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ADDRESS_REWRITING_README	42.68 KB	-rw-r--r--
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BACKSCATTER_README	12.3 KB	-rw-r--r--
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INSTALL	32.72 KB	-rw-r--r--
IPV6_README	11.73 KB	-rw-r--r--
LDAP_README	19.83 KB	-rw-r--r--
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OVERVIEW	18.57 KB	-rw-r--r--
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SMTPD_PROXY_README	10.47 KB	-rw-r--r--
STANDARD_CONFIGURATION_README	25.24 KB	-rw-r--r--
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Code Editor : TLS_README

PPoossttffiixx TTLLSS SSuuppppoorrtt

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

WWAARRNNIINNGG

By turning on TLS support in Postfix, you not only get the ability to encrypt
mail and to authenticate remote SMTP 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.

This document describes a TLS user interface that was introduced with Postfix
version 2.3. Support for an older user interface is documented in
TLS_LEGACY_README, which also describes the differences between Postfix and the
third-party patch on which Postfix version 2.2 TLS support was based.

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
  * 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
                    <-key/cert->             <-key/cert->

/       |    \
                                        |
                              /                \

smtpd           PRNG         smtp
                     session         state        session
                    key cache         file       key cache

BBuuiillddiinngg PPoossttffiixx wwiitthh TTLLSS ssuuppppoorrtt

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

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.

NNOOTTEE:: DDoo nnoott uussee GGnnuu TTLLSS.. IItt wwiillll ssppoonnttaanneeoouussllyy tteerrmmiinnaattee aa PPoossttffiixx ddaaeemmoonn
pprroocceessss wwiitthh eexxiitt ssttaattuuss ccooddee 22,, iinnsstteeaadd ooff aalllloowwiinngg PPoossttffiixx ttoo 11)) rreeppoorrtt tthhee
eerrrroorr ttoo tthhee mmaaiilllloogg ffiillee,, aanndd ttoo 22)) pprroovviiddee ppllaaiinntteexxtt sseerrvviiccee wwhheerree tthhiiss iiss
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 a password. The
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 remote SMTP client will generally not be able to
authenticate the self-signed certificate, but unless the client 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 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, a certificate-less Postfix SMTP server will be unable to receive email
from most TLS enabled clients. To avoid accidental configurations with no
certificates, Postfix enables certificate-less operation only when the
administrator explicitly sets "smtpd_tls_cert_file = none". This ensures that
new Postfix SMTP server configurations will not accidentally run with no
certificates.

RSA, DSA and ECDSA (Postfix >= 2.6) 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
configure all three 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.

To enable a remote SMTP client to verify the Postfix SMTP server certificate,
the issuing CA certificates must be made available to the client. You should
include the required certificates in the server certificate file, the server
certificate first, then the issuing CA(s) (bottom-up order).

Example: the certificate for "server.example.com" 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.

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

Their ECDSA counterparts (Postfix >= 2.6 + OpenSSL >= 0.9.9):

/etc/postfix/main.cf:
        # Most clients will not be ECDSA capable, so you will likely also need
        # an RSA or DSA certificate and private key.
        #
        smtpd_tls_eccert_file = /etc/postfix/server-ecdsa.pem
        smtpd_tls_eckey_file = $smtpd_tls_eccert_file

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 the Postfix SMTP server to request client certificates, the
DNs of certificate authorities 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, the Postfix SMTP server announces STARTTLS support to remote 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 Postfix SMTP 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 remote SMTP
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 remote SMTP clients, these are automatically suppressed
when the Postfix SMTP 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

The client certificate verification depth is specified with the main.cf
smtpd_tls_ccert_verifydepth parameter. The default verification depth is 9 (the
OpenSSL default), for compatibility with Postfix versions before 2.5 where
smtpd_tls_ccert_verifydepth was ignored. 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).

Example:

/etc/postfix/main.cf:
        smtpd_tls_ccert_verifydepth = 2

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:/var/lib/postfix/smtpd_scache

Note: as of version 2.5, Postfix no longer uses root privileges when opening
this file. The file should now be stored under the Postfix-owned
data_directory. As a migration aid, an attempt to open the file under a non-
Postfix directory is 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

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 request if the client certificate
        fingerprint is listed in the client certificate table (see
        relay_clientcerts discussion below).

permit_tls_all_clientcerts
        Allow the remote SMTP client request if the client certificate passes
        trust chain verification. Useful with private-label CAs that only issue
        certificates to trusted clients (and not otherwise).

check_ccert_access type:table
        Use the remote SMTP client certificate fingerprint as the lookup key
        for the specified access(5) table.

The digest algorithm used to construct the client certificate fingerprints is
specified with the main.cf smtpd_tls_fingerprint_digest parameter. The default
is "md5", for compatibility with Postfix versions < 2.5.

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
            ...

Example: 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:

/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.

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 typically "export". The
corresponding smtpd_tls_ciphers parameter (Postfix >= 2.6) controls the cipher
grade used with opportunistic TLS.

By default anonymous ciphers are enabled. They are automatically disabled when
remote SMTP client certificates are requested. If clients are expected to
always verify the Postfix SMTP server certificate you may want to disable
anonymous ciphers by setting "smtpd_tls_mandatory_exclude_ciphers = aNULL" or
"smtpd_tls_exclude_ciphers = aNULL", as appropriate. One can't force a remote
SMTP client to check the server certificate, so excluding anonymous ciphers is
generally unnecessary.

The "smtpd_tls_ciphers" configuration parameter (Postfix >= 2.6) provides
control over the minimum cipher grade for opportunistic TLS. With Postfix <
2.6, the minimum opportunistic TLS cipher grade is always "export".

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. The
mandatory TLS protocol list is specified via the smtpd_tls_mandatory_protocols
configuration parameter. The corresponding smtpd_tls_protocols parameter
(Postfix >= 2.6) controls the SSL/TLS protocols used with opportunistic TLS.

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 or smtpd_tls_eccert_file.

Example, MSA that requires TLSv1, not SSLv2 or SSLv3, 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
        smtpd_tls_security_level = encrypt
        smtpd_tls_mandatory_protocols = TLSv1
        # Also available with Postfix >= 2.5:
        smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3

If you want to take advantage of ciphers with ephemeral Diffie-Hellman (EDH)
key exchange (this offers "forward-secrecy"), DH parameters are needed. Instead
of using the built-in DH parameters for both 1024-bit (non-export ciphers) and
512-bit (export ciphers), 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. Postfix defaults to compiled-in
parameters that are shared by all Postfix users who don't generate their own
settings.

To generate your own set of DH parameters, use:

% ooppeennssssll ggeennddhh --oouutt //eettcc//ppoossttffiixx//ddhh__551122..ppeemm --22 551122
    % ooppeennssssll ggeennddhh --oouutt //eettcc//ppoossttffiixx//ddhh__11002244..ppeemm --22 11002244

Support for elliptic curve cryptography is available with Postfix 2.6 and
OpenSSL 0.9.9 or later. To enable ephemeral elliptic curve Diffie-Hellman
(EECDH) key-exchange, set "smtpd_tls_eecdh_grade = strong" or
"smtpd_tls_eecdh_grade = ultra". The "ultra" setting is substantially more CPU
intensive, and "strong" is sufficiently secure for most situations.

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
        # Postfix >= 2.6:
        smtpd_tls_eecdh_grade = strong

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
  * Configuring TLS in the SMTP/LMTP client
  * 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
  * Client-side SMTPS support
  * Miscellaneous client controls

TTLLSS ssuuppppoorrtt iinn tthhee LLMMTTPP ddeelliivveerryy aaggeenntt

The smtp(8) and lmtp(8) delivery agents are implemented by a single dual-
purpose program. Specifically, 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 Postfix 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 Postfix SMTP 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 =
        # Postfix >= 2.6
        smtp_tls_eccert_file =
        smtp_tls_eckey_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.

RSA, DSA and ECDSA (Postfix >= 2.6) certificates are supported. You can
configure all three 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.

To enable remote SMTP servers to verify the Postfix SMTP client certificate,
the issuing CA certificates must be made available to the server. You should
include the required certificates in the client certificate file, the client
certificate first, then the issuing CA(s) (bottom-up order).

Example: the certificate for "client.example.com" was issued by "intermediate
CA" which itself has a certificate issued by "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.

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

Their ECDSA counterparts (Postfix >= 2.6 + OpenSSL >= 0.9.9):

/etc/postfix/main.cf:
        smtp_tls_eccert_file = /etc/postfix/client-ecdsa.pem
        smtp_tls_eckey_file = $smtp_tls_eccert_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 $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

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:/var/lib/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 Jänicke 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.

CCoonnffiigguurriinngg TTLLSS iinn tthhee SSMMTTPP//LLMMTTPP cclliieenntt

Similar to the Postfix SMTP server, the Postfix SMTP/LMTP client implements
multiple TLS security levels. These levels are described in more detail in the
sections that follow.

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

NNoo TTLLSS eennccrryyppttiioonn

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 and later 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 mostly reduces inter-operability. If you must restrict TLS protocol or
cipher selection even with opportunistic TLS, the "smtp_tls_ciphers" and
"smtp_tls_protocols" configuration parameters (Postfix >= 2.6) provide control
over the protocols and cipher grade used with opportunistic TLS. With earlier
releases the opportunistic TLS cipher grade is always "export" and no protocols
are disabled.

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 and later 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 remote SMTP 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 even 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 and later 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 and later 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 and later, 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 and later, 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

CCeerrttiiffiiccaattee ffiinnggeerrpprriinntt vveerriiffiiccaattiioonn

Certificate fingerprint verification is available with Postfix 2.5 and later.
At this security level ("smtp_tls_security_level = fingerprint"), no trusted
certificate authorities are used or required. The certificate trust chain,
expiration date, ... are not checked. Instead, the
smtp_tls_fingerprint_cert_match parameter or the "match" attribute in the
policy table lists the valid "fingerprints" of the remote SMTP server
certificate.

If certificate fingerprints are exchanged securely, this is the strongest, and
least scalable security level. The administrator needs to securely collect the
fingerprints of the X.509 certificates of each peer server, store them into a
local file, and update this local file whenever the peer server's public
certificate changes. This may be feasible for an SMTP "VPN" connecting a small
number of branch offices over the Internet, or for secure connections to a
central mail hub. It works poorly if the remote SMTP server is managed by a
third party, and its public certificate changes periodically without prior
coordination with the verifying site.

The digest algorithm used to calculate the fingerprint is selected by the
ssmmttpp__ttllss__ffiinnggeerrpprriinntt__ddiiggeesstt parameter. In the policy table multiple
fingerprints can be combined with a "|" delimiter in a single match attribute,
or multiple match attributes can be employed. The ":" character is not used as
a delimiter as it occurs between each pair of fingerprint (hexadecimal) digits.

Example: fingerprint TLS security with an internal mailhub. Two matching
fingerprints are listed. The relayhost may be multiple physical hosts behind a
load-balancer, each with its own private/public key and self-signed
certificate. Alternatively, a single relayhost may be in the process of
switching from one set of private/public keys to another, and both keys are
trusted just prior to the transition.

relayhost = [mailhub.example.com]
        smtp_tls_security_level = fingerprint
        smtp_tls_fingerprint_digest = md5
        smtp_tls_fingerprint_cert_match =
            3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
            EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35

Example: Certificate fingerprint verification with selected destinations. As in
the example above, we show two matching fingerprints:

/etc/postfix/main.cf:
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
        smtp_tls_fingerprint_digest = md5

/etc/postfix/tls_policy:
        example.com	fingerprint
            match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
            match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35

MMaannddaattoorryy sseerrvveerr cceerrttiiffiiccaattee vveerriiffiiccaattiioonn

At the "verify" TLS security level, messages are sent only over TLS encrypted
sessions if the remote SMTP server certificate is valid (not expired or
revoked, and signed by a trusted certificate authority) and where 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 remote SMTP 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 and later 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 and later should use the new TLS
policy settings.

Example:

In this example, the Postfix SMTP 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 remote SMTP 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 and later 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 and
later should use the new TLS policy settings.

Examples:

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

The Postfix SMTP client will encrypt all traffic and verify the destination
name immune from forged DNS responses. MX lookups are still used to find the
hostnames of the SMTP servers for example.com, but these hostnames 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 and later 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 and later, 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

The current TLS policy table was introduced with Postfix 2.3. 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. The optional "ciphers", "exclude" and "protocols"
    attributes (available for opportunistic TLS with Postfix >= 2.6) override
    the "smtp_tls_ciphers", "smtp_tls_exclude_ciphers" and "smtp_tls_protocols"
    configuration parameters.
eennccrryypptt
    Mandatory encryption. Mail is delivered only if the remote SMTP server
    offers STARTTLS and the TLS handshake succeeds. At this level and higher,
    the optional "protocols" attribute overrides the main.cf
    smtp_tls_mandatory_protocols parameter, the optional "ciphers" attribute
    overrides the main.cf smtp_tls_mandatory_ciphers parameter, and the
    optional "exclude" attribute (Postfix >= 2.6) overrides the main.cf
    smtp_tls_mandatory_exclude_ciphers parameter.
ffiinnggeerrpprriinntt
    Certificate fingerprint verification. Available with Postfix 2.5 and later.
    At this security level, there are no trusted certificate authorities. The
    certificate trust chain, expiration date, ... are not checked. Instead, the
    optional mmaattcchh attribute, or else the main.cf
    ssmmttpp__ttllss__ffiinnggeerrpprriinntt__cceerrtt__mmaattcchh parameter, lists the valid fingerprints of
    the server certificate. The digest algorithm used to calculate fingerprints
    is selected by the ssmmttpp__ttllss__ffiinnggeerrpprriinntt__ddiiggeesstt parameter. Multiple
    fingerprints can be combined with a "|" delimiter in a single match
    attribute, or multiple match attributes can be employed. The ":" character
    is not used as a delimiter as it occurs between each pair of fingerprint
    (hexadecimal) digits.
vveerriiffyy
    Mandatory server certificate verification. Mail is delivered only if the
    TLS handshake succeeds, if the remote SMTP 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 certificate verification. Mail is delivered only if the TLS
    handshake succeeds, if the remote SMTP 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.

* The "exclude" attribute (Postfix >= 2.6) is used to disable ciphers that
    cause handshake failures with a specific mandatory TLS destination, without
    disabling the ciphers for all mandatory destinations. Alternatively, you
    can exclude ciphers that cause issues with multiple remote servers in
    main.cf, and selectively enable them on a per-destination basis in the
    policy table by setting a shorter or empty exclusion list. The per-
    destination "exclude" list preempts both the opportunistic and mandatory
    security level exclusions, so that all excluded ciphers can be enabled for
    known-good destinations. For non-mandatory TLS destinations that exhibit
    cipher-specific problems, Postfix will fall back to plain-text delivery. If
    plain-text is not acceptable make TLS mandatory and exclude the problem
    ciphers.

Example:

/etc/postfix/main.cf:
        smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
        # Postfix 2.5 and later
        smtp_tls_fingerprint_digest = md5
    /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
        # Postfix 2.5 and later
        [thumb.example.org]         fingerprint
        	match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
    	match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
        # Postfix 2.6 and later
        example.info            may protocols=!SSLv2 ciphers=medium
    exclude=3DES

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 and later, 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 and later; 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

The server certificate verification depth is specified with the main.cf
smtp_tls_scert_verifydepth parameter. The default verification depth is 9 (the
OpenSSL default), for compatibility with Postfix versions before 2.5 where
smtp_tls_scert_verifydepth was ignored. 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).

Example:

/etc/postfix/main.cf:
        smtp_tls_scert_verifydepth = 2

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
remote SMTP 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.

The "smtp_tls_ciphers" configuration parameter (Postfix >= 2.6) provides
control over the minimum cipher grade for opportunistic TLS. With Postfix <
2.6, the minimum opportunistic TLS cipher grade is always "export".

With mandatory TLS encryption, the Postfix SMTP client will by default only use
SSLv3 or TLSv1. SSLv2 is only used when TLS encryption is optional. The
mandatory TLS protocol list is specified via the smtp_tls_mandatory_protocols
configuration parameter. The corresponding smtp_tls_protocols parameter
(Postfix >= 2.6) controls the SSL/TLS protocols used with opportunistic TLS.

Example:

/etc/postfix/main.cf:
        smtp_tls_mandatory_ciphers = medium
        smtp_tls_mandatory_exclude_ciphers = RC4, MD5
        smtp_tls_exclude_ciphers = aNULL
        smtp_tls_mandatory_protocols = SSLv3, TLSv1
        # Also available with Postfix >= 2.5:
        smtp_tls_mandatory_protocols = !SSLv2
        # Also available with Postfix >= 2.6:
        smtp_tls_ciphers = export
        smtp_tls_protocols = !SSLv2

CClliieenntt--ssiiddee SSMMTTPPSS ssuuppppoorrtt

Although the Postfix SMTP client by itself doesn't support TLS wrapper mode, it
is relatively easy to forward a connection through the stunnel program if
Postfix needs to deliver mail to some legacy system that doesn't support
STARTTLS. Use one of the following two examples, to send only some remote mail,
or to send all remote mail, to an SMTPS server.

SSeennddiinngg aallll rreemmoottee mmaaiill ttoo aann SSMMTTPPSS sseerrvveerr

The first example uses SMTPS to send all remote mail to a provider's mail
server called "mail.example.com".

A minimal stunnel.conf file is sufficient to set up a tunnel from local port
11125 to the remote destination "mail.example.com" and port "smtps". Postfix
will later use this tunnel to connect to the remote server.

/path/to/stunnel.conf:
        [smtp-tls-wrapper]
        accept = 11125
        client = yes
        connect = mail.example.com:smtps

To test this tunnel, use:

$ telnet localhost 11125

This should produce the greeting from the remote SMTP server at
mail.example.com.

On the Postfix side, the relayhost feature sends all remote mail through the
local stunnel listener on port 11125:

/etc/postfix/main.cf:
        relayhost = [127.0.0.1]:11125

Use "postfix reload" to make the change effective.

SSeennddiinngg oonnllyy mmaaiill ffoorr aa ssppeecciiffiicc ddeessttiinnaattiioonn vviiaa SSMMTTPPSS

The second example will use SMTPS to send only mail for "example.com" via
SMTPS. It uses the same stunnel configuration file as the first example, so it
won't be repeated here.

This time, the Postfix side uses a transport map to direct only mail for
"example.com" through the tunnel:

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

/etc/postfix/transport:
        example.com  relay:[127.0.0.1]:11125

Use "postmap hash:/etc/postfix/transport" and "postfix reload" to make the
change effective.

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.

Examples:

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

As of version 2.5, Postfix no longer uses root privileges when opening this
file. The file should now be stored under the Postfix-owned data_directory. As
a migration aid, an attempt to open the file under a non-Postfix directory is
redirected to the Postfix-owned data_directory, and a warning is logged. If you
wish to continue using a pre-existing PRNG state file, move it to the
data_directory and change the ownership to the account specified with the
mail_owner parameter.

With earlier Postfix versions the default file location is under the Postfix
configuration directory, which is not the proper place for information that is
modified by Postfix.

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.porcupine.org 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..ppoorrccuuppiinnee..oorrgg
        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.porcupine.org 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.porcupine.org'
        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/lib/postfix/smtp_tls_session_cache
            smtp_tls_security_level = may
            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/lib/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.

CCrreeddiittss

* TLS support for Postfix was originally developed by Lutz Jänicke 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.
  * Victor Duchovni implemented the fingerprint security level, added more
    sanity checks, and separated TLS connection management from security policy
    enforcement. The latter change simplified the code that verifies
    certificate signatures, certificate names, and certificate fingerprints.

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