`mtu-test` Causes Multiple Errors With dco-win Adapter

[Matthew1471]

Describe the bug

MTU Test (mtu-test) generates a large number of errors before issuing a misleading error, if DCO is in use.

To Reproduce

Connect via DCO and have mtu-test in the configuration.

# To empirically measure MTU on connection startup.
mtu-test

Expected behavior MTU test to either gracefully fail to start with a warning (it's fine if it's just a limitation of DCO adapters) or to perform MTU Test normally (i.e. feature to be implemented on DCO adapters).

Version information (please complete the following information):

• OS: Windows 11
• OpenVPN version: 2.6.6 (Client), 2.4.7 (Server)

Additional context

If it makes any difference, on my system I have deleted all other adapters other than DCO.

Mon Aug 21 09:41:07 2023 NOTE: Beginning empirical MTU test -- results should be available in 3 to 4 minutes.
Mon Aug 21 09:41:07 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:10 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:13 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:16 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:19 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:22 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:25 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:28 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:31 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:34 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:38 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:41 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:44 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:47 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:50 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:53 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:41:56 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:00 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:03 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:06 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:09 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:12 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:15 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:19 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:22 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:25 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:29 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:32 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:35 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:38 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:41 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:44 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:47 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:50 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:53 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:56 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:42:59 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:02 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:05 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:09 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:12 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:15 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:18 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:21 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:24 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:27 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:30 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:33 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:36 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:39 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:42 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:46 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:49 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:52 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:55 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:43:58 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:01 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:04 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:07 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:10 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:14 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:17 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:20 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:24 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:27 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:30 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:33 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:36 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:39 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:43 2023 Attempting to send data packet while data channel offload is in use. Dropping packet
Mon Aug 21 09:44:46 2023 NOTE: failed to empirically measure MTU (requires OpenVPN 1.5 or higher at other end of connection).