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.
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:
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.
/etc/subgid
/etc/subgid-
login.defs(5), newgidmap(1), newuidmap(1), newusers(8), subuid(5), useradd(8), userdel(8), usermod(8), user_namespaces(7).