From b7c15c31519dc44c1f691e0466badd556ffe9423 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 18:18:56 +0200 Subject: Adding upstream version 3.7.10. Signed-off-by: Daniel Baumann --- proto/TLS_LEGACY_README.html | 1606 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 1606 insertions(+) create mode 100644 proto/TLS_LEGACY_README.html (limited to 'proto/TLS_LEGACY_README.html') diff --git a/proto/TLS_LEGACY_README.html b/proto/TLS_LEGACY_README.html new file mode 100644 index 0000000..dacf1c1 --- /dev/null +++ b/proto/TLS_LEGACY_README.html @@ -0,0 +1,1606 @@ + + + + + + +Postfix legacy TLS Support + + + + + + + +

Postfix legacy TLS Support +

+ +
+ +

NOTE

+ +

This document describes an old TLS user interface that is based +on a third-party TLS patch by Lutz Jänicke. As of Postfix +version 2.3, the old user interface still exists to allow migration +from earlier Postfix releases, but its functionality is frozen.

+ +

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, 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.

+ +

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 needs a certificate +and a private key. Both must be in "pem" format. The private key +must not be encrypted, meaning: the key must be accessible without +a password. 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 an 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
+
+
+ +

Server-side TLS activity logging

+ +

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
+
+
+ +

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 +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
+
+
+ +

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.

+ +

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
+
+
+ +

Supporting AUTH over TLS only

+ +

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
+
+
+ +

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
+
+
+ +

As of version 2.5, Postfix will no longer maintain this file +in a directory with non-Postfix ownership. As a migration aid, +attempts to open such files are redirected to the Postfix-owned +$data_directory, and a warning is logged.

+ +

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

+ +

Example:

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

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 a 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

+ +

To influence the Postfix SMTP server cipher selection scheme, +you can give cipherlist string. A detailed description would go +too 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
+
+
+ +

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:

+ + + +

Client-side certificate and private +key configuration

+ +

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 +a 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 an 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 = $smtp_tls_dcert_file
+
+
+ +

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

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

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

+ +

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

+ +

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

+ +

Example:

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

Client-side TLS 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
+
+
+ +

As of version 2.5, Postfix will no longer maintain this file +in a directory with non-Postfix ownership. As a migration aid, +attempts to open such files are redirected to the Postfix-owned +$data_directory, and a warning is logged.

+ +

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

+ +

Example:

+ +
+
+/etc/postfix/main.cf:
+    smtp_tls_session_cache_timeout = 3600s
+
+
+ +

Enabling TLS in the Postfix SMTP +client

+ +

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
+
+
+ +

Requiring TLS encryption +

+ +

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. 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 all names +provided as dNSNames +in the SubjectAlternativeName. If no dNSNames are specified, the +CommonName is checked. Verification may be turned off with the +smtp_tls_enforce_peername option which is discussed below.

+ +

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 only sends email to one specific mailhub that offers +the necessary STARTTLS support.

+ +

Example:

+ +
+
+/etc/postfix/main.cf:
+    smtp_enforce_tls = yes
+
+
+ +

Disabling server certificate +verification

+ +

As of RFC 2487 the requirements for hostname checking for MTA +clients are not set. When TLS is required (smtp_enforce_tls = yes), +the option smtp_tls_enforce_peername can be set to "no" to disable +strict remote SMTP server hostname checking. In this case, the mail +delivery will proceed regardless of the CommonName etc. listed in +the certificate.

+ +

Despite the potential for eliminating "man-in-the-middle" and +other attacks, mandatory certificate/peername verification is not +viable as a default Internet mail delivery policy at this time. A +significant fraction of TLS enabled MTAs uses self-signed certificates, +or certificates that are signed by a private Certification 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 per-site TLS +policies (see below) 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
+
+
+ +

Per-site TLS policies

+ +

A small fraction of servers offer STARTTLS but the negotiation +consistently fails, leading to mail aging out of the queue and +bouncing back to the sender. In such cases, you can use the per-site +policies to disable TLS for the problem sites. Alternatively, you +can enable TLS for just a few specific sites and not enable it for +all sites.

+ + + +

The smtp_tls_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
Don't use TLS at all. 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
Try to use TLS if the server announces support, +otherwise use the unencrypted connection. 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
Require TLS encryption, but do not +require that the remote SMTP server hostname matches the information +in the remote SMTP server certificate, or that the server certificate +was issued by a trusted CA. 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
Require TLS encryption, require that the remote +SMTP server hostname matches the information in the remote SMTP +server certificate, and require that the remote SMTP server certificate +was issued by a trusted CA. 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 + per-site TLS policies

+ +

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:

+ + + +

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
+
+
+ +

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

+ +

Example:

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

Client-side cipher controls

+ +

To influence the Postfix SMTP client cipher selection scheme, +you can give cipherlist string. A detailed description would go +too 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
+
+
+ +

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 Certification Authority, and +Postfix needs to be configured with a list of public key certificates +of Certification 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

+ +

When reporting a problem, please be thorough in the report. +Patches, when possible, are greatly appreciated too.

+ +

Please differentiate when possible between:

+ + + +

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

+ + + + + + -- cgit v1.2.3