|SUBUID(5)||File Formats and Configuration||SUBUID(5)|
subuid - the configuration for subordinate user ids
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.
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:
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.