update default interface feature causes unwanted behaviour with mptcp

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:

https://github.com/SagerNet/sing-box/blob/1ee0b3981750511867bd54a59206dceeed74c5b4/route/network.go#L511-L518

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.

Start with route/network.go at lines 511-518 and reproduce the reported behavior using MPTCP subflows while bringing down the default interface. Compare the connection-reset behavior with the requested alternatives, then verify that active MPTCP traffic is preserved or that the behavior can be disabled.