[CentOS] How to organize your VMs

Tue Apr 13 12:55:33 UTC 2021
Kenneth Porter <shiva at sewingwitch.com>

--On Tuesday, April 13, 2021 1:15 AM -0400 Steven Tardy 
<sjt5atra at gmail.com> wrote:

> IMO each VM should have a singular use/purpose/app. VMs are effectively
> free. And also prevents unintended negative upgrade interactions.
> Think through this to the logical end as each process is it's own
> environment/container/(docker) or each user execution is a unique instance
> (serverless).

My sense is that all the mail apps that touch the same data on disk should 
share a VM. But RoundCubeMail is really an MUA so it can be in a separate 
VM. One VM can hold a caching DNS and the rest can resolve to it. Each web 
server/domain/app should be in its own VM to sandbox it from other domains.

The tricky part with DNS is that outside caching servers (like Google) 
handle short-lived (low TTL) records better (some records have lifetimes of 
seconds!) but mail block lists refuse access from Google because they 
charge for large users, so small mail servers need their own caching DNS. 
Hence, one might split DNS into two servers, one just for mail and one for 
everything else.