transport: add ServerName to TLSConfig and add ValidateSecureEndpoints

ServerName prevents accepting forged SRV records with cross-domain
credentials. ValidateSecureEndpoints prevents downgrade attacks from SRV
records.

pkg/transport/listener.go

@@ -67,6 +67,9 @@ type TLSInfo struct {
 	TrustedCAFile  string
 	ClientCertAuth bool
 
+	// ServerName ensures the cert matches the given host in case of discovery / virtual hosting
+	ServerName string
+
 	selfCert bool
 
 	// parseFunc exists to simplify testing. Typically, parseFunc
@@ -167,6 +170,7 @@ func (info TLSInfo) baseConfig() (*tls.Config, error) {
 	cfg := &tls.Config{
 		Certificates: []tls.Certificate{*tlsCert},
 		MinVersion:   tls.VersionTLS12,
+		ServerName:   info.ServerName,
 	}
 	return cfg, nil
 }
@@ -218,7 +222,7 @@ func (info TLSInfo) ClientConfig() (*tls.Config, error) {
 			return nil, err
 		}
 	} else {
-		cfg = &tls.Config{}
+		cfg = &tls.Config{ServerName: info.ServerName}
 	}
 
 	CAFiles := info.cafiles()
@@ -227,6 +231,8 @@ func (info TLSInfo) ClientConfig() (*tls.Config, error) {
 		if err != nil {
 			return nil, err
 		}
+		// if given a CA, trust any host with a cert signed by the CA
+		cfg.ServerName = ""
 	}
 
 	if info.selfCert {

pkg/transport/tls.go

@@ -0,0 +1,49 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package transport
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ValidateSecureEndpoints scans the given endpoints against tls info, returning only those
+// endpoints that could be validated as secure.
+func ValidateSecureEndpoints(tlsInfo TLSInfo, eps []string) ([]string, error) {
+	t, err := NewTransport(tlsInfo, 5*time.Second)
+	if err != nil {
+		return nil, err
+	}
+	var errs []string
+	var endpoints []string
+	for _, ep := range eps {
+		if !strings.HasPrefix(ep, "https://") {
+			errs = append(errs, fmt.Sprintf("%q is insecure", ep))
+			continue
+		}
+		conn, cerr := t.Dial("tcp", ep[len("https://"):])
+		if cerr != nil {
+			errs = append(errs, fmt.Sprintf("%q failed to dial (%v)", ep, cerr))
+			continue
+		}
+		conn.Close()
+		endpoints = append(endpoints, ep)
+	}
+	if len(errs) != 0 {
+		err = fmt.Errorf("%s", strings.Join(errs, ","))
+	}
+	return endpoints, err
+}