implement a new API to let servers control client address verification

[marten-seemann]

Fixes #3494.

This PR makes 2 changes:

1. It implements the new API suggested in redesign the Retry Token API #3494.
2. It disables address verification by default. This means that, unless configured otherwise, the QUIC handshake will take a single RTT. This is in line with providing safe defaults, as we implement the 3x amplification protection.

@mholt, would be happy about your feedback on this PR!

[mholt]

This looks better!

So basically, if I understand right, I would implement RequireAddressValidation(net.Addr) bool to return true if the server is under load, correct? And I don't need to call any sort of defaultAcceptToken() function in addition?

[marten-seemann]

So basically, if I understand right, I would implement RequireAddressValidation(net.Addr) bool to return true if the server is under load, correct? And I don't need to call any sort of defaultAcceptToken() function in addition?

Exactly! All the token validation logic is now handled under the hood (tunable using the MaxTokenAge and MaxRetryTokenAge). All you have to do is say if you want to perform address validation in the first place (at the cost of 1 additional RTT during the handshake).

[mholt]

Sounds like just what I was hoping for then. Thank you!

[codecov]

Codecov Report

Merging #3501 (bbfb7bd) into master (556a6e2) will decrease coverage by 0.00%.
The diff coverage is 94.74%.

@@            Coverage Diff             @@
##           master    #3501      +/-   ##
==========================================
- Coverage   85.69%   85.69%   -0.00%     
==========================================
  Files         136      136              
  Lines        9995     9992       -3     
==========================================
- Hits         8565     8562       -3     
  Misses       1052     1052              
  Partials      378      378              

Impacted Files	Coverage Δ
server.go	80.60% <88.89%> (-0.56%)	⬇️
config.go	100.00% <100.00%> (ø)
connection.go	77.66% <100.00%> (+0.10%)	⬆️
internal/handshake/token_generator.go	76.60% <100.00%> (-1.84%)	⬇️

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[mholt]

@marten-seemann Excellent! I updated caddyserver/caddy#4707 and it seems to be working (haven't tested the "highLoad" scenario but HTTP/3 works by default).