Set TLS server name from Mumble address

gumble/gumble/client.go

@@ -3,6 +3,7 @@ package gumble
 import (
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"math"
 	"net"
 	"runtime"
@@ -101,6 +102,19 @@ func Dial(config *Config) (*Client, error) {
 	return DialWithDialer(new(net.Dialer), config, nil)
 }
 
+// tlsServerName returns the hostname portion of a Mumble server address for
+// TLS certificate verification and SNI.
+func tlsServerName(address string) (string, error) {
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		return "", fmt.Errorf("gumble: derive TLS server name from %q: %w", address, err)
+	}
+	if host == "" {
+		return "", fmt.Errorf("gumble: derive TLS server name from %q: empty host", address)
+	}
+	return host, nil
+}
+
 // DialWithDialer connects to the Mumble server at the address given in config.
 //
 // The function returns after the connection has been established, the initial
@@ -120,6 +134,23 @@ func DialWithDialer(dialer *net.Dialer, config *Config, tlsConfig *tls.Config) (
 	if err != nil {
 		return nil, err
 	}
+
+	// tls.Client cannot infer a server name from an already-open connection.
+	// Clone the caller's configuration before deriving it so reconnects and
+	// concurrent clients do not mutate a shared configuration.
+	if tlsConfig == nil {
+		tlsConfig = &tls.Config{}
+	} else {
+		tlsConfig = tlsConfig.Clone()
+	}
+	if tlsConfig.ServerName == "" {
+		serverName, err := tlsServerName(config.Address)
+		if err != nil {
+			rawConn.Close()
+			return nil, err
+		}
+		tlsConfig.ServerName = serverName
+	}
 	conn := tls.Client(rawConn, tlsConfig)
 	// net.Dialer.Timeout covers only the TCP dial. Apply the same bounded
 	// deadline to TLS negotiation so a peer that accepts but never responds

gumble/gumble/client_tls_test.go

@@ -0,0 +1,28 @@
+package gumble
+
+import "testing"
+
+func TestTLSServerNameUsesAddressHost(t *testing.T) {
+	for _, test := range []struct {
+		address string
+		want    string
+	}{
+		{"mumble.example:64738", "mumble.example"},
+		{"[2001:db8::1]:64738", "2001:db8::1"},
+	} {
+		got, err := tlsServerName(test.address)
+		if err != nil {
+			t.Errorf("tlsServerName(%q): %v", test.address, err)
+			continue
+		}
+		if got != test.want {
+			t.Errorf("tlsServerName(%q) = %q, want %q", test.address, got, test.want)
+		}
+	}
+}
+
+func TestTLSServerNameRejectsAddressWithoutHost(t *testing.T) {
+	if _, err := tlsServerName(":64738"); err == nil {
+		t.Fatal("tlsServerName accepted an empty host")
+	}
+}