LDAP + KERBEROS + NFS. Why do I need idmapd?

The short of it is that NFSv4 protocol relies a username being shared between the server and client, and not the UID/GID numbers (which were used in the earlier versions) and the UID <==> username mapping can actually be different on the client and the server.

As part of the NFSv4 protocol both the server need to map the common security contexts/permissions, owner and owner_group to something that makes sense for the local file-system operations. That mapping is done by IDMAPD on Linux systems.

On a Linux system many local file-systems operations are UID/GID based but those need to be translated to the shared NFSv4 context before they can be transmitted to the NFS server.

Maybe RFC 3530 can explain it better:

§ 5.8. Interpreting owner and owner_group

The recommended attributes "owner" and "owner_group" (and also users and groups within the "acl" attribute) are represented in terms of a UTF-8 string. To avoid a representation that is tied to a particular underlying implementation at the client or server, the use of the UTF-8 string has been chosen. Note that section 6.1 of [RFC2624] provides additional rationale. It is expected that the client and server will have their own local representation of owner and owner_group that is used for local storage or presentation to the end user. Therefore, it is expected that when these attributes are transferred between the client and server that the local representation is translated to a syntax of the form "user@dns_domain". This will allow for a client and server that do not use the same local representation the ability to translate to a common syntax that can be interpreted by both.

Edit in response to your imapd.conf.

You using a static mapping to local user. You probably want to map the NFSv4 identities to LDAP users, which probably should should happen by the nsswitch option, but apparently is not. You could try to see what is happening by increasing the verbosity of the idmapd on the NFS server.

Alternatively configure idmapd to directly query your LDAP server. The exact syntax may depend on the version you're using, but the man page shows something along the lines of :

[General]

Verbosity = 0
Domain = domain.org
Local-Realms = DOMAIN.ORG,MY.DOMAIN.ORG,YOUR.DOMAIN.ORG

[Mapping]

Nobody-User = nfsnobody
Nobody-Group = nfsnobody

[Translation]

Method = umich_ldap,nsswitch
GSS-Methods = umich_ldap,static

[Static]
[email protected] = johnny

[UMICH_SCHEMA]   
LDAP_server = ldap.domain.org
LDAP_base = dc=org,dc=domain