plugin/kubernetes: Add upstream @self and loop count (#1484)

* add upstream @self and loop count

* 1st round of feedback

* allow argless upstream

* update test

* readmes

* feedback

1 files changed, 35 insertions, 4 deletions

diff --git a/core/dnsserver/server.go b/core/dnsserver/server.go
index a08802204..f10bdb78b 100644
--- a/core/dnsserver/server.go
+++ b/core/dnsserver/server.go
@@ -95,7 +95,7 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 func (s *Server) Serve(l net.Listener) error {
 	s.m.Lock()
 	s.server[tcp] = &dns.Server{Listener: l, Net: "tcp", Handler: dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
-		ctx := context.Background()
+		ctx := context.WithValue(context.Background(), Key{}, s)
 		s.ServeDNS(ctx, w, r)
 	})}
 	s.m.Unlock()
@@ -108,7 +108,7 @@ func (s *Server) Serve(l net.Listener) error {
 func (s *Server) ServePacket(p net.PacketConn) error {
 	s.m.Lock()
 	s.server[udp] = &dns.Server{PacketConn: p, Net: "udp", Handler: dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
-		ctx := context.Background()
+		ctx := context.WithValue(context.Background(), Key{}, s)
 		s.ServeDNS(ctx, w, r)
 	})}
 	s.m.Unlock()
@@ -207,6 +207,12 @@ func (s *Server) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		return
 	}
 
+	ctx, err := incrementDepthAndCheck(ctx)
+	if err != nil {
+		DefaultErrorFunc(w, r, dns.RcodeServerFailure)
+		return
+	}
+
 	q := r.Question[0].Name
 	b := make([]byte, len(q))
 	var off int
@@ -329,11 +335,36 @@ func DefaultErrorFunc(w dns.ResponseWriter, r *dns.Msg, rc int) {
 	w.WriteMsg(answer)
 }
 
+// incrementDepthAndCheck increments the loop counter in the context, and returns an error if
+// the counter exceeds the max number of re-entries
+func incrementDepthAndCheck(ctx context.Context) (context.Context, error) {
+	// Loop counter for self directed lookups
+	loop := ctx.Value(loopKey{})
+	if loop == nil {
+		ctx = context.WithValue(ctx, loopKey{}, 0)
+		return ctx, nil
+	}
+
+	iloop := loop.(int) + 1
+	if iloop > maxreentries {
+		return ctx, fmt.Errorf("too deep")
+	}
+	ctx = context.WithValue(ctx, loopKey{}, iloop)
+	return ctx, nil
+}
+
 const (
-	tcp = 0
-	udp = 1
+	tcp          = 0
+	udp          = 1
+	maxreentries = 10
 )
 
+// Key is the context key for the current server
+type Key struct{}
+
+// loopKey is the context key for counting self loops
+type loopKey struct{}
+
 // enableChaos is a map with plugin names for which we should open CH class queries as
 // we block these by default.
 var enableChaos = map[string]bool{