[dnsdist] Round-robin inside preferred pool with another pool as backup

[Tong Pham]

Hello!

We’ve been very impressed with dnsdist during our testing, and are
considering using it for load balancing our nameservers in AWS.

For an HA setup, we plan to make use of two Availability Zones (AZ) in the
account, with one dnsdist server active in each, and multiple nameservers
in each AZ as well. The nameservers will be grouped by AZ into two pools,
so that:

- each load balancer is aware of, and has access to, both pools;
- the nameserver in each AZ should always prefer to roundrobin **all**
traffic to downstream nameservers in the same AZ (aka the pool in same
subnet);

- only when there are no more nameservers available in the same AZ, should
the load balancer start reaching out to the pool in the other AZ
(load-balancing there)

The main concern here is cost – Amazon charges for cross-AZ traffic. We
would like to minimize that as much as possible.

My questions are:

- Is it important to group the nameservers into pools, by AZ? It seems we
can manually specify nameserver -> pool mapping, but is there an easy way
to match by subnet?

- How would we achieve the load balancing behavior above? Will we have to
write our own load balancing code in Lua, or is there some existing
configurations we can use?

Thanks for reading! Any reply would be appreciated! :")

Regards,

Tong
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