add option to change the MTU of wg interface

[shenjiangqiu]

Add environment option to set the interface MTU

The default MTU is 1420, while it will cause some problems when the MTU of the internet provider is less than 1500. the data will be dropped.

Desired Behavior

add an option MTU to control the MTU of the interface

Current Behavior

Alternatives Considered

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[aptalca]

Can't you just modify it on the client conf?

[shenjiangqiu]

Can't you just modify it on the client conf?

Sure I can, but the issue is related to the server's network environment, it's not convenient to change every peer's configure file.

[aptalca]

You can edit /config/templates/server.conf and add it there

[Taknok]

Yes, but an option to avoid extra steps would be nice. Currently:

1. Configuring
2. Starting container
3. Stopping container
4. Editing server.conf to add postup and postdown
5. Restarting

would become:

1. Configuring
2. Starting container

[aptalca]

This image has too many env vars as it is. I don't plan on adding any more vars unless absolutely necessary (def not for duplicate functionality).

If you'd like to add it, you can customize the image yourselves: https://www.linuxserver.io/blog/2019-09-14-customizing-our-containers