[Pdns-users] DNS Failover

Nikolaos Milas nmilas at admin.noa.gr
Fri Apr 19 12:16:14 UTC 2013

On 19/4/2013 1:12 μμ, kalpesh thaker wrote:

> From my personal experiences, DNS failover does not always work as you 
> would expect... mainly because you wont have much control over 3rd 
> party recursors and of the way they cache TTL values. and by the time 
> DNS does propogate... your mail server CNAMEs could have changed 
> again, so you'd therefor have mail arriving at both servers etc.
> I'd recommend looking into adopting HA strategies rather (with tools 
> such as hearbeat and internal dynamic DNS which can change IPs on the 
> fly within your local network according to availability) than rely on 
> public DNS for failover.


I have been discussing this on the Dovecot mailing list, and Timo 
(Sirainen) said:

    Well, there are two failure reasons:

    a) Dovecot server fails. For that, you could use any kind of a proxy
    that redirects traffic to the other server (even Dovecot proxy on
    another server).

    b) Network connection to the whole data center dies, or the whole
    data center loses power or otherwise dies. There's really no other
    choice to failover that than DNS, unless you manage to route the
    same IP address to two different data centers and just update the route.

    I'd be more worried about b), since you can already fix a) pretty
    quickly with VMs. Or a) could also be switched to become a Dovecot
    proxy on demand if there's a bigger problem that can't be
    immediately fixed.

...and, yes, it's case "b" I want to handle. So that leaves me with DNS 


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