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 useradd 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 (“:”).
       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.

SEE ALSO

       login.defs(5), newgidmap(1), newuidmap(1), newusers(1), subgid(5), useradd(8), userdel(8),
       usermod(8), user_namespaces(7).