Postfix SASL Howto

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How Postfix uses SASL authentication

SMTP servers need to decide whether an SMTP client is authorized to send mail
to remote destinations, or only to destinations that the server itself is
responsible for. Usually, SMTP servers accept mail to remote destinations when
the client's IP address is in the "same network" as the server's IP address.

SMTP clients outside the SMTP server's network need a different way to get
"same network" privileges. To address this need, Postfix supports SASL
authentication (RFC 4954, formerly RFC 2554). With this a remote SMTP client
can authenticate to the Postfix SMTP server, and the Postfix SMTP client can
authenticate to a remote SMTP server. Once a client is authenticated, a server
can give it "same network" privileges.

Postfix does not implement SASL itself, but instead uses existing
implementations as building blocks. This means that some SASL-related
configuration files will belong to Postfix, while other configuration files
belong to the specific SASL implementation that Postfix will use. This document
covers both the Postfix and non-Postfix configuration.

NOTE: People who go to the trouble of installing Postfix may have the
expectation that Postfix is more secure than some other mailers. The Cyrus SASL
library contains a lot of code. With this, Postfix becomes as secure as other
mail systems that use the Cyrus SASL library. Dovecot provides an alternative
that may be worth considering.

You can read more about the following topics:

  * Configuring SASL authentication in the Postfix SMTP server
  * Configuring SASL authentication in the Postfix SMTP/LMTP client
  * Building Postfix with SASL support
  * Using Cyrus SASL version 1.5.x
  * Credits

Configuring SASL authentication in the Postfix SMTP server

As mentioned earlier, SASL is implemented separately from Postfix. For this
reason, configuring SASL authentication in the Postfix SMTP server involves two
different steps:

  * Configuring the SASL implementation to offer a list of mechanisms that are
    suitable for SASL authentication and, depending on the SASL implementation
    used, configuring authentication backends that verify the remote SMTP
    client's authentication data against the system password file or some other
    database.

  * Configuring the Postfix SMTP server to enable SASL authentication, and to
    authorize clients to relay mail or to control what envelope sender
    addresses the client may use.

Successful authentication in the Postfix SMTP server requires a functional SASL
framework. Configuring SASL should therefore always be the first step, before
configuring Postfix.

You can read more about the following topics:

  * Which SASL Implementations are supported?
  * Configuring Dovecot SASL

      o Postfix to Dovecot SASL communication

  * Configuring Cyrus SASL

      o Cyrus SASL configuration file name
      o Cyrus SASL configuration file location
      o Postfix to Cyrus SASL communication

  * Enabling SASL authentication and authorization in the Postfix SMTP server

      o Enabling SASL authentication in the Postfix SMTP server
      o Postfix SMTP Server policy - SASL mechanism properties
      o Enabling SASL authorization in the Postfix SMTP server
      o Additional SMTP Server SASL options

  * Testing SASL authentication in the Postfix SMTP server

Which SASL Implementations are supported?

Currently the Postfix SMTP server supports the Cyrus SASL and Dovecot SASL
implementations.

    Note

    Current Postfix versions have a plug-in architecture that can support
    multiple SASL implementations. Before Postfix version 2.3, Postfix had
    support only for Cyrus SASL.

To find out what SASL implementations are compiled into Postfix, use the
following commands:

    % postconf -a (SASL support in the SMTP server)
    % postconf -A (SASL support in the SMTP+LMTP client)

These commands are available only with Postfix version 2.3 and later.

Configuring Dovecot SASL

Dovecot is a POP/IMAP server that has its own configuration to authenticate
POP/IMAP clients. When the Postfix SMTP server uses Dovecot SASL, it reuses
parts of this configuration. Consult the Dovecot documentation for how to
configure and operate the Dovecot authentication server.

Postfix to Dovecot SASL communication

Communication between the Postfix SMTP server and Dovecot SASL happens over a
UNIX-domain socket or over a TCP socket. We will be using a UNIX-domain socket
for better privacy.

The following fragment for Dovecot version 2 assumes that the Postfix queue is
under /var/spool/postfix/.

     1 conf.d/10-master.conf:
     2     service auth {
     3       ...
     4       unix_listener /var/spool/postfix/private/auth {
     5         mode = 0660
     6         # Assuming the default Postfix user and group
     7         user = postfix
     8         group = postfix
     9       }
    10       ...
    11     }
    12
    13 conf.d/10-auth.conf
    14     auth_mechanisms = plain login

Line 4 places the Dovecot SASL socket in /var/spool/postfix/private/auth, lines
5-8 limit read+write permissions to user and group postfix only, and line 14
provides plain and login as mechanisms for the Postfix SMTP server.

Proceed with the section "Enabling SASL authentication and authorization in the
Postfix SMTP server" to turn on and use SASL in the Postfix SMTP server.

Configuring Cyrus SASL

The Cyrus SASL framework supports a wide variety of applications (POP, IMAP,
SMTP, etc.). Different applications may require different configurations. As a
consequence each application may have its own configuration file.

The first step configuring Cyrus SASL is to determine name and location of a
configuration file that describes how the Postfix SMTP server will use the SASL
framework.

Cyrus SASL configuration file name

The name of the configuration file (default: smtpd.conf) is configurable. It is
a concatenation from a value that the Postfix SMTP server sends to the Cyrus
SASL library, and the suffix .conf, added by Cyrus SASL.

The value sent by Postfix is the name of the server component that will use
Cyrus SASL. It defaults to smtpd and is configured with one of the following
variables:

    /etc/postfix/main.cf:
        # Postfix 2.3 and later
        smtpd_sasl_path = smtpd

        # Postfix < 2.3
        smtpd_sasl_application_name = smtpd

Cyrus SASL configuration file location

The location where Cyrus SASL searches for the named file depends on the Cyrus
SASL version and the OS/distribution used.

You can read more about the following topics:

  * Cyrus SASL version 2.x searches for the configuration file in /usr/lib/
    sasl2/.

  * Cyrus SASL version 2.1.22 and newer additionally search in /etc/sasl2/.

  * With Postfix 2.5 and later you can explicitly configure the search path via
    the cyrus_sasl_config_path configuration parameter. Specify zero or more
    colon-separated directories. If set empty (the default value) the search
    path is the one compiled into the Cyrus SASL library.

  * Some Postfix distributions employ a non-empty default value for
    cyrus_sasl_config_path to look for the Cyrus SASL configuration file in /
    etc/postfix/sasl/, /var/lib/sasl2/ etc. See the output of postconf
    cyrus_sasl_config_path and/or the distribution-specific documentation to
    determine the expected location.

  * Some Debian-based Postfix distributions patch Postfix to hardcode a non-
    default search path, making it impossible to set an alternate search path
    via the "cyrus_sasl_config_path" parameter. This is likely to be the case
    when the distribution documents a Postfix-specific path (e.g. /etc/postfix/
    sasl/) that is different from the default value of "cyrus_sasl_config_path"
    (which then is likely to be empty).

    Note

    Cyrus SASL searches /usr/lib/sasl2/ first. If it finds the specified
    configuration file there, it will not examine other locations.

Postfix to Cyrus SASL communication

As the Postfix SMTP server is linked with the Cyrus SASL library libsasl,
communication between Postfix and Cyrus SASL takes place by calling functions
in the SASL library.

The SASL library may use an external password verification service, or an
internal plugin to connect to authentication backends and verify the SMTP
client's authentication data against the system password file or other
databases.

The following table shows typical combinations discussed in this document:

                                                                     
    |  authentication backend |password verification service / plugin|
    |                         |                                      |
    |/etc/shadow              |saslauthd                             |
    |                         |                                      |
    |PAM                      |saslauthd                             |
    |                         |                                      |
    |IMAP server              |saslauthd                             |
    |                         |                                      |
    |sasldb                   |sasldb                                |
    |                         |                                      |
    |MySQL, PostgreSQL, SQLite|sql                                   |
    |                         |                                      |
    |LDAP                     |ldapdb                                |
    |                         |                                      |

    Note

    Read the Cyrus SASL documentation for other backends it can use.

saslauthd - Cyrus SASL password verification service

Communication between the Postfix SMTP server (read: Cyrus SASL's libsasl) and
the saslauthd server takes place over a UNIX-domain socket.

saslauthd usually establishes the UNIX domain socket in /var/run/saslauthd/ and
waits for authentication requests. The Postfix SMTP server must have
read+execute permission to this directory or authentication attempts will fail.

    Important

    Some distributions require the user postfix to be member of a special group
    e.g. sasl, otherwise it will not be able to access the saslauthd socket
    directory.

The following example configures the Cyrus SASL library to contact saslauthd as
its password verification service:

    /etc/sasl2/smtpd.conf:
        pwcheck_method: saslauthd
        mech_list: PLAIN LOGIN

    Important

    Do not specify any other mechanisms in mech_list than PLAIN or LOGIN when
    using saslauthd! It can only handle these two mechanisms, and
    authentication will fail if clients are allowed to choose other mechanisms.

    Important

    Plaintext mechanisms (PLAIN, LOGIN) send credentials unencrypted. This
    information should be protected by an additional security layer such as a
    TLS-encrypted SMTP session (see: TLS_README).

Additionally the saslauthd server itself must be configured. It must be told
which authentication backend to turn to for password verification. The backend
is selected with a saslauthd command-line option and will be shown in the
following examples.

    Note

    Some distributions use a configuration file to provide saslauthd command
    line options to set e.g. the authentication backend. Typical locations are
    /etc/sysconfig/saslauthd or /etc/default/saslauthd.

Using saslauthd with /etc/shadow

Access to the /etc/shadow system password file requires root privileges. The
Postfix SMTP server (and in consequence libsasl linked to the server) runs with
the least privilege possible. Direct access to /etc/shadow would not be
possible without breaking the Postfix security architecture.

The saslauthd socket builds a safe bridge. Postfix, running as limited user
postfix, can access the UNIX-domain socket that saslauthd receives commands on;
saslauthd, running as privileged user root, has the privileges required to
access the shadow file.

The saslauthd server verifies passwords against the authentication backend /
etc/shadow if started like this:

    % saslauthd -a shadow

See section "Testing saslauthd authentication" for test instructions.

Using saslauthd with PAM

Cyrus SASL can use the PAM framework to authenticate credentials. saslauthd
uses the PAM framework when started like this:

    % saslauthd -a pam

    Note

    PAM configuration for the Postfix SMTP server is usually given in /etc/
    pam.d/smtp and is beyond the scope of this document.

See section "Testing saslauthd authentication" for test instructions.

Using saslauthd with an IMAP server

saslauthd can verify the SMTP client credentials by using them to log into an
IMAP server. If the login succeeds, SASL authentication also succeeds.
saslauthd contacts an IMAP server when started like this:

    % saslauthd -a rimap -O imap.example.com

    Note

    The option "-O imap.example.com" specifies the IMAP server saslauthd should
    contact when it verifies credentials.

    Important

    saslauthd sends IMAP login information unencrypted. Any IMAP session
    leaving the local host should be protected by an additional security layer
    such as an SSL tunnel.

See section "Testing saslauthd authentication" for test instructions.

Testing saslauthd authentication

Cyrus SASL provides the testsaslauthd utility to test saslauthd authentication.
The username and password are given as command line arguments. The example
shows the response when authentication is successful:

    % testsaslauthd -u username -p password
    0: OK "Success."

    Note

    Sometimes the testsaslauthd program is not distributed with a the Cyrus
    SASL main package. In that case, it may be distributed with -devel, -dev or
    -debug packages.

Specify an additional "-s smtp" if saslauthd was configured to contact the PAM
authentication framework, and specify an additional "-f /path/to/socketdir/mux"
if saslauthd establishes the UNIX-domain socket in a non-default location.

If authentication succeeds, proceed with the section "Enabling SASL
authentication and authorization in the Postfix SMTP server".

Cyrus SASL Plugins - auxiliary property plugins

Cyrus SASL uses a plugin infrastructure (called auxprop) to expand libsasl's
capabilities. Currently Cyrus SASL sources provide three authentication
plugins.

                                                                           
    |Plugin|Description                                                    |
    |      |                                                               |
    |sasldb|Accounts are stored stored in a Cyrus SASL Berkeley DB database|
    |      |                                                               |
    |sql   |Accounts are stored in a SQL database                          |
    |      |                                                               |
    |ldapdb|Accounts are stored stored in an LDAP database                 |
    |      |                                                               |

    Important

    These three plugins support shared-secret mechanisms i.e. CRAM-MD5, DIGEST-
    MD5 and NTLM. These mechanisms send credentials encrypted but their
    verification process requires the password to be available in plaintext.
    Consequently passwords cannot (!) be stored in encrypted form.

The sasldb plugin

The sasldb auxprop plugin authenticates SASL clients against credentials that
are stored in a Berkeley DB database. The database schema is specific to Cyrus
SASL. The database is usually located at /etc/sasldb2.

    Note

    The sasldb2 file contains passwords in plaintext, and should have
    read+write access only to user postfix or a group that postfix is member
    of.

The saslpasswd2 command-line utility creates and maintains the database:

    % saslpasswd2 -c -u example.com username
    Password:
    Again (for verification):

This command creates an account username@example.com.

    Important

    users must specify username@example.com as login name, not username.

Run the following command to reuse the Postfix mydomain parameter value as the
login domain:

    % saslpasswd2 -c -u `postconf -h mydomain` username
    Password:
    Again (for verification):

    Note

    Run saslpasswd2 without any options for further help on how to use the
    command.

The sasldblistusers2 command lists all existing users in the sasldb database:

    % sasldblistusers2
    username1@example.com: password1
    username2@example.com: password2

Configure libsasl to use sasldb with the following instructions:

    /etc/sasl2/smtpd.conf:
        pwcheck_method: auxprop
        auxprop_plugin: sasldb
        mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 NTLM

    Note

    In the above example adjust mech_list to the mechanisms that are applicable
    for your environment.

The sql plugin

The sql auxprop plugin is a generic SQL plugin. It provides access to
credentials stored in a MySQL, PostgreSQL or SQLite database. This plugin
requires that SASL client passwords are stored as plaintext.

    Tip

    If you must store encrypted passwords, you cannot use the sql auxprop
    plugin. Instead, see section "Using saslauthd with PAM", and configure PAM
    to look up the encrypted passwords with, for example, the pam_mysql module.
    You will not be able to use any of the methods that require access to
    plaintext passwords, such as the shared-secret methods CRAM-MD5 and DIGEST-
    MD5.

The following example configures libsasl to use the sql plugin and connects it
to a PostgreSQL server:

    /etc/sasl2/smtpd.conf:
        pwcheck_method: auxprop
        auxprop_plugin: sql
        mech_list: PLAIN LOGIN CRAM-MD5 DIGEST-MD5 NTLM
        sql_engine: pgsql
        sql_hostnames: 127.0.0.1, 192.0.2.1
        sql_user: username
        sql_passwd: secret
        sql_database: dbname
        sql_select: SELECT password FROM users WHERE user = '%u@%r'

    Note

    Set appropriate permissions if smtpd.conf contains a password. The file
    should be readable by the postfix user.

    Note

    In the above example, adjust mech_list to the mechanisms that are
    applicable for your environment.

The sql plugin has the following configuration options:

    sql_engine
        Specify mysql to connect to a MySQL server, pgsql for a PostgreSQL
        server or sqlite for an SQLite database

    sql_hostnames
        Specify one or more servers (hostname or hostname:port) separated by
        commas.

            Note

            With MySQL servers, specify localhost to connect over a UNIX-domain
            socket, and specify 127.0.0.1 to connect over a TCP socket.

    sql_user
        The login name to gain access to the database.

    sql_passwd
        The password to gain access to the database.

    sql_database
        The name of the database to connect to.

    sql_select
        The SELECT statement that should retrieve the plaintext password from a
        database table.

            Important

            Do not enclose the statement in quotes! Use single quotes to escape
            macros!

The sql plugin provides macros to build sql_select statements. They will be
replaced with arguments sent from the client. The following macros are
available:

    %u
        The name of the user whose properties are being selected.

    %p
        The name of the property being selected. While this could technically
        be anything, Cyrus SASL will try userPassword and cmusaslsecretMECHNAME
        (where MECHNAME is the name of a SASL mechanism).

    %r
        The name of the realm to which the user belongs. This could be the
        KERBEROS realm, the fully-qualified domain name of the computer the
        SASL application is running on, or the domain after the "@" in a
        username.

The ldapdb plugin

The ldapdb auxprop plugin provides access to credentials stored in an LDAP
server. This plugin requires that SASL client passwords are stored as
plaintext.

    Tip

    If you must store encrypted passwords, you cannot use the ldapdb auxprop
    plugin. Instead, you can use "saslauthd -a ldap" to query the LDAP database
    directly, with appropriate configuration in saslauthd.conf, as described
    here. You will not be able to use any of the methods that require access to
    plaintext passwords, such as the shared-secret methods CRAM-MD5 and DIGEST-
    MD5.

The ldapdb plugin implements proxy authorization. This means that the ldapdb
plugin uses its own username and password to authenticate with the LDAP server,
before it asks the LDAP server for the remote SMTP client's password. The LDAP
server then decides if the ldapdb plugin is authorized to read the remote SMTP
client's password.

In a nutshell: Configuring ldapdb means authentication and authorization must
be configured twice - once in the Postfix SMTP server to authenticate and
authorize the remote SMTP client, and once in the LDAP server to authenticate
and authorize the ldapdb plugin.

This example configures libsasl to use the ldapdb plugin and the plugin to
connect to an LDAP server:

    /etc/sasl2/smtpd.conf:
        pwcheck_method: auxprop
        auxprop_plugin: ldapdb
        mech_list: PLAIN LOGIN NTLM CRAM-MD5 DIGEST-MD5
        ldapdb_uri: ldap://localhost
        ldapdb_id: proxyuser
        ldapdb_pw: password
        ldapdb_mech: DIGEST-MD5

    Important

    Set appropriate permissions if smtpd.conf contains a password. The file
    should be readable by the postfix user.

    Note

    The shared-secret mechanisms (CRAM-MD5, etc.) require that the SASL client
    passwords are stored as plaintext.

The following is a summary of applicable smtpd.conf file entries:

    auxprop_plugin
        Specify ldapdb to enable the plugin.

    ldapdb_uri
        Specify either ldapi:// to connect over a UNIX-domain socket, ldap:/
        / for an unencrypted TCP connection, or ldaps:// for an encrypted TCP
        connection.

    ldapdb_id
        The login name to authenticate the ldapdb plugin to the LDAP server
        (proxy authorization).

    ldapdb_pw
        The password (in plaintext) to authenticate the ldapdb plugin to the
        LDAP server (proxy authorization).

    ldapdb_mech
        The mechanism to authenticate the ldapdb plugin to the LDAP server.

            Note

            Specify a mechanism here that is supported by the LDAP server.

    ldapdb_rc (optional)
        The path to a file containing individual configuration options for the
        ldapdb LDAP client (libldap). This allows to specify a TLS client
        certificate which in turn can be used to use the SASL EXTERNAL
        mechanism.

            Note

            This mechanism supports authentication over an encrypted transport
            layer, which is recommended if the plugin must connect to an
            OpenLDAP server on a remote machine.

    ldapdb_starttls (optional)
        The TLS policy for connecting to the LDAP server. Specify either try or
        demand. If the option is try the plugin will attempt to establish a
        TLS-encrypted connection with the LDAP server, and will fallback to an
        unencrypted connection if TLS fails. If the policy is demand and a TLS-
        encrypted connection cannot be established, the connection fails
        immediately.

When the ldapdb plugin connects to the OpenLDAP server and successfully
authenticates, the OpenLDAP server decides if the plugin user is authorized to
read SASL account information.

The following configuration gives an example of authorization configuration in
the OpenLDAP slapd server:

    /etc/openldap/slapd.conf:
        authz-regexp
        uid=(.*),cn=.*,cn=auth
        ldap:///dc=example,dc=com??sub?cn=$1
        authz-policy to

Here, the authz-regexp option serves for authentication of the ldapdb user. It
maps its login name to a DN in the LDAP directory tree where slapd can look up
the SASL account information. The authz-policy options defines the
authentication policy. In this case it grants authentication privileges "to"
the ldapdb plugin.

The last configuration step is to tell the OpenLDAP slapd server where ldapdb
may search for usernames matching the one given by the mail client. The example
below adds an additional attribute ldapdb user object (here: authzTo because
the authz-policy is "to") and configures the scope where the login name
"proxyuser" may search:

    dn: cn=proxyuser,dc=example,dc=com
    changetype: modify
    add: authzTo
    authzTo: dn.regex:uniqueIdentifier=(.*),ou=people,dc=example,dc=com

Use the ldapmodify or ldapadd command to add the above attribute.

    Note

    Read the chapter "Using SASL" in the OpenLDAP Admin Guide for more detailed
    instructions to set up SASL authentication in OpenLDAP.

Enabling SASL authentication and authorization in the Postfix SMTP server

By default the Postfix SMTP server uses the Cyrus SASL implementation. If the
Dovecot SASL implementation should be used, specify an smtpd_sasl_type value of
dovecot instead of cyrus:

    /etc/postfix/main.cf:
        smtpd_sasl_type = dovecot

Additionally specify how Postfix SMTP server can find the Dovecot
authentication server. This depends on the settings that you have selected in
the section "Postfix to Dovecot SASL communication".

  * If you configured Dovecot for UNIX-domain socket communication, configure
    Postfix as follows:

    /etc/postfix/main.cf:
        smtpd_sasl_path = private/auth

    Note
    This example uses a pathname relative to the Postfix queue directory, so
    that it will work whether or not the Postfix SMTP server runs chrooted.

  * If you configured Dovecot for TCP socket communication, configure Postfix
    as follows. If Dovecot runs on a different machine, replace 127.0.0.1 by
    that machine's IP address.

    /etc/postfix/main.cf:
        smtpd_sasl_path = inet:127.0.0.1:12345

    Note
    If you specify a remote IP address, information will be sent as plaintext
    over the network.

Enabling SASL authentication in the Postfix SMTP server

Regardless of the SASL implementation type, enabling SMTP authentication in the
Postfix SMTP server always requires setting the smtpd_sasl_auth_enable option:

    /etc/postfix/main.cf:
        smtpd_sasl_auth_enable = yes

After a "postfix reload", SMTP clients will see the additional capability AUTH
in an SMTP session, followed by a list of authentication mechanisms the server
supports:

    % telnet server.example.com 25
    ...
    220 server.example.com ESMTP Postfix
    EHLO client.example.com
    250-server.example.com
    250-PIPELINING
    250-SIZE 10240000
    250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
    ...

However not all clients recognize the AUTH capability as defined by the SASL
authentication RFC. Some historical implementations expect the server to send
an "=" as separator between the AUTH verb and the list of mechanisms that
follows it.

The broken_sasl_auth_clients configuration option lets Postfix repeat the AUTH
statement in a form that these broken clients understand:

    /etc/postfix/main.cf:
        broken_sasl_auth_clients = yes

    Note

    Enable this option for Outlook up to and including version 2003 and Outlook
    Express up to version 6. This option does not hurt other clients.

After "postfix reload", the Postfix SMTP server will propagate the AUTH
capability twice - once for compliant and once for broken clients:

    % telnet server.example.com 25
    ...
    220 server.example.com ESMTP Postfix
    EHLO client.example.com
    250-server.example.com
    250-PIPELINING
    250-SIZE 10240000
    250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
    250-AUTH=DIGEST-MD5 PLAIN CRAM-MD5
    ...

Postfix SMTP Server policy - SASL mechanism properties

The Postfix SMTP server supports policies that limit the SASL mechanisms that
it makes available to clients, based on the properties of those mechanisms. The
next two sections give examples of how these policies are used.

                                                                              
    |Property       |Description                                              |
    |               |                                                         |
    |noanonymous    |Don't use mechanisms that permit anonymous               |
    |               |authentication.                                          |
    |               |                                                         |
    |noplaintext    |Don't use mechanisms that transmit unencrypted username  |
    |               |and password information.                                |
    |               |                                                         |
    |nodictionary   |Don't use mechanisms that are vulnerable to dictionary   |
    |               |attacks.                                                 |
    |               |                                                         |
    |forward_secrecy|Require forward secrecy between sessions (breaking one   |
    |               |session does not break earlier sessions).                |
    |               |                                                         |
    |mutual_auth    |Use only mechanisms that authenticate both the client and|
    |               |the server to each other.                                |
    |               |                                                         |

Unencrypted SMTP session

The default policy is to allow any mechanism in the Postfix SMTP server except
for those based on anonymous authentication:

    /etc/postfix/main.cf:
        # Specify a list of properties separated by comma or whitespace
        smtpd_sasl_security_options = noanonymous

    Important

    Always set at least the noanonymous option. Otherwise, the Postfix SMTP
    server can give strangers the same authorization as a properly-
    authenticated client.

Encrypted SMTP session (TLS)

A separate parameter controls Postfix SASL mechanism policy during a TLS-
encrypted SMTP session. The default is to copy the settings from the
unencrypted session:

    /etc/postfix/main.cf:
        smtpd_sasl_tls_security_options = $smtpd_sasl_security_options

A more sophisticated policy allows plaintext mechanisms, but only over a TLS-
encrypted connection:

    /etc/postfix/main.cf:
        smtpd_sasl_security_options = noanonymous, noplaintext
        smtpd_sasl_tls_security_options = noanonymous

To offer SASL authentication only after a TLS-encrypted session has been
established specify this:

    /etc/postfix/main.cf:
        smtpd_tls_auth_only = yes

Enabling SASL authorization in the Postfix SMTP server

After the client has authenticated with SASL, the Postfix SMTP server decides
what the remote SMTP client will be authorized for. Examples of possible SMTP
clients authorizations are:

  * Send a message to a remote recipient.

  * Use a specific envelope sender in the MAIL FROM command.

These permissions are not enabled by default.

Mail relay authorization

With permit_sasl_authenticated the Postfix SMTP server can allow SASL-
authenticated SMTP clients to send mail to remote destinations. Examples:

    # With Postfix 2.10 and later, the mail relay policy is
    # preferably specified under smtpd_relay_restrictions.
    /etc/postfix/main.cf:
        smtpd_relay_restrictions =
            permit_mynetworks
            permit_sasl_authenticated
            reject_unauth_destination

    # Older configurations combine relay control and spam control under
    # smtpd_recipient_restrictions. To use this example with Postfix >=
    # 2.10 specify "smtpd_relay_restrictions=".
    /etc/postfix/main.cf:
        smtpd_recipient_restrictions =
            permit_mynetworks
            permit_sasl_authenticated
            reject_unauth_destination
            ...other rules...

Envelope sender address authorization

By default an SMTP client may specify any envelope sender address in the MAIL
FROM command. That is because the Postfix SMTP server only knows the remote
SMTP client hostname and IP address, but not the user who controls the remote
SMTP client.

This changes the moment an SMTP client uses SASL authentication. Now, the
Postfix SMTP server knows who the sender is. Given a table of envelope sender
addresses and SASL login names, the Postfix SMTP server can decide if the SASL
authenticated client is allowed to use a particular envelope sender address:

    /etc/postfix/main.cf:
        smtpd_sender_login_maps = hash:/etc/postfix/controlled_envelope_senders

        smtpd_recipient_restrictions =
            ...
            reject_sender_login_mismatch
            permit_sasl_authenticated
            ...

The controlled_envelope_senders table specifies the binding between a sender
envelope address and the SASL login names that own that address:

    /etc/postfix/controlled_envelope_senders
        # envelope sender           owners (SASL login names)
        john@example.com            john@example.com
        helpdesk@example.com        john@example.com, mary@example.com
        postmaster                  admin@example.com
        @example.net                barney, fred, john@example.com,
    mary@example.com

With this, the reject_sender_login_mismatch restriction above will reject the
sender address in the MAIL FROM command if smtpd_sender_login_maps does not
specify the SMTP client's login name as an owner of that address.

See also reject_authenticated_sender_login_mismatch,
reject_known_sender_login_mismatch, and
reject_unauthenticated_sender_login_mismatch for additional control over the
SASL login name and the envelope sender.

Additional SMTP Server SASL options

Postfix provides a wide range of SASL authentication configuration options. The
next section lists a few that are discussed frequently. See postconf(5) for a
complete list.

Per-account access control

Postfix can implement policies that depend on the SASL login name (Postfix 2.11
and later). Typically this is used to HOLD or REJECT mail from accounts whose
credentials have been compromised.

    /etc/postfix/main.cf:
        smtpd_recipient_restrictions =
            permit_mynetworks
            check_sasl_access hash:/etc/postfix/sasl_access
            permit_sasl_authenticated
            ...

    /etc/postfix/sasl_access:
         # Use this when smtpd_sasl_local_domain is empty.
         username   HOLD
         # Use this when smtpd_sasl_local_domain=example.com.
         username@example.com HOLD

Default authentication domain

Postfix can append a domain name (or any other string) to a SASL login name
that does not have a domain part, e.g. "john" instead of "john@example.com":

    /etc/postfix/main.cf:
        smtpd_sasl_local_domain = example.com

This is useful as a default setting and safety net for misconfigured clients,
or during a migration to an authentication method/backend that requires an
authentication REALM or domain name, before all SMTP clients are configured to
send such information.

Hiding SASL authentication from clients or networks

Some clients insist on using SASL authentication if it is offered, even when
they are not configured to send credentials - and therefore they will always
fail and disconnect.

Postfix can hide the AUTH capability from these clients/networks:

    /etc/postfix/main.cf:
        smtpd_sasl_exceptions_networks = !192.0.2.171/32, 192.0.2.0/24

Adding the SASL login name to mail headers

To report SASL login names in Received: message headers (Postfix version 2.3
and later):

    /etc/postfix/main.cf:
        smtpd_sasl_authenticated_header = yes

    Note

    The SASL login names will be shared with the entire world.

Testing SASL authentication in the Postfix SMTP Server

To test the server side, connect (for example, with telnet) to the Postfix SMTP
server port and you should be able to have a conversation as shown below.
Information sent by the client (that is, you) is shown in bold font.

    % telnet server.example.com 25
    ...
    220 server.example.com ESMTP Postfix
    EHLO client.example.com
    250-server.example.com
    250-PIPELINING
    250-SIZE 10240000
    250-ETRN
    250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
    250 8BITMIME
    AUTH PLAIN AHRlc3QAdGVzdHBhc3M=
    235 Authentication successful

To test this over a connection that is encrypted with TLS, use openssl s_client
instead of telnet:

    % openssl s_client -connect server.example.com:25 -starttls smtp
    ...
    220 server.example.com ESMTP Postfix
    EHLO client.example.com
    ...see above example for more...

Instead of AHRlc3QAdGVzdHBhc3M=, specify the base64-encoded form of
\0username\0password (the \0 is a null byte). The example above is for a user
named `test' with password `testpass'.

    Caution

    When posting logs of the SASL negotiations to public lists, please keep in
    mind that username/password information is trivial to recover from the
    base64-encoded form.

You can use one of the following commands to generate base64 encoded
authentication information:

  * Using a recent version of the bash shell:

        % echo -ne '\000username\000password' | openssl base64

    Some other shells support similar syntax.

  * Using the printf command:

        % printf '\0%s\0%s' 'username' 'password' | openssl base64
        % printf '\0%s\0%s' 'username' 'password' | mmencode

    The mmencode command is part of the metamail software.

  * Using Perl MIME::Base64 (from http://www.cpan.org/):

        % perl -MMIME::Base64 -e \
            'print encode_base64("\0username\0password");'

    If the username or password contain "@", you must specify "\@".

  * Using the gen-auth script:

        % gen-auth plain
        username: username
        password:

    The gen-auth Perl script was written by John Jetmore and can be found at
    http://jetmore.org/john/code/gen-auth.

Configuring SASL authentication in the Postfix SMTP/LMTP client

The Postfix SMTP and the LMTP client can authenticate with a remote SMTP server
via the Cyrus SASL framework. At this time, the Dovecot SASL implementation
does not provide client functionality.

    Note

    The examples in this section discuss only the SMTP client. Replace smtp_
    with lmtp_ to get the corresponding LMTP client configuration.

You can read more about the following topics:

  * Enabling SASL authentication in the Postfix SMTP/LMTP client
  * Configuring sender-dependent SASL authentication
  * Postfix SMTP/LMTP client policy - SASL mechanism properties
  * Postfix SMTP/LMTP client policy - SASL mechanism names

Enabling SASL authentication in the Postfix SMTP/LMTP client

This section shows a typical scenario where the Postfix SMTP client sends all
messages via a mail gateway server that requires SASL authentication.

    Trouble solving tips:

      * If your SASL logins fail with "SASL authentication failure: No worthy
        mechs found" in the mail logfile, then see the section "Postfix SMTP/
        LMTP client policy - SASL mechanism properties".

      * For a solution to a more obscure class of SASL authentication failures,
        see "Postfix SMTP/LMTP client policy - SASL mechanism names".

To make the example more readable we introduce it in two parts. The first part
takes care of the basic configuration, while the second part sets up the
username/password information.

    /etc/postfix/main.cf:
        smtp_sasl_auth_enable = yes
        smtp_tls_security_level = encrypt
        smtp_sasl_tls_security_options = noanonymous
        relayhost = [mail.isp.example]
        # Alternative form:
        # relayhost = [mail.isp.example]:submission
        smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

  * The smtp_sasl_auth_enable setting enables client-side authentication. We
    will configure the client's username and password information in the second
    part of the example.

  * The smtp_tls_security_level setting ensures that the connection to the
    remote smtp server will be encrypted, and smtp_sasl_tls_security_options
    removes the prohibition on plaintext passwords.

  * The relayhost setting forces the Postfix SMTP to send all remote messages
    to the specified mail server instead of trying to deliver them directly to
    their destination.

  * In the relayhost setting, the "[" and "]" prevent the Postfix SMTP client
    from looking up MX (mail exchanger) records for the enclosed name.

  * The relayhost destination may also specify a non-default TCP port. For
    example, the alternative form [mail.isp.example]:submission tells Postfix
    to connect to TCP network port 587, which is reserved for email client
    applications.

  * The Postfix SMTP client is compatible with SMTP servers that use the non-
    standard "AUTH=method...." syntax in response to the EHLO command; this
    requires no additional Postfix client configuration.

  * With the setting "smtp_tls_wrappermode = yes", the Postfix SMTP client
    supports the "wrappermode" protocol, which uses TCP port 465 on the SMTP
    server (Postfix 3.0 and later).

  * With the smtp_sasl_password_maps parameter, we configure the Postfix SMTP
    client to send username and password information to the mail gateway
    server. As discussed in the next section, the Postfix SMTP client supports
    multiple ISP accounts. For this reason the username and password are stored
    in a table that contains one username/password combination for each mail
    gateway server.

    /etc/postfix/sasl_passwd:
        # destination                   credentials
        [mail.isp.example]              username:password
        # Alternative form:
        # [mail.isp.example]:submission username:password

    Important

    Keep the SASL client password file in /etc/postfix, and make the file
    read+write only for root to protect the username/password combinations
    against other users. The Postfix SMTP client will still be able to read the
    SASL client passwords. It opens the file as user root before it drops
    privileges, and before entering an optional chroot jail.

  * Use the postmap command whenever you change the /etc/postfix/sasl_passwd
    file.

  * If you specify the "[" and "]" in the relayhost destination, then you must
    use the same form in the smtp_sasl_password_maps file.

  * If you specify a non-default TCP Port (such as ":submission" or ":587") in
    the relayhost destination, then you must use the same form in the
    smtp_sasl_password_maps file.

Configuring Sender-Dependent SASL authentication

Postfix supports different ISP accounts for different sender addresses (version
2.3 and later). This can be useful when one person uses the same machine for
work and for personal use, or when people with different ISP accounts share the
same Postfix server.

To make this possible, Postfix supports per-sender SASL passwords and per-
sender relay hosts. In the example below, the Postfix SMTP client will search
the SASL password file by sender address before it searches that same file by
destination. Likewise, the Postfix trivial-rewrite(8) daemon will search the
per-sender relayhost file, and use the default relayhost setting only as a
final resort.

    /etc/postfix/main.cf:
        smtp_sender_dependent_authentication = yes
        sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
        smtp_sasl_auth_enable = yes
        smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
        relayhost = [mail.isp.example]
        # Alternative form:
        # relayhost = [mail.isp.example]:submission

    /etc/postfix/sasl_passwd:
        # Per-sender authentication; see also /etc/postfix/sender_relay.
        user1@example.com               username1:password1
        user2@example.net               username2:password2
        # Login information for the default relayhost.
        [mail.isp.example]              username:password
        # Alternative form:
        # [mail.isp.example]:submission username:password

    /etc/postfix/sender_relay:
        # Per-sender provider; see also /etc/postfix/sasl_passwd.
        user1@example.com               [mail.example.com]:submission
        user2@example.net               [mail.example.net]

  * If you are creative, then you can try to combine the two tables into one
    single MySQL database, and configure different Postfix queries to extract
    the appropriate information.

  * Specify dbm instead of hash if your system uses dbm files instead of db
    files. To find out what lookup tables Postfix supports, use the command
    "postconf -m".

  * Execute the command "postmap /etc/postfix/sasl_passwd" whenever you change
    the sasl_passwd table.

  * Execute the command "postmap /etc/postfix/sender_relay" whenever you change
    the sender_relay table.

Postfix SMTP/LMTP client policy - SASL mechanism properties

Just like the Postfix SMTP server, the SMTP client has a policy that determines
which SASL mechanisms are acceptable, based on their properties. The next two
sections give examples of how these policies are used.

                                                                             
    |Property    |Description                                                |
    |            |                                                           |
    |noanonymous |Don't use mechanisms that permit anonymous authentication. |
    |            |                                                           |
    |noplaintext |Don't use mechanisms that transmit unencrypted username and|
    |            |password information.                                      |
    |            |                                                           |
    |nodictionary|Don't use mechanisms that are vulnerable to dictionary     |
    |            |attacks.                                                   |
    |            |                                                           |
    |mutual_auth |Use only mechanisms that authenticate both the client and  |
    |            |the server to each other.                                  |
    |            |                                                           |

Unencrypted SMTP session

The default policy is stricter than that of the Postfix SMTP server - plaintext
mechanisms are not allowed (nor is any anonymous mechanism):

    /etc/postfix/main.cf:
        smtp_sasl_security_options = noplaintext, noanonymous

This default policy, which allows no plaintext passwords, leads to
authentication failures if the remote server only offers plaintext
authentication mechanisms (the SMTP server announces "AUTH PLAIN LOGIN"). In
such cases the SMTP client will log the following error message:

    SASL authentication failure: No worthy mechs found

    Note

    This same error message will also be logged when the libplain.so or
    liblogin.so modules are not installed in the /usr/lib/sasl2 directory.

The insecure approach is to lower the security standards and permit plaintext
authentication mechanisms:

    /etc/postfix/main.cf:
        smtp_sasl_security_options = noanonymous

The more secure approach is to protect the plaintext username and password with
TLS session encryption. To find out if the remote SMTP server supports TLS,
connect to the server and see if it announces STARTTLS support as shown in the
example. Information sent by the client (that is, you) is shown in bold font.

    % telnet server.example.com 25
    ...
    220 server.example.com ESMTP Postfix
    EHLO client.example.com
    250-server.example.com
    250-PIPELINING
    250-SIZE 10240000
    250-STARTTLS
    ...

Instead of port 25 (smtp), specify port 587 (submission) where appropriate.

Encrypted SMTP session (TLS)

To turn on TLS in the Postfix SMTP client, see TLS_README for configuration
details.

The smtp_sasl_tls_security_options parameter controls Postfix SASL mechanism
policy during a TLS-encrypted SMTP session. The default is to copy the settings
from the unencrypted session:

    /etc/postfix/main.cf:
        smtp_sasl_tls_security_options = $smtp_sasl_security_options

A more sophisticated policy allows plaintext mechanisms, but only over a TLS-
encrypted connection:

    /etc/postfix/main.cf:
        smtp_sasl_security_options = noanonymous, noplaintext
        smtp_sasl_tls_security_options = noanonymous

Postfix SMTP/LMTP client policy - SASL mechanism names

Given the SASL security options of the previous section, the Cyrus SASL library
will choose the most secure authentication mechanism that both the SMTP client
and server implement. Unfortunately, that authentication mechanism may fail
because the client or server is not configured to use that mechanism.

To prevent this, the Postfix SMTP client can filter the names of the
authentication mechanisms from the remote SMTP server. Used correctly, the
filter hides unwanted mechanisms from the Cyrus SASL library, forcing the
library to choose from the mechanisms the Postfix SMTP client filter passes
through.

The following example filters out everything but the mechanisms PLAIN and
LOGIN:

    /etc/postfix/main.cf:
        smtp_sasl_mechanism_filter = plain, login

    Note

    If the remote server does not offer any of the mechanisms on the filter
    list, authentication will fail.

We close this section with an example that passes every mechanism except for
GSSAPI and LOGIN:

    /etc/postfix/main.cf:
        smtp_sasl_mechanism_filter = !gssapi, !login, static:all

Building Postfix with SASL support

As mentioned elsewhere, Postfix supports two SASL implementations: Cyrus SASL
(SMTP client and server) and Dovecot SASL (SMTP server only). Both
implementations can be built into Postfix simultaneously.

  * Building Dovecot SASL support
  * Building Cyrus SASL support

Building Dovecot SASL support

These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

Support for the Dovecot version 1 SASL protocol is available in Postfix 2.3 and
later. At the time of writing, only server-side SASL support is available, so
you can't use it to authenticate the Postfix SMTP client to your network
provider's server.

Dovecot uses its own daemon process for authentication. This keeps the Postfix
build process simple, because there is no need to link extra libraries into
Postfix.

To generate the necessary Makefiles, execute the following in the Postfix top-
level directory:

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH \
        -DDEF_SERVER_SASL_TYPE=\\\"dovecot\\\""

After this, proceed with "make" as described in the INSTALL document.

Note

  * The -DDEF_SERVER_SASL_TYPE=\\\"dovecot\\\" is not necessary; it just makes
    Postfix configuration a little more convenient because you don't have to
    specify the SASL plug-in type in the Postfix main.cf file (but this may
    cause surprises when you switch to a later Postfix version that is built
    with the default SASL type of cyrus).

  * If you also want support for LDAP or TLS (or for Cyrus SASL), you need to
    merge their CCARGS and AUXLIBS options into the above command line; see the
    LDAP_README and TLS_README for details.

        % make tidy # if you have left-over files from a previous build
        % make makefiles CCARGS="-DUSE_SASL_AUTH \
            -DDEF_SERVER_SASL_TYPE=\\\"dovecot\\\" \
            ...CCARGS options for LDAP or TLS etc...." \
           AUXLIBS="...AUXLIBS options for LDAP or TLS etc...."

Building Cyrus SASL support

Building the Cyrus SASL library

Postfix works with cyrus-sasl-1.5.x or cyrus-sasl-2.1.x, which are available
from https://github.com/cyrusimap/cyrus-sasl/releases.

    Important

    If you install the Cyrus SASL libraries as per the default, you will have
    to create a symlink /usr/lib/sasl -> /usr/local/lib/sasl for version 1.5.x
    or /usr/lib/sasl2 -> /usr/local/lib/sasl2 for version 2.1.x.

Reportedly, Microsoft Outlook (Express) requires the non-standard LOGIN and/or
NTLM authentication mechanism. To enable these authentication mechanisms, build
the Cyrus SASL libraries with:

    % ./configure --enable-login --enable-ntlm

Building Postfix with Cyrus SASL support

These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

The following assumes that the Cyrus SASL include files are in /usr/local/
include, and that the Cyrus SASL libraries are in /usr/local/lib.

On some systems this generates the necessary Makefile definitions:

Cyrus SASL version 2.1.x

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
        -I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib -lsasl2"

    If your Cyrus SASL shared library is in a directory that the RUN-TIME
    linker does not know about, add a "-Wl,-R,/path/to/directory" option after
    "-lsasl2".

Cyrus SASL version 1.5.x

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
        -I/usr/local/include" AUXLIBS="-L/usr/local/lib -lsasl"

On Solaris 2.x you need to specify run-time link information, otherwise the
ld.so run-time linker will not find the SASL shared library:

Cyrus SASL version 2.1.x

    % make tidy # remove left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
        -I/usr/local/include/sasl" AUXLIBS="-L/usr/local/lib \
        -R/usr/local/lib -lsasl2"

Cyrus SASL version 1.5.x

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_SASL_AUTH -DUSE_CYRUS_SASL \
        -I/usr/local/include" AUXLIBS="-L/usr/local/lib \
        -R/usr/local/lib -lsasl"

Using Cyrus SASL version 1.5.x

Postfix supports Cyrus SASL version 1.x, but you shouldn't use it unless you
are forced to. The makers of Cyrus SASL write:

    This library is being deprecated and applications should transition to
    using the SASLv2 library (source: Project Cyrus: Downloads).

If you still need to set it up, here's a quick rundown:

Read the regular section on SMTP server configurations for the Cyrus SASL
framework. The differences are:

  * Cyrus SASL version 1.5.x searches for configuration (smtpd.conf) in /usr/
    lib/sasl/ only. You must place the configuration in that directory. Some
    systems may have modified Cyrus SASL and put the files into e.g. /var/lib/
    sasl/.

  * Use the saslpasswd command instead of saslpasswd2 to create users in
    sasldb.

  * Use the sasldblistusers command instead of sasldblistusers2 to find users
    in sasldb.

  * In the smtpd.conf file you can't use mech_list to limit the range of
    mechanisms offered. Instead, remove their libraries from /usr/lib/sasl/
    (and remember remove those files again when a system update re-installs new
    versions).

Credits

  * Postfix SASL support was originally implemented by Till Franke of SuSE
    Rhein/Main AG.
  * Wietse trimmed down the code to only the bare necessities.
  * Support for Cyrus SASL version 2 was contributed by Jason Hoos.
  * Liviu Daia added smtpd_sasl_application_name, separated
    reject_sender_login_mismatch into
    reject_authenticated_sender_login_mismatch and
    reject_unauthenticated_sender_login_mismatch, and revised the docs.
  * Wietse made another iteration through the code to add plug-in support for
    multiple SASL implementations, and for reasons that have been lost, also
    changed smtpd_sasl_application_name into smtpd_sasl_path.
  * The Dovecot SMTP server-only plug-in was originally implemented by Timo
    Sirainen of Procontrol, Finland.
  * Patrick Ben Koetter revised this document for Postfix 2.4 and made much
    needed updates.
  * Patrick Ben Koetter revised this document again for Postfix 2.7 and made
    much needed updates.

