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

Benjamin Bennett ben at psc.edu
Wed Jul 9 13:07:17 PDT 2008

Eric Mei wrote:
> Peter Braam wrote:
>> On 7/8/08 2:38 PM, "Benjamin Bennett" <ben at psc.edu> wrote:
>>> 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?
>> That MIGHT work ... But how would two domains guarantee consistent 
>> updates
>> to the databases?  However, the server - server trust across domains 
>> we need
>> is new to me (and I am not sure if/how it works).
> Practically it's doable, of course. But as Peter pointed out the user 
> database must be the same across all MDSs within a Luster FS. If 2 MDSs 
> could share the user database, why bother putting them into different 
> kerberos realms? So we assume all MDSs should be in a single realm. Does 
> TeraGrid have different requirement?

TeraGrid has a central database of users which could be used to 
consistently generate mappings.

The reason to bother putting MDSs in separate realms is that TeraGrid is 
composed of distinct organizations.  We are trying to distribute a 
filesystem across several organizations, not simply implement a 
centralized fs accessed by several organizations.

>>>> 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... ;-)
>> Restrict yourself, not me or Lustre :)
>>>> 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.
>> Good point.  If so that should be a bug.
>> ===> Eric Mei, what is the story here?
> Yes Ben is right, currently in a same realm any MDS could authenticate 
> with any MDS and OSS. But afaics the problem is nothing to do with 
> Kerberos. It's because currently Lustre have no config information about 
> the server cluster membership, each server target have no idea what 
> other targets are.
> So solve this, we can either place the configuration on each MDS/OST 
> nodes - as Ben proposed in last mail; or probably better centrally 
> managed by MGS, thus MDT/OST would be able to get uptodate server 
> cluster information. Would it work?

Sounds like a good idea.  If I understand correctly...
   A) An MDT/OST is explicitly given the MGS NID by a trusted entity 
(administrator) during mkfs.

   B) The MGS principal name would be derived from its NID (assuming 
lustre_mgs/mgsnode at REALM).  Realm is determined from the usual kerberos 
dns -> realm mapping mechanism?

   C) MDT and OST (or just MDS, OSS) list retrieved via secured MGC -> 
MGS connection.

   D) MDS and OSS principal names are derived from MDS and OSS NIDs. 
Same realm determination as in B?

>> The key (which is manually generated) should authenticate an instance 
>> of an
>> MDS, not a "cluster".   The only case where this might become delicate 
>> is if
>> one MDS node is the server for two file systems.
> GSS/Kerberos is for the a certain kind service on a node, we can tell it 
> simply from the composition of Kerberos principal 
> "service_name/hostname at REALM". As to Lustre, lustre_mds/hostname at REALM 
> it's for MDS, not specific to MDT. So if two MDTs on a MDS serving two 
> different file systems, GSS/Kerberos authentications are performed in 
> the same way for them, further access control should be handled by each 
> target (MDT/OST).
>>>  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?
>> As you pointed out, we need that, and Eric Mei should help you get that.
> Yes that works, probably as temporary solution. As described above, 
> currently OSS don't know that info. we may need a more complete 
> centrally controlled server membership authentication, maybe independent 
> of GSS/Kerberos.

If you're interested, the patch I have is at [1].

>>>> 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.
>> Good.  Does it work across realms, because it seems we need that in any
>> case?
> Yes, Ben had a patch to make it work.

The foreign lustre_root principals have to be mapped on the MDS to allow 
mount.  What are your thoughts on authorizing [squashed] mount to all, 
so as to not require mapping?

>> BTW, thank you for trying this all out in detail, that is very helpful.
>> Perhaps Sheila could talk with you and Eric Mei and get a nice writeup 
>> done
>> for the manual.

np :-)


[1] http://staff.psc.edu/ben/patches/lustre/lustre-explicit-mds-authz.patch

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