SUBUID
Section: File Formats and Configuration (5)
Updated: 02/01/2026
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NAME
subuid - the configuration for subordinate user ids
DESCRIPTION
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.
LOCAL SUBORDINATE DELEGATION
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 (lq:rq). 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.
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(8),
subgid(5),
useradd(8),
userdel(8),
usermod(8),
user_namespaces(7).
Index
- NAME
-
- DESCRIPTION
-
- LOCAL SUBORDINATE DELEGATION
-
- FILES
-
- SEE ALSO
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