ipn/ipnlocal: print warning about DNS servers in bugreport --diagnose

If the user passes the --diagnose flag, print a warning if any of the
default or fallback DNS resolvers are Tailscale IPs. This can interfere
with the ability to connect to the controlplane, and is typically
something to pay attention to if there's a connectivity issue.

Change-Id: Ib14bf6228c037877fbdcd22b069212b1a4b2c456
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

ipn/ipnlocal/local.go

@@ -4626,6 +4626,32 @@ func (b *LocalBackend) Doctor(ctx context.Context, logf logger.Logf) {
 	var checks []doctor.Check
 	checks = append(checks, routetable.Check{})
 
+	// Print a log message if any of the global DNS resolvers are Tailscale
+	// IPs; this can interfere with our ability to connect to the Tailscale
+	// controlplane.
+	checks = append(checks, doctor.CheckFunc("dns-resolvers", func(_ context.Context, logf logger.Logf) error {
+		b.mu.Lock()
+		nm := b.netMap
+		b.mu.Unlock()
+		if nm == nil {
+			return nil
+		}
+
+		for i, resolver := range nm.DNS.Resolvers {
+			ipp, ok := resolver.IPPort()
+			if ok && tsaddr.IsTailscaleIP(ipp.Addr()) {
+				logf("resolver %d is a Tailscale address: %v", i, resolver)
+			}
+		}
+		for i, resolver := range nm.DNS.FallbackResolvers {
+			ipp, ok := resolver.IPPort()
+			if ok && tsaddr.IsTailscaleIP(ipp.Addr()) {
+				logf("fallback resolver %d is a Tailscale address: %v", i, resolver)
+			}
+		}
+		return nil
+	}))
+
 	// TODO(andrew): more
 
 	numChecks := len(checks)