The NFSv4 protocol represents the local system's UID and GID values
on the wire as strings of the form user@domain. The process of
translating from UID to string and string to UID is referred to as
The system derives the user part of the string by performing a
password or group lookup. The lookup mechanism is configured in
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
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
In addition, the -d and -l options are available to help diagnose
misconfigurations. They have no effect on the keyring containing ID
-c Clear the keyring of all the keys.
-d Display the system's effective NFSv4 domain name on stdout.
Revoke the gid key of the given user.
-h Display usage message.
-l Display on stdout all keys currently in the keyring used to
cache ID mapping results. These keys are visible only to the
Revoke both the uid and gid key of the given user.
Set the expiration timer, in seconds, on the key. The default
is 600 seconds (10 mins).
Revoke the uid key of the given user.
-v Increases the verbosity of the output to syslog (can be
specified multiple times).
The file /etc/request-key.conf will need to be modified so
/sbin/request-key can properly direct the upcall. The following line
should be added before a call to keyctl negate:
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
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
This page is part of the nfs-utils (NFS utilities) project.
Information about the project can be found at
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. If you have a bug
report for this manual page, see
⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩. This page was
obtained from the project's upstream Git repository
2020-08-13. (At that time, the date of the most recent commit that
was found in the repository was 2020-08-07.) If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
1 October 2010 nfsidmap(5)