clientv3/credentials: set dial target "Authority" with target address

Overwrite authority when it's IP.

When user dials with "grpc.WithDialer", "grpc.DialContext" "cc.parsedTarget"
update only happens once. This is problematic, because when TLS is enabled,
retries happen through "grpc.WithDialer" with static "cc.parsedTarget" from
the initial dial call.
If the server authenticates by IP addresses, we want to set a new endpoint as
a new authority. Otherwise
"transport: authentication handshake failed: x509: certificate is valid for 127.0.0.1, 192.168.121.180, not 192.168.223.156"
when the new dial target is "192.168.121.180" whose certificate host name is also "192.168.121.180"
but client tries to authenticate with previously set "cc.parsedTarget" field "192.168.223.156"

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

clientv3/credentials/credentials.go

@@ -71,9 +71,36 @@ func newTransportCredential(cfg *tls.Config) *transportCredential {
 }
 
 func (tc *transportCredential) ClientHandshake(ctx context.Context, authority string, rawConn net.Conn) (net.Conn, grpccredentials.AuthInfo, error) {
+	// Only overwrite when authority is an IP address!
+	// Let's say, a server runs SRV records on "etcd.local" that resolves
+	// to "m1.etcd.local", and its SAN field also includes "m1.etcd.local".
+	// But what if SAN does not include its resolved IP address (e.g. 127.0.0.1)?
+	// Then, the server should only authenticate using its DNS hostname "m1.etcd.local",
+	// instead of overwriting it with its IP address.
+	// And we do not overwrite "localhost" either. Only overwrite IP addresses!
+	if isIP(authority) {
+		target := rawConn.RemoteAddr().String()
+		if authority != target {
+			// When user dials with "grpc.WithDialer", "grpc.DialContext" "cc.parsedTarget"
+			// update only happens once. This is problematic, because when TLS is enabled,
+			// retries happen through "grpc.WithDialer" with static "cc.parsedTarget" from
+			// the initial dial call.
+			// If the server authenticates by IP addresses, we want to set a new endpoint as
+			// a new authority. Otherwise
+			// "transport: authentication handshake failed: x509: certificate is valid for 127.0.0.1, 192.168.121.180, not 192.168.223.156"
+			// when the new dial target is "192.168.121.180" whose certificate host name is also "192.168.121.180"
+			// but client tries to authenticate with previously set "cc.parsedTarget" field "192.168.223.156"
+			authority = target
+		}
+	}
 	return tc.gtc.ClientHandshake(ctx, authority, rawConn)
 }
 
+// return true if given string is an IP.
+func isIP(ep string) bool {
+	return net.ParseIP(ep) != nil
+}
+
 func (tc *transportCredential) ServerHandshake(rawConn net.Conn) (net.Conn, grpccredentials.AuthInfo, error) {
 	return tc.gtc.ServerHandshake(rawConn)
 }