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.
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:
The diagram below shows the main elements of the Postfix TLS architecture and their relationships. Colored boxes with numbered names represent Postfix daemon programs. Other colored boxes represent storage elements.
The smtpd(8) server implements the SMTP over TLS server side.
The smtp(8) client implements the SMTP over TLS client side.
The tlsmgr(8) server maintains the pseudo-random number generator (PRNG) that seeds the TLS engines in the smtpd(8) server and smtp(8) client processes, and maintains the TLS session key cache files.
Network-> | smtpd(8) |
<---seed--- <-session-> | tlsmgr(8) |
---seed---> <-session-> | smtp(8) | ->Network | ||||||||||
| | | |
| ||||||||||||||
smtpd session key cache | PRNG state file | smtp session key cache |
To build Postfix with TLS support, first we need to generate the make(1) files with the necessary definitions. This is done by invoking the command "make makefiles" in the Postfix top-level directory and with arguments as shown next.
NOTE: Do not use Gnu TLS. It will spontaneously terminate a Postfix daemon process with exit status code 2, instead of allowing Postfix to 1) report the error to the maillog file, and to 2) provide plaintext service where this is appropriate.
If the OpenSSL include files (such as ssl.h) are in directory /usr/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/lib:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"
If the OpenSSL include files (such as ssl.h) are in directory /usr/local/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/local/lib:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \ AUXLIBS="-L/usr/local/lib -lssl -lcrypto"
On Solaris, specify the -R option as shown below:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \ AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto"
If you need to apply other customizations (such as Berkeley DB databases, MySQL, PosgreSQL, LDAP or SASL), see the respective Postfix README documents, and combine their "make makefiles" instructions with the instructions above:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS \ (other -D or -I options)" \ AUXLIBS="-lssl -lcrypto \ (other -l options for libraries in /usr/lib) \ (-L/path/name + -l options for other libraries)"
To complete the build process, see the Postfix INSTALL instructions. Postfix has TLS support turned off by default, so you can start using Postfix as soon as it is installed.
Topics covered in this section:
In order to use TLS, the Postfix SMTP server needs a certificate and a private key. Both must be in "pem" format. The private key must not be encrypted, meaning: the key must be accessible without password. Both certificate and private key may be in the same file.
Both RSA and DSA certificates are supported. Typically you will only have RSA certificates issued by a commercial CA. In addition, the tools supplied with OpenSSL will by default issue RSA certificates. You can have both at the same time, in which case the cipher used determines which certificate is presented. For Netscape and OpenSSL clients without special cipher choices, the RSA certificate is preferred.
In order for remote SMTP clients to check the Postfix SMTP server certificates, the CA certificate (in case of a certificate chain, all CA certificates) must be available. You should add these certificates to the server certificate, the server certificate first, then the issuing CA(s).
Example: the certificate for "server.dom.ain" was issued by "intermediate CA" which itself has a certificate issued by "root CA". Create the server.pem file with:
% cat server_cert.pem intermediate_CA.pem > server.pem
A Postfix SMTP server certificate supplied here must be usable as SSL server certificate and hence pass the "openssl verify -purpose sslserver ..." test.
A client that trusts the root CA has a local copy of the root CA certificate, so it is not necessary to include the root CA certificate here. Leaving it out of the "server.pem" file reduces the overhead of the TLS exchange.
If you want the Postfix SMTP server to accept remote SMTP client certificates issued by these CAs, append the root certificate to $smtpd_tls_CAfile or install it in the $smtpd_tls_CApath directory. When you configure trust in a root CA, it is not necessary to explicitly trust intermediary CAs signed by the root CA, unless $smtpd_tls_ccert_verifydepth is less than the number of CAs in the certificate chain for the clients of interest. With a verify depth of 1 you can only verify certificates directly signed by a trusted CA, and all trusted intermediary CAs need to be configured explicitly. With a verify depth of 2 you can verify clients signed by a root CA or a direct intermediary CA (so long as the client is correctly configured to supply its intermediate CA certificate).
RSA key and certificate examples:
/etc/postfix/main.cf: smtpd_tls_cert_file = /etc/postfix/server.pem smtpd_tls_key_file = $smtpd_tls_cert_file
Their DSA counterparts:
/etc/postfix/main.cf: smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem smtpd_tls_dkey_file = $smtpd_tls_dcert_file
To verify a remote SMTP client certificate, the Postfix SMTP server needs to trust the certificates of the issuing certification authorities. These certificates in "pem" format can be stored in a single $smtpd_tls_CAfile or in multiple files, one CA per file in the $smtpd_tls_CApath directory. If you use a directory, don't forget to create the necessary "hash" links with:
# $OPENSSL_HOME/bin/c_rehash /path/to/directory
The $smtpd_tls_CAfile contains the CA certificates of one or more trusted CAs. The file is opened (with root privileges) before Postfix enters the optional chroot jail and so need not be accessible from inside the chroot jail.
Additional trusted CAs can be specified via the $smtpd_tls_CApath directory, in which case the certificates are read (with $mail_owner privileges) from the files in the directory when the information is needed. Thus, the $smtpd_tls_CApath directory needs to be accessible inside the optional chroot jail.
When you configure Postfix to request client certificates (by setting $smtpd_tls_ask_ccert = yes), any certificates in $smtpd_tls_CAfile are sent to the client, in order to allow it to choose an identity signed by a CA you trust. If no $smtpd_tls_CAfile is specified, no preferred CA list is sent, and the client is free to choose an identity signed by any CA. Many clients use a fixed identity regardless of the preferred CA list and you may be able to reduce TLS negotiation overhead by installing client CA certificates mostly or only in $smtpd_tls_CApath. In the latter case you need not specify a $smtpd_tls_CAfile.
Note, that unless client certificates are used to allow greater access to TLS authenticated clients, it is best to not ask for client certificates at all, as in addition to increased overhead some clients (notably in some cases qmail) are unable to complete the TLS handshake when client certificates are requested.
Example:
/etc/postfix/main.cf: smtpd_tls_CAfile = /etc/postfix/CAcert.pem smtpd_tls_CApath = /etc/postfix/certs
To get additional information about Postfix SMTP server TLS activity you can increase the loglevel from 0..4. Each logging level also includes the information that is logged at a lower logging level.
0 Disable logging of TLS activity. 1 Log TLS handshake and certificate information. 2 Log levels during TLS negotiation. 3 Log hexadecimal and ASCII dump of TLS negotiation process 4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS
Use loglevel 3 only in case of problems. Use of loglevel 4 is strongly discouraged.
Example:
/etc/postfix/main.cf: smtpd_tls_loglevel = 0
To include information about the protocol and cipher used as well as the client and issuer CommonName into the "Received:" message header, set the smtpd_tls_received_header variable to true. The default is no, as the information is not necessarily authentic. Only information recorded at the final destination is reliable, since the headers may be changed by intermediate servers.
Example:
/etc/postfix/main.cf: smtpd_tls_received_header = yes
By default, TLS is disabled in the Postfix SMTP server, so no difference to plain Postfix is visible. Explicitly switch it on using "smtpd_use_tls = yes".
Example:
/etc/postfix/main.cf: smtpd_use_tls = yes
With this, Postfix SMTP server announces STARTTLS support to SMTP clients, but does not require that clients use TLS encryption.
Note: when an unprivileged user invokes "sendmail -bs", STARTTLS is never offered due to insufficient privileges to access the server private key. This is intended behavior.
You can ENFORCE the use of TLS, so that the Postfix SMTP server announces STARTTLS and accepts no mail without TLS encryption, by setting "smtpd_enforce_tls = yes". According to RFC 2487 this MUST NOT be applied in case of a publicly-referenced Postfix SMTP server. This option is off by default and should only seldom be used.
Example:
/etc/postfix/main.cf: smtpd_enforce_tls = yes
TLS is sometimes used in the non-standard "wrapper" mode where a server always uses TLS, instead of announcing STARTTLS support and waiting for clients to request TLS service. Some clients, namely Outlook [Express] prefer the "wrapper" mode. This is true for OE (Win32 < 5.0 and Win32 >=5.0 when run on a port<>25 and OE (5.01 Mac on all ports).
It is strictly discouraged to use this mode from main.cf. If you want to support this service, enable a special port in master.cf and specify "-o smtpd_tls_wrappermode = yes" as an smtpd(8) command line option. Port 465 (smtps) was once chosen for this feature.
Example:
/etc/postfix/master.cf: smtps inet n - n - - smtpd -o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes
To receive a remote SMTP client certificate, the Postfix SMTP server must explicitly ask for one (any contents of $smtpd_tls_CAfile are also sent to the client as a hint for choosing a certificate from a suitable CA). Unfortunately, Netscape clients will either complain if no matching client certificate is available or will offer the user client a list of certificates to choose from. Additionally some MTAs (notably some versions of qmail) are unable to complete TLS negotiation when client certificates are requested, and abort the SMTP session. So this option is "off" by default. You will however need the certificate if you want to use certificate based relaying with, for example, the permit_tls_clientcerts feature.
Example:
/etc/postfix/main.cf: smtpd_tls_ask_ccert = no
You may also decide to REQUIRE a remote SMTP client certificate before allowing TLS connections. This feature is included for completeness, and implies "smtpd_tls_ask_ccert = yes".
Please be aware, that this will inhibit TLS connections without a proper client certificate and that it makes sense only when non-TLS submission is disabled (smtpd_enforce_tls = yes). Otherwise, clients could bypass the restriction by simply not using STARTTLS at all.
When TLS is not enforced, the connection will be handled as if only "smtpd_tls_ask_ccert = yes" is specified, and a warning is logged.
Example:
/etc/postfix/main.cf: smtpd_tls_req_ccert = no
A client certificate verification depth of 1 is sufficient if the certificate is directly issued by a CA listed in the CA file. The default value (5) should also suffice for longer chains (root CA issues special CA which then issues the actual certificate...)
Example:
/etc/postfix/main.cf: smtpd_tls_ccert_verifydepth = 5
Sending AUTH data over an unencrypted channel poses a security risk. When TLS layer encryption is required (smtpd_enforce_tls = yes), the Postfix SMTP server will announce and accept AUTH only after the TLS layer has been activated with STARTTLS. When TLS layer encryption is optional (smtpd_enforce_tls = no), it may however still be useful to only offer AUTH when TLS is active. To maintain compatibility with non-TLS clients, the default is to accept AUTH without encryption. In order to change this behavior, set "smtpd_tls_auth_only = yes".
Example:
/etc/postfix/main.cf: smtpd_tls_auth_only = no
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
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
To influence the Postfix SMTP server cipher selection scheme, you can give cipherlist string. A detailed description would go to far here; please refer to the OpenSSL documentation. If you don't know what to do with it, simply don't touch it and leave the (openssl-)compiled in default!
DO NOT USE " to enclose the string, specify just the string!!!
Example:
/etc/postfix/main.cf: smtpd_tls_cipherlist = DEFAULT
If you want to take advantage of ciphers with EDH, DH parameters are needed. Instead of using the built-in DH parameters for both 1024bit and 512bit, it is better to generate "own" parameters, since otherwise it would "pay" for a possible attacker to start a brute force attack against parameters that are used by everybody. For this reason, the parameters chosen are already different from those distributed with other TLS packages.
To generate your own set of DH parameters, use:
% openssl gendh -out /etc/postfix/dh_1024.pem -2 -rand /var/run/egd-pool 1024 % openssl gendh -out /etc/postfix/dh_512.pem -2 -rand /var/run/egd-pool 512
Examples:
/etc/postfix/main.cf: smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
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
Topics covered in this section:
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, mere encryption of LMTP traffic over unix-domain sockets is futile, the channel is already secure. TLS is only useful in this context, when it is mandatory and at least one of the server or client is authenticating the other. The eNULL TLS ciphers may be appropriate in this context when available on both client and server, these provide authentication without encryption.
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
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
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
The security properties of TLS communications channels are application specific. While the TLS protocol supports mutual authentication between client and server, not all of its security features are applicable in every case.
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 client, if the client chooses to not verify the server's name, the server is not aware of this. There are many fascinating issues related to browser security, but we shall not dwell on them here. Rather our goal is to understand the security features of TLS in conjuction with SMTP.
An important SMTP-specific observation is that a public MX host is even more at the mercy of the SMTP client. 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 to enforce 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 has 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. Also bounces 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 as implemented in the client. While the server's security role is more significant when it is an MSA that only handles outbound mail from trusted clients, in what follows we focus exclusively 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 (unauthenticated) MX lookups 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, on the other hand, the client verifies the original domain name, an SMTP server that receives mail for multiple domains needs to list all its email domain names in its certificate, and generate a new certificate each time a new domain is 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 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.
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 Jaenicke was designing the first unofficial Postfix TLS patch. See, his original post http://thread.gmane.org/gmane.ietf.apps-tls/304/focus=304 and the first response http://thread.gmane.org/gmane.ietf.apps-tls/304/focus=305. 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.
We are now ready to describe the security levels available to a TLS-enabled SMTP client:
The above security levels can be configured in main.cf as global settings for all destinations, or as per-destination settings via the policy table or obsolete per-site table. The per-destination table syntax is described separately, below we explain the key main.cf settings that come into play at each of TLS security levels.
The client will negotiate a TLS session when possible, at the maximum cryptographic strength supported by the server. The configuration settings are:
/etc/postfix/main.cf: smtp_use_tls = yes
In this case the client encrypts all traffic to example.com (sent via the corresponding MX hosts) and to the non-MX destination example.net (on port 587). All sessions are encrypted via medium or high strength ciphers (128 bit or better). Traffic to example.net will only use TLSv1 and HIGH strength ciphers.
/etc/postfix/main.cf: smtp_tls_cipherlist = HIGH:MEDIUM:!aNULL:+RC4:@STRENGTH smtp_tls_policy_maps = hash:/etc/postfix/tls_policy /etc/postfix/tls_policy: example.com encrypt [example.net]:587 encrypt cipherlist=HIGH:!aNULL:@STRENGTH protocols=TLSv1
In this case the client encrypts all traffic to example.com domain. The peer hostname is verified, but is vulnerable to DNS response forgery. All mandatory encryption uses high strength (better than 128 bit) ciphers.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/trusted-CAs.pem smtp_tls_cipherlist = HIGH:!aNULL:@STRENGTH smtp_tls_policy_maps = hash:/etc/postfix/tls_policy /etc/postfix/tls_policy: example.com verify
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.comdomain, 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. Given that secure channels require at the very least a corresponding policy table entry, adding the transport table entry is not onerous.
Note, there are two ways to handle related domains. The first is to override the next-hop in the transport table, and use a single policy table entry for the common nexthop. The second is to use the default routing for each domain, but add policy table entries for the expected certificates. We choose the second approach, because it works better when domain ownership changes. With the first scenario we securely (mis)deliver mail to the wrong destination, with the second approach, authentication fails and mail stays in the local queue, the second approach is more appropriate in most cases.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/trusted-CAs.pem smtp_tls_cipherlist = HIGH:MEDIUM:!aNULL:+RC4:@STRENGTH 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
In this case traffic to example.com and its related domains is sent to a single logical gateway (ideally, to avoid a single point of failure, its name resolves to one or more load-balancer addresses, or to the combined addresses of multiple hosts). All the physical hosts reachable via the gateway's IP addresses have the 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 before, this approach has the potential to mis-deliver email if the related domains change hands.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/trusted-CAs.pem smtp_tls_cipherlist = HIGH:MEDIUM:!aNULL:+RC4:@STRENGTH 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
By default, TLS is disabled in the Postfix SMTP client, so no difference to plain Postfix is visible. If you enable TLS, the Postfix SMTP client will send STARTTLS when TLS support is announced by the remote SMTP server.
When the server accepts the STARTTLS command, but the subsequent TLS handshake fails, and no other server is available, the Postfix SMTP client defers the delivery attempt, and the mail stays in the queue. After a handshake failure, the communications channel is in an indeterminate state and cannot be used for non-TLS deliveries.
Example:
/etc/postfix/main.cf: smtp_use_tls = yes
You can ENFORCE the use of TLS, so that the Postfix SMTP client will not deliver mail over unencrypted connections. In this mode, the remote SMTP server hostname must match the information in the remote server certificate, and the server certificate must be issued by a CA that is trusted by the Postfix SMTP client (see smtp_tls_CAfile and smtp_tls_CApath). If the remote server certificate doesn't verify or the remote SMTP server hostname doesn't match, and no other server is available, the delivery attempt is deferred and the mail stays in the queue.
The remote SMTP server hostname is verified against any DNS names in the SubjectAlternativeName certificate extension. If no DNS names are specified, the subject CN (CommonName) is used. Verification may be turned off with the smtp_tls_enforce_peername option which is discussed below.
Enforcing TLS in this manner corresponds to a policy setting of verify in the policy table or MUST in the obsolete per-site table. With the new policy table approach, you choose names other the peer hostname to check against the server certificate.
Enforcing the use of TLS is useful if you know that you will only connect to servers that support RFC 2487 and that present server certificates that meet the above requirements. An example would be a client that sends all email to a central mailhub that offers the necessary STARTTLS support. To harden server certificate verification against DNS forgery, see below.
Example:
/etc/postfix/main.cf: smtp_enforce_tls = yes
When TLS encryption is enforced for all connections, the Postfix TLS client also by default enforces server certificate verification. The relevant configuration parameter is smtp_tls_enforce_peername which defaults to yes.
When TLS is required (smtp_enforce_tls = yes), setting smtp_tls_enforce_peername to "no" disables SMTP server certificate checks. In this case, the mail delivery will proceed regardless of whether the signing CAs are trusted or the subject name(s) listed in the certificate.
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. A significant number 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, if you set smtp_enforce_tls = yes, you should probably also set smtp_tls_enforce_peername = no.
You can use the TLS policy table or obsolete per-site table to enable full peer verification for specific destinations that are known to have verifiable TLS server certificates.
Example:
/etc/postfix/main.cf: smtp_enforce_tls = yes smtp_tls_enforce_peername = no
As described in the security levels section above, Postfix 2.3 introduces a new more flexible TLS policy table. 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. It supersedes the obsolete smtp_tls_per_site parameter. When $smtp_tls_policy_maps is not empty, the smtp_tls_per_site parameter is ignored (a warning is written to the logs if it is also 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 level names and attribute syntax:
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 cipherlist=HIGH+aDSS:MEDIUM+aDSS:+RC4:@STRENGTH example.com verify match=hostname:dot-nexthop protocols=SSLv3:TLSv1 cipherlist=kEDH+HIGH:!aNULL:@STRENGTH 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.
This section describes an obsolete per-site TLS policy mechanism. Unlike the new policy table mechanism it implements TLS policy lookup by 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. Additionally, the new policy table makes secure channel configurations easier and provides more control over the cipher and protocol selection for sessions with mandatory encryption.
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_cipherlist and smtp_tls_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 smtp_tls_cipherlist is applied also 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.
- next-hop destination
- This is normally the domain portion of the recipient address, but it may be overruled by information from the transport(5) table, from the relayhost parameter setting, or from the relay_transport setting. When it's not the recipient domain, the next-hop destination can have the Postfix-specific form "[name]", [name]:port", "name" or "name:port".
When both the hostname lookup and the next-hop lookup succeed, the host policy does not automatically override the next-hop policy. Instead, precedence is given to either the more specific or the more secure per-site policy as described below.
The smtp_tls_per_site table uses a simple "name whitespace value" format. Specify host names or next-hop destinations on the left-hand side; no wildcards are allowed. On the right hand side specify one of the following keywords:
- NONE
- 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 TLS verification. This overrides a less secure NONE and MUST_NOPEERMATCH or a less specific MAY lookup result from the alternate host or next-hop lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.
The precedences between global (main.cf) and per-site TLS policies can be summarized as follows:
When neither the remote SMTP server hostname nor the next-hop destination are found in the smtp_tls_per_site table, the policy is based on smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername. Note: "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = yes" imply "smtp_use_tls = yes".
When both hostname and next-hop destination lookups produce a result, the more specific per-site policy (NONE, MUST, etc) overrides the less specific one (MAY), and the more secure per-site policy (MUST, etc) overrides the less secure one (NONE).
After the per-site policy lookups are combined, the result generally overrides the global policy. The exception is the less specific MAY per-site policy, which is overruled by the more specific global "smtp_enforce_tls = yes" with server certificate verification as specified with the smtp_tls_enforce_peername parameter.
For a discussion of hardened TLS secure-channels in Postfix 2.3 and later see the introduction to security levels and the policy table documentation. What follows is applicable with the obsolete Postfix 2.2 per-site table.
As long as no secure DNS lookup mechanism is available, false hostnames in MX or CNAME responses can change the server hostname that Postfix uses for TLS policy lookup and server certificate verification. Even with a perfect match between the server hostname and the server certificate, there is no guarantee that Postfix is connected to the right server. To avoid this loophole take the following steps:
Eliminate MX lookups. Specify local transport(5) table entries for sensitive domains with explicit smtp:[mailhost] or smtp:[mailhost]:port destinations (you can assure security of this table unlike DNS); in the smtp_tls_per_site table specify the value MUST for the key [mailhost] or smtp:[mailhost]:port. This prevents false hostname information in DNS MX records from changing the server hostname that Postfix uses for TLS policy lookup and server certificate verification.
Disallow CNAME hostname overrides. In main.cf specify "smtp_cname_overrides_servername = no". This prevents false hostname information in DNS CNAME records from changing the server hostname that Postfix uses for TLS policy lookup and server certificate verification. This feature requires Postfix 2.2.9 or later. The default value is yes starting with Postfix 2.3.
Example:
/etc/postfix/main.cf: smtp_tls_per_site = hash:/etc/postfix/tls_per_site relayhost = [msa.example.net]:587 /etc/postfix/tls_per_site: # relayhost exact nexthop match [msa.example.net]:587 MUST # TLS should not be used with the example.org MX hosts. example.org NONE # TLS should not be used with the host smtp.example.com. smtp.example.com NONE
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
When verifying a remote SMTP server certificate, a verification depth of 1 is sufficient if the certificate is directly issued by a CA specified with smtp_tls_CAfile or smtp_tls_CApath. The default value of 5 should also suffice for longer chains (root CA issues special CA which then issues the actual certificate...)
Example:
/etc/postfix/main.cf: smtp_tls_scert_verifydepth = 5
To influence the Postfix SMTP client cipher selection scheme, you can give cipherlist string. A detailed description would go to far here; please refer to the OpenSSL documentation. If you don't know what to do with it, simply don't touch it and leave the (openssl-)compiled in default!
DO NOT USE " to enclose the string, specify just the string!!!
Example:
/etc/postfix/main.cf: smtp_tls_cipherlist = DEFAULT
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
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
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.
Become your own Certificate Authority, so that you can sign your own public keys. This example uses the CA.pl script that ships with OpenSSL. By default, OpenSSL installs this as /usr/local/ssl/misc/CA.pl, but your mileage may vary. The script creates a private key in ./demoCA/private/cakey.pem and a public key in ./demoCA/cacert.pem.
% /usr/local/ssl/misc/CA.pl -newca CA certificate filename (or enter to create) Making CA certificate ... Using configuration from /etc/ssl/openssl.cnf Generating a 1024 bit RSA private key ....................++++++ .....++++++ writing new private key to './demoCA/private/cakey.pem' Enter PEM pass phrase:whatever
Create an unpassworded private key for host FOO and create an unsigned public key certificate.
% openssl req -new -nodes -keyout FOO-key.pem -out FOO-req.pem -days 365 Using configuration from /etc/ssl/openssl.cnf Generating a 1024 bit RSA private key ........................................++++++ ....++++++ writing new private key to 'FOO-key.pem' ----- You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank. ----- Country Name (2 letter code) [AU]:US State or Province Name (full name) [Some-State]:New York Locality Name (eg, city) []:Westchester Organization Name (eg, company) [Internet Widgits Pty Ltd]:Porcupine Organizational Unit Name (eg, section) []: Common Name (eg, YOUR name) []:FOO Email Address []:wietse@porcupine.org Please enter the following 'extra' attributes to be sent with your certificate request A challenge password []:whatever An optional company name []:
Sign the public key certificate for host FOO with the Certification Authority private key that we created a few steps ago.
% openssl ca -out FOO-cert.pem -infiles FOO-req.pem Uing configuration from /etc/ssl/openssl.cnf Enter PEM pass phrase:whatever Check that the request matches the signature Signature ok The Subjects Distinguished Name is as follows countryName :PRINTABLE:'US' stateOrProvinceName :PRINTABLE:'New York' localityName :PRINTABLE:'Westchester' organizationName :PRINTABLE:'Porcupine' commonName :PRINTABLE:'FOO' emailAddress :IA5STRING:'wietse@porcupine.org' Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365 days) Sign the certificate? [y/n]:y 1 out of 1 certificate requests certified, commit? [y/n]y Write out database with 1 new entries Data Base Updated
Install the host private key, the host public key certificate, and the Certification Authority certificate files. This requires super-user privileges.
# cp demoCA/cacert.pem FOO-key.pem FOO-cert.pem /etc/postfix # chmod 644 /etc/postfix/FOO-cert.pem /etc/postfix/cacert.pem # chmod 400 /etc/postfix/FOO-key.pem
Configure Postfix, by adding the following to /etc/postfix/main.cf .
smtp_tls_CAfile = /etc/postfix/cacert.pem smtp_tls_cert_file = /etc/postfix/FOO-cert.pem smtp_tls_key_file = /etc/postfix/FOO-key.pem smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache smtp_use_tls = yes smtpd_tls_CAfile = /etc/postfix/cacert.pem smtpd_tls_cert_file = /etc/postfix/FOO-cert.pem smtpd_tls_key_file = /etc/postfix/FOO-key.pem smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:/var/run/smtpd_tls_session_cache smtpd_use_tls = yes tls_random_source = dev:/dev/urandom
When reporting a problem, please be thorough in the report. Patches, when possible, are greatly appreciated too.
Please differentiate when possible between:
Postfix version 2.2 TLS support is based on the Postfix/TLS patch by Lutz Jänicke, but differs in a few minor ways.
main.cf: Specify "btree" instead of "sdbm" for TLS session cache databases.
TLS session cache databases are now accessed only by the tlsmgr(8) process, so there are no more concurrency issues. Although Postfix has an sdbm client, the sdbm library (1000 lines of code) is not included with Postfix.
TLS session caches can use any database that can store objects of several kbytes or more, and that implements the sequence operation. In most cases, btree databases should be adequate.
NOTE: You cannot use dbm databases. TLS session objects are too large.
master.cf: Specify "unix" instead of "fifo" as the tlsmgr service type.
The smtp(8) and smtpd(8) processes now use a client-server protocol in order to access the tlsmgr(8) pseudo-random number generation (PRNG) pool, and in order to access the TLS session cache databases. Such a protocol cannot be run across fifos.
smtp_tls_per_site: the MUST_NOPEERMATCH per-site policy cannot override the global "smtp_tls_enforce_peername = yes" setting.
smtp_tls_per_site: a combined (NONE + MAY) lookup result for (hostname and next-hop destination) produces counter-intuitive results for different main.cf settings. TLS is enabled with "smtp_tls_enforce_peername = no", but it is disabled when both "smtp_enforce_tls = yes" and "smtp_tls_enforce_peername = yes".
The smtp_tls_per_site limitations were removed by the end of the Postfix 2.2 support cycle.