Golang CertChecker hostname validation differs to OpenSSH

[Peter Moody]

On Mon, May 15, 2017 at 3:52 PM, Adam Eijdenberg <adam at continusec.com> wrote:
> On Tue, May 16, 2017 at 2:38 AM, Peter Moody <mindrot at hda3.com> wrote:
>> your proposed patch removes both checks though. I think you'd want to
>> modify knownhosts.go if you want to support not including non-standard
>> ports in IsHostAuthority.
>
> My intention wasn't to modify both checks - I'm currently only
> concerned with principal checking, although I can see how your recent
> patch (as implemented) would also be affected (so if we do change
> anything here, we'll probably need to refactor a little).
>
> Let me give a concrete example, currently our certificates (OpenSSH
> server, and OpenSSH client) look like this and everything works great:
>
> Principals:
>         auth.example.local
>         auth.example.com.au
>
> However, if I write a Go client (which requires a port number be
> specified in their Dial string):
>
>     log.Println(ssh.Dial("tcp", "auth.example.local:10000", &ssh.ClientConfig{
>         HostKeyCallback: (&ssh.CertChecker{}).CheckHostKey,
>     }))
>
> I get the following error, before even attempting to evaluating
> IsHostAuthority():
>
>     ssh: handshake failed: ssh: principal "auth.example.local:10000"
> not in the set of valid principals for given certificate:
> ["auth.example.local" "auth.example.com.au"]
>
>
> If I want a certificate to work with OpenSSH server, and both Go and
> OpenSSH clients, I need to re-generate a certificate like this:
>
> Principals:
>         auth.example.local
>         auth.example.com.au
>         auth.example.local:10000
>         auth.example.com.au:10000
>
>
> That doesn't seem right, and I think the Go principal evaluation is
> incorrect, but I would like a second opinion.

I can kind of see the argument that the port shouldn't be in
principal, but it seems a little odd, especially given the known_hosts
restrictions from the sshd manpage, that a cert would be good for all
2^16 ports.

> (that code in Go also seems to be at least 3 years old)