Remove trailing period from SRV records (#7494)

Recently ran into an issue with Istio in particular, where leaving the
trailing dot on the SRV record returned by `dnssrvnoa` lookups led to an
inability to connect to the endpoint. Removing the trailing dot fixes
this behaviour.

Now, technically, this is a valid URL, and it shouldn't be a problem.
One could definitely argue that Istio should be responsible here for
ensuring that the traffic is delivered. The problem seems rooted in how
Istio attempts to do wildcard matching or URLs it receives - including
the dot leads it to lookup an empty DNS field, which is invalid.

The approach I take here is actually copied from how Prometheus does it.
Therefore I hope we can sneak this through with the argument that 'this
is how Prometheus does it', regardless of whether or not this is
philosophically correct...

Signed-off-by: verejoel <[email protected]>

CHANGELOG.md

@@ -19,6 +19,8 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 ### Changed
 
+- [#7494](https://github.com/thanos-io/thanos/pull/7494) Ruler: remove trailing period from SRV records returned by discovery `dnsnosrva` lookups
+
 ### Removed
 
 ## [v0.36.0](https://github.com/thanos-io/thanos/tree/release-0.36) - in progress

pkg/discovery/dns/resolver.go

@@ -108,7 +108,9 @@ func (s *dnsSD) Resolve(ctx context.Context, name string, qtype QType) ([]string
 			}
 
 			if qtype == SRVNoA {
-				res = append(res, appendScheme(scheme, net.JoinHostPort(rec.Target, resPort)))
+				// Remove the final dot from rooted DNS names (this is for compatibility with Prometheus)
+				target := strings.TrimRight(rec.Target, ".")
+				res = append(res, appendScheme(scheme, net.JoinHostPort(target, resPort)))
 				continue
 			}
 			// Do A lookup for the domain in SRV answer.