man > SUBGID(5)

SUBGID(5)                          File Formats and Configuration                          SUBGID(5)

NAME
       subgid - the configuration for subordinate group ids

DESCRIPTION
       Subgid authorizes a group id to map ranges of group ids from its namespace into child
       namespaces.

       The delegation of the subordinate gids 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 gids to /etc/subgid. 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 gid delegation falls back to files.

       Note, that groupadd will only create entries in /etc/subgid if subid delegation is managed
       via subid files.

LOCAL SUBORDINATE DELEGATION
       Each line in /etc/subgid contains a user name and a range of subordinate group 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 group ID

       •   numerical subordinate group ID count

       This file specifies the group IDs that ordinary users can use, with the newgidmap command, to
       configure gid 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/subgid, 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/subgid
           Per user subordinate group IDs.

       /etc/subgid-
           Backup file for /etc/subgid.

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

shadow-utils 4.13                            05/30/2024                                    SUBGID(5)