Add ExposesUnderlyingConns interface

The interface can be used by DNS handlers to access the underlying TCP
or UDP connection. This can be useful if more information is needed
about the request, such as to what port it was made.

Fixes miekg#1487

server.go

@@ -79,6 +79,26 @@ type response struct {
 	writer         Writer         // writer to output the raw DNS bits
 }
 
+// IncomingConn exposes the underlying TCP connection, if it is used.
+// For UDP connections, it returns nil.
+func (r *response) IncomingConn() net.Conn {
+	return r.tcp
+}
+
+// IncomingPacketConn exposes the underlying UDP connection, if it is used.
+// For TCP connections, it returns nil.
+func (r *response) IncomingPacketConn() net.PacketConn {
+	return r.udp
+}
+
+// The ExposesUnderlyingConns interface is used by a DNS Handler to access underlying connections.
+type ExposesUnderlyingConns interface {
+	// IncomingConn is only valid for TCP connections. It is otherwise nil.
+	IncomingConn() net.Conn
+	// IncomingPackageConn is only valid for UDP connections. It is otherwise nil.
+	IncomingPacketConn() net.PacketConn
+}
+
 // handleRefused returns a HandlerFunc that returns REFUSED for every request it gets.
 func handleRefused(w ResponseWriter, r *Msg) {
 	m := new(Msg)

server_test.go

@@ -17,6 +17,9 @@ import (
 	"golang.org/x/sync/errgroup"
 )
 
+// Assert that the response exposes the underlying connections.
+var _ ExposesUnderlyingConns = &response{}
+
 func HelloServer(w ResponseWriter, req *Msg) {
 	m := new(Msg)
 	m.SetReply(req)