| SUBUID(5) | File Formats and Configuration | SUBUID(5) |
.SH "NAME" subuid - the configuration for subordinate user ids
.SH "DESCRIPTION"
.PP Subuid authorizes a user id to map ranges of user ids from its namespace
into child namespaces.
.PP 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.
.PP Note, that newusers, useradd, and usermod will only
create entries in /etc/subuid if subid delegation is managed via subid
files.
.SH "LOCAL SUBORDINATE DELEGATION"
.PP 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:
.RE
.RE
.RE
.PP This file specifies the user IDs that ordinary users can use, with the
newuidmap command, to configure uid mapping in a user namespace.
.PP Multiple ranges may be specified per user.
.PP 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.
.SH "FILES"
.PP /etc/subuid
Per user subordinate user IDs.
.RE
.PP /etc/subuid-
Backup file for /etc/subuid.
.RE
.SH "SEE ALSO"
.PP login.defs(5), newgidmap(1), newuidmap(1),
newusers(8), subgid(5), useradd(8), userdel(8),
usermod(8), user_namespaces(7).
| 08/29/2026 | shadow-utils 4.20.2 |