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# nss-localuser
- [License](#license)
- [Overview](#overview)
- [Install](#documentation)
## License
This code is published with the license MIT (see LICENSE.txt) for details.
## Overview
`nss-localuser` is a plugin for the GNU Name Service Switch (NSS)
functionality of the GNU C Library (`glibc`) providing host name
resolution for *"localuser"* family of virtual hostnames.
It defines the family *"localuser"* of virtual hostnames as one of the
below names:
- localuser
- localuser-UID
- localuser--APPID
- localuser-UID-APPID
- localuser---APPID
This can be summarized by the following matrix:
|------------------|------------------|---------------------|-------------------|
| | **current user** | **user of UID** | **no user** |
|------------------|------------------|---------------------|-------------------|
| **no APP** | localuser | localuser-UID | |
| **app of APPID** | localuser--APPID | localuser-UID-APPID | localuser---APPID |
|------------------|------------------|---------------------|-------------------|
The delivered NSS service defines one virtual host of name `localuser`
that resolves to an IP address of the localhost loopback that integrates
user ID.
It is intended to enable distinct IP for distinct users, distinct application.
The name *localuser* family is resolved to the IPv4 address range 127.128.0.0/9
The delivered IPv4 address is structured as follow:
```text
+--------+--------+--------+--------+
:01111111:1abbcccc:dddddeee:ffffffff:
+--------+--------+--------+--------+
```
When `a` is `1`, the value 11 bits value `bbccccddddd` encodes the APPID
and the 11 bits value `eeedddddddd` encodes the UID.
This is represented by the following hostnames: `localuser--APPID`
and `localuser-UID-APPID`.
When `abb` is `011`, the 20 bits value `ccccdddddeeeffffffff` encodes the APPID.
This is represented by the following hostnames: `localuser---APPID`.
When `abb` is `010`, the 20 bits value `ccccdddddeeeffffffff` encodes the UID.
This is represented by the following hostnames: `localuser`
and `localuser-UID`.
The values `000` and `001` of `abb` are reserved for futur use.
Examples:
```text
localuser => 127.160.0.0 (when user has UID = 0)
localuser => 127.160.3.233 (when user has UID = 1001)
localuser-0 => 127.160.0.0
localuser-45 => 127.160.0.45
localuser-1024 => 127.160.4.0
localuser-1048575 => 127.175.255.255
localuser---0 => 127.176.0.0
localuser---45 => 127.176.0.45
localuser---1048575 => 127.191.255.255
localuser-0-0 => 127.192.0.0
localuser--78 => 127.194.115.233 (when user has UID = 1001)
localuser-23-54 => 127.193.176.23
localuser-2047-2047 => 127.255.255.255
```
The service also provides the reverse resolution.
This module provides a value for IPv6: it translates to a IPv4-mapped IPv6 address
because IPv6 lacks of loopback range.
Example:
```text
localuser-1024 => ::ffff:127.128.4.0
```
For details about NSS integration, see
[Gnu libc documentation](https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html).
## Install
To install this file:
```sh
make all && sudo install
```
The installation directory is automatically detected by the tiny
script detect-nssdir.sh.
If the script detect-nssdir.sh gives the wrong result, just define the
variable nssdir when calling make, as below:
```sh
make install nssdir=~/lib
```
## Configuration and activation
### Manual setting
To activate the NSS module localuser you have to edit
`/etc/nsswitch.conf` and add `localuser` to the
line starting with "`hosts:`".
Your nsswitch file can then look as below:
<pre># /etc/nsswitch.conf
passwd: files sss systemd
shadow: files sss
group: files sss systemd
hosts: <b>localuser</b> files dns myhostname
ethers: files
netmasks: files
networks: files
protocols: files
services: files sss
aliases: files nisplus</pre>
### Scripted setting
The script activate-localuser.sh can be used to activate,
deactivate or query the status of localuser activation.
It accepts 0, 1 or 2 arguments.
<pre>usage: activate-localuser.sh [command [file]]
command: on, yes, true, 1: activate
off, no, false, 0: deactivate
status, test, check, query: status (default)
file: file to change (default /etc/nsswitch.conf)</pre>
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