Postfix TLS Support


WARNING

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

What Postfix TLS support does for you

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

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

Topics covered in this document:

And last but not least, for the impatient:

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.

Network->
smtpd(8)
 
<---seed---

<-session->

tlsmgr(8)
 
---seed--->

<-session->

smtp(8)
 
->Network
/
/
|
|
\
\
smtpd
session
key cache
PRNG
state
file
smtp
session
key cache

Building Postfix with TLS support

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

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

If 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.

SMTP Server specific settings

Topics covered in this section:

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

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

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

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

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

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

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

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

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

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

RSA key and certificate examples:

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

Their DSA counterparts:

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

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

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

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

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

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

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

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

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

Example:

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

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.

0 Disable logging of TLS activity.
1 Log TLS handshake and certificate information.
2 Log levels during TLS negotiation.
3 Log hexadecimal and ASCII dump of TLS negotiation process
4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS

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

Example:

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

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

Example:

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

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" (Postfix 2.3 and later) or "smtpd_use_tls = yes" (obsolete but still supported).

Example:

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

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

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

You can ENFORCE the use of TLS, so that the Postfix SMTP server announces STARTTLS and accepts no mail without TLS encryption, by setting "smtpd_tls_security_level = encrypt" (Postfix 2.3 and later) or "smtpd_enforce_tls = yes" (obsolete but still supported). According to RFC 2487 this MUST NOT be applied in case of a publicly-referenced Postfix SMTP server. This option is off by default and should only seldom be used.

Example:

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

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

It is strictly discouraged to use this mode from main.cf. If you want to support this service, enable a special port in master.cf and specify "-o smtpd_tls_wrappermode=yes" (note: no space around the "=") as an smtpd(8) command line option. Port 465 (smtps) was once chosen for this feature.

Example:

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

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 2.3 by default offers anonymous ciphers to clients, these are automatically suppressed when the server is configured to ask for client certificates.

Example:

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

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

Example:

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

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

Example:

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

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

Example:

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

Server-side TLS session cache

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

Example:

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

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

Example:

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

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

Example:

    smtpd_tls_always_issue_session_ids = no

Server access control

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

permit_tls_clientcerts

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

permit_tls_all_clientcerts

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

check_ccert_access type:table

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

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

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

Example:

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

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

Example:

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

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

Server-side cipher controls

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

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

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

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

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

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

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

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

To generate your own set of DH parameters, use:

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

Examples:

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

Miscellaneous server controls

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

Example:

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

SMTP Client specific settings

Topics covered in this section:

TLS support in the LMTP delivery agent

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

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

Client-side certificate and private key configuration

Do not configure 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 =

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

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

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

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

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

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

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

RSA key and certificate examples:

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

Their DSA counterparts:

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

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

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

Example:

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

Client-side TLS activity logging

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

0 Disable logging of TLS activity.
1 Log TLS handshake and certificate information.
2 Log levels during TLS negotiation.
3 Log hexadecimal and ASCII dump of TLS negotiation process
4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS

Example:

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

Client-side TLS session cache

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

Example:

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

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

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.

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

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

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

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

Client TLS security levels

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

none
No TLS.
may
Opportunistic TLS.
encrypt
Mandatory TLS encryption.
verify
Mandatory server certificate verification.
secure
Secure-channel TLS.

Disabling TLS in the SMTP/LMTP client

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

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

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

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

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. With Postfix 2.3 and later, opportunistic TLS can be configured by setting "smtp_tls_security_level = may".

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

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

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

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

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

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

Example:

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

Postfix 2.2 syntax:

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

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 server. If no suitable servers are found, the message will be deferred. With Postfix 2.3 and later, mandatory TLS encryption can be configured by setting "smtp_tls_security_level = encrypt". Even though TLS encryption is always used, mail delivery continues if the server certificate is untrusted or bears the wrong name.

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

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

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

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

Examples:

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

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

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

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

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

/etc/postfix/tls_per_site:
    example.com       MUST_NOPEERMATCH

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

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

Postfix 2.2 syntax:

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

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

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

Mandatory server certificate verification

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

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

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

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

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

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

Example:

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

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

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

Postfix 2.2 syntax:

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

/etc/postfix/tls_per_site:
    example.com         MUST

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. With Postfix 2.3 and later, secure-channels can be configured by setting "smtp_tls_security_level = secure". The smtp_tls_secure_cert_match parameter can override the default "nexthop, dot-nexthop" certificate match strategy.

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

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

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

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

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

Examples:

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

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

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

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

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

/etc/postfix/transport:

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

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

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

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

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

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

Postfix 2.2.9+ syntax:

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

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

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

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

TLS policy table

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

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

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

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

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

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

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

none
No TLS. No additional attributes are supported at this level.
may
Opportunistic TLS. No additional attributes are supported at this level.
encrypt
Mandatory TLS encryption. Mail is delivered only if remote SMTP server offers STARTTLS and the TLS handshake succeeds. At this level and higher the optional "ciphers" attribute overrides the main.cf smtp_tls_mandatory_ciphers parameter and the optional "protocols" keyword overrides the main.cf smtp_tls_mandatory_protocols parameter.
verify
Mandatory server certificate verification. Mail is delivered only if the TLS handshake succeeds, if the server certificate can be validated (not expired or revoked, and signed by a trusted certificate authority), and if the server certificate name matches the optional "match" attribute (or the main.cf smtp_tls_verify_cert_match parameter value when no optional "match" attribute is specified).
secure
Secure-channel TLS. Mail is delivered only if the TLS handshake succeeds, if the server certificate can be validated (not expired or revoked, and signed by a trusted certificate authority), and if the server certificate name matches the optional "match" attribute (or the main.cf smtp_tls_secure_cert_match parameter value when no optional "match" attribute is specified).

Notes:

Example:

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

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.

Obsolete per-site TLS policy support

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

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

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

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

When smtp_tls_policy_maps is empty (default) and smtp_tls_per_site is not empty, the per-site table is searched for a policy that matches the following information:

remote SMTP server hostname
This is simply the DNS name of the server that the Postfix SMTP client connects to; this name may be obtained from other DNS lookups, such as MX lookups or CNAME lookups. Use of the hostname lookup key is discouraged; always use the next-hop destination instead.
next-hop destination
This is normally the domain portion of the recipient address, but it may be overridden by information from the transport(5) table, from the relayhost parameter setting, or from the relay_transport setting. When it is not the recipient domain, the next-hop destination can have the Postfix-specific form "[name]", "[name]:port", "name" or "name:port". This is the recommended lookup key for per-site policy lookups (and incidentally for SASL password lookups).

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

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

NONE
No TLS. This overrides a less specific "MAY" lookup result from the alternate host or next-hop lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername settings.
MAY
Opportunistic TLS. This has less precedence than a more specific result (including "NONE") from the alternate host or next-hop lookup key, and has less precedence than the more specific global "smtp_enforce_tls = yes" or "smtp_tls_enforce_peername = yes".
MUST_NOPEERMATCH
Mandatory TLS encryption. This overrides a less secure "NONE" or a less specific "MAY" lookup result from the alternate host or next-hop lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.
MUST
Mandatory server certificate verification. This overrides a less secure "NONE" and "MUST_NOPEERMATCH" or a less specific "MAY" lookup result from the alternate host or next-hop lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.

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

Closing a DNS loophole with obsolete per-site TLS policies

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

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

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

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

  3. Disallow CNAME hostname overrides. In main.cf, specify "smtp_cname_overrides_servername = no". This prevents false hostname information in DNS CNAME records from changing the server hostname that Postfix uses for TLS policy lookup and server certificate verification. This feature requires Postfix 2.2.9 or later. The default value is "no" starting with Postfix 2.3.

Example:

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

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

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

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

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

Discovering servers that support TLS

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

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

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

Example:

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

Server certificate verification depth

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

Example:

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

Client-side cipher controls

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

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

Example:

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

Miscellaneous client controls

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

Example:

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

TLS manager specific settings

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

Example:

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

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

Examples (specify only one in main.cf):

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

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

Example:

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

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

Example:

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

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

Examples:

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

Getting started, quick and dirty

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

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

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.

Compatibility with Postfix < 2.2 TLS support

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

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

Credits