 UTC 2015

Anycast is generally not well-suited for stateful connectivity (e.g. most
things TCP).  The use case for anycast is restricted to simple
challenge-response protocol design.

As such, you typically only see it leveraged for simple services (e.g. DNS,
NTP).

The reason for this, as you suspect, is you can never guarantee that the
path and thus the server will remain consistent across client connections.

Ideally you can leverage DNS to provide a response to a unicast resource
rather than trying to make the service itself anycast.  DNS can be anycast,
and DNS can provide different responses based on geographical location, but
these can happen independently or together.

As you still want failover, you might opt to announce the MX record with
the priorities reversed but still pointing to each server.  For example MX
10 server1, MX 20 server2 on one side, and MX 10 server2, MX 20 server1 on
the other.

Typically you would use a DNS load balancer rather than simple anycast DNS
to achieve this though.

On Mon, Jun 15, 2015 at 1:50 PM, 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?  At first glance anycasting would seem to be a great idea but there is
> a problem of broken sessions when routes change.
>
> Have any of you seen something like this work in the wild?
>
>
> --
> Joe Hamelin, W7COM, Tulalip, WA, 360-474-7474
>

-- 
Ray Patrick Soucy
Network Engineer
University of Maine System

T: 207-561-3526
F: 207-561-3531

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