Avoid to have infinite recursion in DNS lookups when resolving CNAMEs

[pierresouchay]

This will avoid killing Consul when a Service.Address is using CNAME
to a Consul CNAME that creates an infinite recursion.

This will fix #4907

[pierresouchay]

Tests do pass while not updated in github UI

[ShimmerGlass]

LGTM

[pierresouchay]

This also improve quite a bit tests stability ;)

[pierresouchay]

@mkeeler @banks deserves a look since it avoid breaking DNS resolver when loop in services exists

[banks]

Cool thanks, don't have cycles for this immediately although at a glance it looks like a good fix, I'll put it in our backlog of things to look over properly!

[pierresouchay]

will also probably fix #4040

[pierresouchay]

@pearkes this one will probably allow you closing 2 issues at once and fixes a real bug causing DoS

[pierresouchay]

@pearkes @mkeeler do you think you might have a look ?
It avoid breaking the whole Consul DNS when a node does a simple mismatch registering a service causing an infinite recursion as seen in #4907 (for us, it is a big deal as we have very few Consul DNS resolvers per DC, so breaking all of them would happen very quickly in that case)

[mkeeler]

The code looks good to me and I can see that it would fix the problem.

One thing to note is that I don't think it would fix a problem where resolving a CNAME points to another external name which then points back to the original name within Consul. I don't think we can mitigate that scenario in Consul but regardless its good to state the limitations of the fix.

[pierresouchay]

@mkeeler you are right, it would not. But resolvers usually have this kind of protection. It however protect Consul itself from crashing

[pierresouchay]

@mkeeler thank you very much!