From b5896ba9f6047e7031e2bdee0622d543e11a6734 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Mon, 6 May 2024 03:46:30 +0200 Subject: Adding upstream version 3.4.23. Signed-off-by: Daniel Baumann --- html/SASL_README.html | 2258 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 2258 insertions(+) create mode 100644 html/SASL_README.html (limited to 'html/SASL_README.html') diff --git a/html/SASL_README.html b/html/SASL_README.html new file mode 100644 index 0000000..4b12902 --- /dev/null +++ b/html/SASL_README.html @@ -0,0 +1,2258 @@ + + + + +Postfix SASL Howto + + + + + + + +

Postfix SASL Howto

+ +
+ +

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

+ +

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

+ + + +

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?

+ +

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:

+ + + +
+ +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 backendpassword verification service / plugin
/etc/shadowsaslauthd
PAMsaslauthd
IMAP serversaslauthd
sasldbsasldb
MySQL, PostgreSQL, SQLitesql
LDAPldapdb
+ +
+ +
+ +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:// for 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".

+ + + +

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:

+ + + +

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:

+ + + +

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

+ +

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: + + + +
+ +

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
+
+
+ + + +
+
+/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.

+ +
+ + + +

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]
+
+
+ + + +

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

+ +

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 + + + +

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 ftp://ftp.andrew.cmu.edu/pub/cyrus-mail/.

+ +
+ +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"
+
+ +
+ +
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:

+ + + +

Credits

+ + + + + + + -- cgit v1.2.3