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'\" t
.\" Title: subuid
.\" Author: Eric Biederman
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\" Date: 06/21/2024
.\" Manual: File Formats and Configuration Files
.\" Source: shadow-utils 4.15.2
.\" Language: English
.\"
.TH "SUBUID" "5" "06/21/2024" "shadow\-utils 4\&.15\&.2" "File Formats and Configuration"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
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.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
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.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.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)\&.
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