The system derives the user part of the string by performing a password or group lookup. The lookup mechanism is configured in /etc/idmapd.conf.
By default, the domain part of the string is the system's DNS domain name. It can also be specified in /etc/idmapd.conf if the system is multi-homed, or if the system's DNS domain name does not match the name of the system's Kerberos realm.
When the domain is not specified in /etc/idmapd.conf the local DNS server will be queried for the _nfsv4idmapdomain text record. If the record exists that will be used as the domain. When the record does not exist, the domain part of the DNS domain will used.
The /usr/sbin/nfsidmap program performs translations on behalf of the kernel. The kernel uses the request-key mechanism to perform an upcall. /usr/sbin/nfsidmap is invoked by /sbin/request-key, performs the translation, and initializes a key with the resulting information. The kernel then caches the translation results in the key.
nfsidmap can also clear cached ID map results in the kernel, or revoke one particular key. An incorrect cached key can result in file and directory ownership reverting to "nobody" on NFSv4 mount points.
create id_resolver * * /usr/sbin/nfsidmap -t 600 %k %d
This will direct all id_resolver requests to the program /usr/sbin/nfsidmap. The -t 600 defines how many seconds into the future the key will expire. This is an optional parameter for /usr/sbin/nfsidmap and will default to 600 seconds when not specified.
The idmapper system uses four key descriptions:
uid: Find the UID for the given user
gid: Find the GID for the given group
user: Find the user name for the given UID
group: Find the group name for the given GID
You can choose to handle any of these individually, rather than using the generic upcall program. If you would like to use your own program for a uid lookup then you would edit your request-key.conf so it looks similar to this:
create id_resolver uid:* * /some/other/program %k %d
create id_resolver * * /usr/sbin/nfsidmap %k %d
Notice that the new line was added above the line for the generic program. request-key will find the first matching line and run the corresponding program. In this case, /some/other/program will handle all uid lookups, and /usr/sbin/nfsidmap will handle gid, user, and group lookups.