crypto/tls: don't send IPv6 literals and absolute FQDNs as SNI values

This is a followup change to #13111 for filtering out IPv6 literals and
absolute FQDNs from being as the SNI values.

Updates #13111.
Fixes #14404.

Change-Id: I09ab8d2a9153d9a92147e57ca141f2e97ddcef6e
Reviewed-on: https://go-review.googlesource.com/19704Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/tls/handshake_client.go

@@ -16,6 +16,7 @@ import (
 	"io"
 	"net"
 	"strconv"
+	"strings"
 )
 
 type clientHandshakeState struct {
@@ -49,20 +50,13 @@ func (c *Conn) clientHandshake() error {
 		return errors.New("tls: NextProtos values too large")
 	}
 
-	sni := c.config.ServerName
-	// IP address literals are not permitted as SNI values. See
-	// https://tools.ietf.org/html/rfc6066#section-3.
-	if net.ParseIP(sni) != nil {
-		sni = ""
-	}
-
 	hello := &clientHelloMsg{
 		vers:                c.config.maxVersion(),
 		compressionMethods:  []uint8{compressionNone},
 		random:              make([]byte, 32),
 		ocspStapling:        true,
 		scts:                true,
-		serverName:          sni,
+		serverName:          hostnameInSNI(c.config.ServerName),
 		supportedCurves:     c.config.curvePreferences(),
 		supportedPoints:     []uint8{pointFormatUncompressed},
 		nextProtoNeg:        len(c.config.NextProtos) > 0,
@@ -665,3 +659,23 @@ func mutualProtocol(protos, preferenceProtos []string) (string, bool) {
 
 	return protos[0], true
 }
+
+// hostnameInSNI converts name into an approriate hostname for SNI.
+// Literal IP addresses and absolute FQDNs are not permitted as SNI values.
+// See https://tools.ietf.org/html/rfc6066#section-3.
+func hostnameInSNI(name string) string {
+	host := name
+	if len(host) > 0 && host[0] == '[' && host[len(host)-1] == ']' {
+		host = host[1 : len(host)-1]
+	}
+	if i := strings.LastIndex(host, "%"); i > 0 {
+		host = host[:i]
+	}
+	if net.ParseIP(host) != nil {
+		return ""
+	}
+	if len(name) > 0 && name[len(name)-1] == '.' {
+		name = name[:len(name)-1]
+	}
+	return name
+}

src/crypto/tls/handshake_client_test.go

@@ -618,14 +618,35 @@ func TestHandshakClientSCTs(t *testing.T) {
 	runClientTestTLS12(t, test)
 }
 
-func TestNoIPAddressesInSNI(t *testing.T) {
-	for _, ipLiteral := range []string{"1.2.3.4", "::1"} {
+var hostnameInSNITests = []struct {
+	in, out string
+}{
+	// Opaque string
+	{"", ""},
+	{"localhost", "localhost"},
+	{"foo, bar, baz and qux", "foo, bar, baz and qux"},
+
+	// DNS hostname
+	{"golang.org", "golang.org"},
+	{"golang.org.", "golang.org"},
+
+	// Literal IPv4 address
+	{"1.2.3.4", ""},
+
+	// Literal IPv6 address
+	{"::1", ""},
+	{"::1%lo0", ""}, // with zone identifier
+	{"[::1]", ""},   // as per RFC 5952 we allow the [] style as IPv6 literal
+	{"[::1%lo0]", ""},
+}
+
+func TestHostnameInSNI(t *testing.T) {
+	for _, tt := range hostnameInSNITests {
 		c, s := net.Pipe()
 
-		go func() {
-			client := Client(c, &Config{ServerName: ipLiteral})
-			client.Handshake()
-		}()
+		go func(host string) {
+			Client(c, &Config{ServerName: host, InsecureSkipVerify: true}).Handshake()
+		}(tt.in)
 
 		var header [5]byte
 		if _, err := io.ReadFull(s, header[:]); err != nil {
@@ -637,10 +658,20 @@ func TestNoIPAddressesInSNI(t *testing.T) {
 		if _, err := io.ReadFull(s, record[:]); err != nil {
 			t.Fatal(err)
 		}
+
+		c.Close()
 		s.Close()
 
-		if bytes.Index(record, []byte(ipLiteral)) != -1 {
-			t.Errorf("IP literal %q found in ClientHello: %x", ipLiteral, record)
+		var m clientHelloMsg
+		if !m.unmarshal(record) {
+			t.Errorf("unmarshaling ClientHello for %q failed", tt.in)
+			continue
+		}
+		if tt.in != tt.out && m.serverName == tt.in {
+			t.Errorf("prohibited %q found in ClientHello: %x", tt.in, record)
+		}
+		if m.serverName != tt.out {
+			t.Errorf("expected %q not found in ClientHello: %x", tt.out, record)
 		}
 	}
 }