[PATCH 0/3] cifs.upcall: attempt to use AD-style service principals

Andrew Bartlett abartlet at samba.org
Mon Nov 14 15:45:26 MST 2011

On Mon, 2011-11-14 at 09:44 -0500, Jeff Layton wrote:

> The above scheme isn't perfect, but in many cases it will happen to
> work. It's true that there's no reliable mapping between DNS and
> samAccountName, but in a lot of cases the samAccountName *is* the
> capitalized host portion of the DNS name. Does it hurt anything to
> attempt to get a ticket with that name if "cifs/fqdn" fails?

We should never ask for a machine$ name.  It is always the wrong thing
to do, because it will only exist on AD servers, which already do the
mapping between cifs/foo and foo$ internally.  

We should also not map between cifs/ and host/ - cifs/ is a separate
service, just as nfs/ and http/ are. 

> Over the years, we've seen a lot of confused users on the list who are
> not sure what name they need to put in the host portion of the UNC to
> get their krb5 mount to work. This scheme seems like it'll make that a
> bit more forgiving.

I certainly understand the need to make krb5 more forgiving, and
certainly if the KDC indicates that cifs/foo does not exist, then
guessing the DNS domain and asking for cifs/foo.<guessed domain> is

> If the wrong guesses just end up slowing down the upcall, then I'm ok
> with that. If they potentially open a security hole then that's another
> matter entirely. That's my main question here -- are we opening up any
> vulnerabilities with this scheme?

Each time you second-guess the name, you open up a small security hole,
because you potentially allow a connection that was to be to a trusted
host to be impersonated by less trusted member of the same kerberos
realm.  For that reason, any client-side canonicalisation should be
strictly limited.  

Furthermore, you may do more than just slow down the upcall - if you
connect to the right server with the wrong ticket (because you guessed
wrong - cifs vs host etc), the only way to find out is if the server
gives you a LOGON_FAILURE error, and I think this will be even harder to

I do want kerberos to be easier to use, and to 'just work' more often.
I care passionately that Kerberos should be both secure and 'just work'
- falling back to NTLM is an even worse fate.  

I just want to ensure we do not become the source of new expected
behaviour patterns for non-AD domains (such as looking up foo$ or
host/foo for cifs shares), as once we start, it will be very hard to

Andrew Bartlett

Andrew Bartlett                                http://samba.org/~abartlet/
Authentication Developer, Samba Team           http://samba.org

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