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 clients or servers. You also turn on thousands and thousands of lines of OpenSSL library code. Assuming that OpenSSL is written as carefully as Wietse's own code, every 1000 lines introduce one additional bug into Postfix.
Transport Layer Security (TLS, formerly called SSL) provides certificate-based authentication and encrypted sessions. An encrypted session protects the information that is transmitted with SMTP mail or with SASL authentication.
Postfix version 2.2 introduces support for TLS as described in RFC 3207. TLS Support for older Postfix versions was available as an add-on patch. The section "Compatibility with Postfix < 2.2 TLS support" below discusses the differences between these implementations.
Topics covered in this document:
And last but not least, for the impatient:
The diagram below shows the main elements of the Postfix TLS architecture and their relationships. Colored boxes with numbered names represent Postfix daemon programs. Other colored boxes represent storage elements.
The smtpd(8) server implements the SMTP over TLS server side.
The smtp(8) client implements the SMTP over TLS client side.
The tlsmgr(8) server maintains the pseudo-random number generator (PRNG) that seeds the TLS engines in the smtpd(8) server and smtp(8) client processes, and maintains the TLS session key cache files.
| Network-> | smtpd(8) |
<---seed--- <-session-> | tlsmgr(8) |
---seed---> <-session-> | smtp(8) | ->Network | ||||||||||
| | | |
| ||||||||||||||
|
smtpd session key cache | PRNG state file | smtp session key cache |
||||||||||||||
To build Postfix with TLS support, first we need to generate the make(1) files with the necessary definitions. This is done by invoking the command "make makefiles" in the Postfix top-level directory and with arguments as shown next.
If the OpenSSL include files (such as ssl.h) are in directory /usr/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/lib:
% make tidy # if you have left-over files from a previous build % make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"
If the OpenSSL include files (such as ssl.h) are in directory /usr/local/include/openssl, and the OpenSSL libraries (such as libssl.so and libcrypto.so) are in directory /usr/local/lib:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-L/usr/local/lib -lssl -lcrypto"
On Solaris, specify the -R option as shown below:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto"
If you need to apply other customizations (such as Berkeley DB databases, MySQL, PosgreSQL, LDAP or SASL), see the respective Postfix README documents, and combine their "make makefiles" instructions with the instructions above:
% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS \
(other -D or -I options)" \
AUXLIBS="-lssl -lcrypto \
(other -l options for libraries in /usr/lib) \
(-L/path/name + -l options for other libraries)"
To complete the build process, see the Postfix INSTALL instructions. Postfix has TLS support turned off by default, so you can start using Postfix as soon as it is installed.
Topics covered in this section:
In order to use TLS, the Postfix SMTP server needs a certificate and a private key. Both must be in "pem" format. The private key must not be encrypted, meaning: the key must be accessible without password. Both certificate and private key may be in the same file.
Both RSA and DSA certificates are supported. Typically you will only have RSA certificates issued by a commercial CA. In addition, the tools supplied with OpenSSL will by default issue RSA certificates. You can have both at the same time, in which case the cipher used determines which certificate is presented. For Netscape and OpenSSL clients without special cipher choices, the RSA certificate is preferred.
In order for remote SMTP clients to check the Postfix SMTP server certificates, the CA certificate (in case of a certificate chain, all CA certificates) must be available. You should add these certificates to the server certificate, the server certificate first, then the issuing CA(s).
Example: the certificate for "server.dom.ain" was issued by "intermediate CA" which itself has a certificate issued by "root CA". Create the server.pem file with:
% cat server_cert.pem intermediate_CA.pem > server.pem
A Postfix SMTP server certificate supplied here must be usable as SSL server certificate and hence pass the "openssl verify -purpose sslserver ..." test.
A client that trusts the root CA has a local copy of the root CA certificate, so it is not necessary to include the root CA certificate here. Leaving it out of the "server.pem" file reduces the overhead of the TLS exchange.
If you want the Postfix SMTP server to accept remote SMTP client certificates issued by these CAs, append the root certificate to $smtpd_tls_CAfile or install it in the $smtpd_tls_CApath directory. When you configure trust in a root CA, it is not necessary to explicitly trust intermediary CAs signed by the root CA, unless $smtpd_tls_verify_depth 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_asck_ccert = yes), any certificates in $smtpd_tls_CAfile are sent to the client, in order to allow it to choose an identity signed by a CA you trust. If no $smtpd_tls_CAfile is specified, no preferred CA list is sent, and the client is free to choose an identity signed by any CA. Many clients use a fixed identity regardless of the preferred CA list and you may be able to reduce TLS negotiation overhead by installing client CA certificates mostly or only in $smtpd_tls_CApath. In the latter case you need not specify a $smtpd_tls_CAfile.
Note, that unless client certificates are used to allow greater access to TLS authenticated clients, it is best to not ask for client certificates at all, as in addition to increased overhead some clients (notably in some cases qmail) are unable to complete the TLS handshake when client certificates are requested.
Example:
/etc/postfix/main.cf:
smtpd_tls_CAfile = /etc/postfix/CAcert.pem
smtpd_tls_CApath = /etc/postfix/certs
To get additional information about Postfix SMTP server TLS activity you can increase the loglevel from 0..4. Each logging level also includes the information that is logged at a lower logging level.
0 Disable logging of TLS activity. 1 Log TLS handshake and certificate information. 2 Log levels during TLS negotiation. 3 Log hexadecimal and ASCII dump of TLS negotiation process 4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS
Use loglevel 3 only in case of problems. Use of loglevel 4 is strongly discouraged.
Example:
/etc/postfix/main.cf:
smtpd_tls_loglevel = 0
To include information about the protocol and cipher used as well as the client and issuer CommonName into the "Received:" message header, set the smtpd_tls_received_header variable to true. The default is no, as the information is not necessarily authentic. Only information recorded at the final destination is reliable, since the headers may be changed by intermediate servers.
Example:
/etc/postfix/main.cf:
smtpd_tls_received_header = yes
By default, TLS is disabled in the Postfix SMTP server, so no difference to plain Postfix is visible. Explicitly switch it on using "smtpd_use_tls = yes".
Example:
/etc/postfix/main.cf:
smtpd_use_tls = yes
With this, Postfix SMTP server announces STARTTLS support to SMTP clients, but does not require that clients use TLS encryption.
Note: when an unprivileged user invokes "sendmail -bs", STARTTLS is never offered due to insufficient privileges to access the server private key. This is intended behavior.
You can ENFORCE the use of TLS, so that the Postfix SMTP server announces STARTTLS and accepts no mail without TLS encryption, by setting "smtpd_enforce_tls = yes". According to RFC 2487 this MUST NOT be applied in case of a publicly-referenced Postfix SMTP server. This option is off by default and should only seldom be used.
Example:
/etc/postfix/main.cf:
smtpd_enforce_tls = yes
TLS is sometimes used in the non-standard "wrapper" mode where a server always uses TLS, instead of announcing STARTTLS support and waiting for clients to request TLS service. Some clients, namely Outlook [Express] prefer the "wrapper" mode. This is true for OE (Win32 < 5.0 and Win32 >=5.0 when run on a port<>25 and OE (5.01 Mac on all ports).
It is strictly discouraged to use this mode from main.cf. If you want to support this service, enable a special port in master.cf and specify "-o smtpd_tls_wrappermode = yes" as an smtpd(8) command line option. Port 465 (smtps) was once chosen for this feature.
Example:
/etc/postfix/master.cf:
smtps inet n - n - - smtpd
-o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes
To receive a remote SMTP client certificate, the Postfix SMTP server must explicitly ask for one (any contents of $smtpd_tls_CAfile are also sent to the client as a hint for choosing a certificate from a suitable CA). Unfortunately, Netscape clients will either complain if no matching client certificate is available or will offer the user client a list of certificates to choose from. Additionally some MTAs (notably some versions of qmail) are unable to complete TLS negotiation when client certificates are requested, and abort the SMTP session. So this option is "off" by default. You will however need the certificate if you want to use certificate based relaying with, for example, the permit_tls_clientcerts feature.
Example:
/etc/postfix/main.cf:
smtpd_tls_ask_ccert = no
You may also decide to REQUIRE a remote SMTP client certificate before allowing TLS connections. This feature is included for completeness, and implies "smtpd_tls_ask_ccert = yes".
Please be aware, that this will inhibit TLS connections without a proper client certificate and that it makes sense only when non-TLS submission is disabled (smtpd_enforce_tls = yes). Otherwise, clients could bypass the restriction by simply not using STARTTLS at all.
When TLS is not enforced, the connection will be handled as if only "smtpd_tls_ask_ccert = yes" is specified, and a warning is logged.
Example:
/etc/postfix/main.cf:
smtpd_tls_req_ccert = no
A client certificate verification depth of 1 is sufficient if the certificate is directly issued by a CA listed in the CA file. The default value (5) should also suffice for longer chains (root CA issues special CA which then issues the actual certificate...)
Example:
/etc/postfix/main.cf:
smtpd_tls_ccert_verifydepth = 5
Sending AUTH data over an unencrypted channel poses a security risk. When TLS layer encryption is required (smtpd_enforce_tls = yes), the Postfix SMTP server will announce and accept AUTH only after the TLS layer has been activated with STARTTLS. When TLS layer encryption is optional (smtpd_enforce_tls = no), it may however still be useful to only offer AUTH when TLS is active. To maintain compatibility with non-TLS clients, the default is to accept AUTH without encryption. In order to change this behavior, set "smtpd_tls_auth_only = yes".
Example:
/etc/postfix/main.cf:
smtpd_tls_auth_only = no
The Postfix SMTP server and the remote SMTP client negotiate a session, which takes some computer time and network bandwidth. By default, this session information is cached only in the smtpd(8) process actually using this session and is lost when the process terminates. To share the session information between multiple smtpd(8) processes, a persistent session cache can be used. You can specify any database type that can store objects of several kbytes and that supports the sequence operator. DBM databases are not suitable because they can only store small objects. The cache is maintained by the tlsmgr(8) process, so there is no problem with concurrent access. Session caching is highly recommended, because the cost of repeatedly negotiating TLS session keys is high.
Example:
/etc/postfix/main.cf:
smtpd_tls_session_cache_database = btree:/etc/postfix/smtpd_scache
Cached Postfix SMTP server session information expires after a certain amount of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer time of 3600sec (=1 hour). RFC 2246 recommends a maximum of 24 hours.
Example:
/etc/postfix/main.cf:
smtpd_tls_session_cache_timeout = 3600s
Postfix TLS support introduces three additional features for Postfix SMTP server access control:
- permit_tls_clientcerts
Allow the remote SMTP client SMTP request if the client certificate passes verification, and if its fingerprint is listed in the list of client certificates (see relay_clientcerts discussion below).
- permit_tls_all_clientcerts
Allow the remote client SMTP request if the client certificate passes verification.
- check_ccert_access type:table
If the client certificate passes verification, use its fingerprint as a key for the specified access(5) table.
The permit_tls_all_clientcerts feature must be used with caution, because it can result in too many access permissions. Use this feature only if a special CA issues the client certificates, and only if this CA is listed as trusted CA. If other CAs are trusted, any owner of a valid client certificate would be authorized. The permit_tls_all_clientcerts feature can be practical for a specially created email relay server.
It is however recommended to stay with the permit_tls_clientcerts feature and list all certificates via $relay_clientcerts, as permit_tls_all_clientcerts does not permit any control when a certificate must no longer be used (e.g. an employee leaving).
Example:
/etc/postfix/main.cf:
smtpd_recipient_restrictions =
...
permit_tls_clientcerts
reject_unauth_destination
...
The Postfix list manipulation routines give special treatment to whitespace and some other characters, making the use of certificate names unpractical. Instead we use the certificate fingerprints as they are difficult to fake but easy to use for lookup. Postfix lookup tables are in the form of (key, value) pairs. Since we only need the key, the value can be chosen freely, e.g. the name of the user or host.
Example:
/etc/postfix/main.cf:
relay_clientcerts = hash:/etc/postfix/relay_clientcerts
/etc/postfix/relay_clientcerts:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home
To influence the Postfix SMTP server cipher selection scheme, you can give cipherlist string. A detailed description would go to far here; please refer to the OpenSSL documentation. If you don't know what to do with it, simply don't touch it and leave the (openssl-)compiled in default!
DO NOT USE " to enclose the string, specify just the string!!!
Example:
/etc/postfix/main.cf:
smtpd_tls_cipherlist = DEFAULT
If you want to take advantage of ciphers with EDH, DH parameters are needed. Instead of using the built-in DH parameters for both 1024bit and 512bit, it is better to generate "own" parameters, since otherwise it would "pay" for a possible attacker to start a brute force attack against parameters that are used by everybody. For this reason, the parameters chosen are already different from those distributed with other TLS packages.
To generate your own set of DH parameters, use:
% openssl gendh -out /etc/postfix/dh_1024.pem -2 -rand /var/run/egd-pool 1024 % openssl gendh -out /etc/postfix/dh_512.pem -2 -rand /var/run/egd-pool 512
Examples:
/etc/postfix/main.cf:
smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem
smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
The smtpd_starttls_timeout parameter limits the time of Postfix SMTP server write and read operations during TLS startup and shutdown handshake procedures.
Example:
/etc/postfix/main.cf:
smtpd_starttls_timeout = 300s
Topics covered in this section:
During TLS startup negotiation the Postfix SMTP client may present a certificate to the remote SMTP server. The Netscape client is rather clever here and lets the user select between only those certificates that match CA certificates offered by the remote SMTP server. As the Postfix SMTP client uses the "SSL_connect()" function from the OpenSSL package, this is not possible and we have to choose just one certificate. So for now the default is to use _no_ certificate and key unless one is explicitly specified here.
Both RSA and DSA certificates are supported. You can have both at the same time, in which case the cipher used determines which certificate is presented.
It is possible for the Postfix SMTP client to use the same key/certificate pair as the Postfix SMTP server. If a certificate is to be presented, it must be in "pem" format. The private key must not be encrypted, meaning: it must be accessible without password. Both parts (certificate and private key) may be in the same file.
In order for remote SMTP servers to verify the Postfix SMTP client certificates, the CA certificate (in case of a certificate chain, all CA certificates) must be available. You should add these certificates to the client certificate, the client certificate first, then the issuing CA(s).
Example: the certificate for "client.example.com" was issued by "intermediate CA" which itself has a certificate of "root CA". Create the client.pem file with:
% cat client_cert.pem intermediate_CA.pem > client.pem
A Postfix SMTP client certificate supplied here must be usable as SSL client certificate and hence pass the "openssl verify -purpose sslclient ..." test.
A server that trusts the root CA has a local copy of the root CA certificate, so it is not necessary to include the root CA certificate here. Leaving it out of the "client.pem" file reduces the overhead of the TLS exchange.
If you want the Postfix SMTP client to accept remote SMTP server certificates issued by these CAs, append the root certificate to $smtp_tls_CAfile or install it in the $smtp_tls_CApath directory. When you configure trust in a root CA, it is not necessary to explicitly trust intermediary CAs signed by the root CA, unless $smtp_tls_verify_depth is less than the number of CAs in the certificate chain for the servers of interest. With a verify depth of 1 you can only verify certificates directly signed by a trusted CA, and all trusted intermediary CAs need to be configured explicitly. With a verify depth of 2 you can verify servers signed by a root CA or a direct intermediary CA (so long as the server is correctly configured to supply its intermediate CA certificate).
RSA key and certificate examples:
/etc/postfix/main.cf:
smtp_tls_cert_file = /etc/postfix/client.pem
smtp_tls_key_file = $smtp_tls_cert_file
Their DSA counterparts:
/etc/postfix/main.cf:
smtp_tls_dcert_file = /etc/postfix/client-dsa.pem
smtp_tls_dkey_file = $smtpd_tls_cert_file
To verify a remote SMTP server certificate, the Postfix SMTP client needs to trust the certificates of the issuing certification authorities. These certificates in "pem" format can be stored in a single $smtp_tls_CAfile or in multiple files, one CA per file in the $smtp_tls_CApath directory. If you use a directory, don't forget to create the necessary "hash" links with:
# $OPENSSL_HOME/bin/c_rehash /path/to/directory
The $smtp_tls_CAfile contains the CA certificates of one or more trusted CAs. The file is opened (with root privileges) before Postfix enters the optional chroot jail and so need not be accessible from inside the chroot jail.
Additional trusted CAs can be specified via the $smtp_tls_CApath directory, in which case the certificates are read (with $mail_owner privileges) from the files in the directory when the information is needed. Thus, the $smtp_tls_CApath directory needs to be accessible inside the optional chroot jail.
The choice between $smtp_tls_CAfile and $smtpd_tls_CApath is a space/time tradeoff. If there are many trusted CAs, the cost of preloading them all into memory may not pay off in reduced access time when the certificate is needed.
Example:
/etc/postfix/main.cf:
smtp_tls_CAfile = /etc/postfix/CAcert.pem
smtp_tls_CApath = /etc/postfix/certs
To get additional information about Postfix SMTP client TLS activity you can increase the loglevel from 0..4. Each logging level also includes the information that is logged at a lower logging level.
0 Disable logging of TLS activity. 1 Log TLS handshake and certificate information. 2 Log levels during TLS negotiation. 3 Log hexadecimal and ASCII dump of TLS negotiation process 4 Log hexadecimal and ASCII dump of complete transmission after STARTTLS
Example:
/etc/postfix/main.cf:
smtp_tls_loglevel = 0
The remote SMTP server and the Postfix SMTP client negotiate a session, which takes some computer time and network bandwidth. By default, this session information is cached only in the smtp(8) process actually using this session and is lost when the process terminates. To share the session information between multiple smtp(8) processes, a persistent session cache can be used. You can specify any database type that can store objects of several kbytes and that supports the sequence operator. DBM databases are not suitable because they can only store small objects. The cache is maintained by the tlsmgr(8) process, so there is no problem with concurrent access. Session caching is highly recommended, because the cost of repeatedly negotiating TLS session keys is high. Future Postfix SMTP servers may limit the number of sessions that a client is allowed to negotiate per unit time.
Example:
/etc/postfix/main.cf:
smtp_tls_session_cache_database = btree:/etc/postfix/smtp_scache
Cached Postfix SMTP client session information expires after a certain amount of time. Postfix/TLS does not use the OpenSSL default of 300s, but a longer time of 3600s (=1 hour). RFC 2246 recommends a maximum of 24 hours.
Example:
/etc/postfix/main.cf:
smtp_tls_session_cache_timeout = 3600s
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.
WARNING: MS Exchange servers will announce STARTTLS support even when the service is not configured, so that the TLS handshake will fail. It may be wise to not use this option on your central mail hub, as you don't know in advance whether you are going to connect to such a host. Instead, use the smtp_tls_per_site recipient/site specific options that are described below.
When the TLS handshake fails and no other server is available, the Postfix SMTP client defers the delivery attempt, and the mail stays in the queue.
Example:
/etc/postfix/main.cf:
smtp_use_tls = yes
You can ENFORCE the use of TLS, so that the Postfix SMTP client will not deliver mail over unencrypted connections. In this mode, the remote SMTP server hostname must match the information in the remote server certificate, and the server certificate must be issued by a CA that is trusted by the Postfix SMTP client. 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 used in the check is beyond question, as it must be the principal hostname (no CNAME allowed here). Checks are performed against all names provided as dNSNames in the SubjectAlternativeName. If no dNSNames are specified, the CommonName is checked. The behavior may be changed with the smtp_tls_enforce_peername option which is discussed below.
This option is useful only 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 = no
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.
Note: the smtp_tls_enforce_peername setting has no effect on sessions that are controlled via the smtp_tls_per_site table.
Disabling the remote SMTP server hostname verification can make sense in closed environment where special CAs are created. If not used carefully, this option opens the danger of a "man-in-the-middle" attack (the CommonName of this possible attacker is logged).
Example:
/etc/postfix/main.cf:
smtp_tls_enforce_peername = yes
Generally, trying TLS can be a bad idea, as some servers offer STARTTLS but the negotiation will fail leading to unexplainable failures. Instead, it may be a good idea to choose the TLS usage policy based on the recipient or the mailhub to which you are connecting.
Deciding the TLS usage policy per recipient may be difficult, since a single email delivery attempt can involve several recipients. Instead, use of TLS is controlled by the Postfix next-hop destination domain name and by the remote SMTP server hostname. If either of these matches an entry in the smtp_tls_per_site table, appropriate action is taken.
The remote SMTP server hostname is simply the DNS name of the server that the Postfix SMTP client connects to. The next-hop destination is Postfix specific. By default, this is the domain name in the recipient address, but this information can be overruled by the transport(5) table or by the relayhost parameter setting. In these cases the relayhost etc. must be listed in the smtp_tls_per_site table, instead of the recipient domain name.
Format of the table: domain or host names are specified 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.
- MAY
- Try to use STARTTLS if offered, otherwise use the unencrypted connection.
- MUST
- Require usage of STARTTLS, 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.
- MUST_NOPEERMATCH
- Require usage of STARTTLS, 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.
The actual TLS usage policy depends not only on whether the next-hop destination or remote SMTP server hostname are found in the smtp_tls_per_site table, but also on the smtp_enforce_tls setting:
If no match was found, the policy is applied as specified with smtp_enforce_tls.
If a match was found, and the smtp_enforce_tls policy is "enforce", NONE explicitly switches it off; otherwise the "enforce" mode is used even for entries that specify MAY.
Special hint for TLS enforcement mode: since no secure DNS lookup mechanism is available, mail can be delivered to the wrong remote SMTP server. This is not prevented by specifying MUST for the next-hop domain name. The recommended setup is: specify local transport(5) table entries for sensitive domains with explicit smtp:[mailhost] destinations (since you can assure security of this table unlike DNS), then specify MUST for these mail hosts in the smtp_tls_per_site table.
Example:
/etc/postfix/main.cf:
smtp_tls_per_site = hash:/etc/postfix/tls_per_site
As we decide on a "per site" basis whether or not to use TLS, it would be good to have a list of sites that offered "STARTTLS". We can collect it ourselves with this option.
If the smtp_tls_note_starttls_offer feature is enabled and a server offers STARTTLS while TLS is not already enabled for that server, the Postfix SMTP client logs a line as follows:
postfix/smtp[pid]: Host offered STARTTLS: [hostname.example.com]
Example:
/etc/postfix/main.cf:
smtp_tls_note_starttls_offer = yes
When verifying a remote SMTP server certificate, a verification depth of 1 is sufficient if the certificate is directly issued by a CA specified with smtp_tls_CAfile or smtp_tls_CApath. The default value of 5 should also suffice for longer chains (root CA issues special CA which then issues the actual certificate...)
Example:
/etc/postfix/main.cf:
smtp_tls_scert_verifydepth = 5
To influence the Postfix SMTP client cipher selection scheme, you can give cipherlist string. A detailed description would go to far here; please refer to the OpenSSL documentation. If you don't know what to do with it, simply don't touch it and leave the (openssl-)compiled in default!
DO NOT USE " to enclose the string, specify just the string!!!
Example:
/etc/postfix/main.cf:
smtp_tls_cipherlist = DEFAULT
The smtp_starttls_timeout parameter limits the time of Postfix SMTP client write and read operations during TLS startup and shutdown handshake procedures. In case of problems the Postfix SMTP client tries the next network address on the mail exchanger list, and defers delivery if no alternative server is available.
Example:
/etc/postfix/main.cf:
smtp_starttls_timeout = 300s
The security of cryptographic software such as TLS depends critically on the ability to generate unpredictable numbers for keys and other information. To this end, the tlsmgr(8) process maintains a Pseudo Random Number Generator (PRNG) pool. This is queried by the smtp(8) and smtpd(8) processes when they initialize. By default, these daemons request 32 bytes, the equivalent to 256 bits. This is more than sufficient to generate a 128bit (or 168bit) session key.
Example:
/etc/postfix/main.cf:
tls_daemon_random_bytes = 32
In order to feed its in-memory PRNG pool, the tlsmgr(8) reads entropy from an external source, both at startup and during run-time. Specify a good entropy source, like EGD or /dev/urandom; be sure to only use non-blocking sources (on OpenBSD, use /dev/arandom when tlsmgr(8) complains about /dev/urandom timeout errors). If the entropy source is not a regular file, you must prepend the source type to the source name: "dev:" for a device special file, or "egd:" for a source with EGD compatible socket interface.
Examples (specify only one in main.cf):
/etc/postfix/main.cf:
tls_random_source = dev:/dev/urandom
tls_random_source = egd:/var/run/egd-pool
By default, tlsmgr(8) reads 32 bytes from the external entropy source at each seeding event. This amount (256bits) is more than sufficient for generating a 128bit symmetric key. With EGD and device entropy sources, the tlsmgr(8) limits the amount of data read at each step to 255 bytes. If you specify a regular file as entropy source, a larger amount of data can be read.
Example:
/etc/postfix/main.cf:
tls_random_bytes = 32
In order to update its in-memory PRNG pool, the tlsmgr(8) queries the external entropy source again after a pseudo-random amount of time. The time is calculated using the PRNG, and is between 0 and the maximal time specified with tls_random_reseed_period. The default maximal time interval is 1 hour.
Example:
/etc/postfix/main.cf:
tls_random_reseed_period = 3600s
The tlsmgr(8) process saves the PRNG state to a persistent exchange file at regular times and when the process terminates, so that it can recover the PRNG state the next time it starts up. This file is created when it does not exist. Its default location is under the Postfix configuration directory, which is not the proper place for information that is modified by Postfix. Instead, the file location should probably be on the /var partition (but not inside the chroot jail).
Examples:
/etc/postfix/main.cf:
tls_random_exchange_name = /etc/postfix/prng_exch
tls_random_prng_update_period = 3600s
The following steps will get you started quickly. Because you sign your own Postfix public key certificate, you get TLS encryption but no TLS authentication. This is sufficient for testing, and for exchanging email with sites that you have no trust relationship with. For real authentication, your Postfix public key certificate needs to be signed by a recognized Certificate Authority, and Postfix needs to be configured with a list of public key certificates of Certificate Authorities, so that Postfix can verify the public key certificates of remote hosts.
In the examples below, user input is shown in bold font, and a "#" prompt indicates a super-user shell.
Become your own Certificate Authority, so that you can sign your own public keys. This example uses the CA.pl script that ships with OpenSSL. By default, OpenSSL installs this as /usr/local/ssl/misc/CA.pl, but your mileage may vary. The script creates a private key in ./demoCA/private/cakey.pem and a public key in ./demoCA/cacert.pem.
% /usr/local/ssl/misc/CA.pl -newca CA certificate filename (or enter to create) Making CA certificate ... Using configuration from /etc/ssl/openssl.cnf Generating a 1024 bit RSA private key ....................++++++ .....++++++ writing new private key to './demoCA/private/cakey.pem' Enter PEM pass phrase:whatever
Create an unpassworded private key for host FOO and create an unsigned public key certificate.
% openssl req -new -nodes -keyout FOO-key.pem -out FOO-req.pem -days 365 Using configuration from /etc/ssl/openssl.cnf Generating a 1024 bit RSA private key ........................................++++++ ....++++++ writing new private key to 'FOO-key.pem' ----- You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank. ----- Country Name (2 letter code) [AU]:US State or Province Name (full name) [Some-State]:New York Locality Name (eg, city) []:Westchester Organization Name (eg, company) [Internet Widgits Pty Ltd]:Porcupine Organizational Unit Name (eg, section) []: Common Name (eg, YOUR name) []:FOO Email Address []:wietse@porcupine.org Please enter the following 'extra' attributes to be sent with your certificate request A challenge password []:whatever An optional company name []:
Sign the public key certificate for host FOO with the Certification Authority private key that we created a few steps ago.
% openssl ca -out FOO-cert.pem -infiles FOO-req.pem Uing configuration from /etc/ssl/openssl.cnf Enter PEM pass phrase:whatever Check that the request matches the signature Signature ok The Subjects Distinguished Name is as follows countryName :PRINTABLE:'US' stateOrProvinceName :PRINTABLE:'New York' localityName :PRINTABLE:'Westchester' organizationName :PRINTABLE:'Porcupine' commonName :PRINTABLE:'FOO' emailAddress :IA5STRING:'wietse@porcupine.org' Certificate is to be certified until Nov 21 19:40:56 2005 GMT (365 days) Sign the certificate? [y/n]:y 1 out of 1 certificate requests certified, commit? [y/n]y Write out database with 1 new entries Data Base Updated
Install the host private key, the host public key certificate, and the Certification Authority certificate files. This requires super-user privileges.
# cp demoCA/cacert.pem FOO-key.pem FOO-cert.pem /etc/postfix # chmod 644 /etc/postfix/FOO-cert.pem /etc/postfix/cacert.pem # chmod 400 /etc/postfix/FOO-key.pem
Configure Postfix, by adding the following to /etc/postfix/main.cf.
smtp_tls_CAfile = /etc/postfix/cacert.pem smtp_tls_cert_file = /etc/postfix/FOO-cert.pem smtp_tls_key_file = /etc/postfix/FOO-key.pem smtp_tls_session_cache_database = btree:/var/run/smtp_tls_session_cache smtp_use_tls = yes smtpd_tls_CAfile = /etc/postfix/cacert.pem smtpd_tls_cert_file = /etc/postfix/FOO-cert.pem smtpd_tls_key_file = /etc/postfix/FOO-key.pem smtpd_tls_received_header = yes smtpd_tls_session_cache_database = btree:/var/run/smtpd_tls_session_cache smtpd_use_tls = yes tls_random_source = dev:/dev/urandom
When reporting a problem, please be thorough in the report. Patches, when possible, are greatly appreciated too.
Please differentiate when possible between:
Postfix version 2.2 TLS support is based on the Postfix/TLS patch by Lutz Jänicke, but differs in a few minor ways.
main.cf: Specify "btree" instead of "sdbm" for TLS session cache databases.
TLS session cache databases are now accessed only by the tlsmgr(8) process, so there are no more concurrency issues. Although Postfix has an sdbm client, the sdbm library (1000 lines of code) is not included with Postfix.
TLS session caches can use any database that can store objects of several kbytes or more, and that implements the sequence operation. In most cases, btree databases should be adequate.
NOTE: You cannot use dbm databases. TLS session objects are too large.
master.cf: Specify "unix" instead of "fifo" as the tlsmgr service type.
The smtp(8) and smtpd(8) processes now use a client-server protocol in order to access the tlsmgr(8) pseudo-random number generation (PRNG) pool, and in order to access the TLS session cache databases. Such a protocol cannot be run across fifos.