Send the correct SNI from tsh to auth server

SNI is used to indicate which cluster's CA to use for client cert
validation. If SNI is not sent, or set as "teleport.cluster.local"
(which is default in the client config), auth server will attempt to
validate against all known CAs.

The list of CA subjects is sent to the client during handshake, before
client sends its own client cert. If this list is too long, handshake
will fail. The limit is 65535 bytes, because TLS wire encoding uses 2
bytes for a length prefix. In teleport, this fits ~520-540 trusted
cluster CAs.

To avoid handshake failures on such large setups, all clients must send
the correct SNI. In some future version, we should enforce this to catch
such issues early. For now, added a debug log to report clients using
the default ServerName. Also added a check for large number of CAs, to
print a helpful error.

Updates #3870

lib/auth/helpers.go

@@ -366,6 +366,7 @@ func (a *TestAuthServer) NewRemoteClient(identity TestIdentity, addr net.Addr, p
 	}
 	tlsConfig.Certificates = []tls.Certificate{*cert}
 	tlsConfig.RootCAs = pool
+	tlsConfig.ServerName = EncodeClusterName(a.ClusterName)
 	addrs := []utils.NetAddr{{
 		AddrNetwork: addr.Network(),
 		Addr:        addr.String()}}

lib/auth/middleware.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"crypto/tls"
 	"crypto/x509"
+	"math"
 	"net"
 	"net/http"
 
@@ -135,7 +136,16 @@ func (t *TLSServer) Serve(listener net.Listener) error {
 func (t *TLSServer) GetConfigForClient(info *tls.ClientHelloInfo) (*tls.Config, error) {
 	var clusterName string
 	var err error
-	if info.ServerName != "" {
+	switch info.ServerName {
+	case "":
+		// Client does not use SNI, will validate against all known CAs.
+	case teleport.APIDomain:
+		// REMOVE IN 4.4: all 4.3+ clients must specify the correct cluster name.
+		//
+		// Instead, this case should either default to current cluster CAs or
+		// return an error.
+		t.Debugf("Client %q sent %q in SNI, which causes this auth server to send all known CAs in TLS handshake. If this client is version 4.2 or older, this is expected; if this client is version 4.3 or above, please let us know at https://github.com/gravitational/teleport/issues/new", info.Conn.RemoteAddr(), info.ServerName)
+	default:
 		clusterName, err = DecodeClusterName(info.ServerName)
 		if err != nil {
 			if !trace.IsNotFound(err) {
@@ -159,6 +169,26 @@ func (t *TLSServer) GetConfigForClient(info *tls.ClientHelloInfo) (*tls.Config,
 		// this falls back to the default config
 		return nil, nil
 	}
+
+	// Per https://tools.ietf.org/html/rfc5246#section-7.4.4 the total size of
+	// the known CA subjects sent to the client can't exceed 2^16-1 (due to
+	// 2-byte length encoding). The crypto/tls stack will panic if this
+	// happens. To make the error less cryptic, catch this condition and return
+	// a better error.
+	//
+	// This may happen with a very large (>500) number of trusted clusters, if
+	// the client doesn't send the correct ServerName in its ClientHelloInfo
+	// (see the switch at the top of this func).
+	var totalSubjectsLen int64
+	for _, s := range pool.Subjects() {
+		// Each subject in the list gets a separate 2-byte length prefix.
+		totalSubjectsLen += 2
+		totalSubjectsLen += int64(len(s))
+	}
+	if totalSubjectsLen >= int64(math.MaxUint16) {
+		return nil, trace.BadParameter("number of CAs in client cert pool is too large (%d) and cannot be encoded in a TLS handshake; this is due to a large number of trusted clusters; make sure the client sets the correct SNI to match CAs for a specific cluster instead of sending them all", len(pool.Subjects()))
+	}
+
 	tlsCopy := t.TLS.Clone()
 	tlsCopy.ClientCAs = pool
 	for _, cert := range tlsCopy.Certificates {

lib/client/client.go

@@ -19,7 +19,6 @@ package client
 import (
 	"bytes"
 	"context"
-	"crypto/tls"
 	"encoding/json"
 	"io"
 	"io/ioutil"
@@ -352,28 +351,14 @@ func (proxy *ProxyClient) ConnectToCluster(ctx context.Context, clusterName stri
 		})
 	}
 
-	// Because Teleport clients can't be configured (yet), they take the default
-	// list of cipher suites from Go.
-	tlsConfig := utils.TLSConfig(nil)
 	localAgent := proxy.teleportClient.LocalAgent()
-	pool, err := localAgent.GetCerts()
-	if err != nil {
-		return nil, trace.Wrap(err)
-	}
-	tlsConfig.RootCAs = pool
 	key, err := localAgent.GetKey()
 	if err != nil {
 		return nil, trace.Wrap(err, "failed to fetch TLS key for %v", proxy.teleportClient.Username)
 	}
-	if len(key.TLSCert) != 0 {
-		tlsCert, err := tls.X509KeyPair(key.TLSCert, key.Priv)
-		if err != nil {
-			return nil, trace.Wrap(err, "failed to parse TLS cert and key")
-		}
-		tlsConfig.Certificates = append(tlsConfig.Certificates, tlsCert)
-	}
-	if len(tlsConfig.Certificates) == 0 {
-		return nil, trace.BadParameter("no TLS keys found for user %v, please relogin to get new credentials", proxy.teleportClient.Username)
+	tlsConfig, err := key.ClientTLSConfig()
+	if err != nil {
+		return nil, trace.Wrap(err, "failed to generate client TLS config")
 	}
 	clt, err := auth.NewTLSClient(auth.ClientConfig{
 		Dialer: dialer,

lib/client/interfaces.go

@@ -101,6 +101,13 @@ func (k *Key) ClientTLSConfig() (*tls.Config, error) {
 		return nil, trace.Wrap(err, "failed to parse TLS cert and key")
 	}
 	tlsConfig.Certificates = append(tlsConfig.Certificates, tlsCert)
+	// Use Issuer CN from the certificate to populate the correct SNI in
+	// requests.
+	leaf, err := x509.ParseCertificate(tlsCert.Certificate[0])
+	if err != nil {
+		return nil, trace.Wrap(err, "failed to parse TLS cert")
+	}
+	tlsConfig.ServerName = auth.EncodeClusterName(leaf.Issuer.CommonName)
 	return tlsConfig, nil
 }