make CoreDNS DoH Server (#1619)

* WIP: make CoreDNS DoH Server

* It works

* Fix tests

* Review from Tom - on diff. PR

* correct mime type

* Cleanups and use the pkg/nonwriter

* rename and updates

* implement get

* implement GET

* Code review comments

* correct context

* tweaks

* code review

core/dnsserver/server-https.go

@@ -0,0 +1,149 @@
+package dnsserver
+
+import (
+	"context"
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/http"
+	"strconv"
+
+	"github.com/coredns/coredns/plugin/pkg/nonwriter"
+	"github.com/miekg/dns"
+)
+
+// ServerHTTPS represents an instance of a DNS-over-HTTPS server.
+type ServerHTTPS struct {
+	*Server
+	httpsServer *http.Server
+	listenAddr  net.Addr
+	tlsConfig   *tls.Config
+}
+
+// NewServerHTTPS returns a new CoreDNS GRPC server and compiles all plugins in to it.
+func NewServerHTTPS(addr string, group []*Config) (*ServerHTTPS, error) {
+	s, err := NewServer(addr, group)
+	if err != nil {
+		return nil, err
+	}
+	// The *tls* plugin must make sure that multiple conflicting
+	// TLS configuration return an error: it can only be specified once.
+	var tlsConfig *tls.Config
+	for _, conf := range s.zones {
+		// Should we error if some configs *don't* have TLS?
+		tlsConfig = conf.TLSConfig
+	}
+
+	sh := &ServerHTTPS{Server: s, tlsConfig: tlsConfig, httpsServer: new(http.Server)}
+	sh.httpsServer.Handler = sh
+
+	return sh, nil
+}
+
+// Serve implements caddy.TCPServer interface.
+func (s *ServerHTTPS) Serve(l net.Listener) error {
+	s.m.Lock()
+	s.listenAddr = l.Addr()
+	s.m.Unlock()
+
+	if s.tlsConfig != nil {
+		l = tls.NewListener(l, s.tlsConfig)
+	}
+	return s.httpsServer.Serve(l)
+}
+
+// ServePacket implements caddy.UDPServer interface.
+func (s *ServerHTTPS) ServePacket(p net.PacketConn) error { return nil }
+
+// Listen implements caddy.TCPServer interface.
+func (s *ServerHTTPS) Listen() (net.Listener, error) {
+
+	l, err := net.Listen("tcp", s.Addr[len(TransportHTTPS+"://"):])
+	if err != nil {
+		return nil, err
+	}
+	return l, nil
+}
+
+// ListenPacket implements caddy.UDPServer interface.
+func (s *ServerHTTPS) ListenPacket() (net.PacketConn, error) { return nil, nil }
+
+// OnStartupComplete lists the sites served by this server
+// and any relevant information, assuming Quiet is false.
+func (s *ServerHTTPS) OnStartupComplete() {
+	if Quiet {
+		return
+	}
+
+	out := startUpZones(TransportHTTPS+"://", s.Addr, s.zones)
+	if out != "" {
+		fmt.Print(out)
+	}
+	return
+}
+
+// Stop stops the server. It blocks until the server is totally stopped.
+func (s *ServerHTTPS) Stop() error {
+	s.m.Lock()
+	defer s.m.Unlock()
+	if s.httpsServer != nil {
+		s.httpsServer.Shutdown(context.Background())
+	}
+	return nil
+}
+
+// ServeHTTP is the handler that gets the HTTP request and converts to the dns format, calls the plugin
+// chain, converts it back and write it to the client.
+func (s *ServerHTTPS) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+
+	msg := new(dns.Msg)
+	var err error
+
+	if r.URL.Path != pathDOH {
+		http.Error(w, "", http.StatusNotFound)
+		return
+	}
+
+	switch r.Method {
+	case http.MethodPost:
+		msg, err = postRequestToMsg(r)
+	case http.MethodGet:
+		msg, err = getRequestToMsg(r)
+	default:
+		http.Error(w, "", http.StatusMethodNotAllowed)
+		return
+	}
+
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	// Create a non-writer with the correct addresses in it.
+	dw := &nonwriter.Writer{Laddr: s.listenAddr}
+	h, p, _ := net.SplitHostPort(r.RemoteAddr)
+	po, _ := strconv.Atoi(p)
+	ip := net.ParseIP(h)
+	dw.Raddr = &net.TCPAddr{IP: ip, Port: po}
+
+	// We just call the normal chain handler - all error handling is done there.
+	// We should expect a packet to be returned that we can send to the client.
+	s.ServeDNS(context.Background(), dw, msg)
+
+	buf, _ := dw.Msg.Pack()
+
+	w.Header().Set("Content-Type", mimeTypeDOH)
+	w.Header().Set("Cache-Control", "max-age=128") // TODO(issues/1823): implement proper fix.
+	w.Header().Set("Content-Length", strconv.Itoa(len(buf)))
+	w.WriteHeader(http.StatusOK)
+
+	w.Write(buf)
+}
+
+// Shutdown stops the server (non gracefully).
+func (s *ServerHTTPS) Shutdown() error {
+	if s.httpsServer != nil {
+		s.httpsServer.Shutdown(context.Background())
+	}
+	return nil
+}