Add support for custom ALPN

[misery]

Use "bep-relay" as default but allow custom ALPNs if the relay server is used for something unrelated to "bep-relay".

Purpose

This allows to use the perfect syncthing-relay for another usage for different applications without to have conflicts with syncthing application itself and custom application.

Testing

Used "testutil" of strelaysrv and saw that it shows an error in log output if the ALPN does not match. Also used wireshark to see the TLS alert "No application protocol".

Authorship

André Klitzing aklitzing@gmail.com

[calmh]

I'm not sure we want to build general relay infrastructure for non-Syncthing use cases though? I mean, the proposed diff is trivial and harmless for sure, but I question the expansion of scope it implies...

[misery]

Well, maybe syncthing will have benefit, too. The developer community could grow with it.

But I doubt that there will be a big impact because of this.

[misery]

@calmh Did you think about it? :-)

[calmh]

Yeah. Given the unclear implications and the fact that this is just a debug print that doesn't actually prevent you from doing what you want, this is a no from me.

[misery]

It will prevent the client connection as the "NextProtos" in tlsConfig will force it.

It -alpn is defined the testutil cannot connect.

$ ./testutil -test
2024/06/04 14:05:13 main.go:48: ID: LO46K72-PQQ6RGH-CNRKKMS-YGVKCPM-WO5HCCJ-HIOHWS4-DFSZZ2A-IGFSVAD
2024/06/04 14:05:13 main.go:115: FAIL: getting invitation: remote error: tls: no application protocol

[calmh]

Ah, my bad. Nonetheless.