Postfix TLS Support

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WARNING

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.

What Postfix TLS support does for you

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

      NOTE: 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
  * SMTP Server specific settings
  * SMTP Client specific settings
  * TLS manager specific settings
  * Building Postfix with TLS support
  * Reporting problems
  * Credits

And last but not least, for the impatient:

  * Getting started, quick and dirty

How Postfix TLS support works

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

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

  * The smtp(8) client implements the SMTP (and LMTP) 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.

Not shown in the figure are the tlsproxy(8) server and the postscreen(8)
server. These use TLS in the same manner as smtpd(8).

                    <---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

SMTP Server specific settings

Topics covered in this section:

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

Server-side certificate and private key configuration

In order to use TLS, the Postfix SMTP server 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 access restriction applies to the
key file only, and the certificate file may be "world-readable".

Public Internet MX hosts without certificates signed by a well-known public CA
must still 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 verify the self-signed certificate, but unless the client is running
Postfix or similar software, it will only negotiate TLS ciphersuites that
require a server certificate.

For servers that are not 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. Most sites only
have RSA certificates. You can configure all three at the same time, in which
case the ciphersuite negotiated with the remote SMTP client determines which
certificate is used. If your DNS zone is signed, and you want to publish RFC
6698 TLSA records, these must match any of the configured certificates. Since
the best practice is to publish "3 1 1" certificate associations, create a
separate TLSA record for each public-key certificate digest.

Creating the server certificate file

To verify the Postfix SMTP server certificate, the remote SMTP client must
receive the issuing CA certificates via the TLS handshake or via public-key
infrastructure. This means that the Postfix server public-key certificate file
must include the server certificate first, then the issuing CA(s) (bottom-up
order). The Postfix SMTP server certificate must be usable as SSL server
certificate and hence pass the "openssl verify -purpose sslserver ..." test.

The examples that follow show how to create a server certificate file. We
assume that the certificate for "server.example.com" was issued by
"intermediate CA" which itself has a certificate issued by "root CA".

  * With legacy public CA trust verification, you can omit the root certificate
    from the "server.pem" certificate file. If the client trusts the root CA,
    it will already have a local copy of the root CA certificate. Omitting the
    root CA certificate reduces the size of the server TLS handshake.

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

  * If you publish RFC 6698 TLSA "2 0 1" or "2 1 1" records to specify root CA
    certificate digests, you must include the corresponding root CA
    certificates in the "server.pem" certificate file. See the documentation of
    the tls_dane_trust_anchor_digest_enable main.cf parameter.

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

    Remote SMTP clients will be able to use the TLSA record you publish (which
    only contains the certificate digest) only if they have access to the
    corresponding certificate. Failure to verify certificates per the server's
    published TLSA records will typically cause the SMTP client to defer mail
    delivery. The foregoing also applies to "2 0 2" and "2 1 2" TLSA records or
    any other digest of a CA certificate, but it is expected that SHA256 will
    be by far the most common digest for TLSA.

    As a best practice, publish either "3 0 1" or "3 1 1" TLSA associations
    that specify the SHA256 digest of the server certificate public key with
    the alias-expanded hostname of each STARTTLS capable SMTP server. These
    continue to work when a certificate is renewed with the same public/private
    key pair.

For instructions on how to compute the digest of a certificate or its public
key for use in TLSA records, see the documentation of the
smtpd_tls_fingerprint_digest main.cf parameter.

When a new key or certificate is generated, an additional TLSA record with the
new digest must be published in advance of the actual deployment of the new key
or certificate on the server. You must allow sufficient time for any TLSA
RRsets with only the old digest to expire from DNS caches. The safest practice
is to wait until the DNSSEC signature on the previous TLSA RRset expires, and
only then switch the server to use new keys published in the updated TLSA
RRset. Once the new certificate trust chain and private key are in effect, the
DNS should be updated once again to remove the old digest from the TLSA RRset.

If you want the Postfix SMTP server to accept remote SMTP client certificates
issued by one or more root CAs, append the root certificate to
$smtpd_tls_CAfile or install it in the $smtpd_tls_CApath directory.

Configuring the server certificate and key files

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 >= 1.0.0):

    /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:

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

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

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

When you configure the Postfix SMTP server to request client certificates, the
DNs of Certification 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

Server-side TLS activity logging

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.

                                                                             
    |Level|Postfix 2.9 and later             |Earlier releases.              |
    |     |                                  |                               |
    |0    |Disable logging of TLS activity.                                  |
    |     |                                                                  |
    |1    |Log only a summary message on TLS |Log the summary message, peer  |
    |     |handshake completion -- no logging|certificate summary information|
    |     |of client certificate trust-chain |and unconditionally log trust- |
    |     |verification errors if client     |chain verification errors.     |
    |     |certificate verification is not   |                               |
    |     |required.                         |                               |
    |     |                                  |                               |
    |2    |Also log levels during TLS negotiation.                           |
    |     |                                                                  |
    |3    |Also log hexadecimal and ASCII dump of TLS negotiation process.   |
    |     |                                                                  |
    |4    |Also 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

Enabling TLS in the Postfix SMTP server

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

Example:

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

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". According to RFC 2487 this MUST NOT be
applied in case of a publicly-referenced Postfix SMTP server. This option is
off by default and should only seldom be used.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_security_level = encrypt

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

Client certificate verification

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

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
        smtpd_tls_security_level = encrypt

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

Supporting AUTH over TLS only

Sending AUTH data over an unencrypted channel poses a security risk. When TLS
layer encryption is required ("smtpd_tls_security_level = encrypt"), 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"), it may however still be useful to only
offer AUTH when TLS is active. To maintain compatibility with non-TLS clients,
the default is to accept AUTH without encryption. In order to change this
behavior, set "smtpd_tls_auth_only = yes".

Example:

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

Server-side TLS session cache

The Postfix SMTP server and the remote SMTP client negotiate a session, which
takes some computer time and network bandwidth. SSL protocol versions other
than SSLv2 support resumption of cached sessions. Not only is this more CPU and
bandwidth efficient, it also reduces latency as only one network round-trip is
used to resume a session while it takes two round-trips to create a session
from scratch.

Since Postfix uses multiple smtpd(8) service processes, an in-memory cache is
not sufficient for session re-use. Clients store at most one cached session per
server and are very unlikely to repeatedly connect to the same server process.
Thus session caching in the Postfix SMTP server generally requires a shared
cache (an alternative available with Postfix >= 2.11 is described below).

To share the session information between multiple smtpd(8) processes, a session
cache database is 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.

Starting with Postfix 2.11, linked with a compatible OpenSSL library (at least
0.9.8h, preferably 1.0.0 or later) the Postfix SMTP server supports RFC 5077
TLS session resumption without server-side state when the remote SMTP client
also supports RFC 5077. The session is encrypted by the server in a session
ticket returned to client for storage. When a client sends a valid session
ticket, the server decrypts it and resumes the session, provided neither the
ticket nor the session have expired. This makes it possible to resume cached
sessions without allocating space for a shared database on the server.
Consequently, for Postfix >= 2.11 the smtpd_tls_session_cache_database
parameter should generally be left empty. Session caching can be disabled by
setting the session cache timeout to zero, otherwise the timeout must be at
least 2 minutes and at most 100 days.

Note, session tickets can only be negotiated if the client disables SSLv2 and
does not use the legacy SSLv2 compatible HELLO message. This is true by default
with the Postfix >= 2.6 SMTP client.

Example:

    /etc/postfix/main.cf:
        smtpd_tls_session_cache_database = btree:/var/db/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

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0, session
caching is disabled. If set to a positive value less than 2 minutes, the
minimum value of 2 minutes is used instead.

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 reduce waste
of client resources, 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

Server access control

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 or certificate public key fingerprint (Postfix 2.9 and
        later) 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 or public key
        fingerprint (Postfix 2.9 and later) as the lookup key for the specified
        access(5) table.

The digest algorithm used to compute 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:

    # With Postfix 2.10 and later, the mail relay policy is
    # preferably specified under smtpd_relay_restrictions.
    /etc/postfix/main.cf:
        smtpd_relay_restrictions =
            permit_mynetworks
            permit_tls_clientcerts
            reject_unauth_destination

    # Older configurations combine relay control and spam control under
    # smtpd_recipient_restrictions. To use this example with Postfix >=
    # 2.10 specify "smtpd_relay_restrictions=".
    /etc/postfix/main.cf:
        smtpd_recipient_restrictions =
            permit_mynetworks
            permit_tls_clientcerts
            reject_unauth_destination
            ...other rules...

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

To extract the public key fingerprint from an X.509 certificate, you need to
extract the public key from the certificate and compute the appropriate digest
of its DER (ASN.1) encoding. With OpenSSL the "-pubkey" option of the "x509"
command extracts the public key always in "PEM" format. We pipe the result to
another OpenSSL command that converts the key to DER and then to the "dgst"
command to compute the fingerprint.

The actual command to transform the key to DER format depends on the version of
OpenSSL used. With OpenSSL 1.0.0 and later, the "pkey" command supports all key
types. With OpenSSL 0.9.8 and earlier, the key type is always RSA (nobody uses
DSA, and EC keys are not fully supported by 0.9.8), so the "rsa" command is
used.

    # OpenSSL 1.0 with all certificates and SHA-1 fingerprints.
    $ openssl x509 -in cert.pem -noout -pubkey |
        openssl pkey -pubin -outform DER |
        openssl dgst -sha1 -c
    (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

    # OpenSSL 0.9.8 with RSA certificates and MD5 fingerprints.
    $ openssl x509 -in cert.pem -noout -pubkey |
        openssl rsa -pubin -outform DER |
        openssl dgst -md5 -c
    (stdin)= f4:62:60:f6:12:8f:d5:8d:28:4d:13:a7:db:b2:ff:50

Note: Postfix 2.9.0-2.9.5 computed the public key fingerprint incorrectly. To
use public-key fingerprints, upgrade to Postfix 2.9.6 or later.

Server-side cipher controls

The Postfix SMTP server supports 5 distinct cipher grades as specified by the
smtpd_tls_mandatory_ciphers configuration parameter, which determines the
minimum cipher grade with mandatory TLS encryption. The default minimum cipher
grade for mandatory TLS is "medium" which is essentially 128-bit encryption or
better. The smtpd_tls_ciphers parameter (Postfix >= 2.6) controls the minimum
cipher grade used with opportunistic TLS. Here, the default minimum cipher
grade is "medium" for Postfix releases after the middle of 2015, "export" for
older Postfix releases. With Postfix < 2.6, the minimum opportunistic TLS
cipher grade is always "export".

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.

With mandatory and opportunistic TLS encryption, the Postfix SMTP server by
default disables SSLv2 and SSLv3 with Postfix releases after the middle of
2015; older releases only disable SSLv2 for mandatory TLS. The mandatory TLS
protocol list is specified via the smtpd_tls_mandatory_protocols configuration
parameter. The smtpd_tls_protocols parameter (Postfix >= 2.6) controls the SSL/
TLS protocols used with opportunistic TLS.

Note that the OpenSSL library only supports protocol exclusion (not inclusion).
For this reason, Postfix can exclude only protocols that are known at the time
the Postfix software is written. If new protocols are added to the OpenSSL
library, they cannot be excluded without corresponding changes to the Postfix
source code.

For a server that is not a public Internet MX host, Postfix supports
configurations with no server certificates that use only 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 or higher, 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
        # Preferred syntax with Postfix >= 2.5:
        smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
        # Legacy syntax:
        smtpd_tls_mandatory_protocols = TLSv1

If you want to take maximal advantage of ciphers that offer forward secrecy see
the Getting started section of FORWARD_SECRECY_README. The full document
conveniently presents all information about Postfix "perfect" forward secrecy
support in one place: what forward secrecy is, how to tweak settings, and what
you can expect to see when Postfix uses ciphers with forward secrecy.

Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and later allows TLS
servers to preempt the TLS client's cipher-suite preference list. This is
possible only with SSLv3 and later, as in SSLv2 the client chooses the cipher-
suite from a list supplied by the server.

By default, the OpenSSL server selects the client's most preferred cipher-suite
that the server supports. With SSLv3 and later, the server may choose its own
most preferred cipher-suite that is supported (offered) by the client. Setting
"tls_preempt_cipherlist = yes" enables server cipher-suite preferences. The
default OpenSSL behavior applies with "tls_preempt_cipherlist = no".

While server cipher-suite selection may in some cases lead to a more secure or
performant cipher-suite choice, there is some risk of interoperability issues.
In the past, some SSL clients have listed lower priority ciphers that they did
not implement correctly. If the server chooses a cipher that the client prefers
less, it may select a cipher whose client implementation is flawed. Most
notably Windows 2003 Microsoft Exchange servers have flawed implementations of
DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA. Enabling server
cipher-suite selection may create interoperability issues with Windows 2003
Microsoft Exchange clients.

Miscellaneous server controls

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

Example:

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

With Postfix 2.8 and later, the tls_disable_workarounds parameter specifies a
list or bit-mask of OpenSSL bug work-arounds to disable. This may be necessary
if one of the work-arounds enabled by default in OpenSSL proves to pose a
security risk, or introduces an unexpected interoperability issue. Some bug
work-arounds known to be problematic are disabled in the default value of the
parameter when linked with an OpenSSL library that could be vulnerable.

Example:

    /etc/postfix/main.cf:
        tls_disable_workarounds = 0xFFFFFFFF
        tls_disable_workarounds = CVE-2010-4180

With Postfix >= 2.11, the tls_ssl_options parameter specifies a list or bit-
mask of OpenSSL options to enable. Specify one or more of the named options
below, or a hexadecimal bitmask of options found in the ssl.h file
corresponding to the run-time OpenSSL library. While it may be reasonable to
turn off all bug workarounds (see above), it is not a good idea to attempt to
turn on all features.

LEGACY_SERVER_CONNECT
    See SSL_CTX_set_options(3).
NO_TICKET
    See SSL_CTX_set_options(3).
NO_COMPRESSION
    Disable SSL compression even if supported by the OpenSSL library.
    Compression is CPU-intensive, and compression before encryption does not
    always improve security.
Example:

    /etc/postfix/main.cf:
        tls_ssl_options = no_ticket, no_compression

You should only enable features via the hexadecimal mask when the need to
control the feature is critical (to deal with a new vulnerability or a serious
interoperability problem). Postfix DOES NOT promise backwards compatible
behavior with respect to the mask bits. A feature enabled via the mask in one
release may be enabled by other means in a later release, and the mask bit will
then be ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better solution.

SMTP Client specific settings

Topics covered in this section:

  * Configuring TLS in the SMTP/LMTP client
  * Client-side TLS activity logging
  * Client-side certificate and private key configuration
  * Client-side TLS session cache
  * Client TLS limitations
  * Per-destination TLS policy
  * Discovering servers that support TLS
  * Server certificate verification depth
  * Client-side cipher controls
  * Client-side SMTPS support
  * Miscellaneous client controls

Configuring TLS in the SMTP/LMTP client

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.

none
    No TLS.
may
    Opportunistic TLS.
encrypt
    Mandatory TLS encryption.
dane
    Opportunistic DANE TLS.
dane-only
    Mandatory DANE TLS.
fingerprint
    Certificate fingerprint verification.
verify
    Mandatory server certificate verification.
secure
    Secure-channel TLS.

TLS support in the LMTP delivery agent

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 or
loopback TCP connections 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.

No TLS encryption

At the "none" TLS security level, TLS encryption is disabled. This is the
default security level, and can be configured explicitly by setting
"smtp_tls_security_level = none". For LMTP, use the corresponding "lmtp_"
parameter.

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 TLS policy table, specify the "none" security level.

Opportunistic TLS

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. Opportunistic TLS can be
configured by setting "smtp_tls_security_level = may". For LMTP, use the
corresponding "lmtp_" parameter.

The "smtp_tls_ciphers" and "smtp_tls_protocols" configuration parameters
(Postfix >= 2.6) provide control over the cipher grade and protocols used with
opportunistic TLS. With earlier Postfix releases, opportunistic TLS always uses
the cipher grade "export" and enables all protocols.

With opportunistic TLS, mail delivery continues even if the server certificate
is untrusted or bears the wrong name. When the TLS handshake fails for an
opportunistic TLS session, rather than give up on mail delivery, the Postfix
SMTP client retries the transaction 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 TLS policy table, specify the "may" security level.

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

Mandatory TLS encryption

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. 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. For LMTP, use the corresponding "lmtp_" parameter.

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.

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 TLS policy table, specify the "encrypt" security level.

Examples:

In the example below, traffic to example.com and its sub-domains via the
corresponding MX hosts always uses TLS. The SSLv2 protocol will be disabled
(the default setting of smtp_tls_mandatory_protocols excludes SSLv2+3). 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

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.

Note: 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

DANE TLS authentication.

The Postfix SMTP client supports two TLS security levels based on RFC6698 DANE
TLSA records. The opportunistic "dane" level and the mandatory "dane-only"
level.

The "dane" level is a stronger form of opportunistic TLS that is resistant to
man in the middle and downgrade attacks when the destination domain uses DNSSEC
to publish DANE TLSA records for its MX hosts. If a remote SMTP server has
"usable" (see RFC 6698) DANE TLSA records, the server connection will be
authenticated. When DANE authentication fails, there is no fallback to
unauthenticated or plaintext delivery.

If TLSA records are published for a given remote SMTP server (implying TLS
support), but are all "unusable" due to unsupported parameters or malformed
data, the Postfix SMTP client will use mandatory unauthenticated TLS.
Otherwise, when no TLSA records are published, the Postfix SMTP client behavior
is the same as with may.

TLSA records must be published in DNSSEC validated DNS zones. Any TLSA records
in DNS zones not protected via DNSSEC are ignored. The Postfix SMTP client will
not look for TLSA records associated with MX hosts whose "A" or "AAAA" records
lie in an "insecure" DNS zone. Such lookups have been observed to cause
interoperability issues with poorly implemented DNS servers, and are in any
case not expected to ever yield "secure" results, since that would require a
very unlikely DLV DNS trust anchor configured between the host record and the
associated "_25._tcp" child TLSA record.

The "dane-only" level is a form of secure-channel TLS based on the DANE PKI. If
"usable" TLSA records are present these are used to authenticate the remote
SMTP server. Otherwise, or when server certificate verification fails, delivery
via the server in question tempfails.

At both security levels, the TLS policy for the destination is obtained via
TLSA records validated with DNSSEC. For TLSA policy to be in effect, the
destination domain's containing DNS zone must be signed and the Postfix SMTP
client's operating system must be configured to send its DNS queries to a
recursive DNS nameserver that is able to validate the signed records. Each MX
host's DNS zone needs to also be signed, and needs to publish DANE TLSA (RFC
6698) records that specify how that MX host's TLS certificate is to be
verified.

TLSA records do not preempt the normal SMTP MX host selection algorithm, if
some MX hosts support TLSA and others do not, TLS security will vary from
delivery to delivery. It is up to the domain owner to configure their MX hosts
and their DNS sensibly. To configure the Postfix SMTP client for DNSSEC lookups
see the documentation for the smtp_dns_support_level main.cf parameter. The
tls_dane_trust_anchor_digest_enable main.cf parameter controls support for
trust-anchor digest TLSA records. The tls_dane_digests and
tls_dane_digest_agility parameters control the list of supported digests and
digest downgrade attack resistance.

DANE for SMTP MTAs deviates in some details from the baseline DANE protocol in
RFC 6698. Most notably, it is not expected that SMTP MTAs can reasonably
include every public CA that a remote SMTP server's administrator may believe
to be well-known. Nor is there an interactive user to "click OK" when
authentication fails.

Therefore, certificate usages "0" and "1" from RFC 6698 which are intended to
"constrain" existing PKI trust, are not supported. TLSA records with usage "0"
are treated as "unusable". TLSA records with usage "1" are instead treated as
"trust assertions" and mapped to usage "3". Specifically, with certificate
usage "1", Postfix will not require the remote SMTP server's certificate to be
trusted with respect to any locally defined public CAs, it is the domain
owner's responsibility to ensure that the certificate associations in their
TLSA records are appropriate to authenticate their SMTP servers.

The Postfix SMTP client supports only certificate usages "2" and "3" (with "1"
treated as though it were "3"). See tls_dane_trust_anchor_digest_enable for
usage "2" usability considerations. Support for certificate usage "1" is an
experiment, it may be withdrawn in the future. Server operators SHOULD NOT
publish TLSA records with usage "1".

When usable TLSA records are obtained for the remote SMTP server the Postfix
SMTP client sends the SNI TLS extension in its SSL client hello message. This
may help the remote SMTP server live up to its promise to provide a certificate
that matches its TLSA records.

For purposes of protocol and cipher selection, the "dane" security level is
treated like a "mandatory" TLS security level, and weak ciphers and protocols
are disabled. Since DANE authenticates server certificates the "aNULL" cipher-
suites are transparently excluded at this level, no need to configure this
manually. RFC 6698 (DANE) TLS authentication is available with Postfix 2.11 and
later.

When a DANE TLSA record specifies a trust-anchor (TA) certificate (that is an
issuing CA), the strategy used to verify the peername of the server certificate
is unconditionally "nexthop, hostname". Both the nexthop domain and the
hostname obtained from the DNSSEC-validated MX lookup are safe from forgery and
the server certificate must contain at least one of these names.

When a DANE TLSA record specifies an end-entity (EE) certificate, (that is the
actual server certificate), as with the fingerprint security level below, no
name checks or certificate expiration checks are applied. The server
certificate (or its public key) either matches the DANE record or not. Server
administrators should publish such EE records in preference to all other types.

The pre-requisites for DANE support in the Postfix SMTP client are:

  * A compile-time OpenSSL library that supports the TLS SNI extension and
    "SHA-2" message digests.
  * A compile-time DNS resolver library that supports DNSSEC. Postfix binaries
    built on an older system will not support DNSSEC even if deployed on a
    system with an updated resolver library.
  * The "smtp_dns_support_level" must be set to "dnssec".
  * The "smtp_host_lookup" parameter must include "dns".
  * A DNSSEC-validating recursive resolver (see note below).

The above client pre-requisites do not apply to the Postfix SMTP server. It
will support DANE provided it supports TLSv1 and its TLSA records are published
in a DNSSEC signed zone. To receive DANE secured mail for multiple domains, use
the same hostname to add the server to each domain's MX records. There are no
plans to implement SNI in the Postfix SMTP server.

Note: The Postfix SMTP client's internal stub DNS resolver is DNSSEC-aware, but
it does not itself validate DNSSEC records, rather it delegates DNSSEC
validation to the operating system's configured recursive DNS nameserver. The
Postfix DNS client relies on a secure channel to the resolver's cache for
DNSSEC integrity, but does not support TSIG to protect the transmission channel
between itself and the nameserver. Therefore, it is strongly recommended (DANE
security guarantee void otherwise) that each MTA run a local DNSSEC-validating
recursive resolver ("unbound" from nlnetlabs.nl is a reasonable choice)
listening on the loopback interface, and that the system be configured to use
only this local nameserver. The local nameserver may forward queries to an
upstream recursive resolver on another host if desired.

Note: When the operating system's recursive nameserver is not local, enabling
EDNS0 expanded DNS packet sizes and turning on the DNSSEC "DO" bit in the DNS
request and/or the new DNSSEC-specific records returned in the nameserver's
replies may cause problems with older or buggy firewall and DNS server
implementations. Therefore, Postfix does not enable DNSSEC by default. Since MX
lookups happen before the security level is determined, DANE support is
disabled for all destinations unless you set "smtp_dns_support_level = dnssec".
To enable DNSSEC lookups selectively, define a new dedicated transport with a
"-o smtp_dns_support_level=dnssec" override in master.cf and route selected
domains to that transport. If DNSSEC proves to be sufficiently reliable for
these domains, you can enable it for all destinations by changing the global
smtp_dns_support_level in main.cf.

Example: "dane" security for selected destinations, with opportunistic TLS by
default. This is the recommended configuration for early adopters.

  * The "example.com" destination uses DANE, but if TLSA records are not
    present or are unusable, mail is deferred.

  * The "example.org" destination uses DANE if possible, but if no TLSA records
    are found opportunistic TLS is used.

    main.cf:
        indexed = ${default_database_type}:${config_directory}/
        #
        # default: Opportunistic TLS with no DNSSEC lookups.
        #
        smtp_tls_security_level = may
        smtp_dns_support_level = enabled
        #
        # Per-destination TLS policy
        #
        smtp_tls_policy_maps = ${indexed}tls_policy
        #
        # default_transport = smtp, but some destinations are special:
        #
        transport_maps = ${indexed}transport

    transport:
        example.com dane
        example.org dane

    tls_policy:
        example.com dane-only

    master.cf:
        dane       unix  -       -       n       -       -       smtp
          -o smtp_dns_support_level=dnssec
          -o smtp_tls_security_level=dane

Certificate fingerprint verification

At the fingerprint security level, no trusted Certification Authorities are
used or required. The certificate trust chain, expiration date, etc., are not
checked. Instead, the smtp_tls_fingerprint_cert_match parameter or the "match"
attribute in the policy table lists the remote SMTP server certificate
fingerprint or public key fingerprint. Certificate fingerprint verification is
available with Postfix 2.5 and later, public-key fingerprint support is
available with Postfix 2.9 and later.

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. If public key fingerprints are used in place of
fingerprints of the entire certificate, the fingerprints remain valid even
after the certificate is renewed, provided that the same public/private keys
are used to obtain the new certificate.

Fingerprint verification 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
smtp_tls_fingerprint_digest 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

To extract the public key fingerprint from an X.509 certificate, you need to
extract the public key from the certificate and compute the appropriate digest
of its DER (ASN.1) encoding. With OpenSSL the "-pubkey" option of the "x509"
command extracts the public key always in "PEM" format. We pipe the result to
another OpenSSL command that converts the key to DER and then to the "dgst"
command to compute the fingerprint.

The actual command to transform the key to DER format depends on the version of
OpenSSL used. With OpenSSL 1.0.0 and later, the "pkey" command supports all key
types. With OpenSSL 0.9.8 and earlier, the key type is always RSA (nobody uses
DSA, and EC keys are not fully supported by 0.9.8), so the "rsa" command is
used.

    # OpenSSL 1.0 with all certificates and SHA-1 fingerprints.
    $ openssl x509 -in cert.pem -noout -pubkey |
        openssl pkey -pubin -outform DER |
        openssl dgst -sha1 -c
    (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

    # OpenSSL 0.9.8 with RSA certificates and MD5 fingerprints.
    $ openssl x509 -in cert.pem -noout -pubkey |
        openssl rsa -pubin -outform DER |
        openssl dgst -md5 -c
    (stdin)= f4:62:60:f6:12:8f:d5:8d:28:4d:13:a7:db:b2:ff:50

Note: Postfix 2.9.0-2.9.5 computed the public key fingerprint incorrectly. To
use public-key fingerprints, upgrade to Postfix 2.9.6 or later.

Mandatory server certificate verification

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 Certification 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. 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.

With Postfix >= 2.11 the "smtp_tls_trust_anchor_file" parameter or more
typically the corresponding per-destination "tafile" attribute optionally
modifies trust chain verification. If the parameter is not empty the root CAs
in CAfile and CApath are no longer trusted. Rather, the Postfix SMTP client
will only trust certificate-chains signed by one of the trust-anchors contained
in the chosen files. The specified trust-anchor certificates and public keys
are not subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these certificates
also may be found "in the middle" of the trust chain presented by the remote
SMTP server, and any untrusted issuing parent certificates will be ignored.

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 Certification
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 TLS policy table, specify the "verify" security
level.

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

Secure server certificate verification

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.
Postfix 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. 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.

With Postfix >= 2.11 the "smtp_tls_trust_anchor_file" parameter or more
typically the corresponding per-destination "tafile" attribute optionally
modifies trust chain verification. If the parameter is not empty the root CAs
in CAfile and CApath are no longer trusted. Rather, the Postfix SMTP client
will only trust certificate-chains signed by one of the trust-anchors contained
in the chosen files. The specified trust-anchor certificates and public keys
are not subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these certificates
also may be found "in the middle" of the trust chain presented by the remote
SMTP server, and any untrusted issuing parent certificates will be ignored.

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 Certification 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 TLS policy table, specify the "secure" security level.

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.

        /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

Client-side TLS activity logging

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

                                                                             
    |Level|Postfix 2.9 and later             |Earlier releases.              |
    |     |                                  |                               |
    |0    |Disable logging of TLS activity.                                  |
    |     |                                                                  |
    |1    |Log only a summary message on TLS |Log the summary message and    |
    |     |handshake completion -- no logging|unconditionally log trust-chain|
    |     |of remote SMTP server certificate |verification errors.           |
    |     |trust-chain verification errors if|                               |
    |     |server certificate verification is|                               |
    |     |not required.                     |                               |
    |     |                                  |                               |
    |2    |Also log levels during TLS negotiation.                           |
    |     |                                                                  |
    |3    |Also log hexadecimal and ASCII dump of TLS negotiation process.   |
    |     |                                                                  |
    |4    |Also log hexadecimal and ASCII dump of complete transmission after|
    |     |STARTTLS.                                                         |
    |     |                                                                  |

Example:

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

Client-side certificate and private key configuration

Do not configure Postfix SMTP client certificates unless you must 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:

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

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 >= 1.0.0):

    /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:

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

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

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

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

Example:

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

Client-side TLS session cache

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

Example:

    /etc/postfix/main.cf:
        smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_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 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

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0, session
caching is disabled. If set to a positive value less than 2 minutes, the
minimum value of 2 minutes is used instead.

Client TLS limitations

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.

Adoption of DNSSEC and RFC6698 (DANE) may gradually (as domains implement
DNSSEC and publish TLSA records for their MX hosts) address the DNS man-in-the-
middle risk and provide scalable key management for SMTP with TLS. Postfix >=
2.11 supports the new dane and dane-only security levels that take advantage of
these standards.

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. For these reasons secure-channel configurations
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 were new with Postfix 2.3, the issue was well understood
before Postfix 1.0, when Lutz Ja"nicke 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.

TLS policy table

A small fraction of servers offer STARTTLS but the negotiation consistently
fails. As long as encryption is not mandatory, the Postfix SMTP client retries
the delivery immediately with TLS disabled, without any need to explicitly
disable TLS for the problem destinations.

The 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.

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:

none
    No TLS. No additional attributes are supported at this level.
may
    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.
encrypt
    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.
dane
    Opportunistic DANE TLS. The TLS policy for the destination is obtained via
    TLSA records in DNSSEC. If no TLSA records are found, the effective
    security level used is may. If TLSA records are found, but none are usable,
    the effective security level is encrypt. When usable TLSA records are
    obtained for the remote SMTP server, SSLv2+3 are automatically disabled
    (see smtp_tls_mandatory_protocols), and the server certificate must match
    the TLSA records. RFC 6698 (DANE) TLS authentication and DNSSEC support is
    available with Postfix 2.11 and later.
dane-only
    Mandatory DANE TLS. The TLS policy for the destination is obtained via TLSA
    records in DNSSEC. If no TLSA records are found, or none are usable, no
    connection is made to the server. When usable TLSA records are obtained for
    the remote SMTP server, SSLv2+3 are automatically disabled (see
    smtp_tls_mandatory_protocols), and the server certificate must match the
    TLSA records. RFC 6698 (DANE) TLS authentication and DNSSEC support is
    available with Postfix 2.11 and later.
fingerprint
    Certificate fingerprint verification. Available with Postfix 2.5 and later.
    At this security level, there are no trusted Certification Authorities. The
    certificate trust chain, expiration date, ... are not checked. Instead, the
    optional match attribute, or else the main.cf
    smtp_tls_fingerprint_cert_match parameter, lists the server certificate
    fingerprints or public key fingerprints (Postfix 2.9 and later). The digest
    algorithm used to calculate fingerprints is selected by the
    smtp_tls_fingerprint_digest 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.
verify
    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 Certification
    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). With Postfix >= 2.11 the
    "tafile" attribute optionally modifies trust chain verification in the same
    manner as the "smtp_tls_trust_anchor_file" parameter. The "tafile"
    attribute may be specified multiple times to load multiple trust-anchor
    files.
secure
    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 Certification 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). With Postfix >= 2.11 the "tafile"
    attribute optionally modifies trust chain verification in the same manner
    as the "smtp_tls_trust_anchor_file" parameter. The "tafile" attribute may
    be specified multiple times to load multiple trust-anchor files.
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 ciphers=high
        example.com             verify match=hostname:dot-nexthop 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

Note: 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.

Discovering servers that support TLS

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

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

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

Example:

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

Server certificate verification depth

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

Client-side cipher controls

The Postfix SMTP client supports 5 distinct cipher grades 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
cryptanalytic 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. The default
minimum cipher grade for opportunistic TLS is "medium" for Postfix releases
after the middle of 2015, and "export" for older releases. With Postfix < 2.6,
the minimum opportunistic TLS cipher grade is always "export".

With mandatory and opportunistic TLS encryption, the Postfix SMTP client will
by default disable SSLv2 and SSLv3. 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
        # Preferred form with Postfix >= 2.5:
        smtp_tls_mandatory_protocols = !SSLv2
        # Legacy form for Postfix < 2.5:
        smtp_tls_mandatory_protocols = SSLv3, TLSv1
        # Also available with Postfix >= 2.6:
        smtp_tls_ciphers = medium
        smtp_tls_protocols = !SSLv2

Client-side SMTPS support

These sections show how to send mail to a server that does not support
STARTTLS, but that provides the deprecated SMTPS service on TCP port 465.
Depending on the Postfix version, some additional tooling may be required.

Postfix >= 3.0

The Postfix SMTP client has SMTPS support built-in as of version 3.0. Use one
of the following examples, to send all remote mail, or to send only some remote
mail, to an SMTPS server.

Postfix >= 3.0: Sending all remote mail to an SMTPS server

The first example will send all remote mail over SMTPS through a provider's
server called "mail.example.com":

    /etc/postfix/main.cf:
        # Client-side SMTPS requires "encrypt" or stronger.
        smtp_tls_security_level = encrypt
        smtp_tls_wrappermode = yes
        # The [] suppress MX lookups.
        relayhost = [mail.example.com]:465

Use "postfix reload" to make the change effective.

See SOHO_README for additional information about SASL authentication.

Postfix >= 3.0: Sending only mail for a specific destination via SMTPS

The second example will send only mail for "example.com" via SMTPS. This time,
Postfix uses a transport map to deliver only mail for "example.com" via SMTPS:

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

    /etc/postfix/transport:
        example.com  relay-smtps:example.com:465

    /etc/postfix/master.cf:
        relay-smtps  unix  -       -       n       -       -       smtp
            # Client-side SMTPS requires "encrypt" or stronger.
            -o smtp_tls_security_level=encrypt
            -o smtp_tls_wrappermode=yes

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

See SOHO_README for additional information about SASL authentication.

Postfix < 3.0

Although older Postfix SMTP client versions do not 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.

Postfix < 3.0: Sending all remote mail to an SMTPS server

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.

See SOHO_README for additional information about SASL authentication.

Postfix < 3.0: Sending only mail for a specific destination via SMTPS

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.

See SOHO_README for additional information about SASL authentication.

Miscellaneous client controls

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

Example:

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

With Postfix 2.8 and later, the tls_disable_workarounds parameter specifies a
list or bit-mask of OpenSSL bug work-arounds to disable. This may be necessary
if one of the work-arounds enabled by default in OpenSSL proves to pose a
security risk, or introduces an unexpected interoperability issue. Some bug
work-arounds known to be problematic are disabled in the default value of the
parameter when linked with an OpenSSL library that could be vulnerable.

Example:

    /etc/postfix/main.cf:
        tls_disable_workarounds = 0xFFFFFFFF
        tls_disable_workarounds = CVE-2010-4180, LEGACY_SERVER_CONNECT

Note: Disabling LEGACY_SERVER_CONNECT is not wise at this time, lots of servers
are still unpatched and Postfix is not significantly vulnerable to the
renegotiation issue in the TLS protocol.

With Postfix >= 2.11, the tls_ssl_options parameter specifies a list or bit-
mask of OpenSSL options to enable. Specify one or more of the named options
below, or a hexadecimal bitmask of options found in the ssl.h file
corresponding to the run-time OpenSSL library. While it may be reasonable to
turn off all bug workarounds (see above), it is not a good idea to attempt to
turn on all features.

A future version of OpenSSL may by default no longer allow connections to
servers that don't support secure renegotiation. Since the exposure for SMTP is
minimal, and some SMTP servers may remain unpatched, you can add
LEGACY_SERVER_CONNECT to the options to restore the more permissive default of
current OpenSSL releases.

Example:

    /etc/postfix/main.cf:
        tls_ssl_options = NO_TICKET, NO_COMPRESSION, LEGACY_SERVER_CONNECT

You should only enable features via the hexadecimal mask when the need to
control the feature is critical (to deal with a new vulnerability or a serious
interoperability problem). Postfix DOES NOT promise backwards compatible
behavior with respect to the mask bits. A feature enabled via the mask in one
release may be enabled by other means in a later release, and the mask bit will
then be ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better solution.

TLS manager specific settings

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

Example:

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

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

Examples (specify only one in main.cf):

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

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

Example:

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

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

Example:

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

The tlsmgr(8) process saves the PRNG state to a persistent exchange file at
regular times and when the process terminates, so that it can recover the PRNG
state the next time it starts up. This file is created when it does not exist.

Examples:

    /etc/postfix/main.cf:
        tls_random_exchange_name = /var/db/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.

Getting started, quick and dirty

The following steps will get you started quickly. Because you sign your own
Postfix public key certificate, you get TLS encryption but no TLS
authentication. This is sufficient for testing, and for exchanging email with
sites that you have no trust relationship with. For real authentication you
need also enable DNSSEC record signing for your domain and publish TLSA records
and/or your Postfix public key certificate needs to be signed by a recognized
Certification Authority. To authenticate the certificates of remote host you
need a DNSSEC-validating local resolver and to enable DANE authentication and/
or configure the Postfix SMTP client with a list of public key certificates of
Certification Authorities, but make sure to read about the limitations of the
latter approach.

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

  * Quick-start TLS with Postfix >= 3.1.

  * Self-signed server certificate.

  * Private Certification Authority.

Quick-start TLS with Postfix >= 3.1

Postfix 3.1 provides built-in support for enabling TLS in the SMTP client and
server and for ongoing certificate and DANE TLSA record management.

  * Quick-start TLS in the Postfix >= 3.1 SMTP client.

  * Quick-start TLS in the Postfix >= 3.1 SMTP server.

Quick-start TLS in the Postfix >= 3.1 SMTP client.

If you are using Postfix 3.1 or later, and your SMTP client TLS settings are in
their default state, you can enable opportunistic TLS in the SMTP client as
follows:

    # postfix tls enable-client
    # postfix reload

If some of the Postfix SMTP client TLS settings are not in their default state,
this will not make any changes, but will instead suggest the minimal required
settings for SMTP client TLS. The "postfix reload" command is optional, it is
only needed if you want the settings to take effect right away. Note, this does
not enable trust in any public certification authorities, and does not
configure client TLS certificates as these are largely pointless with
opportunistic TLS.

There is not yet a turn-key command for enabling DANE authentication. This is
because DANE requires changes to your resolv.conf file and a corresponding
DNSSEC-validating resolver local to the Postfix host, these changes are
difficult to automate in a portable way.

If you're willing to revert your settings to the defaults and switch to a
"stock" opportunistic TLS configuration, then you can: erase all the SMTP
client TLS settings and then enable client TLS:

    # postconf -X `postconf -nH | egrep '^smtp(_|_enforce_|_use_)tls'`
    # postfix tls enable-client
    # postfix reload

Quick-start TLS in the Postfix >= 3.1 SMTP server.

If you are using Postfix 3.1 or later, and your SMTP server TLS settings are in
their default state, you can enable opportunistic TLS in the SMTP server as
follows:

    # postfix tls enable-server
    # postfix reload

If some of the Postfix SMTP client TLS settings are not in their default state,
this will not make any changes, but will instead suggest the minimal required
settings for SMTP client TLS. The "postfix reload" command is optional, it is
only needed if you want the settings to take effect right away. This will
generate a self-signed private key and certificate and enable TLS in the
Postfix SMTP server.

If you're willing to revert your settings to the defaults and switch to a
"stock" server TLS configuration, then you can: erase all the SMTP server TLS
settings and then enable server TLS:

    # postconf -X `postconf -nH | egrep '^smtpd(_|_enforce_|_use_)tls'`
    # postfix tls enable-server
    # postfix reload

Postfix >= 3.1 provides additional built-in support for ongoing management of
TLS in the SMTP server, via additional "postfix tls" sub-commands. These make
it easy to generate certificate signing requests, create and deploy new keys
and certificates, and generate DANE TLSA records. See the postfix-tls(1)
documentation for details.

Self-signed server certificate

The following commands (credits: Viktor Dukhovni) generate and install a 2048-
bit RSA private key and 10-year self-signed certificate for the local Postfix
system. This requires super-user privileges.

    # dir="$(postconf -h config_directory)"
    # fqdn=$(postconf -h myhostname)
    # case $fqdn in /*) fqdn=$(cat "$fqdn");; esac
    # ymd=$(date +%Y-%m-%d)
    # key="${dir}/key-${ymd}.pem"; rm -f "${key}"
    # cert="${dir}/cert-${ymd}.pem"; rm -f "${cert}"
    # (umask 077; openssl genrsa -out "${key}" 2048) &&
      openssl req -new -key "${key}" \
        -x509 -subj "/CN=${fqdn}" -days 3650 -out "${cert}" &&
      postconf -e \
        "smtpd_tls_cert_file = ${cert}" \
        "smtpd_tls_key_file = ${key}" \
        'smtpd_tls_security_level = may' \
        'smtpd_tls_received_header = yes' \
        'smtpd_tls_loglevel = 1' \
        'smtp_tls_security_level = may' \
        'smtp_tls_loglevel = 1' \
        'smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache'
    \
        'tls_random_source = dev:/dev/urandom'

Note: the last command requires both single (') and double (") quotes.

The postconf(1) command above enables opportunistic TLS for receiving and
sending mail. It also enables logging of TLS connections and recording of TLS
use in the "Received" header. TLS session caching is also enabled in the
Postfix SMTP client. With Postfix >= 2.10, the SMTP server does not need an
explicit session cache since session reuse is better handled via RFC 5077 TLS
session tickets.

Private Certification Authority

  * Become your own Certification Authority, so that you can sign your own
    certificates, and so that your own systems can authenticate certificates
    from your own CA. This example uses the CA.pl script that ships with
    OpenSSL. On some systems, OpenSSL installs this as /usr/local/openssl/misc/
    CA.pl. Some systems install this as part of a package named openssl-perl or
    something similar. The script creates a private key in ./demoCA/private/
    cakey.pem and a public key in ./demoCA/cacert.pem.

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

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

  * Create an unpassworded private key for host foo.porcupine.org and create an
    unsigned public key certificate.

        % (umask 077; openssl req -new -newkey rsa:2048 -nodes -keyout foo-
        key.pem -out foo-req.pem)
        Using configuration from /etc/ssl/openssl.cnf
        Generating a 2048 bit RSA private key
        ........................................++++++
        ....++++++
        writing new private key to 'foo-key.pem'
        -----
        You are about to be asked to enter information that will be
        incorporated
        into your certificate request.
        What you are about to enter is what is called a Distinguished Name or a
        DN.
        There are quite a few fields but you can leave some blank
        For some fields there will be a default value,
        If you enter '.', the field will be left blank.
        -----
        Country Name (2 letter code) [AU]:US
        State or Province Name (full name) [Some-State]:New York
        Locality Name (eg, city) []:Westchester
        Organization Name (eg, company) [Internet Widgits Pty Ltd]:Porcupine
        Organizational Unit Name (eg, section) []:
        Common Name (eg, YOUR name) []:foo.porcupine.org
        Email Address []:wietse@porcupine.org

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

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

        % openssl ca -out foo-cert.pem -days 365 -infiles foo-req.pem
        Using configuration from /etc/ssl/openssl.cnf
        Enter PEM pass phrase:whatever
        Check that the request matches the signature
        Signature ok
        The Subjects Distinguished Name is as follows
        countryName           :PRINTABLE:'US'
        stateOrProvinceName   :PRINTABLE:'New York'
        localityName          :PRINTABLE:'Westchester'
        organizationName      :PRINTABLE:'Porcupine'
        commonName            :PRINTABLE:'foo.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]:y

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

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

    The following commands assume that the key and certificate will be
    installed for the local Postfix MTA. You will need to adjust the commands
    if the Postfix MTA is on a different host.

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

  * Configure Postfix, by adding the following to /etc/postfix/main.cf. 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/db/postfix/smtp_tls_session_cache
            smtp_tls_security_level = may
            smtp_tls_loglevel = 1
            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/db/postfix/smtpd_tls_session_cache
            tls_random_source = dev:/dev/urandom
            smtpd_tls_security_level = may
            smtpd_tls_loglevel = 1

Building Postfix with TLS support

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

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

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

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

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

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

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

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

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

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

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

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

Reporting problems

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.

Credits

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

