Add option to DNS server to ignore them when they are not available via an interface network

service/resolver/main.go

@@ -233,6 +233,9 @@ func AddToDebugInfo(di *debug.Info) {
 		if len(resolver.Search) > 0 {
 			content = append(content, fmt.Sprintf("  Search Domains: %v", strings.Join(resolver.Search, ", ")))
 		}
+		if resolver.LinkLocalUnavailable {
+			content = append(content, "  Link-local, but not available: ignoring")
+		}
 		content = append(content, fmt.Sprintf("  Failing: %v", resolver.Conn.IsFailing()))
 
 		// Add a empty line for all but the last entry.

service/resolver/resolve.go

@@ -422,6 +422,12 @@ func resolveAndCache(ctx context.Context, q *Query, oldCache *RRCache) (rrCache
 			continue
 		}
 
+		// Skip unreachable link-local resolvers.
+		if resolver.LinkLocalUnavailable {
+			log.Tracer(ctx).Tracef("resolver: skipping resolver %s, because it is link-local and not available", resolver)
+			continue
+		}
+
 		// resolve
 		log.Tracer(ctx).Tracef("resolver: sending query for %s to %s", q.ID(), resolver.Info.ID())
 		rrCache, err = resolver.Conn.Query(ctx, q)

service/resolver/resolver.go

@@ -67,6 +67,8 @@ type Resolver struct {
 	Search     []string
 	SearchOnly bool
 	Path       string
+	// Special States
+	LinkLocalUnavailable bool
 
 	// logic interface
 	Conn ResolverConn `json:"-"`

service/resolver/resolvers.go

@@ -34,6 +34,7 @@ const (
 	parameterBlockedIf  = "blockedif"
 	parameterSearch     = "search"
 	parameterSearchOnly = "search-only"
+	parameterLinkLocal  = "link-local"
 )
 
 var (
@@ -179,6 +180,21 @@ func createResolver(resolverURL, source string) (*Resolver, bool, error) {
 		}
 	}
 
+	// Check if this is a link-local resolver.
+	if query.Has(parameterLinkLocal) {
+		if query.Get(parameterLinkLocal) != "" {
+			return nil, false, fmt.Errorf("%s may only be used as an empty parameter", parameterLinkLocal)
+		}
+		// Check if resolver IP is link-local.
+		resolverNet, err := netenv.GetLocalNetwork(newResolver.Info.IP)
+		switch {
+		case err != nil:
+			newResolver.LinkLocalUnavailable = true
+		case resolverNet == nil:
+			newResolver.LinkLocalUnavailable = true
+		}
+	}
+
 	newResolver.Conn = resolverConnFactory(newResolver)
 	return newResolver, false, nil
 }
@@ -208,7 +224,8 @@ func checkAndSetResolverParamters(u *url.URL, resolver *Resolver) error {
 			parameterIP,
 			parameterBlockedIf,
 			parameterSearch,
-			parameterSearchOnly:
+			parameterSearchOnly,
+			parameterLinkLocal:
 			// Known key, continue.
 		default:
 			// Unknown key, abort.