etcidmapdconf

/etc/idmapd.conf is the configuration file for the idmapd daemon, which provides user and group ID mapping for the NFSv4 protocol. It translates UID/GID values between clients and servers across domains or realms, enabling consistent file ownership when accessing NFS shares from machines with different user databases.

The file is part of the NFS server stack and is typically installed with nfs-utils or nfs-kernel-server. On startup, idmapd reads this file to determine how user and group identities should be mapped. The usual path on Linux systems is /etc/idmapd.conf, and the active daemon may be named idmapd or nfs-idmapd depending on the distribution.

The file is a plain text configuration with sections and key=value pairs. Common sections include [General], which defines domain or realm-related settings, and [Mapping], which controls how IDs are translated and what to do when a mapping cannot be found. Values in [General] often specify the domain or realm to be used for mapping, while [Mapping] can designate fallback identities for unmapped IDs, such as Nobody-User and Nobody-Group.

Key considerations include ensuring the domain configuration aligns with the clients and any centralized identity provider (such as Kerberos or LDAP) used in the environment. Proper configuration is essential for correct ownership propagation across NFSv4 mounts; misconfigurations can lead to inaccessible files or incorrect ownership on clients.

After editing, the idmapd service should be restarted to apply changes. Documentation and distribution-specific examples can guide exact option names and acceptable values for a given environment.