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 module requires little configuration. All that is required is to change the listener authenticator module to "PAMAuth".

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

[Primary-Server]
type=server
address=123.456.789.10
port=12345

MaxScale uses the PAM authenticator plugin to authenticate users with plugin set to "pam" in the mysql.user-table. The PAM service name of a user is read from the authentication_string-column. The matching PAM service in the operating system PAM config is used for authenticating the user. If the authentication_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

pam_use_cleartext_plugin

Boolean, default value is "false". If enabled, MaxScale communicates with the client as if using mysql_clear_password. This setting has no effect on MaxScale-to-backend communication, which adapts to either "dialog" or "mysql_clear_password", depending on which one the backend suggests. This setting is meant to be used with the similarly named MariaDB Server setting.

authenticator_options=pam_use_cleartext_plugin=1

pam_mode

This setting defines the authentication mode used. The following values are supported:

• password (default) Normal password-based authentication
• password_2FA Password + 2FA-code based authentication
• suid Authenticate using suid sandbox subprocess

authenticator_options=pam_mode=password_2FA

If set to password_2FA, any users authenticating via PAM will be asked two passwords ("Password" and "Verification code") during login. MaxScale uses the normal password when either the local PAM api or a backend asks for "Password". MaxScale answers any other password prompt (e.g. "Verification code") with the second password. See the limitations section for more details. Two-factor mode is incompatible with pam_use_cleartext_plugin.

If set to suid, MaxScale will launch a separate subprocess for every client to handle pam authentication. This subprocess runs the binary maxscale_pam_auth_tool (installed in the binary directory), which calls the system pam libraries. The binary is installed with the SUID bit set, which means that it runs with root-privileges regardless of the user launching it. This should bypass any file grant issues (e.g. reading etc/shadow) that may arise with the password or password_2FA options. The suid-option may also perform faster if many clients authenticate with pam simultaneously due to better separation of clients. It may also resist buggy pam plugins crashing, as the crash would be limited to the subprocess only. The MariaDB Server uses a similar pam authentication scheme. suid-mode supports two-factor authentication.

pam_backend_mapping

Defines backend authentication mapping, i.e. switch of authentication method between client-to-MaxScale and MaxScale-to-backend. Supported values:

• none (default) No mapping
• mariadb Map users to normal MariaDB accounts

authenticator_options=pam_backend_mapping=mariadb

If set to "mariadb", MaxScale will authenticate clients to backends using standard MariaDB authentication. Authentication to MaxScale itself still uses PAM. MaxScale asks the local PAM system if the client username was mapped to another username during authentication, and use the mapped username when logging in to backends. Passwords for the mapped users can be given in a file, see pam_mapped_pw_file below. If passwords are not given, MaxScale will try to authenticate without a password. Because of this, normal PAM users and mapped users cannot be used on the same listener.

Because the client still needs to authenticate to MaxScale normally, an anonymous user may be required. If the backends do not allow such a user, one can be manually added using the service setting user_accounts_file.

To map usernames, the PAM service needs to use a module such as pam_user_map.so. This module is not a standard Linux component and needs to be installed separately. It is included in recent MariaDB Server packages and can also be compiled from source. See user mapping for more information on how to configure the module. If the goal is to only map users from PAM to MariaDB in MaxScale, then configuring user mapping on just the machine running MaxScale is enough.

Instead of using pam_backend_mapping, consider using the listener setting user_mapping_file, as it is easier to configure. pam_backend_mapping should only be used when the user mapping needs to be defined by pam.

pam_mapped_pw_file

Path to a json-text file with user passwords. Default value is empty, which disables the feature.

authenticator_options=pam_mapped_pw_file=/home/root/passwords.json,pam_backend_mapping=mariadb

This feature only works together with pam_backend_mapping=mariadb. The file is only read during listener creation (typically MaxScale start) or when a listener is modified during runtime. The file should contain passwords for the mapped users. When a client is authenticating, MaxScale searches the password data for a matching username. If one is found, MaxScale uses the supplied password when logging in to backends. Otherwise, MaxScale tries to authenticate without a password.

One array, "users_and_passwords", is read from the file. Each array element in the array must define the following fields: - "user": String. Mapped client username. - "password": String. Backend server password. Can be encrypted with maxpasswd.

An example file is below.

{
    "users_and_passwords": [
        {
            "user": "my_mapped_user1",
            "password": "my_mapped_pw1"
        },
        {
            "user": "my_mapped_user2",
            "password": "A6D4C53619FFFF4DF252A0E595EDB0A12CA44E16AF154D0ED08F687E81604BFF42218B4EBA9F3EF8D907CF35E74ABDAA"
        }
    ]
}

Anonymous user mapping

When backend authenticator mapping is not in use (authenticator_options=pam_backend_mapping=none), the PAM authenticator supports a limited version of user mapping. It requires less configuration but is also less accurate than proper mapping. 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). When a client is logging in using the anonymous user account, MaxScale will log a message starting with "Found matching anonymous user ...".

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. The passwords given to MaxScale need to work as is when MaxScale logs into the backends. This requirement prevents the use of one-time passwords.

The MaxScale PAM authentication module supports two password modes. In normal mode, client authentication begins with MaxScale sending an AuthSwitchRequest packet. In addition to the command, the packet contains the client plugin name ("dialog" or "mysql_clear_password"), 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.

Two-factor authentication support

MaxScale supports a limited form of two-factor authentication with the pam_mode=password_2FA-option. Since MaxScale uses the 2FA-code given by the client to log in to the local PAM api as well as all the backends, the code must be reusable. This prevents the use of any kind of centrally checked one-use codes. Time-based codes work, assuming the backends are checking the codes independently of each other. Automatic reconnection features (e.g. readwritesplit-router) will not work, as the code has likely changed since original authentication.

Optionally, the PAM configuration on the backend servers can be weakened such that the servers only asks for the normal password. This way, MaxScale will check the 2FA-code of the incoming client, while MaxScale logs into the backends using only the password.

Due to technical reasons, MaxScale does not forward the password prompts from the PAM api to the client. MaxScale will always ask for "Password" and "Verification code", even if the PAM api asks for other items. This prevents the use of authentication schemes where a specific question must be answered (e.g. "Input code Nr. 5"). This is not a significant limitation, as such schemes would not work with backend servers anyway.

Test tool

MaxScale binary directory contains the test_pam_login-executable. This simple program asks for a username, password and PAM service and then uses the given credentials to login to the given service. test_pam_login uses the same code as MaxScale itself to communicate with the OS PAM interface and may be useful for diagnosing PAM login issues.