OpenVPN / OpenVPN/openvpn

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

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bug dco-win
Dominant language
C
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Description

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).

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by tracing the mtu-test configuration path and the DCO adapter send path that emits “Attempting to send data packet while data channel offload is in use.” Reproduce the Windows 11 behavior with DCO enabled, then verify that mtu-test either exits with a clear warning or completes normally without repeated misleading errors.

Written by the indexing model from the issue text.

Assessment

Tech stack
c
Domain
networking
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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