x/crypto/ssh: Support GSSAPI AuthMethod

[quentinmit]

As defined in RFC 4462

This is commonly used for (MIT/Heimdal/Active Directory) Kerberos authentication, though other GSSAPI mechanisms exist.

There are several different GSSAPI implementations available for Go (e.g. https://gopkg.in/jcmturner/gokrb5.v5, https://github.com/apcera/gssapi). It probably doesn't make sense to bless one of these external packages in x/crypto/ssh, so I would instead suggest that a callback- or interface-based AuthMethod be defined, so the user can plug in the GSSAPI mechanism of their choice.

I think modern SSH servers only support the gssapi-with-mic auth method, though some patched SSH servers (Debian for instance) also support the gssapi-keyex auth method. The latter auth method is significantly more complicated, so I would suggest starting only with gssapi-with-mic.

[agnivade]

/cc @hanwen

[hanwen]

I don't want to add generic callbacks to the API either.

Is it possible to structure the code such that it effectively becomes a drop-in? Eg. a file that you would add to a local checkout that implements?

Is there a split in the structure that we could apply (eg. putting packetConn into an internal/ package) so you could add a separate gss/ subpackage, without affecting the rest of the SSH code?

[jorgemarey]

Hi,

I just saw this issue. We needed to use kerberos ssh authentication from a golang client.

I did this: golang/crypto@master...jorgemarey:f-gssapi-auth (showing changes)

I don't know if it's ok. I could make some changes it you think the code needs it. I tested it with https://github.com/apcera/gssapi (by implementing the interface) and it worked fine.

[hanwen]

You can try to run this against an existing GSSAPI server.

The tricky bit is that for it to be promoted into the crypto library, I want to see a test. That means either writing a server side support too (which implies integrating with some existing kerberos package), or testing against an OpenSSH server configured to use Kerberos.

[yiya1989]

you can refer to these two:
https://github.com/yiya1989/sshkrb5/blob/main/examples/sshwithkrb5.go
https://github.com/nhywieza/sshgssapi