Postfix TLS Support
Postfix TLS Support
By turning on TLS support in Postfix, you not only get the ability to encrypt mail and to authenticate remote SMTP clients or servers. You also turn on thousands and thousands of lines of OpenSSL library code. Assuming that OpenSSL is written as carefully as Wietse's own code, every 1000 lines introduce one additional bug into Postfix.
Transport Layer Security (TLS, formerly called SSL) provides certificate-based authentication and encrypted sessions. An encrypted session protects the information that is transmitted with SMTP mail or with SASL authentication.
NOTE: This document describes a TLS user interface that was introduced with Postfix version 2.3. Support for an older user interface is documented in TLS_LEGACY_README, which also describes the differences between Postfix and the third-party patch on which Postfix version 2.2 TLS support was based.
Topics covered in this document:
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 (and LMTP) over TLS client side.
The tlsmgr(8) server maintains the pseudo-random number generator (PRNG) that seeds the TLS engines in the smtpd(8) server and smtp(8) client processes, and maintains the TLS session key cache files.
Not shown in the figure are the tlsproxy(8) server and the postscreen(8) server. These use TLS in the same manner as smtpd(8).
| Network-> | smtpd(8) |
<---seed---- <-key/cert-> | tlsmgr(8) |
----seed---> <-key/cert-> | smtp(8) | ->Network | ||||||||||
| | | |
| ||||||||||||||
|
smtpd session key cache | PRNG state file | smtp session key cache |
||||||||||||||
Topics covered in this section:
In order to use TLS, the Postfix SMTP server generally needs a certificate and a private key. Both must be in "PEM" format. The private key must not be encrypted, meaning: the key must be accessible without a password. The certificate and private key may be in the same file, in which case the certificate file should be owned by "root" and not be readable by any other user. If the key is stored separately, this applies to the key file only, and the certificate file may be "world-readable".
Public Internet MX hosts without certificates signed by a "reputable" CA must generate, and be prepared to present to most clients, a self-signed or private-CA signed certificate. The remote SMTP client will generally not be able to authenticate the self-signed certificate, but unless the client is running Postfix or similar software, it will still insist on a server certificate.
For servers that are not public Internet MX hosts, Postfix supports configurations with no certificates. This entails the use of just the anonymous TLS ciphers, which are not supported by typical SMTP clients. Since such clients will not, as a rule, fall back to plain text after a TLS handshake failure, a certificate-less Postfix SMTP server will be unable to receive email from most TLS enabled clients. To avoid accidental configurations with no certificates, Postfix enables certificate-less operation only when the administrator explicitly sets "smtpd_tls_cert_file = none". This ensures that new Postfix SMTP server configurations will not accidentally run with no certificates.
RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported. Typically you will only have RSA certificates issued by a commercial CA. In addition, the tools supplied with OpenSSL will by default issue RSA certificates. You can configure all three at the same time, in which case the cipher used determines which certificate is presented. For Netscape and OpenSSL clients without special cipher choices, the RSA certificate is preferred.
To enable a remote SMTP client to verify the Postfix SMTP server certificate, the issuing CA certificates must be made available to the client. You should include the required certificates in the server certificate file, the server certificate first, then the issuing CA(s) (bottom-up order).
Example: the certificate for "server.example.com" was issued by "intermediate CA" which itself has a certificate issued by "root CA". Create the server.pem file with:
% 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.
RSA key and certificate examples:
/etc/postfix/main.cf:
smtpd_tls_cert_file = /etc/postfix/server.pem
smtpd_tls_key_file = $smtpd_tls_cert_file
Their DSA counterparts:
/etc/postfix/main.cf:
smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem
smtpd_tls_dkey_file = $smtpd_tls_dcert_file
Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 0.9.9):
/etc/postfix/main.cf:
# Most clients will not be ECDSA capable, so you will likely also need
# an RSA or DSA certificate and private key.
#
smtpd_tls_eccert_file = /etc/postfix/server-ecdsa.pem
smtpd_tls_eckey_file = $smtpd_tls_eccert_file
TLS without certificates for servers serving exclusively anonymous-cipher capable clients:
/etc/postfix/main.cf:
smtpd_tls_cert_file = none
To verify a remote SMTP client certificate, the Postfix SMTP server needs to trust the certificates of the issuing certification authorities. These certificates in "PEM" format can be stored in a single $smtpd_tls_CAfile or in multiple files, one CA per file in the $smtpd_tls_CApath directory. If you use a directory, don't forget to create the necessary "hash" links with:
# $OPENSSL_HOME/bin/c_rehash /path/to/directory
The $smtpd_tls_CAfile contains the CA certificates of one or more trusted CAs. The file is opened (with root privileges) before Postfix enters the optional chroot jail and so need not be accessible from inside the chroot jail.
Additional trusted CAs can be specified via the $smtpd_tls_CApath directory, in which case the certificates are read (with $mail_owner privileges) from the files in the directory when the information is needed. Thus, the $smtpd_tls_CApath directory needs to be accessible inside the optional chroot jail.
When you configure the Postfix SMTP server to request client certificates, the DNs of certificate authorities in $smtpd_tls_CAfile are sent to the client, in order to allow it to choose an identity signed by a CA you trust. If no $smtpd_tls_CAfile is specified, no preferred CA list is sent, and the client is free to choose an identity signed by any CA. Many clients use a fixed identity regardless of the preferred CA list and you may be able to reduce TLS negotiation overhead by installing client CA certificates mostly or only in $smtpd_tls_CApath. In the latter case you need not specify a $smtpd_tls_CAfile.
Note, that unless client certificates are used to allow greater access to TLS authenticated clients, it is best to not ask for client certificates at all, as in addition to increased overhead some clients (notably in some cases qmail) are unable to complete the TLS handshake when client certificates are requested.
Example:
/etc/postfix/main.cf:
smtpd_tls_CAfile = /etc/postfix/CAcert.pem
smtpd_tls_CApath = /etc/postfix/certs
To get additional information about Postfix SMTP server TLS activity you can increase the log level from 0..4. Each logging level also includes the information that is logged at a lower logging level.
Level Postfix 2.9 and later Earlier releases. 0 Log only a summary message on TLS handshake completion — no logging of client certificate trust-chain verification errors if client certificate verification is not required. Disable logging of TLS activity. 1 Also log trust-chain verification errors and peer certificate summary information. Also log TLS handshake and certificate information. 2 Also log levels during TLS negotiation. 3 Also log hexadecimal and ASCII dump of TLS negotiation process. 4 Also log hexadecimal and ASCII dump of complete transmission after STARTTLS.
Use log level 3 only in case of problems. Use of log level 4 is strongly discouraged.
Example:
/etc/postfix/main.cf:
smtpd_tls_loglevel = 0
To include information about the protocol and cipher used as well as the client and issuer CommonName into the "Received:" message header, set the smtpd_tls_received_header variable to true. The default is no, as the information is not necessarily authentic. Only information recorded at the final destination is reliable, since the headers may be changed by intermediate servers.
Example:
/etc/postfix/main.cf:
smtpd_tls_received_header = yes
By default, TLS is disabled in the Postfix SMTP server, so no difference to plain Postfix is visible. Explicitly switch it on with "smtpd_tls_security_level = may".
Example:
/etc/postfix/main.cf:
smtpd_tls_security_level = may
With this, the Postfix SMTP server announces STARTTLS support to remote SMTP clients, but does not require that clients use TLS encryption.
Note: when an unprivileged user invokes "sendmail -bs", STARTTLS is never offered due to insufficient privileges to access the Postfix SMTP server private key. This is intended behavior.
You can ENFORCE the use of TLS, so that the Postfix SMTP server announces STARTTLS and accepts no mail without TLS encryption, by setting "smtpd_tls_security_level = encrypt". According to RFC 2487 this MUST NOT be applied in case of a publicly-referenced Postfix SMTP server. This option is off by default and should only seldom be used.
Example:
/etc/postfix/main.cf:
smtpd_tls_security_level = encrypt
TLS is sometimes used in the non-standard "wrapper" mode where a server always uses TLS, instead of announcing STARTTLS support and waiting for remote SMTP clients to request TLS service. Some clients, namely Outlook [Express] prefer the "wrapper" mode. This is true for OE (Win32 < 5.0 and Win32 >=5.0 when run on a port<>25 and OE (5.01 Mac on all ports).
It is strictly discouraged to use this mode from main.cf. If you want to support this service, enable a special port in master.cf and specify "-o smtpd_tls_wrappermode=yes" (note: no space around the "=") as an smtpd(8) command line option. Port 465 (smtps) was once chosen for this feature.
Example:
/etc/postfix/master.cf:
smtps inet n - n - - smtpd
-o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes
To receive a remote SMTP client certificate, the Postfix SMTP server must explicitly ask for one (any contents of $smtpd_tls_CAfile are also sent to the client as a hint for choosing a certificate from a suitable CA). Unfortunately, Netscape clients will either complain if no matching client certificate is available or will offer the user client a list of certificates to choose from. Additionally some MTAs (notably some versions of qmail) are unable to complete TLS negotiation when client certificates are requested, and abort the SMTP session. So this option is "off" by default. You will however need the certificate if you want to use certificate based relaying with, for example, the permit_tls_clientcerts feature. A server that wants client certificates must first present its own certificate. While Postfix by default offers anonymous ciphers to remote SMTP clients, these are automatically suppressed when the Postfix SMTP server is configured to ask for client certificates.
Example:
/etc/postfix/main.cf:
smtpd_tls_ask_ccert = yes
smtpd_tls_security_level = may
When TLS is enforced you may also decide to REQUIRE a remote SMTP client certificate for all TLS connections, by setting "smtpd_tls_req_ccert = yes". This feature implies "smtpd_tls_ask_ccert = yes". When TLS is not enforced, "smtpd_tls_req_ccert = yes" is ignored and a warning is logged.
Example:
/etc/postfix/main.cf:
smtpd_tls_req_ccert = yes
smtpd_tls_security_level = encrypt
The client certificate verification depth is specified with the main.cf smtpd_tls_ccert_verifydepth parameter. The default verification depth is 9 (the OpenSSL default), for compatibility with Postfix versions before 2.5 where smtpd_tls_ccert_verifydepth was ignored. When you configure trust in a root CA, it is not necessary to explicitly trust intermediary CAs signed by the root CA, unless $smtpd_tls_ccert_verifydepth is less than the number of CAs in the certificate chain for the clients of interest. With a verify depth of 1 you can only verify certificates directly signed by a trusted CA, and all trusted intermediary CAs need to be configured explicitly. With a verify depth of 2 you can verify clients signed by a root CA or a direct intermediary CA (so long as the client is correctly configured to supply its intermediate CA certificate).
Example:
/etc/postfix/main.cf:
smtpd_tls_ccert_verifydepth = 2
Sending AUTH data over an unencrypted channel poses a security risk. When TLS layer encryption is required ("smtpd_tls_security_level = encrypt"), the Postfix SMTP server will announce and accept AUTH only after the TLS layer has been activated with STARTTLS. When TLS layer encryption is optional ("smtpd_tls_security_level = may"), it may however still be useful to only offer AUTH when TLS is active. To maintain compatibility with non-TLS clients, the default is to accept AUTH without encryption. In order to change this behavior, set "smtpd_tls_auth_only = yes".
Example:
/etc/postfix/main.cf:
smtpd_tls_auth_only = no
The Postfix SMTP server and the remote SMTP client negotiate a session, which takes some computer time and network bandwidth. By default, this session information is cached only in the smtpd(8) process actually using this session and is lost when the process terminates. To share the session information between multiple smtpd(8) processes, a persistent session cache can be used. You can specify any database type that can store objects of several kbytes and that supports the sequence operator. DBM databases are not suitable because they can only store small objects. The cache is maintained by the tlsmgr(8) process, so there is no problem with concurrent access. Session caching is highly recommended, because the cost of repeatedly negotiating TLS session keys is high.
Example:
/etc/postfix/main.cf:
smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_scache
Note: as of version 2.5, Postfix no longer uses root privileges when opening this file. The file should now be stored under the Postfix-owned data_directory. As a migration aid, an attempt to open the file under a non-Postfix directory is redirected to the Postfix-owned data_directory, and a warning is logged.
Cached Postfix SMTP server session information expires after a certain amount of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer time of 3600sec (=1 hour). RFC 2246 recommends a maximum of 24 hours.
Example:
/etc/postfix/main.cf:
smtpd_tls_session_cache_timeout = 3600s
When the Postfix SMTP server does not save TLS sessions to an external cache database, client-side session caching is unlikely to be useful. To prevent such wastage, the Postfix SMTP server can be configured to not issue TLS session ids. By default the Postfix SMTP server always issues TLS session ids. This works around known interoperability issues with some MUAs, and prevents possible interoperability issues with other MTAs.
Example:
smtpd_tls_always_issue_session_ids = no
Postfix TLS support introduces three additional features for Postfix SMTP server access control:
- permit_tls_clientcerts
Allow the remote SMTP client request if the client certificate fingerprint or certificate public key fingerprint (Postfix 2.9 and later) is listed in the client certificate table (see relay_clientcerts discussion below).
- permit_tls_all_clientcerts
Allow the remote SMTP client request if the client certificate passes trust chain verification. Useful with private-label CAs that only issue certificates to trusted clients (and not otherwise).
- check_ccert_access type:table
Use the remote SMTP client certificate fingerprint or public key fingerprint (Postfix 2.9 and later) as the lookup key for the specified access(5) table.
The digest algorithm used to compute the client certificate fingerprints is specified with the main.cf smtpd_tls_fingerprint_digest parameter. The default is "md5", for compatibility with Postfix versions < 2.5.
The permit_tls_all_clientcerts feature must be used with caution, because it can result in too many access permissions. Use this feature only if a special CA issues the client certificates, and only if this CA is listed as trusted CA. If other CAs are trusted, any owner of a valid client certificate would be authorized. The permit_tls_all_clientcerts feature can be practical for a specially created email relay server.
It is however recommended to stay with the permit_tls_clientcerts feature and list all certificates via $relay_clientcerts, as permit_tls_all_clientcerts does not permit any control when a certificate must no longer be used (e.g. an employee leaving).
Example:
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
permit_tls_clientcerts
reject_unauth_destination
...
Example: Postfix lookup tables are in the form of (key, value) pairs. Since we only need the key, the value can be chosen freely, e.g. the name of the user or host:
/etc/postfix/main.cf:
relay_clientcerts = hash:/etc/postfix/relay_clientcerts
/etc/postfix/relay_clientcerts:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home
The Postfix SMTP server supports 5 distinct cipher security levels as specified by the smtpd_tls_mandatory_ciphers configuration parameter, which determines the cipher grade with mandatory TLS encryption. The default value is "medium" which is essentially 128-bit encryption or better. With opportunistic TLS encryption, the minimum accepted cipher grade is typically "export". The corresponding smtpd_tls_ciphers parameter (Postfix ≥ 2.6) controls the cipher grade used with opportunistic TLS.
By default anonymous ciphers are enabled. They are automatically disabled when remote SMTP client certificates are requested. If clients are expected to always verify the Postfix SMTP server certificate you may want to disable anonymous ciphers by setting "smtpd_tls_mandatory_exclude_ciphers = aNULL" or "smtpd_tls_exclude_ciphers = aNULL", as appropriate. One can't force a remote SMTP client to check the server certificate, so excluding anonymous ciphers is generally unnecessary.
The "smtpd_tls_ciphers" configuration parameter (Postfix ≥ 2.6) provides control over the minimum cipher grade for opportunistic TLS. With Postfix < 2.6, the minimum opportunistic TLS cipher grade is always "export".
With mandatory TLS encryption, the Postfix SMTP server will by default disable SSLv2. SSLv2 is used only when TLS encryption is optional. The mandatory TLS protocol list is specified via the smtpd_tls_mandatory_protocols configuration parameter. The corresponding smtpd_tls_protocols parameter (Postfix ≥ 2.6) controls the SSL/TLS protocols used with opportunistic TLS.
Note that the OpenSSL library only supports protocol exclusion (not inclusion). For this reason, Postfix can exclude only protocols that are known at the time the Postfix software is written. If new protocols are added to the OpenSSL library, they cannot be excluded without corresponding changes to the Postfix source code.
For a server that is not a public Internet MX host, Postfix supports configurations with no server certificates that use only the anonymous ciphers. This is enabled by explicitly setting "smtpd_tls_cert_file = none" and not specifying an smtpd_tls_dcert_file or smtpd_tls_eccert_file.
Example, MSA that requires TLSv1, not SSLv2 or SSLv3, with high grade ciphers:
/etc/postfix/main.cf:
smtpd_tls_cert_file = /etc/postfix/cert.pem
smtpd_tls_key_file = /etc/postfix/key.pem
smtpd_tls_mandatory_ciphers = high
smtpd_tls_mandatory_exclude_ciphers = aNULL, MD5
smtpd_tls_security_level = encrypt
# Preferred form with Postfix ≥ 2.5:
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
# Alternative form.
smtpd_tls_mandatory_protocols = TLSv1
If you want to take advantage of ciphers with ephemeral Diffie-Hellman (EDH) key exchange (this offers "forward-secrecy"), DH parameters are needed. Instead of using the built-in DH parameters for both 1024-bit (non-export ciphers) and 512-bit (export ciphers), it is better to generate your own parameters, since otherwise it would "pay" for a possible attacker to start a brute force attack against parameters that are used by everybody. Postfix defaults to compiled-in parameters that are shared by all Postfix users who don't generate their own settings.
To generate your own set of DH parameters, use:
% openssl gendh -out /etc/postfix/dh_512.pem -2 512 % openssl gendh -out /etc/postfix/dh_1024.pem -2 1024
Support for elliptic curve cryptography is available with Postfix 2.6 and OpenSSL 0.9.9 or later. To enable ephemeral elliptic curve Diffie-Hellman (EECDH) key-exchange, set "smtpd_tls_eecdh_grade = strong" or "smtpd_tls_eecdh_grade = ultra". The "ultra" setting is substantially more CPU intensive, and "strong" is sufficiently secure for most situations.
Examples:
/etc/postfix/main.cf:
smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem
smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
# Postfix ≥ 2.6:
smtpd_tls_eecdh_grade = strong
Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and later allows TLS servers to preempt the TLS client's cipher preference list. This is possible only with SSLv3 and later, as in SSLv2 the client chooses the cipher from a list supplied by the server.
By default, the OpenSSL server selects the client's most preferred cipher that the server supports. With SSLv3 and later, the server may choose its own most preferred cipher that is supported (offered) by the client. Setting "tls_preempt_cipherlist = yes" enables server cipher preferences. The default OpenSSL behaviour applies with "tls_preempt_cipherlist = no".
While server cipher selection may in some cases lead to a more secure or performant cipher choice, there is some risk of interoperability issues. In the past, some SSL clients have listed lower priority ciphers that they did not implement correctly. If the server chooses a cipher that the client prefers less, it may select a cipher whose client implementation is flawed.
The smtpd_starttls_timeout parameter limits the time of Postfix SMTP server write and read operations during TLS startup and shutdown handshake procedures.
Example:
/etc/postfix/main.cf:
smtpd_starttls_timeout = 300s
With Postfix 2.8 and later, the tls_disable_workarounds parameter specifies a list or bit-mask of OpenSSL bug work-arounds to disable. This may be necessary if one of the work-arounds enabled by default in OpenSSL proves to pose a security risk, or introduces an unexpected interoperability issue. Some bug work-arounds known to be problematic are disabled in the default value of the parameter when linked with an OpenSSL library that could be vulnerable.
Example:
/etc/postfix/main.cf:
tls_disable_workarounds = 0xFFFFFFFF
tls_disable_workarounds = CVE-2010-4180, LEGACY_SERVER_CONNECT
Note: Disabling LEGACY_SERVER_CONNECT is not wise at this time, lots of servers are still unpatched and Postfix is not significantly vulnerable to the renegotiation issue in the TLS protocol.
Topics covered in this section:
Similar to the Postfix SMTP server, the Postfix SMTP/LMTP client implements multiple TLS security levels. These levels are described in more detail in the sections that follow.
The smtp(8) and lmtp(8) delivery agents are implemented by a single dual-purpose program. Specifically, all the TLS features described below apply equally to SMTP and LMTP, after replacing the "smtp_" prefix of the each parameter name with "lmtp_".
The Postfix LMTP delivery agent can communicate with LMTP servers listening on UNIX-domain sockets. When server certificate verification is enabled and the server is listening on a UNIX-domain socket, the $myhostname parameter is used to set the TLS verification nexthop and hostname.
NOTE: Opportunistic encryption of LMTP traffic over UNIX-domain sockets or loopback TCP connections is futile. TLS is only useful in this context when it is mandatory, typically to allow at least one of the server or the client to authenticate the other. The "null" cipher grade may be appropriate in this context, when available on both client and server. The "null" ciphers provide authentication without encryption.
At the "none" TLS security level, TLS encryption is disabled. This is the default security level, and can be configured explicitly by setting "smtp_tls_security_level = none". For LMTP, use the corresponding "lmtp_" parameter.
Per-destination settings may override this default setting, in which case TLS is used selectively, only with destinations explicitly configured for TLS.
You can disable TLS for a subset of destinations, while leaving it enabled for the rest. With the Postfix TLS policy table, specify the "none" security level.
At the "may" TLS security level, TLS encryption is opportunistic. The SMTP transaction is encrypted if the STARTTLS ESMTP feature is supported by the server. Otherwise, messages are sent in the clear. Opportunistic TLS can be configured by setting "smtp_tls_security_level = may". For LMTP, use the corresponding "lmtp_" parameter.
Since sending in the clear is acceptable, demanding stronger than default TLS security mostly reduces inter-operability. If you must restrict TLS protocol or cipher selection even with opportunistic TLS, the "smtp_tls_ciphers" and "smtp_tls_protocols" configuration parameters (Postfix ≥ 2.6) provide control over the protocols and cipher grade used with opportunistic TLS. With earlier releases the opportunistic TLS cipher grade is always "export" and no protocols are disabled.
With opportunistic TLS, mail delivery continues even if the server certificate is untrusted or bears the wrong name. When the TLS handshake fails for an opportunistic TLS session, rather than give up on mail delivery, the Postfix SMTP client retries the transaction with TLS disabled. Trying an unencrypted connection makes it possible to deliver mail to sites with non-interoperable server TLS implementations.
Opportunistic encryption is never used for LMTP over UNIX-domain sockets. The communications channel is already confidential without TLS, so the only potential benefit of TLS is authentication. Do not configure opportunistic TLS for LMTP deliveries over UNIX-domain sockets. Only configure TLS for LMTP over UNIX-domain sockets at the encrypt security level or higher. Attempts to configure opportunistic encryption of LMTP sessions will be ignored with a warning written to the mail logs.
You can enable opportunistic TLS just for selected destinations. With the Postfix TLS policy table, specify the "may" security level.
This is the most common security level for TLS protected SMTP sessions, stronger security is not generally available and, if needed, is typically only configured on a per-destination basis. See the section on TLS limitations above.
Example:
/etc/postfix/main.cf:
smtp_tls_security_level = may
At the "encrypt" TLS security level, messages are sent only over TLS encrypted sessions. The SMTP transaction is aborted unless the STARTTLS ESMTP feature is supported by the remote SMTP server. If no suitable servers are found, the message will be deferred. Mandatory TLS encryption can be configured by setting "smtp_tls_security_level = encrypt". Even though TLS encryption is always used, mail delivery continues even if the server certificate is untrusted or bears the wrong name. For LMTP, use the corresponding "lmtp_" parameter.
At this security level and higher, the smtp_tls_mandatory_protocols and smtp_tls_mandatory_ciphers configuration parameters determine the list of sufficiently secure SSL protocol versions and the minimum cipher strength. If the protocol or cipher requirements are not met, the mail transaction is aborted. The documentation for these parameters includes useful interoperability and security guidelines.
Despite the potential for eliminating passive eavesdropping attacks, mandatory TLS encryption is not viable as a default security level for mail delivery to the public Internet. Most MX hosts do not support TLS at all, and some of those that do have broken implementations. On a host that delivers mail to the Internet, you should not configure mandatory TLS encryption as the default security level.
You can enable mandatory TLS encryption just for specific destinations. With the Postfix TLS policy table, specify the "encrypt" security level.
Examples:
In the example below, traffic to example.com and its sub-domains via the corresponding MX hosts always uses TLS. The SSLv2 protocol will be disabled (the default setting of smtp_tls_mandatory_protocols excludes "SSLv2"). Only high- or medium-strength (i.e. 128 bit or better) ciphers will be used by default for all "encrypt" security level sessions.
/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
/etc/postfix/tls_policy:
example.com encrypt
.example.com encrypt
In the next example, secure message submission is configured via the MSA "[example.net]:587". TLS sessions are encrypted without authentication, because this MSA does not possess an acceptable certificate. This MSA is known to be capable of "TLSv1" and "high" grade ciphers, so these are selected via the policy table.
Note: the policy table lookup key is the verbatim next-hop specification from the recipient domain, transport(5) table or relayhost parameter, with any enclosing square brackets and optional port. Take care to be consistent: the suffixes ":smtp" or ":25" or no port suffix result in different policy table lookup keys, even though they are functionally equivalent nexthop specifications. Use at most one of these forms for all destinations. Below, the policy table has multiple keys, just in case the transport table entries are not specified consistently.
/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
/etc/services:
submission 587/tcp msa # mail message submission
/etc/postfix/tls_policy:
[example.net]:587 encrypt protocols=TLSv1 ciphers=high
[example.net]:msa encrypt protocols=TLSv1 ciphers=high
[example.net]:submission encrypt protocols=TLSv1 ciphers=high
Certificate fingerprint verification is available with Postfix 2.5 and later. At this security level ("smtp_tls_security_level = fingerprint"), no trusted certificate authorities are used or required. The certificate trust chain, expiration date, ... are not checked. Instead, the smtp_tls_fingerprint_cert_match parameter or the "match" attribute in the policy table lists the remote SMTP server certificate fingerprint or public key fingerprint (Postfix 2.9 and later).
If certificate fingerprints are exchanged securely, this is the strongest, and least scalable security level. The administrator needs to securely collect the fingerprints of the X.509 certificates of each peer server, store them into a local file, and update this local file whenever the peer server's public certificate changes. If public key fingerprints are used in place of fingerprints of the entire certificate, the fingerprints remain valid even after the certificate is renewed, provided that the same public/private keys are used to obtain the new certificate.
Fingerprint verification may be feasible for an SMTP "VPN" connecting a small number of branch offices over the Internet, or for secure connections to a central mail hub. It works poorly if the remote SMTP server is managed by a third party, and its public certificate changes periodically without prior coordination with the verifying site.
The digest algorithm used to calculate the fingerprint is selected by the smtp_tls_fingerprint_digest parameter. In the policy table multiple fingerprints can be combined with a "|" delimiter in a single match attribute, or multiple match attributes can be employed. The ":" character is not used as a delimiter as it occurs between each pair of fingerprint (hexadecimal) digits.
Example: fingerprint TLS security with an internal mailhub. Two matching fingerprints are listed. The relayhost may be multiple physical hosts behind a load-balancer, each with its own private/public key and self-signed certificate. Alternatively, a single relayhost may be in the process of switching from one set of private/public keys to another, and both keys are trusted just prior to the transition.
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = md5
smtp_tls_fingerprint_cert_match =
3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
Example: Certificate fingerprint verification with selected destinations. As in the example above, we show two matching fingerprints:
/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
smtp_tls_fingerprint_digest = md5
/etc/postfix/tls_policy:
example.com fingerprint
match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
At the "verify" TLS security level, messages are sent only over TLS encrypted sessions if the remote SMTP server certificate is valid (not expired or revoked, and signed by a trusted certificate authority) and where the server certificate name matches a known pattern. Mandatory server certificate verification can be configured by setting "smtp_tls_security_level = verify". The smtp_tls_verify_cert_match parameter can override the default "hostname" certificate name matching strategy. Fine-tuning the matching strategy is generally only appropriate for secure-channel destinations. For LMTP use the corresponding "lmtp_" parameters.
If the server certificate chain is trusted (see smtp_tls_CAfile and smtp_tls_CApath), any DNS names in the SubjectAlternativeName certificate extension are used to verify the remote SMTP server name. If no DNS names are specified, the certificate CommonName is checked. If you want mandatory encryption without server certificate verification, see above.
Despite the potential for eliminating "man-in-the-middle" and other attacks, mandatory certificate trust chain and subject name verification is not viable as a default Internet mail delivery policy. Most MX hosts do not support TLS at all, and a significant portion of TLS enabled MTAs use self-signed certificates, or certificates that are signed by a private certificate authority. On a machine that delivers mail to the Internet, you should not configure mandatory server certificate verification as a default policy.
Mandatory server certificate verification as a default security level may be appropriate if you know that you will only connect to servers that support RFC 2487 and that present verifiable server certificates. An example would be a client that sends all email to a central mailhub that offers the necessary STARTTLS support. In such cases, you can often use a secure-channel configuration instead.
You can enable mandatory server certificate verification just for specific destinations. With the Postfix TLS policy table, specify the "verify" security level.
Example:
In this example, the Postfix SMTP client encrypts all traffic to the example.com domain. The peer hostname is verified, but verification is vulnerable to DNS response forgery. Mail transmission to example.com recipients uses "high" grade ciphers.
/etc/postfix/main.cf:
indexed = ${default_database_type}:${config_directory}/
smtp_tls_CAfile = ${config_directory}/CAfile.pem
smtp_tls_policy_maps = ${indexed}tls_policy
/etc/postfix/tls_policy:
example.com verify ciphers=high
At the secure TLS security level, messages are sent only over secure-channel TLS sessions where DNS forgery resistant server certificate verification succeeds. If no suitable servers are found, the message will be deferred. Postfix secure-channels can be configured by setting "smtp_tls_security_level = secure". The smtp_tls_secure_cert_match parameter can override the default "nexthop, dot-nexthop" certificate match strategy. For LMTP, use the corresponding "lmtp_" parameters.
If the server certificate chain is trusted (see smtp_tls_CAfile and smtp_tls_CApath), any DNS names in the SubjectAlternativeName certificate extension are used to verify the remote SMTP server name. If no DNS names are specified, the CommonName is checked. If you want mandatory encryption without server certificate verification, see above.
Despite the potential for eliminating "man-in-the-middle" and other attacks, mandatory secure server certificate verification is not viable as a default Internet mail delivery policy. Most MX hosts do not support TLS at all, and a significant portion of TLS enabled MTAs use self-signed certificates, or certificates that are signed by a private certificate authority. On a machine that delivers mail to the Internet, you should not configure secure TLS verification as a default policy.
Mandatory secure server certificate verification as a default security level may be appropriate if you know that you will only connect to servers that support RFC 2487 and that present verifiable server certificates. An example would be a client that sends all email to a central mailhub that offers the necessary STARTTLS support.
You can enable secure TLS verification just for specific destinations. With the Postfix TLS policy table, specify the "secure" security level.
Examples:
Secure-channel TLS without transport(5) table overrides:
The Postfix SMTP client will encrypt all traffic and verify the destination name immune from forged DNS responses. MX lookups are still used to find the hostnames of the SMTP servers for example.com, but these hostnames are not used when checking the names in the server certificate(s). Rather, the requirement is that the MX hosts for example.com have trusted certificates with a subject name of example.com or a sub-domain, see the documentation for the smtp_tls_secure_cert_match parameter.
The related domains example.co.uk and example.co.jp are hosted on the same MX hosts as the primary example.com domain, and traffic to these is secured by verifying the primary example.com domain in the server certificates. This frees the server administrator from needing the CA to sign certificates that list all the secondary domains. The downside is that clients that want secure channels to the secondary domains need explicit TLS policy table entries.
Note, there are two ways to handle related domains. The first is to use the default routing for each domain, but add policy table entries to override the expected certificate subject name. The second is to override the next-hop in the transport table, and use a single policy table entry for the common nexthop. We choose the first approach, because it works better when domain ownership changes. With the second approach we securely deliver mail to the wrong destination, with the first approach, authentication fails and mail stays in the local queue, the first approach is more appropriate in most cases.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/CAfile.pem smtp_tls_policy_maps = hash:/etc/postfix/tls_policy /etc/postfix/transport: /etc/postfix/tls_policy: example.com secure example.co.uk secure match=example.com:.example.com example.co.jp secure match=example.com:.example.com
Secure-channel TLS with transport(5) table overrides:
In this case traffic to example.com and its related domains is sent to a single logical gateway (to avoid a single point of failure, its name may resolve to one or more load-balancer addresses, or to the combined addresses of multiple physical hosts). All the physical hosts reachable via the gateway's IP addresses have the logical gateway name listed in their certificates.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/CAfile.pem transport_maps = hash:/etc/postfix/transport smtp_tls_policy_maps = hash:/etc/postfix/tls_policy /etc/postfix/transport: example.com smtp:[tls.example.com] example.co.uk smtp:[tls.example.com] example.co.jp smtp:[tls.example.com] /etc/postfix/tls_policy: [tls.example.com] secure match=tls.example.com
To get additional information about Postfix SMTP client TLS activity you can increase the loglevel from 0..4. Each logging level also includes the information that is logged at a lower logging level.
Level Postfix 2.9 and later Earlier releases. 0 Log only a summary message on TLS handshake completion — no logging of remote SMTP server certificate trust-chain verification errors if server certificate verification is not required. Disable logging of TLS activity. 1 Also log remote SMTP server trust-chain verification errors and peer certificate summary information. Also log TLS handshake and certificate information. 2 Also log levels during TLS negotiation. 3 Also log hexadecimal and ASCII dump of TLS negotiation process. 4 Also log hexadecimal and ASCII dump of complete transmission after STARTTLS.
Example:
/etc/postfix/main.cf:
smtp_tls_loglevel = 0
Do not configure Postfix SMTP client certificates unless you must present client TLS certificates to one or more servers. Client certificates are not usually needed, and can cause problems in configurations that work well without them. The recommended setting is to let the defaults stand:
smtp_tls_cert_file =
smtp_tls_dcert_file =
smtp_tls_key_file =
smtp_tls_dkey_file =
# Postfix ≥ 2.6
smtp_tls_eccert_file =
smtp_tls_eckey_file =
The best way to use the default settings is to comment out the above parameters in main.cf if present.
During TLS startup negotiation the Postfix SMTP client may present a certificate to the remote SMTP server. The Netscape client is rather clever here and lets the user select between only those certificates that match CA certificates offered by the remote SMTP server. As the Postfix SMTP client uses the "SSL_connect()" function from the OpenSSL package, this is not possible and we have to choose just one certificate. So for now the default is to use _no_ certificate and key unless one is explicitly specified here.
RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported. You can configure all three at the same time, in which case the cipher used determines which certificate is presented.
It is possible for the Postfix SMTP client to use the same key/certificate pair as the Postfix SMTP server. If a certificate is to be presented, it must be in "PEM" format. The private key must not be encrypted, meaning: it must be accessible without password. Both parts (certificate and private key) may be in the same file.
To enable remote SMTP servers to verify the Postfix SMTP client certificate, the issuing CA certificates must be made available to the server. You should include the required certificates in the client certificate file, the client certificate first, then the issuing CA(s) (bottom-up order).
Example: the certificate for "client.example.com" was issued by "intermediate CA" which itself has a certificate issued by "root CA". Create the client.pem file with:
% cat client_cert.pem intermediate_CA.pem > client.pem
A Postfix SMTP client certificate supplied here must be usable as SSL client certificate and hence pass the "openssl verify -purpose sslclient ..." test.
A server that trusts the root CA has a local copy of the root CA certificate, so it is not necessary to include the root CA certificate here. Leaving it out of the "client.pem" file reduces the overhead of the TLS exchange.
If you want the Postfix SMTP client to accept remote SMTP server certificates issued by these CAs, append the root certificate to $smtp_tls_CAfile or install it in the $smtp_tls_CApath directory.
RSA key and certificate examples:
/etc/postfix/main.cf:
smtp_tls_cert_file = /etc/postfix/client.pem
smtp_tls_key_file = $smtp_tls_cert_file
Their DSA counterparts:
/etc/postfix/main.cf:
smtp_tls_dcert_file = /etc/postfix/client-dsa.pem
smtp_tls_dkey_file = $smtp_tls_dcert_file
Their ECDSA counterparts (Postfix ≥ 2.6 + OpenSSL ≥ 0.9.9):
/etc/postfix/main.cf:
smtp_tls_eccert_file = /etc/postfix/client-ecdsa.pem
smtp_tls_eckey_file = $smtp_tls_eccert_file
To verify a remote SMTP server certificate, the Postfix SMTP client needs to trust the certificates of the issuing certification authorities. These certificates in "pem" format can be stored in a single $smtp_tls_CAfile or in multiple files, one CA per file in the $smtp_tls_CApath directory. If you use a directory, don't forget to create the necessary "hash" links with:
# $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
The remote SMTP server and the Postfix SMTP client negotiate a session, which takes some computer time and network bandwidth. By default, this session information is cached only in the smtp(8) process actually using this session and is lost when the process terminates. To share the session information between multiple smtp(8) processes, a persistent session cache can be used. You can specify any database type that can store objects of several kbytes and that supports the sequence operator. DBM databases are not suitable because they can only store small objects. The cache is maintained by the tlsmgr(8) process, so there is no problem with concurrent access. Session caching is highly recommended, because the cost of repeatedly negotiating TLS session keys is high. Future Postfix SMTP servers may limit the number of sessions that a client is allowed to negotiate per unit time.
Example:
/etc/postfix/main.cf:
smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_scache
Note: as of version 2.5, Postfix no longer uses root privileges when opening this file. The file should now be stored under the Postfix-owned data_directory. As a migration aid, an attempt to open the file under a non-Postfix directory is redirected to the Postfix-owned data_directory, and a warning is logged.
Cached Postfix SMTP client session information expires after a certain amount of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer time of 3600s (=1 hour). RFC 2246 recommends a maximum of 24 hours.
Example:
/etc/postfix/main.cf:
smtp_tls_session_cache_timeout = 3600s
The security properties of TLS communication channels are application specific. While the TLS protocol can provide a confidential, tamper-resistant, mutually authenticated channel between client and server, not all of these security features are applicable to every communication.
For example, while mutual TLS authentication between browsers and web servers is possible, it is not practical, or even useful, for web-servers that serve the public to verify the identity of every potential user. In practice, most HTTPS transactions are asymmetric: the browser verifies the HTTPS server's identity, but the user remains anonymous. Much of the security policy is up to the client. If the client chooses to not verify the server's name, the server is not aware of this. There are many interesting browser security topics, but we shall not dwell on them here. Rather, our goal is to understand the security features of TLS in conjunction with SMTP.
An important SMTP-specific observation is that a public MX host is even more at the mercy of the SMTP client than is an HTTPS server. Not only can it not enforce due care in the client's use of TLS, but it cannot even enforce the use of TLS, because TLS support in SMTP clients is still the exception rather than the rule. One cannot, in practice, limit access to one's MX hosts to just TLS-enabled clients. Such a policy would result in a vast reduction in one's ability to communicate by email with the world at large.
One may be tempted to try enforcing TLS for mail from specific sending organizations, but this, too, runs into obstacles. One such obstacle is that we don't know who is (allegedly) sending mail until we see the "MAIL FROM:" SMTP command, and at that point, if TLS is not already in use, a potentially sensitive sender address (and with SMTP PIPELINING one or more of the recipients) has (have) already been leaked in the clear. Another obstacle is that mail from the sender to the recipient may be forwarded, and the forwarding organization may not have any security arrangements with the final destination. Bounces also need to be protected. These can only be identified by the IP address and HELO name of the connecting client, and it is difficult to keep track of all the potential IP addresses or HELO names of the outbound email servers of the sending organization.
Consequently, TLS security for mail delivery to public MX hosts is almost entirely the client's responsibility. The server is largely a passive enabler of TLS security, the rest is up to the client. While the server has a greater opportunity to mandate client security policy when it is a dedicated MSA that only handles outbound mail from trusted clients, below we focus on the client security policy.
On the SMTP client, there are further complications. When delivering mail to a given domain, in contrast to HTTPS, one rarely uses the domain name directly as the target host of the SMTP session. More typically, one uses MX lookups - these are usually unauthenticated - to obtain the domain's SMTP server hostname(s). When, as is current practice, the client verifies the insecurely obtained MX hostname, it is subject to a DNS man-in-the-middle attack.
If clients instead attempted to verify the recipient domain name, an SMTP server for multiple domains would need to list all its email domain names in its certificate, and generate a new certificate each time a new domain were added. At least some CAs set fairly low limits (20 for one prominent CA) on the number of names that server certificates can contain. This approach is not consistent with current practice and does not scale.
It is regrettably the case that TLS secure-channels (fully authenticated and immune to man-in-the-middle attacks) impose constraints on the sending and receiving sites that preclude ubiquitous deployment. One needs to manually configure this type of security for each destination domain, and in many cases implement non-default TLS policy table entries for additional domains hosted at a common secured destination. For these reasons secure-channel configurations will never be the norm. For the generic domain with which you have made no specific security arrangements, this security level is not a good fit.
Given that strong authentication is not generally possible, and that verifiable certificates cost time and money, many servers that implement TLS use self-signed certificates or private CAs. This further limits the applicability of verified TLS on the public Internet.
Historical note: while the documentation of these issues and many of the related features were new with Postfix 2.3, the issue was well understood before Postfix 1.0, when Lutz Jänicke was designing the first unofficial Postfix TLS patch. See his original post http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html and the first response http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html. The problem is not even unique to SMTP or even TLS, similar issues exist for secure connections via aliases for HTTPS and Kerberos. SMTP merely uses indirect naming (via MX records) more frequently.
A small fraction of servers offer STARTTLS but the negotiation consistently fails. As long as encryption is not mandatory, the Postfix SMTP client retries the delivery immediately with TLS disabled, without any need to explicitly disable TLS for the problem destinations.
The policy table is specified via the smtp_tls_policy_maps parameter. This lists optional lookup tables with the Postfix SMTP client TLS security policy by next-hop destination.
The TLS policy table is indexed by the full next-hop destination, which is either the recipient domain, or the verbatim next-hop specified in the transport table, $local_transport, $virtual_transport, $relay_transport or $default_transport. This includes any enclosing square brackets and any non-default destination server port suffix. The LMTP socket type prefix (inet: or unix:) is not included in the lookup key.
Only the next-hop domain, or $myhostname with LMTP over UNIX-domain sockets, is used as the nexthop name for certificate verification. The port and any enclosing square brackets are used in the table lookup key, but are not used for server name verification.
When the lookup key is a domain name without enclosing square brackets or any :port suffix (typically the recipient domain), and the full domain is not found in the table, just as with the transport(5) table, the parent domain starting with a leading "." is matched recursively. This allows one to specify a security policy for a recipient domain and all its sub-domains.
The lookup result is a security level, followed by an optional list of whitespace and/or comma separated name=value attributes that override related main.cf settings. The TLS security levels are described above. Below, we describe the corresponding table syntax:
Notes:
The "match" attribute is especially useful to verify TLS certificates for domains that are hosted on a shared server. In that case, specify "match" rules for the shared server's name. While secure verification can also be achieved with manual routing overrides in Postfix transport(5) tables, that approach can deliver mail to the wrong host when domains are assigned to new gateway hosts. The "match" attribute approach avoids the problems of manual routing overrides; mail is deferred if verification of a new MX host fails.
When a policy table entry specifies multiple match patterns, multiple match strategies, or multiple protocols, these must be separated by colons.
The "exclude" attribute (Postfix ≥ 2.6) is used to disable ciphers that cause handshake failures with a specific mandatory TLS destination, without disabling the ciphers for all mandatory destinations. Alternatively, you can exclude ciphers that cause issues with multiple remote servers in main.cf, and selectively enable them on a per-destination basis in the policy table by setting a shorter or empty exclusion list. The per-destination "exclude" list preempts both the opportunistic and mandatory security level exclusions, so that all excluded ciphers can be enabled for known-good destinations. For non-mandatory TLS destinations that exhibit cipher-specific problems, Postfix will fall back to plain-text delivery. If plain-text is not acceptable make TLS mandatory and exclude the problem ciphers.
Example:
/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
# Postfix 2.5 and later
smtp_tls_fingerprint_digest = md5
/etc/postfix/tls_policy:
example.edu none
example.mil may
example.gov encrypt protocols=SSLv3:TLSv1 ciphers=high
example.com verify
match=hostname:dot-nexthop protocols=SSLv3:TLSv1 ciphers=high
example.net secure
.example.net secure match=.example.net:example.net
[mail.example.org]:587 secure match=nexthop
# Postfix 2.5 and later
[thumb.example.org] fingerprint
match=EC:3B:2D:B0:5B:B1:FB:6D:20:A3:9D:72:F6:8D:12:35
match=3D:95:34:51:24:66:33:B9:D2:40:99:C0:C1:17:0B:D1
# Postfix 2.6 and later
example.info may protocols=!SSLv2 ciphers=medium exclude=3DES
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.
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
The server certificate verification depth is specified with the main.cf smtp_tls_scert_verifydepth parameter. The default verification depth is 9 (the OpenSSL default), for compatibility with Postfix versions before 2.5 where smtp_tls_scert_verifydepth was ignored. When you configure trust in a root CA, it is not necessary to explicitly trust intermediary CAs signed by the root CA, unless $smtp_tls_scert_verifydepth is less than the number of CAs in the certificate chain for the servers of interest. With a verify depth of 1 you can only verify certificates directly signed by a trusted CA, and all trusted intermediary CAs need to be configured explicitly. With a verify depth of 2 you can verify servers signed by a root CA or a direct intermediary CA (so long as the server is correctly configured to supply its intermediate CA certificate).
Example:
/etc/postfix/main.cf:
smtp_tls_scert_verifydepth = 2
The Postfix SMTP client supports 5 distinct cipher security levels as specified by the smtp_tls_mandatory_ciphers configuration parameter. This setting controls the minimum acceptable SMTP client TLS cipher grade for use with mandatory TLS encryption. The default value "medium" is suitable for most destinations with which you may want to enforce TLS, and is beyond the reach of today's cryptanalytic methods. See smtp_tls_policy_maps for information on how to configure ciphers on a per-destination basis.
By default anonymous ciphers are allowed, and automatically disabled when remote SMTP server certificates are verified. If you want to disable anonymous ciphers even at the "encrypt" security level, set "smtp_tls_mandatory_exclude_ciphers = aNULL"; and to disable anonymous ciphers even with opportunistic TLS, set "smtp_tls_exclude_ciphers = aNULL". There is generally no need to take these measures. Anonymous ciphers save bandwidth and TLS session cache space, if certificates are ignored, there is little point in requesting them.
The "smtp_tls_ciphers" configuration parameter (Postfix ≥ 2.6) provides control over the minimum cipher grade for opportunistic TLS. With Postfix < 2.6, the minimum opportunistic TLS cipher grade is always "export".
With mandatory TLS encryption, the Postfix SMTP client will by default disable SSLv2. SSLv2 is used only when TLS encryption is optional. The mandatory TLS protocol list is specified via the smtp_tls_mandatory_protocols configuration parameter. The corresponding smtp_tls_protocols parameter (Postfix ≥ 2.6) controls the SSL/TLS protocols used with opportunistic TLS.
Example:
/etc/postfix/main.cf:
smtp_tls_mandatory_ciphers = medium
smtp_tls_mandatory_exclude_ciphers = RC4, MD5
smtp_tls_exclude_ciphers = aNULL
# Preferred form with Postfix ≥ 2.5:
smtp_tls_mandatory_protocols = !SSLv2
# Alternative form.
smtp_tls_mandatory_protocols = SSLv3, TLSv1
# Also available with Postfix ≥ 2.6:
smtp_tls_ciphers = export
smtp_tls_protocols = !SSLv2
Although the Postfix SMTP client by itself doesn't support TLS wrapper mode, it is relatively easy to forward a connection through the stunnel program if Postfix needs to deliver mail to some legacy system that doesn't support STARTTLS. Use one of the following two examples, to send only some remote mail, or to send all remote mail, to an SMTPS server.
The first example uses SMTPS to send all remote mail to a provider's mail server called "mail.example.com".
A minimal stunnel.conf file is sufficient to set up a tunnel from local port 11125 to the remote destination "mail.example.com" and port "smtps". Postfix will later use this tunnel to connect to the remote server.
/path/to/stunnel.conf:
[smtp-tls-wrapper]
accept = 11125
client = yes
connect = mail.example.com:smtps
To test this tunnel, use:
$ telnet localhost 11125
This should produce the greeting from the remote SMTP server at mail.example.com.
On the Postfix side, the relayhost feature sends all remote mail through the local stunnel listener on port 11125:
/etc/postfix/main.cf:
relayhost = [127.0.0.1]:11125
Use "postfix reload" to make the change effective.
The second example will use SMTPS to send only mail for "example.com" via SMTPS. It uses the same stunnel configuration file as the first example, so it won't be repeated here.
This time, the Postfix side uses a transport map to direct only mail for "example.com" through the tunnel:
/etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport
/etc/postfix/transport:
example.com relay:[127.0.0.1]:11125
Use "postmap hash:/etc/postfix/transport" and "postfix reload" to make the change effective.
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.
Examples:
/etc/postfix/main.cf:
tls_random_exchange_name = /var/lib/postfix/prng_exch
tls_random_prng_update_period = 3600s
As of version 2.5, Postfix no longer uses root privileges when opening this file. The file should now be stored under the Postfix-owned data_directory. As a migration aid, an attempt to open the file under a non-Postfix directory is redirected to the Postfix-owned data_directory, and a warning is logged. If you wish to continue using a pre-existing PRNG state file, move it to the data_directory and change the ownership to the account specified with the mail_owner parameter.
With earlier Postfix versions the default file location is under the Postfix configuration directory, which is not the proper place for information that is modified by Postfix.
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.porcupine.org 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.porcupine.org Email Address []:wietse@porcupine.org Please enter the following 'extra' attributes to be sent with your certificate request A challenge password []:whatever An optional company name []:
Sign the public key certificate for host foo.porcupine.org with the Certification Authority private key that we created a few steps ago.
% openssl ca -out foo-cert.pem -infiles foo-req.pem Using configuration from /etc/ssl/openssl.cnf Enter PEM pass phrase:whatever Check that the request matches the signature Signature ok The Subjects Distinguished Name is as follows countryName :PRINTABLE:'US' stateOrProvinceName :PRINTABLE:'New York' localityName :PRINTABLE:'Westchester' organizationName :PRINTABLE:'Porcupine' commonName :PRINTABLE:'foo.porcupine.org' emailAddress :IA5STRING:'wietse@porcupine.org' Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365 days) Sign the certificate? [y/n]:y 1 out of 1 certificate requests certified, commit? [y/n]y Write out database with 1 new entries Data Base Updated
Install the host private key, the host public key certificate, and the Certification Authority certificate files. This requires super-user privileges.
# cp demoCA/cacert.pem foo-key.pem foo-cert.pem /etc/postfix # chmod 644 /etc/postfix/foo-cert.pem /etc/postfix/cacert.pem # chmod 400 /etc/postfix/foo-key.pem
Configure Postfix, by adding the following to /etc/postfix/main.cf . It is generally best to not configure client certificates, unless there are servers which authenticate your mail submission via client certificates. Often servers that perform TLS client authentication will issue the required certificates signed by their own CA. If you configure the client certificate and key incorrectly, you will be unable to send mail to sites that request client certificate, but don't require them from all clients.
/etc/postfix/main.cf: smtp_tls_CAfile = /etc/postfix/cacert.pem smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_tls_session_cache smtp_tls_security_level = may smtpd_tls_CAfile = /etc/postfix/cacert.pem smtpd_tls_cert_file = /etc/postfix/foo-cert.pem smtpd_tls_key_file = /etc/postfix/foo-key.pem smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_tls_session_cache tls_random_source = dev:/dev/urandom smtpd_tls_security_level = may
These instructions assume that you build Postfix from source code as described in the INSTALL document. Some modification may be required if you build Postfix from a vendor-specific source package.
To build Postfix with TLS support, first we need to generate the make(1) files with the necessary definitions. This is done by invoking the command "make makefiles" in the Postfix top-level directory and with arguments as shown next.
NOTE: Do not use Gnu TLS. It will spontaneously terminate a Postfix daemon process with exit status code 2, instead of allowing Postfix to 1) report the error to the maillog file, and to 2) provide plaintext service where this is appropriate.
If the OpenSSL include files (such as ssl.h) are in directory /usr/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/lib:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"
If the OpenSSL include files (such as ssl.h) are in directory /usr/local/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/local/lib:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-L/usr/local/lib -lssl -lcrypto"
On Solaris, specify the -R option as shown below:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto"
If you need to apply other customizations (such as Berkeley DB databases, MySQL, PostgreSQL, LDAP or SASL), see the respective Postfix README documents, and combine their "make makefiles" instructions with the instructions above:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS \
(other -D or -I options)" \
AUXLIBS="-lssl -lcrypto \
(other -l options for libraries in /usr/lib) \
(-L/path/name + -l options for other libraries)"
To complete the build process, see the Postfix INSTALL instructions. Postfix has TLS support turned off by default, so you can start using Postfix as soon as it is installed.
Problems are preferably reported via <postfix-users@postfix.org>. See http://www.postfix.org/lists.html for subscription information. When reporting a problem, please be thorough in the report. Patches, when possible, are greatly appreciated too.