PPoossttffiixx SSAASSLL HHoowwttoo
-------------------------------------------------------------------------------
WWaarrnniinngg
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.
HHooww PPoossttffiixx uusseess SSAASSLL aauutthheennttiiccaattiioonn
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.
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
CCoonnffiigguurriinngg SSAASSLL aauutthheennttiiccaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP sseerrvveerr
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.
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
WWhhiicchh SSAASSLL IImmpplleemmeennttaattiioonnss aarree ssuuppppoorrtteedd??
Currently the Postfix SMTP server supports the Cyrus SASL and Dovecot SASL
implementations.
NNoottee
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:
% ppoossttccoonnff --aa (SASL support in the SMTP server)
% ppoossttccoonnff --AA (SASL support in the SMTP+LMTP client)
These commands are available only with Postfix version 2.3 and later.
CCoonnffiigguurriinngg DDoovveeccoott SSAASSLL
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.
PPoossttffiixx ttoo DDoovveeccoott SSAASSLL ccoommmmuunniiccaattiioonn
Communication between the Postfix SMTP server and Dovecot SASL happens over a
UNIX-domain socket or over a TCP socket. Dovecot 1 supports UNIX-domain socket
communication only.
UUNNIIXX--ddoommaaiinn ssoocckkeett ccoommmmuunniiccaattiioonn
The socket pathname and the list of mechanisms offered to Postfix need to be
specified on the Dovecot server side in dovecot.conf.
The following example assumes that the Postfix queue is under /var/spool/
postfix/.
1 /etc/dovecot.conf:
2 auth default {
3 mechanisms = plain login
4 passdb pam {
5 }
6 userdb passwd {
7 }
8 socket listen {
9 client {
10 path = /var/spool/postfix/private/auth
11 mode = 0660
12 user = postfix
13 group = postfix
14 }
15 }
16 }
Line 3 provides plain and login as mechanisms for the Postfix SMTP server, line
10 places the Dovecot SASL socket in /var/spool/postfix/private/auth, and lines
11-13 limit read+write permissions to user and group postfix only.
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.
TTCCPP ssoocckkeett ccoommmmuunniiccaattiioonn
The TCP port and the list of mechanisms offered to Postfix need to be specified
on the Dovecot server side in 10-auth.conf and 10-master.conf.
The following examples assume that Postfix should communicate with Dovecot on
TCP port 12345.
1 /etc/dovecot/conf.d/10-auth.conf:
2 auth_mechanisms = plain login
Line 2 provides plain and login as mechanisms for the Postfix SMTP server.
1 /etc/dovecot/conf.d/10-master.conf:
2 service auth {
3 unix_listener auth-userdb {
4 }
5 inet_listener {
6 port = 12345
7 }
8 }
Line 5 creates a new TCP socket and line 6 specifies port 12345 where Dovecot
SASL should wait for Postfix authentication requests.
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.
CCoonnffiigguurriinngg CCyyrruuss SSAASSLL
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.
CCyyrruuss SSAASSLL ccoonnffiigguurraattiioonn ffiillee nnaammee
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
CCyyrruuss SSAASSLL ccoonnffiigguurraattiioonn ffiillee llooccaattiioonn
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/.
* Some Postfix distributions are modified and look for the Cyrus SASL
configuration file in /etc/postfix/sasl/, /var/lib/sasl2/ etc. See the
distribution-specific documentation to determine the expected location.
NNoottee
Cyrus SASL searches /usr/lib/sasl2/ first. If it finds the specified
configuration file there, it will not examine other locations.
PPoossttffiixx ttoo CCyyrruuss SSAASSLL ccoommmmuunniiccaattiioonn
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:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
| aauutthheennttiiccaattiioonn bbaacckkeenndd |ppaasssswwoorrdd vveerriiffiiccaattiioonn sseerrvviiccee // pplluuggiinn|
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|/etc/shadow |saslauthd |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|PAM |saslauthd |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|IMAP server |saslauthd |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|sasldb |sasldb |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|MySQL, PostgreSQL, SQLite|sql |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|LDAP |ldapdb |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
NNoottee
Read the Cyrus SASL documentation for other backends it can use.
ssaassllaauutthhdd -- CCyyrruuss SSAASSLL ppaasssswwoorrdd vveerriiffiiccaattiioonn sseerrvviiccee
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.
IImmppoorrttaanntt
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
IImmppoorrttaanntt
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.
IImmppoorrttaanntt
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.
NNoottee
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.
UUssiinngg ssaassllaauutthhdd wwiitthh //eettcc//sshhaaddooww
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:
% ssaassllaauutthhdd --aa sshhaaddooww
See section "Testing saslauthd authentication" for test instructions.
UUssiinngg ssaassllaauutthhdd wwiitthh PPAAMM
Cyrus SASL can use the PAM framework to authenticate credentials. saslauthd
uses the PAM framework when started like this:
% ssaassllaauutthhdd --aa ppaamm
NNoottee
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.
UUssiinngg ssaassllaauutthhdd wwiitthh aann IIMMAAPP sseerrvveerr
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:
% ssaassllaauutthhdd --aa rriimmaapp --OO iimmaapp..eexxaammppllee..ccoomm
NNoottee
The option "-O imap.example.com" specifies the IMAP server saslauthd should
contact when it verifies credentials.
IImmppoorrttaanntt
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.
TTeessttiinngg ssaassllaauutthhdd aauutthheennttiiccaattiioonn
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:
% tteessttssaassllaauutthhdd --uu uusseerrnnaammee --pp ppaasssswwoorrdd
0: OK "Success."
NNoottee
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 //ppaatthh//ttoo//ssoocckkeettddiirr//mmuuxx"
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".
CCyyrruuss SSAASSLL PPlluuggiinnss -- aauuxxiilliiaarryy pprrooppeerrttyy pplluuggiinnss
Cyrus SASL uses a plugin infrastructure (called auxprop) to expand libsasl's
capabilities. Currently Cyrus SASL sources provide three authentication
plugins.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
|PPlluuggiinn|DDeessccrriippttiioonn |
|_ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|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 |
|_ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
IImmppoorrttaanntt
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.
TThhee ssaassllddbb pplluuggiinn
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.
NNoottee
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:
% ssaassllppaasssswwdd22 --cc --uu eexxaammppllee..ccoomm uusseerrnnaammee
Password:
Again (for verification):
This command creates an account uusseerrnnaammee@@eexxaammppllee..ccoomm.
IImmppoorrttaanntt
users must specify uusseerrnnaammee@@eexxaammppllee..ccoomm as login name, not uusseerrnnaammee.
Run the following command to reuse the Postfix mydomain parameter value as the
login domain:
% ssaassllppaasssswwdd22 --cc --uu ``ppoossttccoonnff --hh mmyyddoommaaiinn`` uusseerrnnaammee
Password:
Again (for verification):
NNoottee
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:
% ssaassllddbblliissttuusseerrss22
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
NNoottee
In the above example adjust mech_list to the mechanisms that are applicable
for your environment.
TThhee ssqqll pplluuggiinn
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.
TTiipp
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'
NNoottee
Set appropriate permissions if smtpd.conf contains a password. The file
should be readable by the postfix user.
NNoottee
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.
NNoottee
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.
IImmppoorrttaanntt
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.
TThhee llddaappddbb pplluuggiinn
The ldapdb auxprop plugin provides access to credentials stored in an LDAP
server. This plugin requires that SASL client passwords are stored as
plaintext.
TTiipp
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. This may be
documented in a later version of this document. 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
IImmppoorrttaanntt
Set appropriate permissions if smtpd.conf contains a password. The file
should be readable by the postfix user.
NNoottee
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.
NNoottee
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.
NNoottee
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.
NNoottee
Read the chapter "Using SASL" in the OpenLDAP Admin Guide for more detailed
instructions to set up SASL authentication in OpenLDAP.
EEnnaabblliinngg SSAASSLL aauutthheennttiiccaattiioonn aanndd aauutthhoorriizzaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP sseerrvveerr
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
NNoottee
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
NNoottee
If you specify a remote IP address, information will be sent as plaintext
over the network.
EEnnaabblliinngg SSAASSLL aauutthheennttiiccaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP sseerrvveerr
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:
% tteellnneett sseerrvveerr..eexxaammppllee..ccoomm 2255
...
220 server.example.com ESMTP Postfix
EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm
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
NNoottee
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:
% tteellnneett sseerrvveerr..eexxaammppllee..ccoomm 2255
...
220 server.example.com ESMTP Postfix
EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250-AUTH=DIGEST-MD5 PLAIN CRAM-MD5
...
PPoossttffiixx SSMMTTPP SSeerrvveerr ppoolliiccyy -- SSAASSLL mmeecchhaanniissmm pprrooppeerrttiieess
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.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
|PPrrooppeerrttyy |DDeessccrriippttiioonn |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|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. |
|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
UUnneennccrryypptteedd SSMMTTPP sseessssiioonn
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
IImmppoorrttaanntt
Always set at least the noanonymous option. Otherwise, the Postfix SMTP
server can give strangers the same authorization as a properly-
authenticated client.
EEnnccrryypptteedd SSMMTTPP sseessssiioonn ((TTLLSS))
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
EEnnaabblliinngg SSAASSLL aauutthhoorriizzaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP sseerrvveerr
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.
MMaaiill rreellaayy aauutthhoorriizzaattiioonn
The permit_sasl_authenticated restriction allows SASL-authenticated SMTP
clients to send mail to remote destinations. Add it to the list of
smtpd_recipient_restrictions as follows:
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
permit_mynetworks
ppeerrmmiitt__ssaassll__aauutthheennttiiccaatteedd
reject_unauth_destination
...
EEnnvveellooppee sseennddeerr aaddddrreessss aauutthhoorriizzaattiioonn
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:
ssmmttppdd__sseennddeerr__llooggiinn__mmaappss == hhaasshh:://eettcc//ppoossttffiixx//ccoonnttrroolllleedd__eennvveellooppee__sseennddeerrss
smtpd_recipient_restrictions =
...
rreejjeecctt__sseennddeerr__llooggiinn__mmiissmmaattcchh
permit_sasl_authenticated
permit_mynetworks
reject_unauth_destination
...
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 and
reject_unauthenticated_sender_login_mismatch for additional control over the
SASL login name and the envelope sender.
AAddddiittiioonnaall SSMMTTPP SSeerrvveerr SSAASSLL ooppttiioonnss
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.
DDeeffaauulltt aauutthheennttiiccaattiioonn ddoommaaiinn
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.
HHiiddiinngg SSAASSLL aauutthheennttiiccaattiioonn ffrroomm cclliieennttss oorr nneettwwoorrkkss
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
AAddddiinngg tthhee SSAASSLL llooggiinn nnaammee ttoo mmaaiill hheeaaddeerrss
To report SASL login names in Received: message headers (Postfix version 2.3
and later):
/etc/postfix/main.cf:
smtpd_sasl_authenticated_header = yes
NNoottee
The SASL login names will be shared with the entire world.
TTeessttiinngg SSAASSLL aauutthheennttiiccaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP SSeerrvveerr
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 bboolldd font.
% tteellnneett sseerrvveerr..eexxaammppllee..ccoomm 2255
...
220 server.example.com ESMTP Postfix
EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-ETRN
250-AUTH DIGEST-MD5 PLAIN CRAM-MD5
250 8BITMIME
AAUUTTHH PPLLAAIINN AAHHRRllcc33QQAAddGGVVzzddHHBBhhcc33MM==
235 Authentication successful
To test this over a connection that is encrypted with TLS, use openssl s_client
instead of telnet:
% ooppeennssssll ss__cclliieenntt --ccoonnnneecctt sseerrvveerr..eexxaammppllee..ccoomm::2255 --ssttaarrttttllss ssmmttpp
...
220 server.example.com ESMTP Postfix
EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm
...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'.
CCaauuttiioonn
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 bbaasshh shell:
% eecchhoo --nnee ''\\000000uusseerrnnaammee\\000000ppaasssswwoorrdd'' || ooppeennssssll bbaassee6644
Some other shells support similar syntax.
* Using the pprriinnttff command:
% pprriinnttff ''\\00%%ss\\00%%ss'' ''uusseerrnnaammee'' ''ppaasssswwoorrdd'' || ooppeennssssll bbaassee6644
% pprriinnttff ''\\00%%ss\\00%%ss'' ''uusseerrnnaammee'' ''ppaasssswwoorrdd'' || mmmmeennccooddee
The mmmmeennccooddee command is part of the metamail software.
* Using Perl MMIIMMEE::::BBaassee6644:
% ppeerrll --MMMMIIMMEE::::BBaassee6644 --ee \\
''pprriinntt eennccooddee__bbaassee6644((""\\00uusseerrnnaammee\\00ppaasssswwoorrdd""));;''
MIME::Base64 is available from http://www.cpan.org/.
* Using the ggeenn--aauutthh script:
% ggeenn--aauutthh ppllaaiinn
username: uusseerrnnaammee
password:
The ggeenn--aauutthh Perl script was written by John Jetmore and can be found at
http://jetmore.org/john/code/gen-auth.
CCoonnffiigguurriinngg SSAASSLL aauutthheennttiiccaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP//LLMMTTPP cclliieenntt
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.
NNoottee
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 pprrooppeerrttiieess
* Postfix SMTP/LMTP client policy - SASL mechanism nnaammeess
EEnnaabblliinngg SSAASSLL aauutthheennttiiccaattiioonn iinn tthhee PPoossttffiixx SSMMTTPP//LLMMTTPP cclliieenntt
This section shows a typical scenario where the Postfix SMTP client sends all
messages via a mail gateway server that requires SASL authentication.
TTrroouubbllee ssoollvviinngg ttiippss::
* 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 pprrooppeerrttiieess".
* For a solution to a more obscure class of SASL authentication failures,
see "Postfix SMTP/LMTP client policy - SASL mechanism nnaammeess".
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
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 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=mmeetthhoodd....." syntax in response to the EHLO command; this
requires no additional Postfix client configuration.
* The Postfix SMTP client does not support the obsolete "wrappermode"
protocol, which uses TCP port 465 on the SMTP server. See TLS_README for a
solution that uses the stunnel command.
* 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
IImmppoorrttaanntt
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.
CCoonnffiigguurriinngg SSeennddeerr--DDeeppeennddeenntt SSAASSLL aauutthheennttiiccaattiioonn
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 username2:password2
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.
PPoossttffiixx SSMMTTPP//LLMMTTPP cclliieenntt ppoolliiccyy -- SSAASSLL mmeecchhaanniissmm pprrooppeerrttiieess
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.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
|PPrrooppeerrttyy |DDeessccrriippttiioonn |
|_ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
|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. |
|_ _ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ |
UUnneennccrryypptteedd SSMMTTPP sseessssiioonn
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
NNoottee
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 bboolldd font.
% tteellnneett sseerrvveerr..eexxaammppllee..ccoomm 2255
...
220 server.example.com ESMTP Postfix
EEHHLLOO cclliieenntt..eexxaammppllee..ccoomm
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-STARTTLS
...
Instead of port 25 (smtp), specify port 587 (submission) where appriopriate.
EEnnccrryypptteedd SSMMTTPP sseessssiioonn ((TTLLSS))
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
PPoossttffiixx SSMMTTPP//LLMMTTPP cclliieenntt ppoolliiccyy -- SSAASSLL mmeecchhaanniissmm nnaammeess
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
NNoottee
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
BBuuiillddiinngg PPoossttffiixx wwiitthh SSAASSLL ssuuppppoorrtt
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
BBuuiillddiinngg DDoovveeccoott SSAASSLL ssuuppppoorrtt
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:
% mmaakkee ttiiddyy # if you have left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==''--DDUUSSEE__SSAASSLL__AAUUTTHH \\
--DDDDEEFF__SSEERRVVEERR__SSAASSLL__TTYYPPEE==\\""ddoovveeccoott\\""''
After this, proceed with "make" as described in the INSTALL document.
NNoottee
* 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 sasl).
* 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.
% mmaakkee ttiiddyy # if you have left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==''--DDUUSSEE__SSAASSLL__AAUUTTHH \\
--DDDDEEFF__SSEERRVVEERR__SSAASSLL__TTYYPPEE==\\""ddoovveeccoott\\"" \\
......CCCCAARRGGSS ooppttiioonnss ffoorr LLDDAAPP oorr TTLLSS eettcc........'' \\
AAUUXXLLIIBBSS==''......AAUUXXLLIIBBSS ooppttiioonnss ffoorr LLDDAAPP oorr TTLLSS eettcc........''
BBuuiillddiinngg CCyyrruuss SSAASSLL ssuuppppoorrtt
BBuuiillddiinngg tthhee CCyyrruuss SSAASSLL lliibbrraarryy
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/.
IImmppoorrttaanntt
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:
% ..//ccoonnffiigguurree ----eennaabbllee--llooggiinn ----eennaabbllee--nnttllmm
BBuuiillddiinngg PPoossttffiixx wwiitthh CCyyrruuss SSAASSLL ssuuppppoorrtt
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
% mmaakkee ttiiddyy # if you have left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__SSAASSLL__AAUUTTHH --DDUUSSEE__CCYYRRUUSS__SSAASSLL \\
--II//uussrr//llooccaall//iinncclluuddee//ssaassll"" AAUUXXLLIIBBSS==""--LL//uussrr//llooccaall//lliibb --llssaassll22""
Cyrus SASL version 1.5.x
% mmaakkee ttiiddyy # if you have left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__SSAASSLL__AAUUTTHH --DDUUSSEE__CCYYRRUUSS__SSAASSLL \\
--II//uussrr//llooccaall//iinncclluuddee"" AAUUXXLLIIBBSS==""--LL//uussrr//llooccaall//lliibb --llssaassll""
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
% mmaakkee ttiiddyy # remove left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__SSAASSLL__AAUUTTHH --DDUUSSEE__CCYYRRUUSS__SSAASSLL \\
--II//uussrr//llooccaall//iinncclluuddee//ssaassll"" AAUUXXLLIIBBSS==""--LL//uussrr//llooccaall//lliibb \\
--RR//uussrr//llooccaall//lliibb --llssaassll22""
Cyrus SASL version 1.5.x
% mmaakkee ttiiddyy # if you have left-over files from a previous build
% mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__SSAASSLL__AAUUTTHH --DDUUSSEE__CCYYRRUUSS__SSAASSLL \\
--II//uussrr//llooccaall//iinncclluuddee"" AAUUXXLLIIBBSS==""--LL//uussrr//llooccaall//lliibb \\
--RR//uussrr//llooccaall//lliibb --llssaassll""
UUssiinngg CCyyrruuss SSAASSLL vveerrssiioonn 11..55..xx
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).
CCrreeddiittss
* 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.