---
name: "subuid"
section: "5"
platform: "linux"
package: "shadow"
version: "4.20.0.arch1-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/subuid"
---

# subuid(5)

the configuration for subordinate user ids

## 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 (“:”). 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)](/login.defs/5.md), [**newgidmap**(1)](/newgidmap/1.md), [**newuidmap**(1)](/newuidmap/1.md), [**newusers**(8)](/newusers/8.md), [**subgid**(5)](/subgid/5.md), [**useradd**(8)](/useradd/8.md), [**userdel**(8)](/userdel/8.md), [**usermod**(8)](/usermod/8.md), [**user\_namespaces**(7)](/user_namespaces/7.md).