[Lustre-devel] GSS cross-realm on MDT -> OST

Benjamin Bennett ben at psc.edu
Tue Jul 8 13:38:21 PDT 2008

Peter Braam wrote:
> Hmm. Perhaps there are implementation issues here that overshadow the
> architecture.
> To interact with MDS nodes that are part of one file system, the MDS needs
> to be part of a realm.  The MDS performs authorization based on a principal
> to MDS (i.e. Lustre) user/group database.   Within one Lustre file system
> each MDS MUST HAVE the same user group database.  We will likely want to
> place MDS's distributedly in the longer term future, so take clear note of
> this: one Kerberos realm owns the entire MDS cluster for a file system.

Could you explain more on why this requires a single realm and not just 
consistent mappings across all MDSs?

> There can be multiple MDS clusters, i.e. Lustre file systems, in a single
> realm, each serving their own file system.  Each Lustre file system can have
> its own user/group database.  No restrictions here.

Well, that's the problem with multiple clusters in a single realm, lack 
of restriction... ;-)

> For a given file system the MDS nodes produce capabilities which the OSS
> nodes use for authorization.   It is important that the MDS can maken
> authenticated RPC's to the OSS nodes in its file system and for this we use
> Kerberos (this is not a "must have" - it could have been done with a
> different key sharing mechanism).

With multiple clusters in a single realm an MDS from any cluster could 
authenticate and authorize as an MDS to an OSS in any cluster.  This 
would allow an MDS in one cluster to change the key used for 
capabilities on the OSSs in another cluster, no?

> ==> So the first issue you have to become clear about is how you authorize
> an MDS to contact one of its OSS nodes, wherever these are place.

I've changed lsvcgssd on the OSSs to take an arbitrary number of '-M 
lustre_mds/mdshost at REALM' and use this list to determine MDS 
authorization.  Is there a way in which an OSS is already aware of its 
appropriate MDSs?

> Similarly the Kerberos connections are used by the clients to connect to the
> OSS, but they are not used to authenticate anything (but optionally the
> node), they are used merely to provide privacy and/or authenticity for
> transporting data between the client and the OSS nodes.  With relatively
> little effort this could be done without Kerberos at all, on the other hand,
> probably using Kerberos for this leads to a more easily understood
> architecture.
> So, to repeat,  the authorization uses capabilities, which authenticate the
> requestor and contain authorization information, independent of a server
> user/group database on the OSS.
> ==> The second issue you need to be clear about is how you authenticate
> client NODES (NOT users) to OSS nodes.

Client nodes are issued lustre_root/host credentials from their local 
realm.  This works just fine for Client -> OST since the only 
[kerberos-related] authorization check is a "lustre_root" service part.

> Peter


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