Postfix legacy TLS Support

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NOTE

This document describes an old TLS user interface that is based on a third-
party TLS patch by Lutz Ja"nicke. As of Postfix version 2.3, the old user
interface still exists to allow migration from earlier Postfix releases, but
its functionality is frozen.

What Postfix TLS support does for you

Transport Layer Security (TLS, formerly called SSL) provides certificate-based
authentication and encrypted sessions. An encrypted session protects the
information that is transmitted with SMTP mail or with SASL authentication.

Postfix version 2.2 introduces support for TLS as described in RFC 3207. TLS
Support for older Postfix versions was available as an add-on patch. The
section "Compatibility with Postfix < 2.2 TLS support" below discusses the
differences between these implementations.

Topics covered in this document:

  * How Postfix TLS support works
  * Building Postfix with TLS support
  * SMTP Server specific settings
  * SMTP Client specific settings
  * TLS manager specific settings
  * Reporting problems
  * Compatibility with Postfix < 2.2 TLS support
  * Credits

And last but not least, for the impatient:

  * Getting started, quick and dirty

How Postfix TLS support works

The diagram below shows the main elements of the Postfix TLS architecture and
their relationships. Colored boxes with numbered names represent Postfix daemon
programs. Other colored boxes represent storage elements.

  * The smtpd(8) server implements the SMTP over TLS server side.

  * The smtp(8) client implements the SMTP over TLS client side.

  * The tlsmgr(8) server maintains the pseudo-random number generator (PRNG)
    that seeds the TLS engines in the smtpd(8) server and smtp(8) client
    processes, and maintains the TLS session key cache files.

                    <---seed---              ---seed--->
Network-> smtpd(8)                tlsmgr(8)                 smtp(8)  ->Network
                    <-session->              <-session->

                                /       |    \
                                        |
                              /                \

                      smtpd           PRNG         smtp
                     session         state        session
                    key cache         file       key cache

Building Postfix with TLS support

To build Postfix with TLS support, first we need to generate the make(1) files
with the necessary definitions. This is done by invoking the command "make
makefiles" in the Postfix top-level directory and with arguments as shown next.

NOTE: Do not use Gnu TLS. It will spontaneously terminate a Postfix daemon
process with exit status code 2, instead of allowing Postfix to 1) report the
error to the maillog file, and to 2) provide plaintext service where this is
appropriate.

  * If the OpenSSL include files (such as ssl.h) are in directory /usr/include/
    openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are
    in directory /usr/lib:

        % make tidy # if you have left-over files from a previous build
        % make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"

  * If the OpenSSL include files (such as ssl.h) are in directory /usr/local/
    include/openssl, and the OpenSSL libraries (such as libssl.so and
    libcrypto.so) are in directory /usr/local/lib:

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

    On Solaris, specify the -R option as shown below:

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

If you need to apply other customizations (such as Berkeley DB databases,
MySQL, PosgreSQL, LDAP or SASL), see the respective Postfix README documents,
and combine their "make makefiles" instructions with the instructions above:

    % make tidy # if you have left-over files from a previous build
    % make makefiles CCARGS="-DUSE_TLS \
        (other -D or -I options)" \
        AUXLIBS="-lssl -lcrypto \
        (other -l options for libraries in /usr/lib) \
        (-L/path/name + -l options for other libraries)"

To complete the build process, see the Postfix INSTALL instructions. Postfix
has TLS support turned off by default, so you can start using Postfix as soon
as it is installed.

SMTP Server specific settings

Topics covered in this section:

  * Server-side certificate and private key configuration
  * Server-side TLS activity logging
  * Enabling TLS in the Postfix SMTP server
  * Client certificate verification
  * Supporting AUTH over TLS only
  * Server-side TLS session cache
  * Server access control
  * Server-side cipher controls
  * Miscellaneous server controls

Server-side certificate and private key configuration

In order to use TLS, the Postfix SMTP server needs a certificate and a private
key. Both must be in "pem" format. The private key must not be encrypted,
meaning: the key must be accessible without a password. Both certificate and
private key may be in the same file.

Both RSA and DSA certificates are supported. Typically you will only have RSA
certificates issued by a commercial CA. In addition, the tools supplied with
OpenSSL will by default issue RSA certificates. You can have both at the same
time, in which case the cipher used determines which certificate is presented.
For Netscape and OpenSSL clients without special cipher choices, the RSA
certificate is preferred.

In order for remote SMTP clients to check the Postfix SMTP server certificates,
the CA certificate (in case of a certificate chain, all CA certificates) must
be available. You should add these certificates to the server certificate, the
server certificate first, then the issuing CA(s).

Example: the certificate for "server.dom.ain" was issued by "intermediate CA"
which itself has a certificate issued by "root CA". Create the server.pem file
with:

    % cat server_cert.pem intermediate_CA.pem > server.pem

A Postfix SMTP server certificate supplied here must be usable as an SSL server
certificate and hence pass the "openssl verify -purpose sslserver ..." test.

A client that trusts the root CA has a local copy of the root CA certificate,
so it is not necessary to include the root CA certificate here. Leaving it out
of the "server.pem" file reduces the overhead of the TLS exchange.

If you want the Postfix SMTP server to accept remote SMTP client certificates
issued by these CAs, append the root certificate to $smtpd_tls_CAfile or
install it in the $smtpd_tls_CApath directory. When you configure trust in a
root CA, it is not necessary to explicitly trust intermediary CAs signed by the
root CA, unless $smtpd_tls_ccert_verifydepth is less than the number of CAs in
the certificate chain for the clients of interest. With a verify depth of 1 you
can only verify certificates directly signed by a trusted CA, and all trusted
intermediary CAs need to be configured explicitly. With a verify depth of 2 you
can verify clients signed by a root CA or a direct intermediary CA (so long as
the client is correctly configured to supply its intermediate CA certificate).

RSA key and certificate examples:

    /etc/postfix/main.cf:
        smtpd_tls_cert_file = /etc/postfix/server.pem
        smtpd_tls_key_file = $smtpd_tls_cert_file

Their DSA counterparts:

    /etc/postfix/main.cf:
        smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem
        smtpd_tls_dkey_file = $smtpd_tls_dcert_file

To verify a remote SMTP client certificate, the Postfix SMTP server needs to
trust the certificates of the issuing Certification Authorities. These
certificates in "pem" format can be stored in a single $smtpd_tls_CAfile or in
multiple files, one CA per file in the $smtpd_tls_CApath directory. If you use
a directory, don't forget to create the necessary "hash" links with:

    # $OPENSSL_HOME/bin/c_rehash /path/to/directory

The $smtpd_tls_CAfile contains the CA certificates of one or more trusted CAs.
The file is opened (with root privileges) before Postfix enters the optional
chroot jail and so need not be accessible from inside the chroot jail.

Additional trusted CAs can be specified via the $smtpd_tls_CApath directory, in
which case the certificates are read (with $mail_owner privileges) from the
files in the directory when the information is needed. Thus, the
$smtpd_tls_CApath directory needs to be accessible inside the optional chroot
jail.

When you configure Postfix to request client certificates (by setting
$smtpd_tls_ask_ccert = yes), any certificates in $smtpd_tls_CAfile are sent to
the client, in order to allow it to choose an identity signed by a CA you
trust. If no $smtpd_tls_CAfile is specified, no preferred CA list is sent, and
the client is free to choose an identity signed by any CA. Many clients use a
fixed identity regardless of the preferred CA list and you may be able to
reduce TLS negotiation overhead by installing client CA certificates mostly or
only in $smtpd_tls_CApath. In the latter case you need not specify a
$smtpd_tls_CAfile.

Note, that unless client certificates are used to allow greater access to TLS
authenticated clients, it is best to not ask for client certificates at all, as
in addition to increased overhead some clients (notably in some cases qmail)
are unable to complete the TLS handshake when client certificates are
requested.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_CAfile = /etc/postfix/CAcert.pem
        smtpd_tls_CApath = /etc/postfix/certs

Server-side TLS activity logging

To get additional information about Postfix SMTP server TLS activity you can
increase the loglevel from 0..4. Each logging level also includes the
information that is logged at a lower logging level.

    0 Disable logging of TLS activity.

    1 Log TLS handshake and certificate information.

    2 Log levels during TLS negotiation.

    3 Log hexadecimal and ASCII dump of TLS negotiation process

    4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS

Use loglevel 3 only in case of problems. Use of loglevel 4 is strongly
discouraged.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_loglevel = 0

To include information about the protocol and cipher used as well as the client
and issuer CommonName into the "Received:" message header, set the
smtpd_tls_received_header variable to true. The default is no, as the
information is not necessarily authentic. Only information recorded at the
final destination is reliable, since the headers may be changed by intermediate
servers.

Example:

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

Enabling TLS in the Postfix SMTP server

By default, TLS is disabled in the Postfix SMTP server, so no difference to
plain Postfix is visible. Explicitly switch it on using "smtpd_use_tls = yes".

Example:

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

With this, Postfix SMTP server announces STARTTLS support to SMTP clients, but
does not require that clients use TLS encryption.

Note: when an unprivileged user invokes "sendmail -bs", STARTTLS is never
offered due to insufficient privileges to access the server private key. This
is intended behavior.

You can ENFORCE the use of TLS, so that the Postfix SMTP server announces
STARTTLS and accepts no mail without TLS encryption, by setting
"smtpd_enforce_tls = yes". According to RFC 2487 this MUST NOT be applied in
case of a publicly-referenced Postfix SMTP server. This option is off by
default and should only seldom be used.

Example:

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

TLS is sometimes used in the non-standard "wrapper" mode where a server always
uses TLS, instead of announcing STARTTLS support and waiting for clients to
request TLS service. Some clients, namely Outlook [Express] prefer the
"wrapper" mode. This is true for OE (Win32 < 5.0 and Win32 >=5.0 when run on a
port<>25 and OE (5.01 Mac on all ports).

It is strictly discouraged to use this mode from main.cf. If you want to
support this service, enable a special port in master.cf and specify "-
o smtpd_tls_wrappermode = yes" as an smtpd(8) command line option. Port 465
(smtps) was once chosen for this feature.

Example:

    /etc/postfix/master.cf:
        smtps    inet  n       -       n       -       -       smtpd
          -o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes

Client certificate verification

To receive a remote SMTP client certificate, the Postfix SMTP server must
explicitly ask for one (any contents of $smtpd_tls_CAfile are also sent to the
client as a hint for choosing a certificate from a suitable CA). Unfortunately,
Netscape clients will either complain if no matching client certificate is
available or will offer the user client a list of certificates to choose from.
Additionally some MTAs (notably some versions of qmail) are unable to complete
TLS negotiation when client certificates are requested, and abort the SMTP
session. So this option is "off" by default. You will however need the
certificate if you want to use certificate based relaying with, for example,
the permit_tls_clientcerts feature.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_ask_ccert = no

You may also decide to REQUIRE a remote SMTP client certificate before allowing
TLS connections. This feature is included for completeness, and implies
"smtpd_tls_ask_ccert = yes".

Please be aware, that this will inhibit TLS connections without a proper client
certificate and that it makes sense only when non-TLS submission is disabled
(smtpd_enforce_tls = yes). Otherwise, clients could bypass the restriction by
simply not using STARTTLS at all.

When TLS is not enforced, the connection will be handled as if only
"smtpd_tls_ask_ccert = yes" is specified, and a warning is logged.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_req_ccert = no

A client certificate verification depth of 1 is sufficient if the certificate
is directly issued by a CA listed in the CA file. The default value (5) should
also suffice for longer chains (root CA issues special CA which then issues the
actual certificate...)

Example:

    /etc/postfix/main.cf:
        smtpd_tls_ccert_verifydepth = 5

Supporting AUTH over TLS only

Sending AUTH data over an unencrypted channel poses a security risk. When TLS
layer encryption is required (smtpd_enforce_tls = yes), the Postfix SMTP server
will announce and accept AUTH only after the TLS layer has been activated with
STARTTLS. When TLS layer encryption is optional (smtpd_enforce_tls = no), it
may however still be useful to only offer AUTH when TLS is active. To maintain
compatibility with non-TLS clients, the default is to accept AUTH without
encryption. In order to change this behavior, set "smtpd_tls_auth_only = yes".

Example:

    /etc/postfix/main.cf:
        smtpd_tls_auth_only = no

Server-side TLS session cache

The Postfix SMTP server and the remote SMTP client negotiate a session, which
takes some computer time and network bandwidth. By default, this session
information is cached only in the smtpd(8) process actually using this session
and is lost when the process terminates. To share the session information
between multiple smtpd(8) processes, a persistent session cache can be used.
You can specify any database type that can store objects of several kbytes and
that supports the sequence operator. DBM databases are not suitable because
they can only store small objects. The cache is maintained by the tlsmgr(8)
process, so there is no problem with concurrent access. Session caching is
highly recommended, because the cost of repeatedly negotiating TLS session keys
is high.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_session_cache_database = btree:/etc/postfix/smtpd_scache

As of version 2.5, Postfix will no longer maintain this file in a directory
with non-Postfix ownership. As a migration aid, attempts to open such files are
redirected to the Postfix-owned $data_directory, and a warning is logged.

Cached Postfix SMTP server session information expires after a certain amount
of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer
time of 3600sec (=1 hour). RFC 2246 recommends a maximum of 24 hours.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_session_cache_timeout = 3600s

Server access control

Postfix TLS support introduces three additional features for Postfix SMTP
server access control:

    permit_tls_clientcerts
        Allow the remote SMTP client SMTP request if the client certificate
        passes verification, and if its fingerprint is listed in the list of
        client certificates (see relay_clientcerts discussion below).

    permit_tls_all_clientcerts
        Allow the remote client SMTP request if the client certificate passes
        verification.

    check_ccert_access type:table
        If the client certificate passes verification, use its fingerprint as a
        key for the specified access(5) table.

The permit_tls_all_clientcerts feature must be used with caution, because it
can result in too many access permissions. Use this feature only if a special
CA issues the client certificates, and only if this CA is listed as a trusted
CA. If other CAs are trusted, any owner of a valid client certificate would be
authorized. The permit_tls_all_clientcerts feature can be practical for a
specially created email relay server.

It is however recommended to stay with the permit_tls_clientcerts feature and
list all certificates via $relay_clientcerts, as permit_tls_all_clientcerts
does not permit any control when a certificate must no longer be used (e.g. an
employee leaving).

Example:

    /etc/postfix/main.cf:
        smtpd_recipient_restrictions =
            ...
            permit_tls_clientcerts
            reject_unauth_destination
            ...

The Postfix list manipulation routines give special treatment to whitespace and
some other characters, making the use of certificate names impractical. Instead
we use the certificate fingerprints as they are difficult to fake but easy to
use for lookup. Postfix lookup tables are in the form of (key, value) pairs.
Since we only need the key, the value can be chosen freely, e.g. the name of
the user or host.

Example:

    /etc/postfix/main.cf:
        relay_clientcerts = hash:/etc/postfix/relay_clientcerts

    /etc/postfix/relay_clientcerts:
        D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home

Server-side cipher controls

To influence the Postfix SMTP server cipher selection scheme, you can give
cipherlist string. A detailed description would go too far here; please refer
to the OpenSSL documentation. If you don't know what to do with it, simply
don't touch it and leave the (openssl-)compiled in default!

DO NOT USE " to enclose the string, specify just the string!!!

Example:

    /etc/postfix/main.cf:
        smtpd_tls_cipherlist = DEFAULT

If you want to take advantage of ciphers with EDH, DH parameters are needed.
Instead of using the built-in DH parameters for both 1024bit and 512bit, it is
better to generate "own" parameters, since otherwise it would "pay" for a
possible attacker to start a brute force attack against parameters that are
used by everybody. For this reason, the parameters chosen are already different
from those distributed with other TLS packages.

To generate your own set of DH parameters, use:

    % openssl gendh -out /etc/postfix/dh_1024.pem -2 -rand /var/run/egd-pool
    1024
    % openssl gendh -out /etc/postfix/dh_512.pem -2 -rand /var/run/egd-pool 512

Examples:

    /etc/postfix/main.cf:
        smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem
        smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem

Miscellaneous server controls

The smtpd_starttls_timeout parameter limits the time of Postfix SMTP server
write and read operations during TLS startup and shutdown handshake procedures.

Example:

    /etc/postfix/main.cf:
        smtpd_starttls_timeout = 300s

SMTP Client specific settings

Topics covered in this section:

  * Client-side certificate and private key configuration
  * Client-side TLS activity logging
  * Client-side TLS session cache
  * Enabling TLS in the Postfix SMTP client
  * Requiring TLS encryption
  * Disabling server certificate verification
  * Per-site TLS policies
  * Closing a DNS loophole with  per-site TLS policies
  * Discovering servers that support TLS
  * Server certificate verification depth
  * Client-side cipher controls
  * Miscellaneous client controls

Client-side certificate and private key configuration

During TLS startup negotiation the Postfix SMTP client may present a
certificate to the remote SMTP server. The Netscape client is rather clever
here and lets the user select between only those certificates that match CA
certificates offered by the remote SMTP server. As the Postfix SMTP client uses
the "SSL_connect()" function from the OpenSSL package, this is not possible and
we have to choose just one certificate. So for now the default is to use _no_
certificate and key unless one is explicitly specified here.

Both RSA and DSA certificates are supported. You can have both at the same
time, in which case the cipher used determines which certificate is presented.

It is possible for the Postfix SMTP client to use the same key/certificate pair
as the Postfix SMTP server. If a certificate is to be presented, it must be in
"pem" format. The private key must not be encrypted, meaning: it must be
accessible without a password. Both parts (certificate and private key) may be
in the same file.

In order for remote SMTP servers to verify the Postfix SMTP client
certificates, the CA certificate (in case of a certificate chain, all CA
certificates) must be available. You should add these certificates to the
client certificate, the client certificate first, then the issuing CA(s).

Example: the certificate for "client.example.com" was issued by "intermediate
CA" which itself has a certificate of "root CA". Create the client.pem file
with:

    % cat client_cert.pem intermediate_CA.pem > client.pem

A Postfix SMTP client certificate supplied here must be usable as an SSL client
certificate and hence pass the "openssl verify -purpose sslclient ..." test.

A server that trusts the root CA has a local copy of the root CA certificate,
so it is not necessary to include the root CA certificate here. Leaving it out
of the "client.pem" file reduces the overhead of the TLS exchange.

If you want the Postfix SMTP client to accept remote SMTP server certificates
issued by these CAs, append the root certificate to $smtp_tls_CAfile or install
it in the $smtp_tls_CApath directory. When you configure trust in a root CA, it
is not necessary to explicitly trust intermediary CAs signed by the root CA,
unless $smtp_tls_scert_verifydepth is less than the number of CAs in the
certificate chain for the servers of interest. With a verify depth of 1 you can
only verify certificates directly signed by a trusted CA, and all trusted
intermediary CAs need to be configured explicitly. With a verify depth of 2 you
can verify servers signed by a root CA or a direct intermediary CA (so long as
the server is correctly configured to supply its intermediate CA certificate).

RSA key and certificate examples:

    /etc/postfix/main.cf:
        smtp_tls_cert_file = /etc/postfix/client.pem
        smtp_tls_key_file = $smtp_tls_cert_file

Their DSA counterparts:

    /etc/postfix/main.cf:
        smtp_tls_dcert_file = /etc/postfix/client-dsa.pem
        smtp_tls_dkey_file = $smtp_tls_dcert_file

To verify a remote SMTP server certificate, the Postfix SMTP client needs to
trust the certificates of the issuing Certification Authorities. These
certificates in "pem" format can be stored in a single $smtp_tls_CAfile or in
multiple files, one CA per file in the $smtp_tls_CApath directory. If you use a
directory, don't forget to create the necessary "hash" links with:

    # $OPENSSL_HOME/bin/c_rehash /path/to/directory

The $smtp_tls_CAfile contains the CA certificates of one or more trusted CAs.
The file is opened (with root privileges) before Postfix enters the optional
chroot jail and so need not be accessible from inside the chroot jail.

Additional trusted CAs can be specified via the $smtp_tls_CApath directory, in
which case the certificates are read (with $mail_owner privileges) from the
files in the directory when the information is needed. Thus, the
$smtp_tls_CApath directory needs to be accessible inside the optional chroot
jail.

The choice between $smtp_tls_CAfile and $smtp_tls_CApath is a space/time
tradeoff. If there are many trusted CAs, the cost of preloading them all into
memory may not pay off in reduced access time when the certificate is needed.

Example:

    /etc/postfix/main.cf:
        smtp_tls_CAfile = /etc/postfix/CAcert.pem
        smtp_tls_CApath = /etc/postfix/certs

Client-side TLS activity logging

To get additional information about Postfix SMTP client TLS activity you can
increase the loglevel from 0..4. Each logging level also includes the
information that is logged at a lower logging level.

    0 Disable logging of TLS activity.

    1 Log TLS handshake and certificate information.

    2 Log levels during TLS negotiation.

    3 Log hexadecimal and ASCII dump of TLS negotiation process

    4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS

Example:

    /etc/postfix/main.cf:
        smtp_tls_loglevel = 0

Client-side TLS session cache

The remote SMTP server and the Postfix SMTP client negotiate a session, which
takes some computer time and network bandwidth. By default, this session
information is cached only in the smtp(8) process actually using this session
and is lost when the process terminates. To share the session information
between multiple smtp(8) processes, a persistent session cache can be used. You
can specify any database type that can store objects of several kbytes and that
supports the sequence operator. DBM databases are not suitable because they can
only store small objects. The cache is maintained by the tlsmgr(8) process, so
there is no problem with concurrent access. Session caching is highly
recommended, because the cost of repeatedly negotiating TLS session keys is
high. Future Postfix SMTP servers may limit the number of sessions that a
client is allowed to negotiate per unit time.

Example:

    /etc/postfix/main.cf:
        smtp_tls_session_cache_database = btree:/etc/postfix/smtp_scache

As of version 2.5, Postfix will no longer maintain this file in a directory
with non-Postfix ownership. As a migration aid, attempts to open such files are
redirected to the Postfix-owned $data_directory, and a warning is logged.

Cached Postfix SMTP client session information expires after a certain amount
of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer
time of 3600s (=1 hour). RFC 2246 recommends a maximum of 24 hours.

Example:

    /etc/postfix/main.cf:
        smtp_tls_session_cache_timeout = 3600s

Enabling TLS in the Postfix SMTP client

By default, TLS is disabled in the Postfix SMTP client, so no difference to
plain Postfix is visible. If you enable TLS, the Postfix SMTP client will send
STARTTLS when TLS support is announced by the remote SMTP server.

When the server accepts the STARTTLS command, but the subsequent TLS handshake
fails, and no other server is available, the Postfix SMTP client defers the
delivery attempt, and the mail stays in the queue. After a handshake failure,
the communications channel is in an indeterminate state and cannot be used for
non-TLS deliveries.

Example:

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

Requiring TLS encryption

You can ENFORCE the use of TLS, so that the Postfix SMTP client will not
deliver mail over unencrypted connections. In this mode, the remote SMTP server
hostname must match the information in the remote server certificate, and the
server certificate must be issued by a CA that is trusted by the Postfix SMTP
client. If the remote server certificate doesn't verify or the remote SMTP
server hostname doesn't match, and no other server is available, the delivery
attempt is deferred and the mail stays in the queue.

The remote SMTP server hostname is verified against all names provided as
dNSNames in the SubjectAlternativeName. If no dNSNames are specified, the
CommonName is checked. Verification may be turned off with the
smtp_tls_enforce_peername option which is discussed below.

Enforcing the use of TLS is useful if you know that you will only connect to
servers that support RFC 2487 _and_ that present server certificates that meet
the above requirements. An example would be a client only sends email to one
specific mailhub that offers the necessary STARTTLS support.

Example:

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

Disabling server certificate verification

As of RFC 2487 the requirements for hostname checking for MTA clients are not
set. When TLS is required (smtp_enforce_tls = yes), the option
smtp_tls_enforce_peername can be set to "no" to disable strict remote SMTP
server hostname checking. In this case, the mail delivery will proceed
regardless of the CommonName etc. listed in the certificate.

Despite the potential for eliminating "man-in-the-middle" and other attacks,
mandatory certificate/peername verification is not viable as a default Internet
mail delivery policy at this time. A significant fraction of TLS enabled MTAs
uses self-signed certificates, or certificates that are signed by a private
Certification Authority. On a machine that delivers mail to the Internet, if
you set smtp_enforce_tls = yes, you should probably also set
smtp_tls_enforce_peername = no. You can use the per-site TLS policies (see
below) to enable full peer verification for specific destinations that are
known to have verifiable TLS server certificates.

Example:

    /etc/postfix/main.cf:
        smtp_enforce_tls = yes
        smtp_tls_enforce_peername = no

Per-site TLS policies

A small fraction of servers offer STARTTLS but the negotiation consistently
fails, leading to mail aging out of the queue and bouncing back to the sender.
In such cases, you can use the per-site policies to disable TLS for the problem
sites. Alternatively, you can enable TLS for just a few specific sites and not
enable it for all sites.

The smtp_tls_per_site table is searched for a policy that matches the following
information:

    remote SMTP server hostname
        This is simply the DNS name of the server that the Postfix SMTP client
        connects to; this name may be obtained from other DNS lookups, such as
        MX lookups or CNAME lookups.
    next-hop destination
        This is normally the domain portion of the recipient address, but it
        may be overruled by information from the transport(5) table, from the
        relayhost parameter setting, or from the relay_transport setting. When
        it's not the recipient domain, the next-hop destination can have the
        Postfix-specific form "[name]", [name]:port", "name" or "name:port".

When both the hostname lookup and the next-hop lookup succeed, the host policy
does not automatically override the next-hop policy. Instead, precedence is
given to either the more specific or the more secure per-site policy as
described below.

The smtp_tls_per_site table uses a simple "name whitespace value" format.
Specify host names or next-hop destinations on the left-hand side; no wildcards
are allowed. On the right hand side specify one of the following keywords:

    NONE
        Don't use TLS at all. This overrides a less specific MAY lookup result
        from the alternate host or next-hop lookup key, and overrides the
        global smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername
        settings.
    MAY
        Try to use TLS if the server announces support, otherwise use the
        unencrypted connection. This has less precedence than a more specific
        result (including NONE) from the alternate host or next-hop lookup key,
        and has less precedence than the more specific global "smtp_enforce_tls
        = yes" or "smtp_tls_enforce_peername = yes".
    MUST_NOPEERMATCH
        Require TLS encryption, but do not require that the remote SMTP server
        hostname matches the information in the remote SMTP server certificate,
        or that the server certificate was issued by a trusted CA. This
        overrides a less secure NONE or a less specific MAY lookup result from
        the alternate host or next-hop lookup key, and overrides the global
        smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.
    MUST
        Require TLS encryption, require that the remote SMTP server hostname
        matches the information in the remote SMTP server certificate, and
        require that the remote SMTP server certificate was issued by a trusted
        CA. This overrides a less secure NONE and MUST_NOPEERMATCH or a less
        specific MAY lookup result from the alternate host or next-hop lookup
        key, and overrides the global smtp_use_tls, smtp_enforce_tls and
        smtp_tls_enforce_peername settings.

The precedences between global (main.cf) and per-site TLS policies can be
summarized as follows:

  * When neither the remote SMTP server hostname nor the next-hop destination
    are found in the smtp_tls_per_site table, the policy is based on
    smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername. Note:
    "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = yes" imply
    "smtp_use_tls = yes".

  * When both hostname and next-hop destination lookups produce a result, the
    more specific per-site policy (NONE, MUST, etc.) overrides the less
    specific one (MAY), and the more secure per-site policy (MUST, etc.)
    overrides the less secure one (NONE).

  * After the per-site policy lookups are combined, the result generally
    overrides the global policy. The exception is the less specific MAY per-
    site policy, which is overruled by the more specific global
    "smtp_enforce_tls = yes" with server certificate verification as specified
    with the smtp_tls_enforce_peername parameter.

Closing a DNS loophole with  per-site TLS policies

As long as no secure DNS lookup mechanism is available, false hostnames in MX
or CNAME responses can change the server hostname that Postfix uses for TLS
policy lookup and server certificate verification. Even with a perfect match
between the server hostname and the server certificate, there is no guarantee
that Postfix is connected to the right server. To avoid this loophole take the
following steps:

  * Eliminate MX lookups. Specify local transport(5) table entries for
    sensitive domains with explicit smtp:[mailhost] or smtp:[mailhost]:port
    destinations (you can assure security of this table unlike DNS); in the
    smtp_tls_per_site table specify the value MUST for the key [mailhost] or
    smtp:[mailhost]:port. This prevents false hostname information in DNS MX
    records from changing the server hostname that Postfix uses for TLS policy
    lookup and server certificate verification.

  * Disallow CNAME hostname overrides. In main.cf specify
    "smtp_cname_overrides_servername = no". This prevents false hostname
    information in DNS CNAME records from changing the server hostname that
    Postfix uses for TLS policy lookup and server certificate verification.
    This feature requires Postfix 2.2.9 or later.

Example:

    /etc/postfix/main.cf:
        smtp_tls_per_site = hash:/etc/postfix/tls_per_site
        relayhost = [msa.example.net]:587

    /etc/postfix/tls_per_site:
        # relayhost exact nexthop match
        [msa.example.net]:587       MUST

        # TLS should not be used with the example.org MX hosts.
        example.org                 NONE

        # TLS should not be used with the host smtp.example.com.
        [smtp.example.com]          NONE

Discovering servers that support TLS

As we decide on a "per site" basis whether or not to use TLS, it would be good
to have a list of sites that offered "STARTTLS". We can collect it ourselves
with this option.

If the smtp_tls_note_starttls_offer feature is enabled and a server offers
STARTTLS while TLS is not already enabled for that server, the Postfix SMTP
client logs a line as follows:

    postfix/smtp[pid]: Host offered STARTTLS: [hostname.example.com]

Example:

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

Server certificate verification depth

When verifying a remote SMTP server certificate, a verification depth of 1 is
sufficient if the certificate is directly issued by a CA specified with
smtp_tls_CAfile or smtp_tls_CApath. The default value of 5 should also suffice
for longer chains (root CA issues special CA which then issues the actual
certificate...)

Example:

    /etc/postfix/main.cf:
        smtp_tls_scert_verifydepth = 5

Client-side cipher controls

To influence the Postfix SMTP client cipher selection scheme, you can give
cipherlist string. A detailed description would go too far here; please refer
to the OpenSSL documentation. If you don't know what to do with it, simply
don't touch it and leave the (openssl-)compiled in default!

DO NOT USE " to enclose the string, specify just the string!!!

Example:

    /etc/postfix/main.cf:
        smtp_tls_cipherlist = DEFAULT

Miscellaneous client controls

The smtp_starttls_timeout parameter limits the time of Postfix SMTP client
write and read operations during TLS startup and shutdown handshake procedures.
In case of problems the Postfix SMTP client tries the next network address on
the mail exchanger list, and defers delivery if no alternative server is
available.

Example:

    /etc/postfix/main.cf:
        smtp_starttls_timeout = 300s

TLS manager specific settings

The security of cryptographic software such as TLS depends critically on the
ability to generate unpredictable numbers for keys and other information. To
this end, the tlsmgr(8) process maintains a Pseudo Random Number Generator
(PRNG) pool. This is queried by the smtp(8) and smtpd(8) processes when they
initialize. By default, these daemons request 32 bytes, the equivalent to 256
bits. This is more than sufficient to generate a 128bit (or 168bit) session
key.

Example:

    /etc/postfix/main.cf:
        tls_daemon_random_bytes = 32

In order to feed its in-memory PRNG pool, the tlsmgr(8) reads entropy from an
external source, both at startup and during run-time. Specify a good entropy
source, like EGD or /dev/urandom; be sure to only use non-blocking sources (on
OpenBSD, use /dev/arandom when tlsmgr(8) complains about /dev/urandom timeout
errors). If the entropy source is not a regular file, you must prepend the
source type to the source name: "dev:" for a device special file, or "egd:" for
a source with EGD compatible socket interface.

Examples (specify only one in main.cf):

    /etc/postfix/main.cf:
        tls_random_source = dev:/dev/urandom
        tls_random_source = egd:/var/run/egd-pool

By default, tlsmgr(8) reads 32 bytes from the external entropy source at each
seeding event. This amount (256bits) is more than sufficient for generating a
128bit symmetric key. With EGD and device entropy sources, the tlsmgr(8) limits
the amount of data read at each step to 255 bytes. If you specify a regular
file as entropy source, a larger amount of data can be read.

Example:

    /etc/postfix/main.cf:
        tls_random_bytes = 32

In order to update its in-memory PRNG pool, the tlsmgr(8) queries the external
entropy source again after a pseudo-random amount of time. The time is
calculated using the PRNG, and is between 0 and the maximal time specified with
tls_random_reseed_period. The default maximal time interval is 1 hour.

Example:

    /etc/postfix/main.cf:
        tls_random_reseed_period = 3600s

The tlsmgr(8) process saves the PRNG state to a persistent exchange file at
regular times and when the process terminates, so that it can recover the PRNG
state the next time it starts up. This file is created when it does not exist.
Its default location is under the Postfix configuration directory, which is not
the proper place for information that is modified by Postfix. Instead, the file
location should probably be on the /var partition (but not inside the chroot
jail).

Examples:

    /etc/postfix/main.cf:
        tls_random_exchange_name = /etc/postfix/prng_exch
        tls_random_prng_update_period = 3600s

Getting started, quick and dirty

The following steps will get you started quickly. Because you sign your own
Postfix public key certificate, you get TLS encryption but no TLS
authentication. This is sufficient for testing, and for exchanging email with
sites that you have no trust relationship with. For real authentication, your
Postfix public key certificate needs to be signed by a recognized Certification
Authority, and Postfix needs to be configured with a list of public key
certificates of Certification Authorities, so that Postfix can verify the
public key certificates of remote hosts.

In the examples below, user input is shown in bold font, and a "#" prompt
indicates a super-user shell.

  * Become your own Certification Authority, so that you can sign your own
    public keys. This example uses the CA.pl script that ships with OpenSSL. By
    default, OpenSSL installs this as /usr/local/ssl/misc/CA.pl, but your
    mileage may vary. The script creates a private key in ./demoCA/private/
    cakey.pem and a public key in ./demoCA/cacert.pem.

        % /usr/local/ssl/misc/CA.pl -newca
        CA certificate filename (or enter to create)

        Making CA certificate ...
        Using configuration from /etc/ssl/openssl.cnf
        Generating a 1024 bit RSA private key
        ....................++++++
        .....++++++
        writing new private key to './demoCA/private/cakey.pem'
        Enter PEM pass phrase:whatever

  * Create an unpassworded private key for host FOO and create an unsigned
    public key certificate.

        % openssl req -new -nodes -keyout FOO-key.pem -out FOO-req.pem -days
        365
        Using configuration from /etc/ssl/openssl.cnf
        Generating a 1024 bit RSA private key
        ........................................++++++
        ....++++++
        writing new private key to 'FOO-key.pem'
        -----
        You are about to be asked to enter information that will be
        incorporated
        into your certificate request.
        What you are about to enter is what is called a Distinguished Name or a
        DN.
        There are quite a few fields but you can leave some blank
        For some fields there will be a default value,
        If you enter '.', the field will be left blank.
        -----
        Country Name (2 letter code) [AU]:US
        State or Province Name (full name) [Some-State]:New York
        Locality Name (eg, city) []:Westchester
        Organization Name (eg, company) [Internet Widgits Pty Ltd]:Porcupine
        Organizational Unit Name (eg, section) []:
        Common Name (eg, YOUR name) []:FOO
        Email Address []:wietse@porcupine.org

        Please enter the following 'extra' attributes
        to be sent with your certificate request
        A challenge password []:whatever
        An optional company name []:

  * Sign the public key certificate for host FOO with the Certification
    Authority private key that we created a few steps ago.

        % openssl ca -out FOO-cert.pem -infiles FOO-req.pem
        Uing configuration from /etc/ssl/openssl.cnf
        Enter PEM pass phrase:whatever
        Check that the request matches the signature
        Signature ok
        The Subjects Distinguished Name is as follows
        countryName           :PRINTABLE:'US'
        stateOrProvinceName   :PRINTABLE:'New York'
        localityName          :PRINTABLE:'Westchester'
        organizationName      :PRINTABLE:'Porcupine'
        commonName            :PRINTABLE:'FOO'
        emailAddress          :IA5STRING:'wietse@porcupine.org'
        Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365
        days)
        Sign the certificate? [y/n]:y

        1 out of 1 certificate requests certified, commit? [y/n]y
        Write out database with 1 new entries
        Data Base Updated

  * Install the host private key, the host public key certificate, and the
    Certification Authority certificate files. This requires super-user
    privileges.

        # cp demoCA/cacert.pem FOO-key.pem FOO-cert.pem /etc/postfix
        # chmod 644 /etc/postfix/FOO-cert.pem /etc/postfix/cacert.pem
        # chmod 400 /etc/postfix/FOO-key.pem

  * Configure Postfix, by adding the following to /etc/postfix/main.cf.

        smtp_tls_CAfile = /etc/postfix/cacert.pem
        smtp_tls_cert_file = /etc/postfix/FOO-cert.pem
        smtp_tls_key_file = /etc/postfix/FOO-key.pem
        smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache
        smtp_use_tls = yes
        smtpd_tls_CAfile = /etc/postfix/cacert.pem
        smtpd_tls_cert_file = /etc/postfix/FOO-cert.pem
        smtpd_tls_key_file = /etc/postfix/FOO-key.pem
        smtpd_tls_received_header = yes
        smtpd_tls_session_cache_database = btree:/var/run/
        smtpd_tls_session_cache
        smtpd_use_tls = yes
        tls_random_source = dev:/dev/urandom

Reporting problems

When reporting a problem, please be thorough in the report. Patches, when
possible, are greatly appreciated too.

Please differentiate when possible between:

  * Problems in the TLS code: <postfix_tls@aet.tu-cottbus.de>
  * Problems in vanilla Postfix: <postfix-users@postfix.org>

Compatibility with Postfix < 2.2 TLS support

Postfix version 2.2 TLS support is based on the Postfix/TLS patch by Lutz
Ja"nicke, but differs in a few minor ways.

  * main.cf: Specify "btree" instead of "sdbm" for TLS session cache databases.

    TLS session cache databases are now accessed only by the tlsmgr(8) process,
    so there are no more concurrency issues. Although Postfix has an sdbm
    client, the sdbm library (1000 lines of code) is not included with Postfix.

    TLS session caches can use any database that can store objects of several
    kbytes or more, and that implements the sequence operation. In most cases,
    btree databases should be adequate.

    NOTE: You cannot use dbm databases. TLS session objects are too large.

  * master.cf: Specify "unix" instead of "fifo" as the tlsmgr service type.

    The smtp(8) and smtpd(8) processes now use a client-server protocol in
    order to access the tlsmgr(8) pseudo-random number generation (PRNG) pool,
    and in order to access the TLS session cache databases. Such a protocol
    cannot be run across fifos.

  * smtp_tls_per_site: the MUST_NOPEERMATCH per-site policy cannot override the
    global "smtp_tls_enforce_peername = yes" setting.

  * smtp_tls_per_site: a combined (NONE + MAY) lookup result for (hostname and
    next-hop destination) produces counter-intuitive results for different
    main.cf settings. TLS is enabled with "smtp_tls_enforce_peername = no", but
    it is disabled when both "smtp_enforce_tls = yes" and
    "smtp_tls_enforce_peername = yes".

The smtp_tls_per_site limitations were removed by the end of the Postfix 2.2
support cycle.

Credits

  * TLS support for Postfix was originally developed by Lutz Ja"nicke at
    Cottbus Technical University.
  * Wietse Venema adopted the code, did some restructuring, and compiled this
    part of the documentation from Lutz's documents.
  * Victor Duchovni was instrumental with the re-implementation of the
    smtp_tls_per_site code in terms of enforcement levels, which simplified the
    implementation greatly.

