Hello sssd-developers,

Allow me to introduce myself and the suggested contribution to the SSSD project. PrimaryData Inc. is a startup company in the field of distributed NAS. As such, we would like to contribute to the SSSD project by developing support for rpc.idmapd.

Below is our proposed design for this new feature.

Best regards,
Noam Meltzer


SSS NFS Client (rpc.idmapd plugin) - Design

We named the client "sss_nfs" (althogh "sss_idmap" or "idmap" might have been better names, the term "idmap" is already occupied in the SSSD world).

rpc.idmapd - background

rpc.idmapd runs on NFSv4 servers as a userspace daemon (part of nfs-utils). It's role is to assist knfsd by providing the following 6 mapping functions:

1. (user) name to uid

2. (group) name to gid

3. uid to (user) name

4. gid to (group) name

5. principal (user) name to ids (uid + gid)

6. principal (user) name to grouplist (groups which user are member of)

rpc.idmapd provides API for developing plugins (loaded by dlopen(3)) which implements the actual mapping process.

* note: 5 + 6 are relevant only for kerberised NFSv4 servers. At the first stage we only purpose a design without kerberos support.

* note2: On the kernel level, there's a caching mechanism for the responses from the userspace daemon.

SSSD - Responder

The functionality required from the Responder side is a subset of the functionality provided by existing NSS Responder's commands.

As you can see below (on the client part of the design) - no changes are needed in the NSS Responder.

SSSD - NFS Client

Responder-Facing Interactions (existing NSS Responder commands)

SSS_NSS_GETPWNAM - map (user) name to uid requests
SSS_NSS_GETGRNAM - map (group) name to gid requests
SSS_NSS_GETPWUID - map uid to (user) name requests
SSS_NSS_GETGRGID - map gid to (group) name requests

The request & reply sent to & from the responder is "standard" in terms of the NSS Responder.

The client only needs a portion of the reply. Only this portion will be extracted from the packet (i.e. uid/gid/user name/group name).

Optimisation Techniques

The optimisation techniques used for the NSS client will be used here as well. i.e. Fast Cache (memcache) & negative-cache.

It will be possible for the user to disable Fast Cache from the configuration file. (see below)

Configuration File

The configuration of the client will be part of rpc.idmap config file (/etc/idmapd.conf).


Noam Meltzer

Linux Software Engineer


P.O. Box 12650, Herzliya Pituach 4673300
9 Hamenofim St. Akerstein Towers, Tower A, 5th fl. Herzliya

Office: +972-77-8981888 | Fax: +972-3-7617140 | Mobile: +972-54-5873843

E-mail: noam@primarydata.com