Check if the device is valid on every third connection attempt

[dlon]

This PR changes the default constraints somewhat. WireGuard is used three times instead of two: once on a random port, once on port 53, and once using udp2tcp (if obfuscation is auto).

In general, when the tunnel protocol is WireGuard and no port is set, the relay selector simply alternates between a random port and port 53.

The device validity check occurs on every third attempt instead of on every other attempt.

Fix DES-486.

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This change is 

[linear]

DES-486 Make daemon check if device is valid on every third failed connection attempt

[MarkusPettersson98]

Reviewed 4 of 4 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @dlon)

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mullvad-relay-selector/src/lib.rs line 1054 at r2 (raw file):

    const fn get_auto_obfuscator_retry_attempt(retry_attempt: u32) -> Option<u32> {
        match retry_attempt % 4 {
            0 | 1 => None,

What are these numbers, and why were they chosen? 😊

Code quote:

        match retry_attempt % 4 {
            0 | 1 => None,

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mullvad-relay-selector/src/lib.rs line 1056 at r2 (raw file):

            0 | 1 => None,
            // when the retry attempt is 2-3, 6-7, 10-11 ... obfuscation will be used
            filtered_retry => Some(retry_attempt / 4 + filtered_retry - 2),

It's nice that there's an explanation of when obfuscation will be used, but what is it that is being calculated in case Some is returned, and why is it defined this way?:)

Code quote:

            // when the retry attempt is 2-3, 6-7, 10-11 ... obfuscation will be used
            filtered_retry => Some(retry_attempt / 4 + filtered_retry - 2),

[dlon]

Reviewable status: 3 of 4 files reviewed, 2 unresolved discussions (waiting on @MarkusPettersson98)

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mullvad-relay-selector/src/lib.rs line 1054 at r2 (raw file):

Previously, MarkusPettersson98 (Markus Pettersson) wrote…

What are these numbers, and why were they chosen? 😊

I've added some documentation.

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mullvad-relay-selector/src/lib.rs line 1056 at r2 (raw file):

Previously, MarkusPettersson98 (Markus Pettersson) wrote…

It's nice that there's an explanation of when obfuscation will be used, but what is it that is being calculated in case Some is returned, and why is it defined this way?:)

I've explained what this does in more detail now.

Turns out the expression was actually wrong, so thanks for pointing my attention to this. :)

[MarkusPettersson98]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @dlon)

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mullvad-relay-selector/src/lib.rs line 1056 at r2 (raw file):

Previously, dlon (David Lönnhager) wrote…

I've explained what this does in more detail now.

Turns out the expression was actually wrong, so thanks for pointing my attention to this. :)

Nice, thanks! Glad I could help :D