update default interface feature causes unwanted behaviour with mptcp

[arinc9]

Operating system

Linux

System version

6.12

Installation type

Original sing-box Command Line

If you are using a graphical client, please provide the version of the client.

No response

Version

sing-box version 1.12.12

Description

The feature that changes the default interface when the default interface is no longer available resets active connections:

sing-box/route/network.go

Lines 511 to 518 in 1ee0b39

	r.logger.Info("updated default interface ", defaultInterface.Name, ", ", strings.Join(options, ", "))
	r.UpdateWIFIState()
	if !r.started {
	return
	}
	r.ResetNetwork()
	}

This is unwanted in case of MPTCP in use as it causes all traffic to drop for a while, which MPTCP can handle with subflows.

So I request either an option to be able to disable this feature or this feature should be disabled when MPTCP is in use.

Reproduction

Use MPTCP with subflows and then set down the default interface.

Logs

Supporter

• I am a sponsor

Integrity requirements

• I confirm that I have read the documentation, understand the meaning of all the configuration items I wrote, and did not pile up seemingly useful options or default values.
• I confirm that I have provided the server and client configuration files and process that can be reproduced locally, instead of a complicated client configuration file that has been stripped of sensitive data.
• I confirm that I have provided the simplest configuration that can be used to reproduce the error I reported, instead of depending on remote servers, TUN, graphical interface clients, or other closed-source software.
• I confirm that I have provided the complete configuration files and logs, rather than just providing parts I think are useful out of confidence in my own intelligence.

[nekohasekai]

How do you make MPTCP work? First I need to know it makes sense.

[arinc9]

To set it up: Enable MPTCP on sing-box outbound configuration. Add default routes with a different interface and a metric value to the routing table. Add every interface with a default route as an MPTCP subflow endpoint (ip mptcp endpoint add).

The MPTCP code in the kernel will establish an MPTCP stream using the route with the smallest metric and establish a subflow using the other endpoints on the endpoint table.