feat(commands): implement RFC 7151 (File Transfer Protocol HOST Command for Virtual Hosts)

[lezsakdomi]

Fix #114

Todo:

• Add handler for command
• Add listener
• Provide option for checking hostname just at login time
• Checking for hostname (domain name / ip literal) syntax according to section 3.1, including port number persistence
• Checking if IP address matches server IP (in case supplied) (section 3.1)
• When TLS used, checking if supplied hostname matches with one from TLS (section 3.2.2)
• CLI options (specifying a map between virtualhosts and user/pass/root combos)
• Tests

[lezsakdomi]

[BUG] It looks like passing motd and/or anonymous has no effect :(

[trs]

Thanks for the start of this PR! I'll take a look into giving you a hand with it when I get a chance. I'll read up on the HOST command first.

[lezsakdomi]

RFC 7151 TL;DR:

The parameter of a HOST command is the hostname as the server was accessed, obviously.
The hostname SHOULD NOT contain a port.
If it's an IPv6, the hostname SHOULD be encapsulated in []s. If an IPv4, it MAY be enclosed between brackets or provided in the IPv6 form.

According to the specs, the server should:

• Validate the syntax (I was afraid to implement this, because validating IPv6 addresses is hard)
• Validate hostname validity if possible
  • If it's a bare IP (the IP should be our public IP)
  • If TLS used and hostname was in certificate

[lezsakdomi]

I think this project shouldn't be so strict about validation. If ever implemented, maybe it's a good idea to provide an option to turn validation off.

Okay, syntax check could come any time :)

[lezsakdomi]

Oh, and I was able to test my program using WinSCP - the only client implementing HOST I found so far