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

subuid - the subordinate uid file

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.

/etc/subuid
Per user subordinate user IDs.
/etc/subuid-
Backup file for /etc/subuid.

login.defs(5), newgidmap(1), newuidmap(1), newusers(1), subgid(5), useradd(8), userdel(8), usermod(8), user_namespaces(7).
07/28/2018 shadow-utils 4.6