'\" t
.\"     Title: subuid
.\"    Author: Eric Biederman
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 07/02/2024
.\"    Manual: File Formats and Configuration Files
.\"    Source: shadow-utils 4.16.0
.\"  Language: English
.\"
.TH "SUBUID" "5" "07/02/2024" "shadow\-utils 4\&.16\&.0" "File Formats and Configuration"
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.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
\fIsubid\fR
field in
/etc/nsswitch\&.conf
file\&. Only one value can be set as the delegation source\&. Setting this field to
\fIfiles\fR
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
\fIlibsubid_$value\&.so\fR\&. If the value or plugin is missing, then the subordinate uid delegation falls back to
\fIfiles\fR\&.
.PP
Note, that
\fBuseradd\fR
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 (\(lq:\(rq)\&. These fields are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
login name or UID
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
numerical subordinate user ID
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
numerical subordinate user ID count
.RE
.PP
This file specifies the user IDs that ordinary users can use, with the
\fBnewuidmap\fR
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
.RS 4
Per user subordinate user IDs\&.
.RE
.PP
/etc/subuid\-
.RS 4
Backup file for /etc/subuid\&.
.RE
.SH "SEE ALSO"
.PP
\fBlogin.defs\fR(5),
\fBnewgidmap\fR(1),
\fBnewuidmap\fR(1),
\fBnewusers\fR(8),
\fBsubgid\fR(5),
\fBuseradd\fR(8),
\fBuserdel\fR(8),
\fBusermod\fR(8),
\fBuser_namespaces\fR(7)\&.