PPoossttffiixx TTLLSS SSuuppppoorrtt

-------------------------------------------------------------------------------

WWAARRNNIINNGG

By turning on TLS support in Postfix, you not only get the ability to encrypt
mail and to authenticate clients or servers. You also turn on thousands and
thousands of lines of OpenSSL library code. Assuming that OpenSSL is written as
carefully as Wietse's own code, every 1000 lines introduce one additional bug
into Postfix.

WWhhaatt PPoossttffiixx TTLLSS ssuuppppoorrtt ddooeess ffoorr yyoouu

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

HHooww PPoossttffiixx TTLLSS ssuuppppoorrtt wwoorrkkss

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

BBuuiillddiinngg PPoossttffiixx wwiitthh TTLLSS ssuuppppoorrtt

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.

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pprroocceessss wwiitthh eexxiitt ssttaattuuss ccooddee 22,, iinnsstteeaadd ooff aalllloowwiinngg PPoossttffiixx ttoo 11)) rreeppoorrtt tthhee
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aapppprroopprriiaattee..

  * 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:

        % mmaakkee ttiiddyy # if you have left-over files from a previous build
        % mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__TTLLSS"" AAUUXXLLIIBBSS==""--llssssll --llccrryyppttoo""

  * 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:

        % mmaakkee ttiiddyy # if you have left-over files from a previous build
        % mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__TTLLSS --II//uussrr//llooccaall//iinncclluuddee"" \\
            AAUUXXLLIIBBSS==""--LL//uussrr//llooccaall//lliibb --llssssll --llccrryyppttoo""

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

        % mmaakkee ttiiddyy # if you have left-over files from a previous build
        % mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__TTLLSS --II//uussrr//llooccaall//iinncclluuddee"" \\
            AAUUXXLLIIBBSS==""--RR//uussrr//llooccaall//lliibb --LL//uussrr//llooccaall//lliibb --llssssll --llccrryyppttoo""

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

    % mmaakkee ttiiddyy # if you have left-over files from a previous build
    % mmaakkee mmaakkeeffiilleess CCCCAARRGGSS==""--DDUUSSEE__TTLLSS \\
        ((ootthheerr --DD oorr --II ooppttiioonnss))"" \\
        AAUUXXLLIIBBSS==""--llssssll --llccrryyppttoo \\
        ((ootthheerr --ll ooppttiioonnss ffoorr lliibbrraarriieess iinn //uussrr//lliibb)) \\
        ((--LL//ppaatthh//nnaammee ++ --ll ooppttiioonnss ffoorr ootthheerr lliibbrraarriieess))""

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.

SSMMTTPP SSeerrvveerr ssppeecciiffiicc sseettttiinnggss

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

SSeerrvveerr--ssiiddee cceerrttiiffiiccaattee aanndd pprriivvaattee kkeeyy ccoonnffiigguurraattiioonn

In order to use TLS, the Postfix SMTP server generally 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 password. Both
certificate and private key may be in the same file, in which case the
certificate file should be owned by "root" and not be readable by any other
user. If the key is stored separately, this applies to the key file only, and
the certificate file may be "world-readable".

Public Internet MX hosts without certificates signed by a "reputable" CA must
generate, and be prepared to present to most clients, a self-signed or private-
CA signed certificate. The client will not be able to authenticate the server,
but unless it is running Postfix 2.3 or similar software, it will still insist
on a server certificate.

For servers that are nnoott public Internet MX hosts, Postfix 2.3 supports
configurations with no certificates. This entails the use of just the anonymous
TLS ciphers, which are not supported by typical SMTP clients. Since such
clients will not, as a rule, fall back to plain text after a TLS handshake
failure, the server will be unable to receive email from most TLS enabled
clients. To avoid accidental configurations with no certificates, Postfix 2.3
enables certificate-less operation only when the administrator explicitly sets
"smtpd_tls_cert_file = none". This ensures that new Postfix configurations will
not accidentally run with no certificates.

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 any intermediate CA certificates to the server
certificate: the server certificate first, then the intermediate 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:

    % ccaatt sseerrvveerr__cceerrtt..ppeemm iinntteerrmmeeddiiaattee__CCAA..ppeemm >> sseerrvveerr..ppeemm

A Postfix SMTP server certificate supplied here must be usable as 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

Postfix 2.3 and later, TLS without certificates for servers serving exclusively
anonymous-cipher capable clients:

    /etc/postfix/main.cf:
        smtpd_tls_cert_file = none

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:

    # $$OOPPEENNSSSSLL__HHOOMMEE//bbiinn//cc__rreehhaasshh //ppaatthh//ttoo//ddiirreeccttoorryy

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, any CA 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

SSeerrvveerr--ssiiddee TTLLSS aaccttiivviittyy llooggggiinngg

To get additional information about Postfix SMTP server TLS activity you can
increase the log level 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 log level 3 only in case of problems. Use of log level 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

EEnnaabblliinngg TTLLSS iinn tthhee PPoossttffiixx SSMMTTPP sseerrvveerr

By default, TLS is disabled in the Postfix SMTP server, so no difference to
plain Postfix is visible. Explicitly switch it on with
"smtpd_tls_security_level = may" (Postfix 2.3 and later) or "smtpd_use_tls =
yes" (obsolete but still supported).

Example:

    /etc/postfix/main.cf:
        # Postfix 2.3 and later
        smtpd_tls_security_level = may
        # Obsolete, but still supported
        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_tls_security_level = encrypt" (Postfix 2.3 and later) or
"smtpd_enforce_tls = yes" (obsolete but still supported). 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:
        # Postfix 2.3 and later
        smtpd_tls_security_level = encrypt
        # Obsolete, but still supported
        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" (note: no space around the "=") 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

CClliieenntt cceerrttiiffiiccaattee vveerriiffiiccaattiioonn

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. A server that wants client certificates
must first present its own certificate. While Postfix 2.3 by default offers
anonymous ciphers to clients, these are automatically suppressed when the
server is configured to ask for client certificates.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_ask_ccert = yes
        # Postfix 2.3 and later
        smtpd_tls_security_level = may
        # Obsolete, but still supported
        smtpd_use_tls = yes

When TLS is enforced you may also decide to REQUIRE a remote SMTP client
certificate for all TLS connections, by setting "smtpd_tls_req_ccert = yes".
This feature implies "smtpd_tls_ask_ccert = yes". When TLS is not enforced,
"smtpd_tls_req_ccert = yes" is ignored and a warning is logged.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_req_ccert = yes
        # Postfix 2.3 and later
        smtpd_tls_security_level = encrypt
        # Obsolete, but still supported
        smtpd_enforce_tls = yes

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

SSuuppppoorrttiinngg AAUUTTHH oovveerr TTLLSS oonnllyy

Sending AUTH data over an unencrypted channel poses a security risk. When TLS
layer encryption is required ("smtpd_tls_security_level = encrypt" or the
obsolete "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_tls_security_level = may" or the obsolete
"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

SSeerrvveerr--ssiiddee TTLLSS sseessssiioonn ccaacchhee

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

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

When the Postfix SMTP server does not save TLS sessions to an external cache
database, client-side session caching is unlikely to be useful. To prevent such
wastage, the Postfix SMTP server can be configured to not issue TLS session
ids. By default the Postfix SMTP server always issues TLS session ids. This
works around known interoperability issues with some MUAs, and prevents
possible interoperability issues with other MTAs.

Example:

        smtpd_tls_always_issue_session_ids = no

SSeerrvveerr aacccceessss ccoonnttrrooll

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 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

SSeerrvveerr--ssiiddee cciipphheerr ccoonnttrroollss

The description below is for Postfix 2.3; for Postfix < 2.3 the
smtpd_tls_cipherlist parameter specifies the acceptable ciphers as an explicit
OpenSSL cipherlist. The obsolete setting applies even when TLS encryption is
not enforced. Use of this control on public MX hosts is strongly discouraged.

With mandatory TLS encryption, the Postfix SMTP server will by default only use
SSLv3 or TLSv1. SSLv2 is only used when TLS encryption is optional. This is
controlled by the smtpd_tls_mandatory_protocols configuration parameter.

The Postfix SMTP server supports 5 distinct cipher security levels as specified
by the smtpd_tls_mandatory_ciphers configuration parameter, which determines
the cipher grade with mandatory TLS encryption. The default value is "medium"
which is essentially 128-bit encryption or better. With opportunistic TLS
encryption, the minimum accepted cipher grade is always "export".

By default anonymous ciphers are allowed, and automatically disabled when
client certificates are requested. If clients are expected to always verify the
server certificate you may want to exclude anonymous ciphers by setting
"smtpd_tls_mandatory_exclude_ciphers = aNULL". One can't force a client to
check the server certificate, so excluding anonymous ciphers is generally
unnecessary.

For a server that is not a public Internet MX host, Postfix 2.3 supports
configurations with no server certificates that use oonnllyy the anonymous ciphers.
This is enabled by explicitly setting "smtpd_tls_cert_file = none" and not
specifying an smtpd_tls_dcert_file.

Example: (MSA that requires TLS with high grade ciphers)

    /etc/postfix/main.cf:
        smtpd_tls_cert_file = /etc/postfix/cert.pem
        smtpd_tls_key_file = /etc/postfix/key.pem
        smtpd_tls_mandatory_ciphers = high
        smtpd_tls_mandatory_exclude_ciphers = aNULL, MD5
        # Postfix 2.3 and later
        smtpd_tls_security_level = encrypt
        # Obsolete, but still supported
        smtpd_enforce_tls = yes

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 your 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 default parameters chosen by OpenSSL
are already different from those distributed with other TLS packages.

To generate your own set of DH parameters, use:

    % ooppeennssssll ggeennddhh --oouutt //eettcc//ppoossttffiixx//ddhh__11002244..ppeemm --22 --rraanndd //vvaarr//rruunn//eeggdd--ppooooll
    11002244
    % ooppeennssssll ggeennddhh --oouutt //eettcc//ppoossttffiixx//ddhh__551122..ppeemm --22 --rraanndd //vvaarr//rruunn//eeggdd--ppooooll 551122

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

MMiisscceellllaanneeoouuss sseerrvveerr ccoonnttrroollss

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

SSMMTTPP CClliieenntt ssppeecciiffiicc sseettttiinnggss

Topics covered in this section:

  * TLS support in the LMTP delivery agent
  * Client-side certificate and private key configuration
  * Client-side TLS activity logging
  * Client-side TLS session cache
  * Client TLS limitations
  * Client TLS security levels
  * Disabling TLS in the SMTP/LMTP client
  * Enabling TLS in the SMTP/LMTP client
  * Mandating TLS encryption
  * Mandating server certificate verification
  * Secure server certificate verification
  * Per-destination TLS policy
  * Obsolete per-site TLS policy support
  * Closing a DNS loophole with obsolete per-site TLS policies
  * Discovering servers that support TLS
  * Server certificate verification depth
  * Client-side cipher controls
  * Miscellaneous client controls

TTLLSS ssuuppppoorrtt iinn tthhee LLMMTTPP ddeelliivveerryy aaggeenntt

In Postfix 2.3, the smtp(8) and lmtp(8) delivery agents have been merged into a
single dual-purpose program. As a result the lmtp(8) delivery agent is no
longer the poor cousin of the more extensively used smtp(8). Specifically, as
of Postfix 2.3, all the TLS features described below apply equally to SMTP and
LMTP, after replacing the "smtp_" prefix of the each parameter name with
"lmtp_".

The LMTP delivery agent can communicate with LMTP servers listening on UNIX-
domain sockets. When server certificate verification is enabled and the server
is listening on a UNIX-domain socket, the $myhostname parameter is used to set
the TLS verification nexthop and hostname. Note, opportunistic encryption of
LMTP traffic over UNIX-domain sockets is futile. TLS is only useful in this
context when it is mandatory, typically to allow at least one of the server or
the client to authenticate the other. The "null" cipher grade may be
appropriate in this context, when available on both client and server. The
"null" ciphers provide authentication without encryption.

CClliieenntt--ssiiddee cceerrttiiffiiccaattee aanndd pprriivvaattee kkeeyy ccoonnffiigguurraattiioonn

Do not configure client certificates unless you mmuusstt present client TLS
certificates to one or more servers. Client certificates are not usually
needed, and can cause problems in configurations that work well without them.
The recommended setting is to let the defaults stand:

        smtp_tls_cert_file =
        smtp_tls_dcert_file =
        smtp_tls_key_file =
        smtp_tls_dkey_file =

The best way to use the default settings is to comment out the above parameters
in main.cf if present.

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 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:

    % ccaatt cclliieenntt__cceerrtt..ppeemm iinntteerrmmeeddiiaattee__CCAA..ppeemm >> cclliieenntt..ppeemm

A Postfix SMTP client certificate supplied here must be usable as 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 = $smtpd_tls_cert_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:

    # $$OOPPEENNSSSSLL__HHOOMMEE//bbiinn//cc__rreehhaasshh //ppaatthh//ttoo//ddiirreeccttoorryy

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 $smtpd_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

CClliieenntt--ssiiddee TTLLSS aaccttiivviittyy llooggggiinngg

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

CClliieenntt--ssiiddee TTLLSS sseessssiioonn ccaacchhee

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

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

CClliieenntt TTLLSS lliimmiittaattiioonnss

The security properties of TLS communication channels are application specific.
While the TLS protocol can provide a confidential, tamper-resistant, mutually
authenticated channel between client and server, not all of these security
features are applicable to every communication.

For example, while mutual TLS authentication between browsers and web servers
is possible, it is not practical, or even useful, for web-servers that serve
the public to verify the identity of every potential user. In practice, most
HTTPS transactions are asymmetric: the browser verifies the HTTPS server's
identity, but the user remains anonymous. Much of the security policy is up to
the client. If the client chooses to not verify the server's name, the server
is not aware of this. There are many interesting browser security topics, but
we shall not dwell on them here. Rather, our goal is to understand the security
features of TLS in conjunction with SMTP.

An important SMTP-specific observation is that a public MX host is even more at
the mercy of the SMTP client than is an HTTPS server. Not only can it not
enforce due care in the client's use of TLS, but it cannot even enforce the use
of TLS, because TLS support in SMTP clients is still the exception rather than
the rule. One cannot, in practice, limit access to one's MX hosts to just TLS-
enabled clients. Such a policy would result in a vast reduction in one's
ability to communicate by email with the world at large.

One may be tempted to try enforcing TLS for mail from specific sending
organizations, but this, too, runs into obstacles. One such obstacle is that we
don't know who is (allegedly) sending mail until we see the "MAIL FROM:" SMTP
command, and at that point, if TLS is not already in use, a potentially
sensitive sender address (and with SMTP PIPELINING one or more of the
recipients) has (have) already been leaked in the clear. Another obstacle is
that mail from the sender to the recipient may be forwarded, and the forwarding
organization may not have any security arrangements with the final destination.
Bounces also need to be protected. These can only be identified by the IP
address and HELO name of the connecting client, and it is difficult to keep
track of all the potential IP addresses or HELO names of the outbound email
servers of the sending organization.

Consequently, TLS security for mail delivery to public MX hosts is almost
entirely the client's responsibility. The server is largely a passive enabler
of TLS security, the rest is up to the client. While the server has a greater
opportunity to mandate client security policy when it is a dedicated MSA that
only handles outbound mail from trusted clients, below we focus on the client
security policy.

On the SMTP client, there are further complications. When delivering mail to a
given domain, in contrast to HTTPS, one rarely uses the domain name directly as
the target host of the SMTP session. More typically, one uses MX lookups -
these are usually unauthenticated - to obtain the domain's SMTP server hostname
(s). When, as is current practice, the client verifies the insecurely obtained
MX hostname, it is subject to a DNS man-in-the-middle attack.

If clients instead attempted to verify the recipient domain name, an SMTP
server for multiple domains would need to list all its email domain names in
its certificate, and generate a new certificate each time a new domain were
added. At least some CAs set fairly low limits (20 for one prominent CA) on the
number of names that server certificates can contain. This approach is not
consistent with current practice and does not scale.

It is regrettably the case that TLS secure-channels (fully authenticated and
immune to man-in-the-middle attacks) impose constraints on the sending and
receiving sites that preclude ubiquitous deployment. One needs to manually
configure this type of security for each destination domain, and in many cases
implement non-default TLS policy table entries for additional domains hosted at
a common secured destination. With Postfix 2.3, we make secure-channel
configurations substantially easier to configure, but they will never be the
norm. For the generic domain with which you have made no specific security
arrangements, this security level is not a good fit.

Given that strong authentication is not generally possible, and that verifiable
certificates cost time and money, many servers that implement TLS use self-
signed certificates or private CAs. This further limits the applicability of
verified TLS on the public Internet.

Historical note: while the documentation of these issues and many of the
related features are new with Postfix 2.3, the issue was well understood before
Postfix 1.0, when Lutz Jnicke was designing the first unofficial Postfix TLS
patch. See his original post http://www.imc.org/ietf-apps-tls/mail-archive/
msg00304.html and the first response http://www.imc.org/ietf-apps-tls/mail-
archive/msg00305.html. The problem is not even unique to SMTP or even TLS,
similar issues exist for secure connections via aliases for HTTPS and Kerberos.
SMTP merely uses indirect naming (via MX records) more frequently.

CClliieenntt TTLLSS sseeccuurriittyy lleevveellss

The TLS security levels listed below are described in more detail in the
sections that follow.

nnoonnee
    No TLS.
mmaayy
    Opportunistic TLS.
eennccrryypptt
    Mandatory TLS encryption.
vveerriiffyy
    Mandatory server certificate verification.
sseeccuurree
    Secure-channel TLS.

DDiissaabblliinngg TTLLSS iinn tthhee SSMMTTPP//LLMMTTPP cclliieenntt

At the "none" TLS security level, TLS encryption is disabled. This is the
default security level. With Postfix 2.3 and later, it can be configured
explicitly by setting "smtp_tls_security_level = none".

With Postfix 2.2 and earlier, or when smtp_tls_security_level is set to its
default (backwards compatible) empty value, the appropriate configuration
settings are "smtp_use_tls = no" and "smtp_enforce_tls = no". With either
approach, TLS is not used even if supported by the server. For LMTP, use the
corresponding "lmtp_" parameters.

Per destination settings may override this default setting, in which case TLS
is used selectively, only with destinations explicitly configured for TLS.

You can disable TLS for a subset of destinations, while leaving it enabled for
the rest. With the Postfix 2.3+ TLS policy table, specify the "none" security
level. With the obsolete per-site table, specify the "NONE" keyword.

OOppppoorrttuunniissttiicc TTLLSS

At the "may" TLS security level, TLS encryption is opportunistic. The SMTP
transaction is encrypted if the STARTTLS ESMTP feature is supported by the
server. Otherwise, messages are sent in the clear. With Postfix 2.3 and later,
opportunistic TLS can be configured by setting "smtp_tls_security_level = may".

Since sending in the clear is acceptable, demanding stronger than default TLS
security merely reduces inter-operability. For this reason, Postfix 2.3 and
later ignore the smtp_tls_mandatory_ciphers and smtp_tls_mandatory_protocols
parameters at the "may" security level: all protocols are allowed, and "export"
grade or better ciphers are used.

With Postfix 2.2 and earlier, or when smtp_tls_security_level is set to its
default (backwards compatible) empty value, the appropriate configuration
settings are "smtp_use_tls = yes" and "smtp_enforce_tls = no". For LMTP use the
corresponding "lmtp_" parameters.

With opportunistic TLS, mail delivery continues even if the server certificate
is untrusted or bears the wrong name. Starting with Postfix 2.3, when the TLS
handshake fails for an opportunistic TLS session, rather than give up on mail
delivery, the transaction is retried with TLS disabled. Trying an unencrypted
connection makes it possible to deliver mail to sites with non-interoperable
server TLS implementations.

Opportunistic encryption is never used for LMTP over UNIX-domain sockets. The
communications channel is already confidential without TLS, so the only
potential benefit of TLS is authentication. Do not configure opportunistic TLS
for LMTP deliveries over UNIX-domain sockets. Only configure TLS for LMTP over
UNIX-domain sockets at the encrypt security level or higher. Attempts to
configure opportunistic encryption of LMTP sessions will be ignored with a
warning written to the mail logs.

You can enable opportunistic TLS just for selected destinations. With the
Postfix 2.3+ TLS policy table, specify the "may" security level. With the
obsolete per-site table, specify the "MAY" keyword.

This is the most common security level for TLS protected SMTP sessions,
stronger security is not generally available and, if needed, is typically only
configured on a per-destination basis. See the section on TLS limitations
above.

Example:

    /etc/postfix/main.cf:
        smtp_tls_security_level = may

Postfix 2.2 syntax:

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

MMaannddaattoorryy TTLLSS eennccrryyppttiioonn

At the "encrypt" TLS security level, messages are sent only over TLS encrypted
sessions. The SMTP transaction is aborted unless the STARTTLS ESMTP feature is
supported by the server. If no suitable servers are found, the message will be
deferred. With Postfix 2.3 and later, mandatory TLS encryption can be
configured by setting "smtp_tls_security_level = encrypt". Even though TLS
encryption is always used, mail delivery continues if the server certificate is
untrusted or bears the wrong name.

At this security level and higher, the smtp_tls_mandatory_protocols and
smtp_tls_mandatory_ciphers configuration parameters determine the list of
sufficiently secure SSL protocol versions and the minimum cipher strength. If
the protocol or cipher requirements are not met, the mail transaction is
aborted. The documentation for these parameters includes useful
interoperability and security guidelines.

With Postfix 2.2 and earlier, or when smtp_tls_security_level is set to its
default (backwards compatible) empty value, the appropriate configuration
settings are "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = no". For
LMTP use the corresponding "lmtp_" parameters.

Despite the potential for eliminating passive eavesdropping attacks, mandatory
TLS encryption is not viable as a default security level for mail delivery to
the public Internet. Most MX hosts do not support TLS at all, and some of those
that do have broken implementations. On a host that delivers mail to the
Internet, you should not configure mandatory TLS encryption as the default
security level.

You can enable mandatory TLS encryption just for specific destinations. With
the Postfix 2.3+ TLS policy table, specify the "encrypt" security level. With
the obsolete per-site table, specify the "MUST_NOPEERMATCH" keyword. While the
obsolete approach still works with Postfix 2.3, it is strongly discouraged:
users of Postfix 2.3+ should use the new TLS policy settings.

Examples:

In the example below, traffic to example.com and its sub-domains via the
corresponding MX hosts always uses TLS. The protocol version will be "SSLv3" or
"TLSv1" (the default setting of smtp_tls_mandatory_protocols excludes "SSLv2").
Only high or medium strength (i.e. 128 bit or better) ciphers will be used by
default for all "encrypt" security level sessions.

    /etc/postfix/main.cf:
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy

    /etc/postfix/tls_policy:
        example.com       encrypt
        .example.com      encrypt

Postfix 2.2 syntax (no support for sub-domains without resorting to regexp
tables). With Postfix 2.3+, do not use the obsolete per-site table.

    /etc/postfix/main.cf:
        smtp_tls_per_site = hash:/etc/postfix/tls_per_site

    /etc/postfix/tls_per_site:
        example.com       MUST_NOPEERMATCH

In the next example, secure message submission is configured via the MSA "
[example.net]:587". TLS sessions are encrypted without authentication, because
this MSA does not possess an acceptable certificate. This MSA is known to be
capable of "TLSv1" and "high" grade ciphers, so these are selected via the
policy table.

NNoottee:: the policy table lookup key is the verbatim next-hop specification from
the recipient domain, transport(5) table or relayhost parameter, with any
enclosing square brackets and optional port. Take care to be consistent: the
suffixes ":smtp" or ":25" or no port suffix result in different policy table
lookup keys, even though they are functionally equivalent nexthop
specifications. Use at most one of these forms for all destinations. Below, the
policy table has multiple keys, just in case the transport table entries are
not specified consistently.

    /etc/postfix/main.cf:
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy

    /etc/services:
        submission      587/tcp         msa             # mail message
    submission

    /etc/postfix/tls_policy:
        [example.net]:587 encrypt protocols=TLSv1 ciphers=high
        [example.net]:msa encrypt protocols=TLSv1 ciphers=high
        [example.net]:submission encrypt protocols=TLSv1 ciphers=high

Postfix 2.2 syntax:

NNoottee:: Avoid policy lookups with the bare hostname (for example, "example.net").
Instead, use the destination (for example, "[example.net]:587"), as the per-
site table lookup key (a recipient domain or MX-enabled transport nexthop with
no port suffix may look like a bare hostname, but is still a suitable
destination). With Postfix 2.3+, do not use the obsolete per-site table; use
the new policy table instead.

    /etc/postfix/main.cf:
        smtp_tls_per_site = hash:/etc/postfix/tls_per_site

    /etc/postfix/tls_per_site:
        [example.net]:587   MUST_NOPEERMATCH

MMaannddaattoorryy sseerrvveerr cceerrttiiffiiccaattee vveerriiffiiccaattiioonn

At the "verify" TLS security level, messages are sent only over TLS encrypted
sessions if the server certificate is valid (not expired or revoked, and signed
by a trusted certificate authority) and if the server certificate name matches
a known pattern. Mandatory server certificate verification can be configured by
setting "smtp_tls_security_level = verify". The smtp_tls_verify_cert_match
parameter can override the default "hostname" certificate name matching
strategy. Fine-tuning the matching strategy is generally only appropriate for
secure-channel destinations.

With Postfix 2.2 and earlier, or when smtp_tls_security_level is set to its
default (backwards compatible) empty value, the appropriate configuration
settings are "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = yes".
For LMTP use the corresponding "lmtp_" parameters.

If the server certificate chain is trusted (see smtp_tls_CAfile and
smtp_tls_CApath), any DNS names in the SubjectAlternativeName certificate
extension are used to verify the server name. If no DNS names are specified,
the certificate CommonName is checked. If you want mandatory encryption without
server certificate verification, see above.

Despite the potential for eliminating "man-in-the-middle" and other attacks,
mandatory certificate trust chain and subject name verification is not viable
as a default Internet mail delivery policy. Most MX hosts do not support TLS at
all, and a significant portion of TLS enabled MTAs use self-signed
certificates, or certificates that are signed by a private certificate
authority. On a machine that delivers mail to the Internet, you should not
configure mandatory server certificate verification as a default policy.

Mandatory server certificate verification as a default security level may be
appropriate if you know that you will only connect to servers that support RFC
2487 and that present verifiable server certificates. An example would be a
client that sends all email to a central mailhub that offers the necessary
STARTTLS support. In such cases, you can often use a secure-channel
configuration instead.

You can enable mandatory server certificate verification just for specific
destinations. With the Postfix 2.3+ TLS policy table, specify the "verify"
security level. With the obsolete per-site table, specify the "MUST" keyword.
While the obsolete approach still works with Postfix 2.3, it is strongly
discouraged: users of Postfix 2.3+ should use the new TLS policy settings.

Example:

In this example, the client encrypts all traffic to the example.com domain. The
peer hostname is verified, but verification is vulnerable to DNS response
forgery. Mail transmission to example.com recipients uses "high" grade ciphers.

    /etc/postfix/main.cf:
        indexed = ${default_database_type}:${config_directory}/
        smtp_tls_CAfile = ${config_directory}/CAfile.pem
        smtp_tls_policy_maps = ${indexed}tls_policy

    /etc/postfix/tls_policy:
        example.com       verify ciphers=high

Postfix 2.2 syntax:

    /etc/postfix/main.cf:
        indexed = ${default_database_type}:${config_directory}/
        smtp_tls_CAfile = ${config_directory}/CAfile.pem
        smtp_tls_per_site = ${indexed}tls_per_site

    /etc/postfix/tls_per_site:
        example.com         MUST

SSeeccuurree sseerrvveerr cceerrttiiffiiccaattee vveerriiffiiccaattiioonn

At the secure TLS security level, messages are sent only over secure-channel
TLS sessions where DNS forgery resistant server certificate verification
succeeds. If no suitable servers are found, the message will be deferred. With
Postfix 2.3 and later, secure-channels can be configured by setting
"smtp_tls_security_level = secure". The smtp_tls_secure_cert_match parameter
can override the default "nexthop, dot-nexthop" certificate match strategy.

With Postfix 2.2 and earlier, or when smtp_tls_security_level is set to its
default (backwards compatible) empty value, the appropriate configuration
settings are "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = yes"
with additional settings to harden peer certificate verification against forged
DNS data. For LMTP, use the corresponding "lmtp_" parameters.

If the server certificate chain is trusted (see smtp_tls_CAfile and
smtp_tls_CApath), any DNS names in the SubjectAlternativeName certificate
extension are used to verify the server name. If no DNS names are specified,
the CommonName is checked. If you want mandatory encryption without server
certificate verification, see above.

Despite the potential for eliminating "man-in-the-middle" and other attacks,
mandatory secure server certificate verification is not viable as a default
Internet mail delivery policy. Most MX hosts do not support TLS at all, and a
significant portion of TLS enabled MTAs use self-signed certificates, or
certificates that are signed by a private certificate authority. On a machine
that delivers mail to the Internet, you should not configure secure TLS
verification as a default policy.

Mandatory secure server certificate verification as a default security level
may be appropriate if you know that you will only connect to servers that
support RFC 2487 and that present verifiable server certificates. An example
would be a client that sends all email to a central mailhub that offers the
necessary STARTTLS support.

You can enable secure TLS verification just for specific destinations. With the
Postfix 2.3+ TLS policy table, specify the "secure" security level. With the
obsolete per-site table, specify the "MUST" keyword and harden the certificate
verification against DNS forgery. While the obsolete approach still works with
Postfix 2.3, it is strongly discouraged: users of Postfix 2.3+ should use the
new TLS policy settings.

Examples:

Secure-channel TLS without transport(5) table overrides:

The client will encrypt all traffic and verify the destination name immune from
forged DNS responses. MX lookups are still used to find the SMTP servers for
example.com, but these are not used when checking the names in the server
certificate(s). Rather, the requirement is that the MX hosts for example.com
have trusted certificates with a subject name of example.com or a sub-domain,
see the documentation for the smtp_tls_secure_cert_match parameter.

The related domains example.co.uk and example.co.jp are hosted on the same MX
hosts as the primary example.com domain, and traffic to these is secured by
verifying the primary example.com domain in the server certificates. This frees
the server administrator from needing the CA to sign certificates that list all
the secondary domains. The downside is that clients that want secure channels
to the secondary domains need explicit TLS policy table entries.

Note, there are two ways to handle related domains. The first is to use the
default routing for each domain, but add policy table entries to override the
expected certificate subject name. The second is to override the next-hop in
the transport table, and use a single policy table entry for the common
nexthop. We choose the first approach, because it works better when domain
ownership changes. With the second approach we securely deliver mail to the
wrong destination, with the first approach, authentication fails and mail stays
in the local queue, the first approach is more appropriate in most cases.

    /etc/postfix/main.cf:
        smtp_tls_CAfile = /etc/postfix/CAfile.pem
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy

    /etc/postfix/transport:

    /etc/postfix/tls_policy:
        example.com     secure
        example.co.uk   secure match=example.com:.example.com
        example.co.jp   secure match=example.com:.example.com

Secure-channel TLS with transport(5) table overrides:

In this case traffic to example.com and its related domains is sent to a single
logical gateway (to avoid a single point of failure, its name may resolve to
one or more load-balancer addresses, or to the combined addresses of multiple
physical hosts). All the physical hosts reachable via the gateway's IP
addresses have the logical gateway name listed in their certificates. This
secure-channel configuration can also be implemented via a hardened variant of
the MUST policy in the obsolete per-site table. As stated above, this approach
has the potential to mis-deliver email if the related domains change hands.

    /etc/postfix/main.cf:
        smtp_tls_CAfile = /etc/postfix/CAfile.pem
        transport_maps = hash:/etc/postfix/transport
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy

    /etc/postfix/transport:
        example.com     smtp:[tls.example.com]
        example.co.uk   smtp:[tls.example.com]
        example.co.jp   smtp:[tls.example.com]

    /etc/postfix/tls_policy:
        [tls.example.com] secure match=tls.example.com

Postfix 2.2.9+ syntax:

NNoottee:: Avoid policy lookups with the bare hostname (for example,
"tls.example.com"). Instead, use the destination (for example, "
[tls.example.com]") as the per-site table lookup key (a recipient domain or MX-
enabled transport nexthop with no port suffix may look like a bare hostname,
but is still a suitable destination). With Postfix 2.3+, do not use the
obsolete per-site table; use the new policy table instead.

    /etc/postfix/main.cf:
        smtp_cname_overrides_servername = no
        smtp_tls_CAfile = /etc/postfix/CAfile.pem
        transport_maps = hash:/etc/postfix/transport
        smtp_tls_per_site = hash:/etc/postfix/tls_per_site

    /etc/postfix/transport:
        example.com     smtp:[tls.example.com]
        example.co.uk   smtp:[tls.example.com]
        example.co.jp   smtp:[tls.example.com]

    /etc/postfix/tls_per_site:
        [tls.example.com]       MUST

TTLLSS ppoolliiccyy ttaabbllee

Postfix 2.3 introduces a new more flexible TLS policy table. For earlier
releases, read the description of the obsolete Postfix 2.2 per-site table.

A small fraction of servers offer STARTTLS but the negotiation consistently
fails. With Postfix 2.3, so long as encryption is not enforced, the delivery is
immediately retried with TLS disabled. You no longer need to explicitly disable
TLS for the problem destinations. As soon as their TLS software or
configuration is repaired, encryption will be used.

The new policy table is specified via the smtp_tls_policy_maps parameter. This
lists optional lookup tables with the Postfix SMTP client TLS security policy
by next-hop destination. When $smtp_tls_policy_maps is not empty, the obsolete
smtp_tls_per_site parameter is ignored (a warning is written to the logs if
both parameter values are non-empty).

The TLS policy table is indexed by the full next-hop destination, which is
either the recipient domain, or the verbatim next-hop specified in the
transport table, $local_transport, $virtual_transport, $relay_transport or
$default_transport. This includes any enclosing square brackets and any non-
default destination server port suffix. The LMTP socket type prefix (inet: or
unix:) is not included in the lookup key.

Only the next-hop domain, or $myhostname with LMTP over UNIX-domain sockets, is
used as the nexthop name for certificate verification. The port and any
enclosing square brackets are used in the table lookup key, but are not used
for server name verification.

When the lookup key is a domain name without enclosing square brackets or any :
port suffix (typically the recipient domain), and the full domain is not found
in the table, just as with the transport(5) table, the parent domain starting
with a leading "." is matched recursively. This allows one to specify a
security policy for a recipient domain and all its sub-domains.

The lookup result is a security level, followed by an optional list of
whitespace and/or comma separated name=value attributes that override related
main.cf settings. The TLS security levels are described above. Below, we
describe the corresponding table syntax:

nnoonnee
    No TLS. No additional attributes are supported at this level.
mmaayy
    Opportunistic TLS. No additional attributes are supported at this level.
eennccrryypptt
    Mandatory TLS encryption. Mail is delivered only if remote SMTP server
    offers STARTTLS and the TLS handshake succeeds. At this level and higher
    the optional "ciphers" attribute overrides the main.cf
    smtp_tls_mandatory_ciphers parameter and the optional "protocols" keyword
    overrides the main.cf smtp_tls_mandatory_protocols parameter.
vveerriiffyy
    Mandatory server certificate verification. Mail is delivered only if the
    TLS handshake succeeds, if the server certificate can be validated (not
    expired or revoked, and signed by a trusted certificate authority), and if
    the server certificate name matches the optional "match" attribute (or the
    main.cf smtp_tls_verify_cert_match parameter value when no optional "match"
    attribute is specified).
sseeccuurree
    Secure-channel TLS. Mail is delivered only if the TLS handshake succeeds,
    if the server certificate can be validated (not expired or revoked, and
    signed by a trusted certificate authority), and if the server certificate
    name matches the optional "match" attribute (or the main.cf
    smtp_tls_secure_cert_match parameter value when no optional "match"
    attribute is specified).
Notes:

  * The "match" attribute is especially useful to verify TLS certificates for
    domains that are hosted on a shared server. In that case, specify "match"
    rules for the shared server's name. While secure verification can also be
    achieved with manual routing overrides in Postfix transport(5) tables, that
    approach can deliver mail to the wrong host when domains are assigned to
    new gateway hosts. The "match" attribute approach avoids the problems of
    manual routing overrides; mail is deferred if verification of a new MX host
    fails.

  * When a policy table entry specifies multiple match patterns, multiple match
    strategies, or multiple protocols, these must be separated by colons.

Example:

    /etc/postfix/main.cf:
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
    /etc/postfix/tls_policy:
        example.edu             none
        example.mil             may
        example.gov             encrypt protocols=SSLv3:TLSv1 ciphers=high
        example.com             verify
                match=hostname:dot-nexthop protocols=SSLv3:TLSv1 ciphers=high
        example.net             secure
        .example.net            secure match=.example.net:example.net
        [mail.example.org]:587  secure match=nexthop

NNoottee:: The "hostname" strategy if listed in a non-default setting of
smtp_tls_secure_cert_match or in the "match" attribute in the policy table can
render the "secure" level vulnerable to DNS forgery. Do not use the "hostname"
strategy for secure-channel configurations in environments where DNS security
is not assured.

OObbssoolleettee ppeerr--ssiittee TTLLSS ppoolliiccyy ssuuppppoorrtt

This section describes an obsolete per-site TLS policy mechanism. Unlike the
Postfix 2.3 policy table mechanism, this uses as a policy lookup key a
potentially untrusted server hostname, and lacks control over what names can
appear in server certificates. Because of this, the obsolete mechanism is
typically vulnerable to false DNS hostname information in MX or CNAME records.
These attacks can be eliminated only with great difficulty. The new policy
table makes secure-channel configurations easier and provides more control over
the cipher and protocol selection for sessions with mandatory encryption.

Avoid policy lookups with the bare hostname. Instead, use the full destination
nexthop (enclosed in [] with a possible ":port" suffix) as the per-site table
lookup key (a recipient domain or MX-enabled transport nexthop with no port
suffix may look like a bare hostname, but is still a suitable destination).
With Postfix 2.3+, use of the obsolete approach documented here is strongly
discouraged: use the new policy table instead.

Starting with Postfix 2.3, the underlying TLS enforcement levels are common to
the obsolete per-site table and the new policy table. The main.cf
smtp_tls_mandatory_ciphers and smtp_tls_mandatory_protocols parameters control
the TLS ciphers and protocols for mandatory encryption regardless of which
table is used. The smtp_tls_verify_cert_match parameter determines the match
strategy for the obsolete "MUST" keyword in the same way as for the "verify"
level in the new policy.

With Postfix < 2.3, the obsolete smtp_tls_cipherlist parameter is also applied
for opportunistic TLS sessions, and should be used with care, or not at all.
Setting cipherlist restrictions that are incompatible with a remote SMTP server
render that server unreachable, TLS handshakes are always attempted and always
fail.

When smtp_tls_policy_maps is empty (default) and smtp_tls_per_site is not
empty, the 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. Use of the hostname lookup key is
        discouraged; always use the next-hop destination instead.
    next-hop destination
        This is normally the domain portion of the recipient address, but it
        may be overridden by information from the transport(5) table, from the
        relayhost parameter setting, or from the relay_transport setting. When
        it is not the recipient domain, the next-hop destination can have the
        Postfix-specific form "[name]", "[name]:port", "name" or "name:port".
        This is the recommended lookup key for per-site policy lookups (and
        incidentally for SASL password lookups).

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
        No TLS. 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
        Opportunistic TLS. 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
        Mandatory TLS encryption. 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
        Mandatory server certificate verification. 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.

CClloossiinngg aa DDNNSS lloooopphhoollee wwiitthh oobbssoolleettee ppeerr--ssiittee TTLLSS ppoolliicciieess

For a general discussion of TLS security for SMTP see TLS limitations above.
What follows applies only to Postfix 2.2.9 and subsequent Postfix 2.2 patch
levels. Do not use this approach with Postfix 2.3+; instead see the
instructions under secure server certificate verification.

As long as no secure DNS lookup mechanism is available, false hostnames in MX
or CNAME responses can change Postfix's notion of the server hostname that is
used 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 all of the following steps:

 1. Use a dedicated message delivery transport (for example, "securetls") as
    illustrated below.

 2. Eliminate MX lookups. Specify local transport(5) table entries for
    sensitive domains with explicit securetls:[mailhost] or securetls:
    [mailhost]:port destinations (you can assure security of this table unlike
    DNS). This prevents false hostname information in DNS MX records from
    changing Postfix's notion of the server hostname that is used for TLS
    policy lookup and server certificate verification. The "securetls"
    transport is configured to enforce TLS with peername verification, and to
    disable the SMTP connection cache which could interfere with enforcement of
    smtp_tls_per_site policies.

 3. 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. The default value is "no"
    starting with Postfix 2.3.

Example:

We give the non-default "securetls" transport an explicit master.cf process
limit, so that we don't raise its process limit when raising
$default_process_limit. The total process limit for *all* transports should
stay somewhat under 1024 (the typical select() file descriptor limit);
otherwise transports may be throttled under steady high load, compounding
congestion. It is not uncommon at high volume sites to set the default process
limit to 500 or more.

We also default the "securetls" transport TLS security level to MUST, obviating
the need for per-site table entries for secure-channel destinations.

    /etc/postfix/main.cf:
        transport_maps = hash:/etc/postfix/transport

    /etc/postfix/transport:
        example.com         securetls:[tls.example.com]

    /etc/postfix/master.cf:
        securetls unix  -       -       n       -       100     smtp
            -o smtp_enforce_tls=yes
            -o smtp_tls_enforce_peername=yes

DDiissccoovveerriinngg sseerrvveerrss tthhaatt ssuuppppoorrtt TTLLSS

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

SSeerrvveerr cceerrttiiffiiccaattee vveerriiffiiccaattiioonn ddeepptthh

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 (where the root CA issues a special CA certificate which then
issues the actual certificate).

Example:

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

CClliieenntt--ssiiddee cciipphheerr ccoonnttrroollss

The Postfix SMTP client supports 5 distinct cipher security levels as specified
by the smtp_tls_mandatory_ciphers configuration parameter. This setting
controls the minimum acceptable SMTP client TLS cipher grade for use with
mandatory TLS encryption. The default value "medium" is suitable for most
destinations with which you may want to enforce TLS, and is beyond the reach of
today's crypt-analytic methods. See smtp_tls_policy_maps for information on how
to configure ciphers on a per-destination basis.

By default anonymous ciphers are allowed, and automatically disabled when
server certificates are verified. If you want to disable anonymous ciphers even
at the "encrypt" security level, set "smtp_tls_mandatory_exclude_ciphers =
aNULL"; and to disable anonymous ciphers even with opportunistic TLS, set
"smtp_tls_exclude_ciphers = aNULL". There is generally no need to take these
measures. Anonymous ciphers save bandwidth and TLS session cache space, if
certificates are ignored, there is little point in requesting them.

Example:

    /etc/postfix/main.cf:
        smtp_tls_mandatory_ciphers = medium
        smtp_tls_mandatory_exclude_ciphers = RC4, MD5
        smtp_tls_exclude_ciphers = aNULL

MMiisscceellllaanneeoouuss cclliieenntt ccoonnttrroollss

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

TTLLSS mmaannaaggeerr ssppeecciiffiicc sseettttiinnggss

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 nnoott inside the chroot
jail).

Examples:

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

GGeettttiinngg ssttaarrtteedd,, qquuiicckk aanndd ddiirrttyy

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 Certificate
Authority, and Postfix needs to be configured with a list of public key
certificates of Certificate Authorities, so that Postfix can verify the public
key certificates of remote hosts.

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

  * Become your own Certificate 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.

        % //uussrr//llooccaall//ssssll//mmiisscc//CCAA..ppll --nneewwccaa
        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:wwhhaatteevveerr

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

        % ooppeennssssll rreeqq --nneeww --nnooddeess --kkeeyyoouutt FFOOOO--kkeeyy..ppeemm --oouutt FFOOOO--rreeqq..ppeemm --ddaayyss
        336655
        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]:UUSS
        State or Province Name (full name) [Some-State]:NNeeww YYoorrkk
        Locality Name (eg, city) []:WWeessttcchheesstteerr
        Organization Name (eg, company) [Internet Widgits Pty Ltd]:PPoorrccuuppiinnee
        Organizational Unit Name (eg, section) []:
        Common Name (eg, YOUR name) []:FFOOOO
        Email Address []:wwiieettssee@@ppoorrccuuppiinnee..oorrgg

        Please enter the following 'extra' attributes
        to be sent with your certificate request
        A challenge password []:wwhhaatteevveerr
        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.

        % ooppeennssssll ccaa --oouutt FFOOOO--cceerrtt..ppeemm --iinnffiilleess FFOOOO--rreeqq..ppeemm
        Using configuration from /etc/ssl/openssl.cnf
        Enter PEM pass phrase:wwhhaatteevveerr
        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]:yy

        1 out of 1 certificate requests certified, commit? [y/n]yy
        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.

        # ccpp ddeemmooCCAA//ccaacceerrtt..ppeemm FFOOOO--kkeeyy..ppeemm FFOOOO--cceerrtt..ppeemm //eettcc//ppoossttffiixx
        # cchhmmoodd 664444 //eettcc//ppoossttffiixx//FFOOOO--cceerrtt..ppeemm //eettcc//ppoossttffiixx//ccaacceerrtt..ppeemm
        # cchhmmoodd 440000 //eettcc//ppoossttffiixx//FFOOOO--kkeeyy..ppeemm

  * Configure Postfix, by adding the following to /etc/postfix/main.cf. It is
    generally best to not configure client certificates, unless there are
    servers which authenticate your mail submission via client certificates.
    Often servers that perform TLS client authentication will issue the
    required certificates signed by their own CA. If you configure the client
    certificate and key incorrectly, you will be unable to send mail to sites
    that request client certificate, but don't require them from all clients.

        /etc/postfix/main.cf:
            smtp_tls_CAfile = /etc/postfix/cacert.pem
            smtp_tls_session_cache_database =
        	btree:/var/spool/postfix/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/spool/postfix/smtpd_tls_session_cache
            tls_random_source = dev:/dev/urandom
            # Postfix 2.3 and later
            smtpd_tls_security_level = may
            # Obsolete, but still supported
            smtpd_use_tls = yes

RReeppoorrttiinngg pprroobblleemmss

Problems are preferably reported via <postfix-users@postfix.org>. See http://
www.postfix.org/lists.html for subscription information. When reporting a
problem, please be thorough in the report. Patches, when possible, are greatly
appreciated too.

CCoommppaattiibbiilliittyy wwiitthh PPoossttffiixx << 22..22 TTLLSS ssuuppppoorrtt

Postfix version 2.2 TLS support is based on the Postfix/TLS patch by Lutz
Jnicke, 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.

CCrreeddiittss

  * TLS support for Postfix was originally developed by Lutz Jnicke 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.