enhancement: support DNS query for different domains at different datacenter

[leslie-wang]

my use case is :
dc1: app1.dc1.consul.local
dc2: app2.dc2.xyz.com

I want app2@dc2 can resolve app1.dc1.consul.local, and app1@dc1 can resolve app2.dc2.xyz.com.

Seems like dns query is only bound with each DC's domain name as below codes.

• https://github.com/hashicorp/consul/blob/master/command/agent/dns.go

• 95 // Register mux handlers

• 96 mux.HandleFunc(domain, srv.handleQuery)

It may be useful to support different domain name query cross multiple datacenter.

[slackpad]

Hi @leslie-wang thanks for opening an issue. I think right now app2.dc2.consul.local would be supported, but I can see how that's a bit weird. We'd need to figure out the best way to configure something like this. The simplest code-wise would be at the agent level, but you probably want something more centrally-managed (might be something prepared queries could do).