# phpman > man > nfsidmap(5) [nfsidmap(5)](https://www.chedong.com/phpMan.php/man/nfsidmap/5/markdown) File Formats Manual [nfsidmap(5)](https://www.chedong.com/phpMan.php/man/nfsidmap/5/markdown) ## NAME nfsidmap - The NFS idmapper upcall program ## SYNOPSIS ### nfsidmap [-v] [-t timeout] key desc ### nfsidmap [-v] [-c] ### nfsidmap [-v] [-u|-g|-r user] ### nfsidmap -d ### nfsidmap -l ### nfsidmap -h ## DESCRIPTION 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 re‐ ferred to as "ID mapping." 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 infor‐ mation. 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 misconfigurations. They have no effect on the keyring containing ID mapping results. ## OPTIONS ### -c ### -d ### -g user Revoke the gid key of the given user. ### -h ### -l These keys are visible only to the superuser. ### -r user Revoke both the uid and gid key of the given user. ### -t timeout Set the expiration timer, in seconds, on the key. The default is 600 seconds (10 mins). ### -u user Revoke the uid key of the given user. ### -v ## CONFIGURING The file _/etc/request-key.conf_ will need to be modified so _/sbin/request-key_ can properly di‐ rect 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** de‐ fines 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 pro‐ gram. 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. ## FILES _/etc/idmapd.conf_ ID mapping configuration file _/etc/request-key.conf_ Request key configuration file ## SEE ALSO [**idmapd.conf**(5)](https://www.chedong.com/phpMan.php/man/idmapd.conf/5/markdown), [**request-key**(8)](https://www.chedong.com/phpMan.php/man/request-key/8/markdown) ## AUTHOR Bryan Schumaker, <> 1 October 2010 [nfsidmap(5)](https://www.chedong.com/phpMan.php/man/nfsidmap/5/markdown)