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NAME | DESCRIPTION | LOCAL SUBORDINATE DELEGATION | FILES | SEE ALSO | COLOPHON |
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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 newusers, useradd, and usermod 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:
• 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(8), subgid(5),
useradd(8), userdel(8), usermod(8), user_namespaces(7).
This page is part of the shadow-utils (utilities for managing
accounts and shadow password files) project. Information about
the project can be found at
⟨https://github.com/shadow-maint/shadow⟩. If you have a bug report
for this manual page, send it to
pkg-shadow-devel@alioth-lists.debian.net. This page was obtained
from the project's upstream Git repository
⟨https://github.com/shadow-maint/shadow⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
shadow-utils 4.18.0 08/11/2025 SUBUID(5)
Pages that refer to this page: getsubids(1), newuidmap(1), unshare(1), nsswitch.conf(5), subgid(5), user_namespaces(7), newusers(8), useradd(8), userdel(8), usermod(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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