Add support for ALPN

Closes: https://todo.sr.ht/~emersion/tlstunnel/11

directives.go

@@ -51,6 +51,11 @@ func parseFrontend(srv *Server, d *scfg.Directive) error {
 		}
 	}
 
+	protocolDirective := d.Children.Get("protocol")
+	if protocolDirective != nil {
+		frontend.Protocols = protocolDirective.Params
+	}
+
 	for _, addr := range d.Params {
 		host, port, err := net.SplitHostPort(addr)
 		if err != nil {

server.go

@@ -133,35 +133,68 @@ func (ln *Listener) handle(conn net.Conn) error {
 	defer conn.Close()
 
 	// TODO: setup timeouts
-	tlsConn := tls.Server(conn, ln.Server.ACMEConfig.TLSConfig())
+	tlsConfig := ln.Server.ACMEConfig.TLSConfig()
+	getConfigForClient := tlsConfig.GetConfigForClient
+	tlsConfig.GetConfigForClient = func(hello *tls.ClientHelloInfo) (*tls.Config, error) {
+		// Call previous GetConfigForClient function, if any
+		var tlsConfig *tls.Config
+		if getConfigForClient != nil {
+			var err error
+			tlsConfig, err = getConfigForClient(hello)
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			tlsConfig = ln.Server.ACMEConfig.TLSConfig()
+		}
+
+		fe, err := ln.matchFrontend(hello.ServerName)
+		if err != nil {
+			return nil, err
+		}
+
+		tlsConfig.NextProtos = fe.Protocols
+		return tlsConfig, nil
+	}
+	tlsConn := tls.Server(conn, tlsConfig)
 	if err := tlsConn.Handshake(); err != nil {
 		return err
 	}
 
 	tlsState := tlsConn.ConnectionState()
+	fe, err := ln.matchFrontend(tlsState.ServerName)
+	if err != nil {
+		return err
+	}
 
-	fe, ok := ln.Frontends[tlsState.ServerName]
+	return fe.handle(tlsConn, &tlsState)
+}
+
+func (ln *Listener) matchFrontend(serverName string) (*Frontend, error) {
+	fe, ok := ln.Frontends[serverName]
 	if !ok {
-		// match wildcard certificates, allowing only a single, non-partial wildcard, in the left-most label
-		i := strings.IndexByte(tlsState.ServerName, '.')
-		// don't allow wildcards with only a TLD (eg *.com)
-		if i >= 0 && strings.IndexByte(tlsState.ServerName[i+1:], '.') >= 0 {
-			fe, ok = ln.Frontends["*"+tlsState.ServerName[i:]]
+		// Match wildcard certificates, allowing only a single, non-partial
+		// wildcard, in the left-most label
+		i := strings.IndexByte(serverName, '.')
+		// Don't allow wildcards with only a TLD (e.g. *.com)
+		if i >= 0 && strings.IndexByte(serverName[i+1:], '.') >= 0 {
+			fe, ok = ln.Frontends["*"+serverName[i:]]
 		}
 	}
 	if !ok {
 		fe, ok = ln.Frontends[""]
 	}
 	if !ok {
-		return fmt.Errorf("can't find frontend for server name %q", tlsState.ServerName)
+		return nil, fmt.Errorf("can't find frontend for server name %q", serverName)
 	}
 
-	return fe.handle(tlsConn, &tlsState)
+	return fe, nil
 }
 
 type Frontend struct {
-	Server  *Server
-	Backend Backend
+	Server    *Server
+	Backend   Backend
+	Protocols []string
 }
 
 func (fe *Frontend) handle(downstream net.Conn, tlsState *tls.ConnectionState) error {
@@ -184,6 +217,9 @@ func (fe *Frontend) handle(downstream net.Conn, tlsState *tls.ConnectionState) e
 		if tlsState.ServerName != "" {
 			tlvs = append(tlvs, authorityTLV(tlsState.ServerName))
 		}
+		if tlsState.NegotiatedProtocol != "" {
+			tlvs = append(tlvs, alpnTLV(tlsState.NegotiatedProtocol))
+		}
 		if tlv, err := sslTLV(tlsState); err != nil {
 			return fmt.Errorf("failed to set PROXY protocol header SSL TLV: %v", err)
 		} else {
@@ -228,6 +264,13 @@ func authorityTLV(name string) proxyproto.TLV {
 	}
 }
 
+func alpnTLV(proto string) proxyproto.TLV {
+	return proxyproto.TLV{
+		Type: proxyproto.PP2_TYPE_ALPN,
+		Value: []byte(proto),
+	}
+}
+
 func sslTLV(state *tls.ConnectionState) (proxyproto.TLV, error) {
 	pp2ssl := tlvparse.PP2SSL{
 		Client: tlvparse.PP2_BITFIELD_CLIENT_SSL, // all of our connections are TLS

tlstunnel.1.scd

@@ -66,6 +66,21 @@ The following directives are supported:
 
 			This disables automatic TLS.
 
+	*protocol* <name>...
+		List of supported application-layer protocols.
+
+		The first protocol which is also supported by the client is negociated.
+
+		The protocols will be advertised via the TLS ALPN extension. See the
+		IANA registry for a list of protocol names:
+		https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids
+
+		For instance, for an HTTP server supporting HTTP/1 and HTTP/2:
+
+		```
+		protocol h2 http/1.1 http/1.0
+		```
+
 *tls* { ... }
 	Customise global TLS configuration.