rpc.idmapd is the NFSv4 ID <-> name mapping daemon. It provides functionality to the NFSv4 kernel client
and server, to which it communicates via upcalls, by translating user and group IDs to names, and vice
versa.
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.
Note that on more recent kernels only the NFSv4 server uses rpc.idmapd. The NFSv4 client instead uses
nfsidmap(8), and only falls back to rpc.idmapd if there was a problem running the nfsidmap(8) program.
The options are as follows:
-h Display usage message.
-v Increases the verbosity level (can be specified multiple times).
-f Runs rpc.idmapd in the foreground and prints all output to the terminal.
-ppath Specifies the location of the RPC pipefs to be path. The default value is
"/var/lib/nfs/rpc_pipefs".
-cpath Use configuration file path. This option is deprecated.
-C Client-only: perform no idmapping for any NFS server, even if one is detected.
-S Server-only: perform no idmapping for any NFS client, even if one is detected.