feat: extended TLS configuration

[trutx]

This PR extends the server's TLS configuration. While the default TLS settings should be enough for most use cases, if you get a bit paranoid about security you'd probably want to tweak things a bit. This PR allows for that.

The motivation in my case is passing the FedRAMP certification, which required some TLS vulnerabilities to be addressed. Here's a FedRAMP compliant (as of Sep 5th, 2019) configuration example:

server:
  addr: :5001
  certificate: /path/to/server.pem
  key: /path/to/server.key
  hsts: true
  tls_min_version: 0x0303
  tls_curve_preferences:
    - 25
    - 24
    - 23
  tls_cipher_suites:
    - 0x009c
    - 0x009d
    - 0xc02f
    - 0xc02b
    - 0xc030
    - 0xc02c
    - 0xcca8
    - 0xcca9

If you're curious I used this tool to test both the current server and the patched one with the settings above, and the differences are there.

[rojer]

no objections in general, but for readability, can we use string names for the ciphers and curve names instead?

[rojer]

TLS_RSA_WITH_AES_128_GCM_SHA256 is much more readable than 0x009c
if there is no existing method to look up cipher suite number by name, you could use reflect to look up symbol by name in the tls package.

[trutx]

I'm not sure I understand what you mean by using reflect to lookup constants from a remote package. If it can be done I'd definitely love to learn how.

My goal was to keep the code simple and to avoid hardcoding (I guess that was the goal in https://github.com/cesanta/docker_auth/pull/232/files too). I see no way around either a hardcoded map to "translate" user provided strings to actual tls.CipherSuites or some sort of pre-build go generate that does a similar thing, but again I'd love to learn a new solution. I totally agree that the actual ciphersuite name is much more readable and user friendly than a hex value.

[rojer]

ok, scratch that, you cannot use reflect to lookup package constants.

so, how about we support string representation for existing list of ciphers as map[string]uint16 but also allow numeric values.

that way:

tls_cipher_suites:
  - TLS_RSA_WITH_RC4_128_SHA
  - 0x0005

will both work. all you need to do is make that list of strings, look up value in the hard-coded map and then fall back to strconv.Atoi.

same applies to curve names.

[trutx]

That should do it. It was slightly trickier than strconv.Atoi because we're dealing with uint16 here. Now you can pass in either a string or an hex value to configure the TLS min version, the TLS Curves and the TLS CipherSuites.

[rojer]

getting there, some minor comments

[trutx]

Some validation logic could be added in config.go to check that all the passed values exist in the value maps, though this would mean doing the conversion dance twice -- one in server.validate() and another one in ServeOnce(). Not sure if it's worth it.

[rojer]

merged, thank you!