dave.daugherty at centrify.com
Mon Jun 19 20:32:32 GMT 2006
>From simo: Monday, June 19, 2006 1:03 PM
> After the reject of my previous proposed unix socket based protocol
> external idmap caches I thought some more about the problem I have
> the current implementation of idmap and on ways to fix it in a
> reasonable way.
> How do we allow the change of uid/gid to sid mappings while
> the current idmap model and the tdb cache ?
> Some modules already allow administrators to easily and externally
> change mappings (idmap_ad, idmap_ldap) but actually the tdb cache will
> not acknowledge this changes resulting, eventually, in inconsistent
> situations across multiple servers. It is not my desire to discuss
> whether changing mappings is a good policy or not.
> Proposed Solution:
> Introduce an date timestamp on cached mappings so that entries can be
> Actually idmap_tdb implement a persistent cache where values, once
> assigned, are never again validated against the remote central mapping
> facility. Adding a creation date timestamp to each mapping entry will
> easily allow to expire the mapping regularly so that it can be checked
> against the remote servers. This behavior will be regulated by a new
> parameter called "idmap expire time" set by default to 0 meaning
> and only checked when an idmap backend is actually configured so that
> standalone installations will continue to work as usual.
> In case a remote backend is configured and idmap exipre time is not 0,
> each time a mapping is requested idmap_tdb will check if the retrieved
> mapping is still valid or expired. In case it is expired it will
> the remote backend to return the current mapping.
> If the mapping is confirmed the mapping timestamp in the cache will be
> renewed, otherwise the old mapping will be deleted and a new mapping
> will be returned.
> In case the remote server is temporarily not available expired entries
> will be returned as valid until they can be validated against a
> remote server. They will not be removed, but the timestamp will not be
> updated until the validation can be performed.
> This schema requires to add a new SID-timestamp set of entries in the
> idmap tdb database (winbind_idmap.tdb) and to make sure remote
> return a different error for invalid mappings and unreachable servers.
> Actually the idmap module returns NT_STATUS_UNSUCCESSFUL in most cases
> when an operation fails.
> I propose to change the modules to return NT_STATUS_UNSUCCESSFUL in
> of any generic error (like memory allocation errors or connection
> errors), and NT_STATUS_NONE_MAPPED only when it is clear that the
> mapping is not valid anymore and no mapping has been found. (This may
> used later on to implement some form of negative caching).
> I will not be able to work on this before next week but any comment
> be really appreciated.
>  Changing mappings is not recommended. Changing a mapping means
> file ownerships may change or become invalid. It is also understood
> on little installations the situation can be fixed just by deleting
> local tdb cache and restarting samba.
> But in the case the administrator has carefully considered all his
> options and thinks that changing uid mappings is the best thing to do,
> and the installation consist of a big number of samba servers I am
> convinced that a way to cope with this situation without requiring
> prone human intervention is a much better option.
Since idmap_ad and idmap_ldap do not have a local cache, I can see why
you would want to do this to help performance, understanding that it
complicates the implementation of the TDB caching. However I still
think the best thing is to bypass the TDB cache completely (especially
if the idmapper does its own local caching). Please continue to allow
for this too.
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