nfsidmap(5) File Formats Manual nfsidmap(5)
nfsidmap - The NFS idmapper upcall program
nfsidmap [-v] [-t timeout] key desc
nfsidmap [-v] [-c]
nfsidmap [-v] [-u|-g|-r user]
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 translat‐
ing from UID to string and string to UID is referred to as "ID map‐
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 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.
In addition, the -d and -l options are available to help diagnose mis‐
configurations. They have no effect on the keyring containing ID map‐
-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 speci‐
fied 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 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 pro‐
gram. request-key will find the first matching line and run the corre‐
sponding program. In this case, /some/other/program will handle all
uid lookups, and /usr/sbin/nfsidmap will handle gid, user, and group
ID mapping configuration file
Request key configuration file
Bryan Schumaker, <email@example.com>
1 October 2010 nfsidmap(5)