linux man page for subuid

SUBUID(5)                                                  File Formats and Conversions                                                  SUBUID(5)

NAME
       subuid - the subordinate uid file

DESCRIPTION
       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:

       o   login name or UID

       o   numerical subordinate user ID

       o   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).

shadow-utils 4.5                                                    01/25/2018                                                           SUBUID(5)