Add DefaultServerConfigCallback option for create custom default ServerConfigs

[binwiederhier]

This callback allows a more granular control of the SSH server configuration, such as for the key exchange / cipher / MAC algorithms.

Example usage:

server := ssh.Server{
  Addr: ":2222",
  // ...
  DefaultServerConfigCallback: func (ctx ssh.Context) *gossh.ServerConfig {
    config := &gossh.ServerConfig{}

    config.KeyExchanges = []string{"curve25519-sha256@libssh.org"}
    config.MACs = []string{"hmac-sha2-256-etm@openssh.com"}
    config.Ciphers = []string{
      "aes128-gcm@openssh.com",
      "ssh-ed25519",
      "ssh-rsa",
    }

    return config
  },
}

server.ListenAndServe()

[progrium]

LGTM

[belak]

Huh, interesting. I like this approach. This would also replace #86

[belak]

Alternatively, this could be done by adding a config hook which would take a context and the ServerConfig and allow the user to modify it before actually using it.

[binwiederhier]

I didn't see that #86 existed. Whatever you guys think is best. :-)

[progrium]

I like callbacks. Eventually will clean up the API into single delegate that can implement various interfaces. We/somebody can change this to pass in a default config later.

[binwiederhier]

Awesome. You guys are super quick. How long does it usually take to get things merged / into a tag?

[progrium]

I'd have merged already if @belak explicitly gave thumbs up. Tagging is done by request right now.

[belak]

As long as we understand that the API may change later, I'm fine with this in its current form. It looks good to me!

[progrium]

Pre-1.0 the API will be changing a lot

[belak]

Then I'm good with this. 👍 from me!

[binwiederhier]

Awesome thanks guys. Thank was so quick!
Can I get a tag too? ❤️

[belak]

Tagged as v0.1.3. Thanks for contributing!