LibNSS-AAD: Linux Name Switch Service plugin for passwd and group lookups into Azure Active Directory

Introduction

This is a glibc NSS plugin that will query Azure Active Directory for information about users, written in Rust. It is very, very simple, and does not even go so far as to properly use OAuth2. It implements the following libc functions:

• getpwnam
• getgrnam
• getgrgid
• initgroups_dyn

This plugin works best with (and basically, though not actually, requires) nscd. Because nscd can have unintended effects on hosts, it is highly recommended that you refer to nscd.conf(5) and enable nscd caching only for passwd and group services, and set the various configuration options appropriately for your environment.

This plugin was written as a compliment to OpenSSH certificate authentication.

Security Considerations

This plugin provides a limited set of information to the system, and has some aspects that are worth mentioning.

• UIDs are not sanity-checked, except for uid 0 (which the plugin will refuse to honor and return NSS_STATUS_NOTFOUND).
• As currently implemented, UIDs are retrieved from the immutableId user attribute. This may not be appropriate in your environment.
• GIDs are not checked, not even for GID 0.
• The user password field returned is ., because OpenSSH considers a password field of * to indicate a locked account.

The /etc/nssaad.conf file must be readable by any user (privileged or not) that wants to obtain information from AAD (akin to the passwd service), and thus the Azure AD Application's client secret will be world-readable. Careful use of NSCD may alleviate this (as it may when using the bindpw option libnss-ldap), but very well may not. It is recommended that you limit the permissions granted to the Application, and not grant shell access to users whom you do not want querying the Directory.

Configuration

Plugin Configuration

The plugin expects to read from /etc/nssaad.conf, which is a YAML file:

client_id: "..."
client_secret: "..."
tenant: "..."
group_ids:
  ...: ###
  ...

• client_id: is the Application ID of the AAD Application that you have created, and to which you have granted it the necessary permissions (namely, Directory.Read.All, or a combination of User.ReadBasic.All and Group.Read.All) to query data from the Graph API.
• client_secret: is a key that the client can use to obtain an OAuth2 bearer token.
• tenant: is your Azure AD tenant name, or its GUID.
• group_ids: This library does not (yet) make allowances for storing GIDs in AAD. As a workaround, you must specify GIDs within the config file. The config expects a dict named group_ids that maps group names to GIDs:

NSS Configuration

Add the aad service to the /etc/nsswitch.conf file. Probably something like:

passwd:         compat aad
group:          compat aad

Installation

Upon building the library, copy the target/release/libnss_aad.so file to /lib/???-linux-gnu/libnss_aad.so.2.

Known Issues

• Every call to the plugin results in at least one connection to the OAuth2 endpoint for a new token.
• OpenSSH may consider a user account with a password field of * to be locked, and thus this plugin returns . instead.