PAM Authenticator

Pluggable authentication module (PAM) is a general purpose authentication API. An application using PAM can authenticate a user without knowledge about the underlying authentication implementation. The actual authentication scheme is defined in the operating system PAM config (e.g. /etc/pam.d/), and can be quite elaborate. MaxScale supports a very limited form of the PAM protocol, which this document details.

Configuration

The MaxScale PAM modules themselves have no configuration. All that is required is to change the listener and backend authenticator modules to PAMAuth and PAMBackendAuth, respectively.

[Read-Write-Listener]
type=listener
address=::
service=Read-Write-Service
protocol=MariaDBClient
authenticator=PAMAuth

[Master-Server]
type=server
address=123.456.789.10
port=12345
protocol=MariaDBBackend
authenticator=PAMBackendAuth

The PAM authenticator fetches user entries with plugin='pam' from the mysql.user table of a backend. The user accounts also need to have either the global SELECT-privilege or a database or a table-level privilege. Privileges through a default role are also detected. The PAM service name of a user is read from the authetication_string-column. The matching PAM service in the operating system PAM config is used for authenticating the user. If the authetication_string for a user is empty, the fallback service mysql is used.

PAM service configuration is out of the scope of this document, see The Linux-PAM System Administrators' Guide for more information. A simple service definition used for testing this module is below.

auth            required        pam_unix.so
account         required        pam_unix.so

Anonymous user mapping

The MaxScale PAM authenticator supports a limited version of user mapping. It requires less configuration but is also less accurate than the server authentication. Anonymous mapping is enabled in MaxScale if the following user exists: - Empty username (e.g. ''@'%' or ''@'myhost.com') - plugin = 'pam' - Proxy grant is on (The query SHOW GRANTS FOR user@host; returns at least one row with GRANT PROXY ON ...)

When the authenticator detects such users, anonymous account mapping is enabled for the hosts of the anonymous users. To verify this, enable the info log (log_info=1 in MaxScale config file) and look for messages such as "Found 2 anonymous PAM user(s) ..." and "Added anonymous PAM user ..." during MaxScale startup.

When mapping is on, the MaxScale PAM authenticator does not require client accounts to exist in the mysql.user-table received from the backend. MaxScale only requires that the hostname of the incoming client matches the host field of one of the anonymous users (comparison performed using LIKE). If a match is found, MaxScale attempts to authenticate the client to the local machine with the username and password supplied. The PAM service used for authentication is read from the authentication_string-field of the anonymous user. If authentication was successful, MaxScale then uses the username and password to log to the backends.

Anonymous mapping is only attempted if the client username is not found in the mysql.user-table as explained in Configuration. This means, that if a user is found and the authentication fails, anonymous authentication is not attempted even when it could use a different PAM service with a different outcome.

Setting up PAM group mapping for the MariaDB server is a more involved process as the server requires details on which Unix user or group is mapped to which MariaDB user. See this guide for more details. Performing all the steps in the guide also on the MaxScale machine is not required, as the MaxScale PAM plugin only checks that the client host matches an anonymous user and that the client (with the username and password it provided) can log into the local PAM configuration. If using normal password authentication, simply generating the Unix user and password should be enough.

Implementation details and limitations

The general PAM authentication scheme is difficult for a proxy such as MaxScale. An application using the PAM interface needs to define a conversation function to allow the OS PAM modules to communicate with the client, possibly exchanging multiple messages. This works when a client logs in to a normal server, but not with MaxScale since it needs to autonomously log into multiple backends. For MaxScale to successfully log into the servers, the messages and answers need to be predefined. This requirement denies the use of more exotic schemes such as one-time passwords or two-factor authentication.

The MaxScale PAM authentication module only supports a simple password exchange. On the client side, the authentication begins with MaxScale sending an AuthSwitchRequest packet. In addition to the command, the packet contains the client plugin name dialog, a message type byte 4 and the message Password:. In the next packet, the client should send the password, which MaxScale will forward to the PAM API running on the local machine. If the password is correct, an OK packet is sent to the client. If the local PAM API asks for additional credentials as is typical in two-factor authentication schemes, authentication fails. Informational messages such as password expiration notifications are allowed. These are simply printed to the log.

On the backend side, MaxScale expects the servers to act as MaxScale did towards the client. The servers should send an AuthSwitchRequest packet as defined above, MaxScale responds with the password received by the client authenticator and finally backend replies with OK. Informational messages from backends are only printed to the info-log.

Building the module

The PAM authenticator modules require the PAM and SQLite3 development libraries (libpam0g-dev and sqlite3-dev on Ubuntu).