fix #774

Don't advertise sasl=EXTERNAL to clients who aren't connected over native TLS,
since even if they have TLS configured locally and they enable a client
certificate, that information will be discarded by the reverse proxy.

irc/client.go

@@ -195,6 +195,15 @@ type MultilineBatch struct {
 	tags          map[string]string
 }
 
+// is this session connected in a way that could, in principle, allow certfp authentication?
+// see #774
+func (s *Session) IsCertFPCapable() bool {
+	conn := s.socket.conn.UnderlyingConn()
+	// no client certs on websockets:
+	// https://bugs.chromium.org/p/chromium/issues/detail?id=329884
+	return conn.Config.TLSConfig != nil && !conn.Config.WebSocket
+}
+
 // Starts a multiline batch, failing if there's one already open
 func (s *Session) StartMultilineBatch(label, target, responseLabel string, tags map[string]string) (err error) {
 	if s.batch.label != "" {

irc/config.go

@@ -591,6 +591,7 @@ type Config struct {
 		supportedCaps            *caps.Set
 		supportedCapsWithoutSTS  *caps.Set
 		capValues                caps.Values
+		capValuesNoExternal      caps.Values
 		Casemapping              Casemapping
 		EnforceUtf8              bool                `yaml:"enforce-utf8"`
 		OutputPath               string              `yaml:"output-path"`
@@ -1388,7 +1389,7 @@ func LoadConfig(filename string) (config *Config, err error) {
 		config.Accounts.VHosts.validRegexp = defaultValidVhostRegex
 	}
 
-	saslCapValue := "PLAIN,EXTERNAL,SCRAM-SHA-256"
+	saslCapValue := "PLAIN,SCRAM-SHA-256,EXTERNAL"
 	// TODO(#1782) clean this up:
 	if !config.Accounts.AdvertiseSCRAM {
 		saslCapValue = "PLAIN,EXTERNAL"
@@ -1544,6 +1545,9 @@ func LoadConfig(filename string) (config *Config, err error) {
 	config.Server.supportedCapsWithoutSTS.Union(config.Server.supportedCaps)
 	config.Server.supportedCapsWithoutSTS.Disable(caps.STS)
 
+	config.Server.capValuesNoExternal = utils.CopyMap(config.Server.capValues)
+	config.Server.capValuesNoExternal[caps.SASL] = strings.TrimSuffix(saslCapValue, ",EXTERNAL")
+
 	return config, nil
 }
 

irc/handlers.go

@@ -591,7 +591,11 @@ func capHandler(server *Server, client *Client, msg ircmsg.Message, rb *Response
 				rb.session.capVersion = newVersion
 			}
 		}
-		sendCapLines(supportedCaps, config.Server.capValues)
+		capValues := config.Server.capValues
+		if !rb.session.IsCertFPCapable() {
+			capValues = config.Server.capValuesNoExternal
+		}
+		sendCapLines(supportedCaps, capValues)
 
 	case "LIST":
 		// values not sent on LIST