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SUBUID

Section: File Formats and Configuration (5)
Updated: 02/01/2026
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NAME

subuid - the configuration for subordinate user ids

DESCRIPTION

Subuid authorizes a user id to map ranges of user ids from its namespace into child namespaces.

The delegation of the subordinate uids can be configured via the subid field in /etc/nsswitch.conf file. Only one value can be set as the delegation source. Setting this field to files configures the delegation of uids to /etc/subuid. Setting any other value treats the delegation as a plugin following with a name of the form libsubid_$value.so. If the value or plugin is missing, then the subordinate uid delegation falls back to files.

Note, that newusers, useradd, and usermod will only create entries in /etc/subuid if subid delegation is managed via subid files.

LOCAL SUBORDINATE DELEGATION

Each line in /etc/subuid contains a user name and a range of subordinate user ids that user is allowed to use. This is specified with three fields delimited by colons (lq:rq). These fields are:

: * login name or UID

: * numerical subordinate user ID

: * numerical subordinate user ID count

This file specifies the user IDs that ordinary users can use, with the newuidmap command, to configure uid mapping in a user namespace.

Multiple ranges may be specified per user.

When large number of entries (10000-100000 or more) are defined in /etc/subuid, parsing performance penalty will become noticeable. In this case it is recommended to use UIDs instead of login names. Benchmarks have shown speed-ups up to 20x.

FILES

/etc/subuid

: Per user subordinate user IDs.

/etc/subuid-

: Backup file for /etc/subuid.