GSSAPI Proxy initiative

Nico Williams nico at
Fri Nov 4 12:20:08 EDT 2011

On Fri, Nov 4, 2011 at 10:55 AM, Adamson, Andy
<William.Adamson at> wrote:
> On Nov 4, 2011, at 11:13 AM, Nico Williams wrote:
>> Ideally we could store in each RPCSEC_GSS context (not GSS context)
>> enough state on the client side to recover quickly when the server
>> reboots.
> You mean not to use the user Kerberos credential to re-establish the GSS context with the server?

Kerberos has tickets.  Other GSS mechanisms don't.  The GSS-API
completely abstracts this, so there's no way to extract a service
ticket and store it alongside the context (RPCSEC_GSS, in this case)
where you might need it in the future.  Storing all of a GSS-API
credential (think of a whole ccache) in kernel memory is not an option
either (ccaches have unbounded size).

Moreover, if we do this in a light-weight enough fashion we might be
able to handle all of the recovery path in kernel-mode, with no
dependence on upcalls.  But if we didn't by somehow extracting the
service ticket and storing it in the RPCSEC_GSS context we'd probably
still need to upcall to make use of it.

>> How would we do this?  Suppose the server gives the client a
>> "ticket", and a key much like the Kerberos ticket session key is
>> agreed upon or sent by the server -- that could be stored in the
>> RPCSEC_GSS context and could be used to recover it quickly for
>> recovery from server reboot.  I'm eliding a lot of details here, but I
>> believe this is fundamentally workable.
> So re-establish the RPCSEC_GSS session lost at the server on server reboot by storing enough additional info on the client?

Yes.  And not just server reboot.  The server is free to lose
RPCSEC_GSS contexts any time it wants to.

Basically, we need a fast re-authentication facility that is easy to
code entirely in kernel-mode.  Thinking of it this way I would not
reuse any Kerberos tech for this.  The server would return a ticket in
RPCSEC_GSS context establishment, but the ticket would consist of
{secret key index, encrypted octet string} and the server and client
would both compute a "session key" (for proving ticket possession)
with GSS_Pseudo_random() (this way we can make this work even when the
GSS mech only does MICs and not wrap tokens).  To re-authenticate the
client would send the ticket and an authenticator just like in
Kerberos, but simpler.


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