Postfix TLS Support
Postfix TLS Support
This document describes how to configure the Transport Layer Security (TLS) support in the Postfix SMTP client and Postfix SMTP server, and how to configure the TLS manager daemon that maintains the Pseudo Random Number Generator (PRNG) pool and the TLS session cache information.
Topics covered in this document:
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 root_CA.pem > server.pem
If you want the Postfix SMTP server to accept remote SMTP client certificates issued by these CAs, you can also add the CA certificates to the smtpd_tls_CAfile, in which case it is not necessary to have them in the smtpd_tls_cert_file or smtpd_tls_dcert_file.
A Postfix SMTP server certificate supplied here must be usable as SSL server certificate and hence pass the "openssl verify -purpose sslserver ..." test.
RSA key and certificate examples:
smtpd_tls_cert_file = /etc/postfix/server.pem smtpd_tls_key_file = $smtpd_tls_cert_file
Their DSA counterparts:
smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem smtpd_tls_dkey_file = $smtpd_tls_dcert_file
The Postfix SMTP server certificate was issued by a certification authority (CA), the CA-cert of which must be provided with the CA file if it is not already provided in the certificate file. The CA file may also contain the CA certificates of other trusted CAs. You must use this file for the list of trusted CAs if you want to use chroot-mode. No default is supplied for this value as of now.
Example:
smtpd_tls_CAfile = /etc/postfix/CAcert.pem
To verify a remote SMTP client certificate, the Postfix SMTP server needs to know the certificates of the issuing certification authorities. These certificates in "pem" format are collected in a directory. The same CA certificates are offered to clients for client verification. Don't forget to create the necessary "hash" links with $OPENSSL_HOME/bin/c_rehash /etc/postfix/certs. A typical place for the CA certificates may also be $OPENSSL_HOME/certs, so there is no default and you explicitly have to set the value here!
To use this option in chroot mode, this directory itself or a copy of it must be inside the chroot jail. Please note also, that the CAs in this directory are not listed to the client, so that e.g. Netscape might not offer certificates issued by them. For this reason, the use of this feature is discouraged.
Example:
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:
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:
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:
smtpd_use_tls = yes
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 accepts no commands (except QUIT of course) 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. So this option is off by default and should only seldom be used. Using this option implies "smtpd_use_tls = yes".
Example:
smtpd_enforce_tls = yes
Besides RFC 2487 some clients, namely Outlook [Express] prefer to run the non-standard "wrapper" mode, not the STARTTLS enhancement to SMTP. 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:
smtpd_tls_wrappermode = no
To receive a remote SMTP client certificate, the Postfix SMTP server must explicitly ask for one by sending the $smtpd_tls_CAfile certificates to the client. 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. This might be annoying, 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_client_certs feature.
Example:
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:
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:
smtpd_tls_ccert_verifydepth = 5
Sending AUTH data over an un-encrypted 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:
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 based on the SDBM databases (routines included in Postfix/TLS). Since concurrent writing must be supported, only SDBM can be used.
Example:
smtpd_tls_session_cache_database = sdbm:/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:
smtpd_tls_session_cache_timeout = 3600s
Postfix TLS support introduces two 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.
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:
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:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home
Example:
relay_clientcerts = hash:/etc/postfix/relay_clientcerts
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:
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
Your source for "entropy" might vary; some systems have /dev/random; on other systems you might consider the "Entropy Gathering Daemon EGD", available at http://www.lothar.com/tech/crypto/.
Examples:
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:
smtpd_starttls_timeout = 300s
Topics covered in this section:
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.dom.ain" 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 root_CA.pem > client.pem
If you want the Postfix SMTP client to accept certificates issued by these CAs, you can also add the CA certificates to the smtp_tls_CAfile, in which case it is not necessary to have them in the smtp_tls_cert_file or smtp_tls_dcert_file.
A Postfix SMTP client certificate supplied here must be usable as SSL client certificate and hence pass the "openssl verify -purpose sslclient ..." test.
RSA key and certificate examples:
smtp_tls_cert_file = /etc/postfix/client.pem smtp_tls_key_file = $smtp_tls_cert_file
Their DSA counterparts:
smtp_tls_dcert_file = /etc/postfix/client-dsa.pem smtp_tls_dkey_file = $smtpd_tls_cert_file
The Postfix SMTP client certificate was issued by a certification authority (CA), the CA-cert of which must be provided with the CA file if it is not already provided in the certificate file. The CA file may also contain the CA certificates of other trusted CAs. You must use this file for the list of trusted CAs if you want to use chroot-mode. No default is supplied for this value as of now.
Example:
smtp_tls_CAfile = /etc/postfix/CAcert.pem
To verify a remote SMTP server certificate, the Postfix SMTP client needs to know the certificates of the issuing certification authorities. These certificates in "pem" format are collected in a directory. Don't forget to create the necessary "hash" links with $OPENSSL_HOME/bin/c_rehash /etc/postfix/certs. A typical place for the CA certificates may also be $OPENSSL_HOME/certs, so there is no default and you explicitly have to set the value here!
To use this option in chroot mode, this directory itself or a copy of it must be inside the chroot jail.
Example:
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:
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 based on the SDBM databases (routines included in Postfix/TLS). Since concurrent writing must be supported, only SDBM can be used.
Example:
smtp_tls_session_cache_database = sdbm:/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:
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:
smtp_use_tls = yes
You can ENFORCE the use of TLS, so that the Postfix SMTP client will not deliver mail over un-encrypted 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:
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:
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 un-encrypted 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:
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:
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:
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:
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:
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 a fixed-size 1024-byte exchange file that is read by the smtp(8) and smtpd(8) processes when they initialize. These processes also add some more entropy to the file by stirring in their own time and process id information.
The tlsmgr(8) process creates the file if it does not already exist, and rewrites the file at random time intervals with information from its in-memory PRNG pool. The 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).
Example:
tls_random_exchange_name = /etc/postfix/prng_exch
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. 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):
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:
tls_random_bytes = 32
In order to update its in-memory PRNG pool, the tlsmgr(8) queries the external entropy source again after a 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:
tls_random_reseed_period = 3600s
The tlsmgr(8) re-generates the 1024 byte seed exchange file after a random amount of time. The time is calculated using the PRNG, and is between 0 and the maximal time specified with tls_random_update_period. The default maximal time interval is 60 seconds.
Example:
tls_random_prng_update_period = 60s
If you have an entropy source available that is not easily drained (like /dev/urandom), the smtp(8) and smtpd(8) daemons can load additional entropy on startup. By default, an amount of 32 bytes is read, the equivalent to 256 bits. This is more than sufficient to generate a 128bit (or 168bit) session key. However, when Postfix needs to generate more than one key it can drain the EGD. Consider the case of 50 smtp(8) processes starting up with a full queue; this will request 1600bytes of entropy. This is however not fatal, as long as "entropy" data can still be read from the seed file that is maintained by tlsmgr(8).
Examples:
tls_daemon_random_source = dev:/dev/urandom tls_daemon_random_source = egd:/var/run/egd-pool tls_daemon_random_bytes = 32
When reporting a problem, please be thorough in the report. Patches, when possible, are greatly appreciated too.
Please differentiate when possible between: