Golang CertChecker hostname validation differs to OpenSSH

[Peter Moody]

On Mon, May 15, 2017 at 2:01 AM, Adam Eijdenberg <adam at continusec.com> wrote:
> On Mon, May 15, 2017 at 11:39 AM, Peter Moody <mindrot at hda3.com> wrote:
>> my reading of the sshd manpage is that ssh is more permissive than it should be
>>
>> SSH_KNOWN_HOSTS FILE FORMAT :
>>   ...
>>
>>   A hostname or address may optionally be enclosed within `[' and `]'
>> brackets then followed by `:' and a non-standard port number.
>
> Hi Peter, I'm not sure that quite answers the same question.
>
> ie at one level there is a decision that is made about whether a line
> in the known hosts file should be evaluated for a given host/port -
> and I think that's what you are referring to above.
>
> However once a line from known hosts is allowed for evaluation for a
> host/port, there's a second matter of checking whether the certificate
> presented contains the appropriate principal.
>
> I think this what "check_host_cert()" does, and as far as I can tell,
> OpenSSH only passes it the hostname (not "host:port"). See:
> https://github.com/openssh/openssh-portable/blob/f382362e8dfb6b277f16779ab1936399d7f2af78/sshconnect.c#L866
>
> (for better or for worse, this would be roughly inline with X.509v3
> cert host matching, which also doesn't match on port numbers)

possibly.

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.

Note, you can also write your own IsHostAuthority that ignores the
port, I think this just affects the HostKeyCallback provided by
golang.org/x/crypto/ssh/knownhosts.