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 client PAM authenticator will fetch user entries with plugin='pam' from the mysql.user table. The entries should also have a PAM service name set in the authetication_string column. The matching PAM service in the operating system PAM config will be used for authenticating a user. If the authetication_string for an entry is empty, a fallback service (e.g. other) is used. If a username@host has multiple matching entries, they will all be attempted until authentication succeeds or all fail.

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. Anonymous mapping is enabled in MaxScale if the following user exists:

Empty username and wildcard host (''@'%')
plugin = 'pam'
Proxy grant is on (The query SHOW GRANTS FOR ''@'%'; returns GRANT PROXY ON ...

When the authenticator detects such a user, anonymous account mapping is enabled. To verify this, search the MaxScale log for "Anonymous PAM user with proxy grant found. User account mapping enabled." When mapping is on, the PAM authenticator does not require client accounts to exist in the mysql.user-table received from the backend. It will simply 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.

Implementation details and limitations

The PAM general 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 current version of 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. No additional PAM-related messaging is allowed, as this would indicate a more complicated authentication scheme.

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.

SSL support

PAM Authenticator supports SSL connections from client to MaxScale, but not from MaxScale to backends.

Building the module

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

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