clientv3/credentials: overwrite authority when it's IP

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

clientv3/credentials/credentials.go

@@ -76,18 +76,26 @@ func newTransportCredential(cfg *tls.Config) *transportCredential {
 }
 
 func (tc *transportCredential) ClientHandshake(ctx context.Context, authority string, rawConn net.Conn) (net.Conn, grpccredentials.AuthInfo, error) {
-	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
+	// 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.
+	if net.ParseIP(authority) != nil {
+		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)
 }