Using e-mail servers (211.1)
Using e-mail servers (211.1)¶
Candidates should be able to manage an e-mail server, including the configuration of e-mail aliases, e-mail quotas and virtual e-mail domains. This objective includes configuring internal e-mail relays and monitoring e-mail servers.
Key Knowledge Areas¶
Configuration files for postfix
Basic knowledge of the SMTP protocol
Awareness of sendmail and exim
Terms and Utilities¶
Configuration files and commands for postfix
sendmail emulation layer commands
mail-related logs in /var/log
Basic knowledge of the SMTP protocol¶
Since October 2008, RFC 5321 is used to describe the SMTPprotocol.
When an SMTPclienthas a message to transmit, it establishes a two-way transmission channel to an SMTPserver. The responsibility of the client is to transfer the message to one or more SMTP servers, or to report a failure to do so.
The means by which an e-mail message is presented to an SMTP client, and how the domain name to transfer the message to is determined, is a local matter that will not be addressed here.
Detailed information can be found at:RFC5321.
For demonstration and testing purposes a plain text SMTP session can be
ncat: nc ncat telnet
1 2 3 4 5 6 7 8 9 10 11
ncat when performing the commands above, an
Enter keystroke may be necessary. This is due to the
differences in return characters that may exist between
other, similar tools.
telnet <smtp servername>
25 initial contact with the SMTP server is established. In
the example above mx1.unix.nl was used as the server. The server
responds with the maximum message size, the pipelining, and the
authentication capabilities of the server, in this case AUTH PLAIN
LOGIN. The server is also able to use STARTTLS, and HELP.
SIZE: when the size of the message the client wishes to send exceeds the set limit, the SMTP transaction will be aborted with an ERROR code pipelining. Also, when enabled, an SMTP client can transmit a group of SMTP commands (e.g., RSET, MAIL FROM:, RCPT TO:) without waiting for a response from the server.
AUTH PLAIN LOGIN: the SMTP server is capable of handling a plain password/username authentication. This can be handy for mobile devices using the SMTP server while originating from different IP addresses.
STARTTLS: The SMTP protocol itself doesn't include any form of encryption. With STARTTLS the communciation can be encrypted using certificates. This is fully described in RFC 3207.
Since we have established the initial connection we can now proceed:
1 2 3 4
The server responds with "250 OK" but when, for example, the "MAIL FROM:" command is followed by a misformatted or incomplete email address the server responds with "501: sender address must contain domain" or a similar error. With "RCPT TO: \<email address>" the destination of the message is given. If the recipient's address is accepted the server responds with: "250 Accepted".
Now the mail serverknows who we are and to whom we wish to transmit a message. The email client also knows what the SMTP server is capable of. We then proceed with the transmission:
1 2 3 4 5 6 7 8 9 10 11 12
With the DATA command we proceed with the contents of the message. The server responds with: "354 Enter message, ending with \".\" on a line by itself".
Then the message contents is entered, starting with the message headers. These headers are used by the email client. In the example the commands Mail From:, To:, Subject:, Date: and MessageID: are used. MessageID: is a unique identifier generated by the SMTP client. These headers are required as described in RFC 5322.
After the headers have been entered, a blank line indicates that the actual text of the message commences. The message ends, as the DATA command response has indicated, with a "." (without the quotes) on a line by itself. The server responds with: "250 OK id=1OjReS-0005kT-Jj". The id is a unique SMTP server identifier and can be used for troubleshooting.
Note To fight spam, some SMTP servers can check whether a message is RFC 5322 compliant. Regular SMTP clients are RFC 5322 compliant, but spammers often use not so regular SMTP clients and hence could send malformed messages.
Basic sendmail configuration steps are: sendmail
Set up Sendmail
Build the Sendmail User Table
Add your domain names to Sendmail
Test your configuration file
Basic configuration of sendmail is described at: Sendmail basic installation.
An installation and operation guide for sendmail 8.12 can be found at: Sendmail installation and operation guide.
Sendmail can either be built from source or installed as sendmail binaries. With sendmail you must build a run-time configuration file. This is a file that sendmail reads when it starts which describes the mailers it knows about, how to parse addresses, how to rewrite the message header, and the settings of various options. Although it can be quite complex, it can usually be built using an m4-based configuration language (m4 is a general-purpose macro processor). Assuming you have the standard sendmail distribution, see cf/README for more information.
The sendmail configuration files are processed by m4 to facilitate local customization; the directory cf in the distribution directory contains the source files. It contains several subdirectories:
Both site-dependent and site-independent descriptions of hosts. The files can be named after hosts (e.g., ucbvax.mc) when these are gateways, or after categories (such as generic-solaris2.mc) as a general description of an SMTP-connected host running Solaris 2.x. Files ending with .mc ("M4 Configuration") are input definitions; the output is in the corresponding .cf file. The files' general structure is described below.
Site-dependent subdomain descriptions. These are tied to the way your
organization wants to do addressing. For example,
domain/CS.Berkeley.EDU.m4 is our description for hosts in the
CS.Berkeley.EDU subdomain. These are referenced using the DOMAIN m4
macro in the .mc file.
Definitions of specific features that a particular host at your site
might need. These are referenced using the FEATURE m4 macro. An
example is use_cw_file that tells sendmail to read an
/etc/mail/local-host-names file on startup to determine the set of
Local hacks, referenced using the HACK m4 macro. Try to avoid these. The point of having them here is to make it clear that they smell.
Site-independent m4(1) include files that have information common to all configuration files. This can be thought of as an #include directory.
Definitions of mailers, referenced using the MAILER m4 macro. The types that are known in this distribution are fax, local, smtp, uucp, and usenet. For example, to include support for UUCP-based mailers, use MAILER(uucp).
Definitions describing various operating system environments (such as the location of support files). These are referenced using the OSTYPE m4 macro.
Shell files used by the m4 build process. You probably don't have to change these.
Local UUCP connectivity information. This directory has been replaced by the mailertable feature; any new configurations should use that feature to do UUCP (and other) routing. The use of the siteconfig directory is deprecated.
Make sure the m4 utility is available at the server. With it the .mc macro files can be converted to a sendmail.cf file. The recommended method to configure sendmail is to edit the macro files and not the sendmail.cf file itself.
Details of sendmail installation files can be seen at: Sendmail installation and operation guide.
Important sendmail files
/etc/mail/sendmail.cf - Main sendmail configuration file. sendmail.cf
/etc/mail/access - The sendmail access database file
can be created to accept or reject mail from selected domains, systems
and users. Since it is a database, after editing the text file you have
makemap to create or update it.
To create a new access map:
- Accept mail even if other rules in the running ruleset would reject it, for example, if the domain name is unresolvable. \"Accept\" does not mean \"relay\", but at most acceptance for local recipients. Thus, OK allows less than RELAY.
- Accept mail addressed to the indicated domain or received from the indicated domain for relaying through your SMTP server. RELAY also serves as an implicit OK for the other checks.
- Reject the sender or recipient with a general purpose message.
- Discard the message completely using the $#discard mailer. If it is used in check_compat, it affects only the designated recipient, not the whole message as it does in all other cases. This should only be used if really necessary.
- This can only be used for host/domain names and IP addresses/nets. It will abort the current search for this entry without accepting or rejecting it but causing the default action.
/etc/mail/local-host-names - Local host names can be defined in the
local-host-names file. Add each domain that should be considered local
/etc/mail/virtusertable - Used to map incoming email to a local
account. With virtusertable, messages sent to one account can be
distributed to two users. Also a \"catch all\" email address can be set
up to route all mistyped email to one particular user to create a new
To generate the virtusertable database map:
/etc/mail/genericstable - Used for outbound mail. Can be used to
rewrite local usernames so they appear to have originated from a
different host or domain.
/etc/mail/genericsdomain - Populate this file with the
hostname --long command.
/etc/mail/mailertable - Used to route email from remote systems.
/etc/mail/domaintable - Can be used to transition from an old domain
name to a new one.
/etc/mail/aliases - Used to redirect mail for local
recepients. Each line of
/etc/aliases has the format of alias: user.
Two system aliases must always be present: mailer_daemon: postmaster
and postmaster: root. You can use aliases for all kind of daemons, for
example use ntp: root. Now you can add a line to redirect all mail to
root to a specific user or group of administrators, for example root:
newaliases needs to be run after any change to this file.
If any update is made to one of the configuration files, sendmail needs to reload its' configuration:
Starting with version 8.9, sendmail does not relay by default. When
using an older sendmail version, make changes to the
access files to make sure that it does not relay. Antirelaying tips
are described at: Controlling SMTP relaying tips -
Sendmail test option¶
Sendmail can be run in test mode. Use the
-t options to do
this. You need to run this as root.
Sendmail and DNS¶
Make sure that the MX records for the MTA's are available in DNS. This can be checked with: MX records
Manual entries in sendmail.cf¶
m4 As mentioned before this is not the recommended way. Change the m4
files instead and use
m4 to generate
Exim is a mail transfer agent(MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. Exim can be installed instead of Sendmail, although the configuration of Exim is quite different. See exim.org for more detailed information.
The exim4-config_files man page contains descriptions for each of
them. Exim comes with an
exim.conf template. Just edit this config
file for your environment. The Exim Wiki on
GitHub provides additional
configuration help, FAQ, etc.
Postfix is another, widely adopted, MTA. It focusses on speed, ease of administration and security. The author of the software, Wietse Venema, was dependant on long computer calculations during his study. It was during that time that he developed a habit of creating software that would be as fault-free and fault-resistant as possible. Only if he could trust the software to run for two weeks straight without errors, there would be no necessity to sleep on the floor for those two weeks while babysitting it. This coding habit stuck with the author for the years to come. When the Postfix program was first written, it was done with the same habit as those earlier study assignments. This habit then became a coding philosophy. By writing the code in a structured manner, only adding neccessary bits and making sure execution flows as efficient as possible, the resulting software became fast, easy to administer and secure. Today, Postfix is still under active development. Many people have contributed to it, and many still do. Despite this common effort, the original author still assures that only necessary code gets added. And that it is added in a way that does not interfere with the philosophy that has been part of Postfix from the start. This approach, in combination with the way that Postfix consists of various small programs working together instead of one big program, makes Postfix a very attractive alternative to other MTA solutions.
At the time of writing Postfix version 3.1 is the latest stable release.
Postfix version 3.2 is (still) considered experimental. Experimental
Postfix releases can be recognized by their names. These will carry a
date in them. An example of an experimental release is the following
postfix-3.2-20161204.tar.gz. Stable Postfix versions only
show (major and minor) version numbers without the date at the end. Due
to its wide adoption, most Linux distributions offer Postfix. However
there is a wide variety in versions being deployed. According to its
website http://www.postfix.org, versions prior to Postfix 2.10 are
End-Of-Support (EOS). CentOS 6.8 still ships with Postfix 2.6 though.
Debian 7 did not get beyond Postfix 2.9. More recent Linux distribution
versions like CentOS 7 and Debian 8 ship with Postfix >2.10, while some
bleeding edge Linux distribution versions offer Postfix >3.1. There is
absolutely no certainty that more recent versions of Postfix will have
fewer bugs than older versions of Postfix. It is regarded as a 'best
practice' to be aware of which version of Postfix you are exposing to
the Internet, while at the same time keeping up to date about known
Postfix vulnerabilities. There is no reason to restrict this best
practice to Postfix alone though.
To address the Postfix objectives that are defined for the LPIC-2 exam, the following will provide some examples. These asume you are using Postfix installed from packages on either a Debian-based or a Red Hat based Linux distribution. There may still be circumstances that require Postfix to be build from source. That subject is beyond the scope of this chapter but the topic in general is covered by LPIC objective 206.1.
Postfix main.cf file format¶
The default location for the postfix configuration files is
/etc/postfix. Amongst others there are two main ones that determine
master.cf. By default, the main.cf
configuration file specifies a subset of all the parameters that control
the operation of the Postfix mail system. Parameters not explicitly
specified are left at their default values. Refer to the
manpage for a full listing of all parameters. The
contains useful comments as well as references to other documentation
sources at the top. On Debian-based systems, the
file you encounter may be a stripped-down version. The
/usr/share/postfix directory should then contain a
that is more elaborate. The following line count operation gives an
estimate about the differences between the different
main.cf files on
a Debian-based system:
1 2 3 4 5 6
The example above demonstrates that the
/usr/share/postfix/main.cf.dist file holds more than 600 lines,
whereas the default
/etc/postfix/main.cf file only holds 43. Keeping
the main.cf file clean and tidy is a good habit. It is also a good habit
to use a pager to read through the more elaborate
main.cf.dist file on
Debian-based systems. Red Hat based systems keep well documented
configuration files within the
By default, postfix will only read configuration files from the
/etc/postfix directory. Other configuration directories may be
specified by using the
alternate_config_directories directive in
/etc/postfix/main.cf. Red Hat based distributions may have
directories. These directories exist to ease the life of the Postfix
package creators and should be left alone unless you are really sure
about what you are doing.
The postfix configuration directory
/etc/postfix is "hard-coded" in
compilation. Chapter 4.6 of the following document explains how to
change this value when building Postfix from source: Postfix
Installation From Source Code
The general format of the main.cf file is as follows:
Each logical line is in the form
parameter = value. Whitespace around the "=" is ignored, as is whitespace at the end of a logical line.
Empty lines and whitespace-only lines are ignored, as are lines whose first non-whitespace character is a "#".
A logical line starts with non-whitespace text. A line that starts with whitespace continues a logical line.
A parameter value may refer to other parameters.
When the same parameter is defined multiple times, only the last instance is remembered.
Otherwise, the order of
main.cfparameter definitions does not matter.
Also see Postfix Configuration
Parameters as an alternative to
postconf man page. The remainder of that document is a description
of all Postfix configuration parameters. Default values are shown after
the parameter name in parentheses. These default values can also be
looked up with the
sudo /usr/sbin/postconf -d command. Beware though,
since this command will produce 877 lines of output on a Postfix 3.1
instance. Because it is an administrative command and usually located
sudo or a shell with root privileges should be
used to invoke this command.
One of the "best practices" to use with Postfix is to disable the
SMTP VRFY command on a publicly accessible mail server. This VeRiFY
command can be abused by an attacker to enumerate valid user accounts or
email adddresses as follows:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
The 200-category response codes expose valid recipients. Just like HTTP
servers do, a SMTP server can respond with status codes. It is
recommended to be familiar with the most common response codes. This
will aid when debugging system performance or issues. It is usually not
necessary to know the difference between 220 and 250 at first sight. But
the difference between 2xx and 5xx status codes should be noticable and
familiar. The VRFY command is enabled on Postfix by default. By enabling
or adding the following line to
main.cf the VRFY feature is disabled:
Let's try the
VRFY root command from above again after both having
disabled the VRFY command and having reloaded Postfix:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
By disabling the possibility to distinguish between valid (2xx) and invalid (5xx) email recipients, the Internet has become a safer place.
postfix The Postfix developers recommend to change no more than 2-3
parameters at a time, and test if Postfix still works after every
change. After making changes to
main.cf you need to reload postfix.
Depending on the Linux distribution used, this can be accomplished using
one of the following commands using
sudo or a shell with root
postfix reload or
systemctl restart postfix or even
Postfix master.cf file format¶
The Postfix mail system follows a modular approach by implementing a
small number of (mostly) client commands that are invoked by users, and
by a larger number of services that run in the background. Postfix
services are implemented by daemon processes. These run in the
background and are controlled by the master process. The
configuration file defines how a client program connects to a service,
and what daemon program runs when a service is requested.
The general format of the
master.cf file is as follows:
Empty lines and whitespace-only lines are ignored, as are lines whose first non-whitespace character is a "#".
A logical line starts with non-whitespace text. A line that starts with whitespace continues a logical line.
Each logical line defines a single Postfix service. Each service is identified by its name and type as described below. When multiple lines specify the same service name and type, only the last one is remembered. Otherwise, the order of master.cf service definitions does not matter.
Each logical line consists of eight fields separated by whitespace. These are described below in the order as they appear in the master.cf file. Where applicable a field of "-" requests that the built-in default value be used. For boolean fields specify "y" or "n" to override the default value.
Chroot option (default: Postfix >= 3.0: n, Postfix \<3.0: y) - Whether
or not the service runs chrooted to the mail queue directory (pathname
is controlled by the queue_directory configuration variable in de
Before postfix can be used, it needs to know:
what domains to receive mail for (mydestination)
which domain name to use for outbound mail (myorigin)
which domain(s) postfix is allowed to relay mail for (relay_domains)
what delivery method to use (relayhost)
myorigin parameter specifies the domain that appears as the
"From:" domain in outgoing email. The
myorigin option is subject to
other options, which define whether the "From:" domain gets appended or
replaced by the
myorigin value. Refer to the
main.cf file or
postconf man page for details. By default, the
myorigin value is
$myhostname. Changing the
myorigin value to the
configured domain name can be done as follows:
$mydomain are replaced by postfix with the
hostname or domain of the server it is running on.
Postfix needs to know for which domain(s) it will receive mail. The
mydestination is used for this. More than one domain may be
specified. The domain names can be separated using whitespace or a
comma. Also, a pattern can be used to point to a lookup table (hash,
btree, nis, ldap or mysql).
You have to include
$mydomain when the server is used as a mail server
for the entire domain.
The default configuration of postfix will try to deliver incoming mail
to authorized destinations only. The
controls for which domains postfix will accept and forward emails.
1 2 3 4
By default postfix tries to deliver directly to the internet depending
on the domain name of the destination address in the mail message. Using
relayhost parameter we can specify to use another SMTP server as
This is the default, direct delivery to the internet, or using another ISP SMTP server:
maillog Postfix uses the syslog daemon for its logging. When
/etc/syslog.conf is configured like in the example below, Postfix log
events are written to
/var/log/maillog. Error messages are, in the
example below, redirected to the console.
/var/log/maillog will help you to find any problems
postfix encountered. There are also third party utilities like
pflogsumm that can generate statistics out of Postfix logging.
Generally a postfix server is the final destination for a limited number
of domains. But postfix can also be configured to handle mail for
additional domains which are different from, for example, the domain in
which the postfix server is located. These destinations are called
virtual hosts. Using the
virtual_alias_domains parameter we can
specify for which virtual hosts we wish to receive mail. The format of
the parameters is the same as in the samples above. Separate multiple
virtual hosts using a space or a comma. Also a link to a (hashed) file
on disk is possible:
postmap or when using a hashed file (using the
The content of
/etc/postfix/virtual can be:
1 2 3 4
In the above example peter receives the postmaster\@example.com email. Gerda receives the info\@sue.nl email and the sales\@example.com goes to petra. The last line is a "catch all" rule, all email for example.com without a valid destination goes to jim.
postmap /etc/postfix/virtual to create the hashed file and issue a
postfix reload afterwards.
Sendmail emulation layer commands
newaliases Since Sendmail has been the de facto mail delivery standard on Unix-like systems for years, replacements like Postfix have felt compelled to implement sendmail emulation layer commands in order to maintain compatibility with outside programs. That is to say that certain commands that were originally included with the sendmail package are available with Postfix as well.
mailqis available on most systems to check the mail queue. It is equivalent to
sendmail -bp, which works with Postfix too. Its native alternative is
postqueue -p. postqueue
newaliasesis required to generate a binary aliases file with both sendmail and postfix. Both employ
/etc/aliasesas the default input file.
On Postfix systems
man sendmail will provide more details on the
"Postfix to Sendmail compatibility interface".
Specifies the default mail drop directory. By default all mail is
delivered to the
LPIC objective 208.2 covers HTTP over SSL/TLS. The SMTP protocol is similar to the HTTP protocol in that both protocols use plain text network commands. This comes in handy when debugging, because it allows for command execution through telnet- or netcat-like software. But it does not come in handy when transferring sensitive information across a range of systems. Due to the nature of the SMTP protocol, multiple machines may be involved to transfer an e-mail message from the source to the final destination. And there is no way of excluding the possibility that a message has been altered during transfer. This is where encryption comes in. By using encrypted communication, at least some form of confidentiality and authenticity can be added to SMTP transactions. Just like Apache needs to be set up properly for HTTPS transactions, Postfix needs to be configured for use with TLS as well. Unlike Apache, we leave out the SSL acronym and only refer to TLS for Postfix. The following paragraphs will explain more about TLS-specific Postfix directives and about configuring Postfix for use with TLS.
At the time of writing, TLSv1.3 is the latest TLS version. TLSv1.3 has reached DRAFT status, and it will be a matter of time before it hits the streets. There will be some noteworthy changes between TLSv1.2 and TLSv1.3. Up to TLSv1.2, a ciphersuite consisted of an authentication algorithm, an encryption algorithm, a message digest algorithm and a key exchange algorithm. These four algorithms work together and are defined by their acronyms. From TLSv1.3 onward, the ciphersuite will consist of only the encryption and message authentication algorithm.
The following paragraphs are ment to assist you in getting Postfix with
TLS up and running. Encryption is an ever expanding subject, and it is
recommended to take the time to read and understand the available
documentation with regard to Postfix and the (proper) use of TLS.
Postfix comes with extensive documentation that may be found within the
/usr/share/doc/postfix directory (on Debian-based systems, you might
have to install the
postfix-doc package for completeness as is
explained on the bottom of this page). The
TLS_README document is
definitely worth reading. It can be viewed using a pager like
first expanded using a utility like
zcat if the file is compressed.
TLS_README.txt document is also available on the Postix website
openssl When creating a keypair for use with Apache, it may be preferred
to use an encrypted private key. An encrypted key needs to be
decrypted using a password before it can be used. OpenSSL will encrypt
the private key during creation by default, unless specified otherwise.
Postfix requires the private key used for TLS encrypted communication to
be unencrypted, in other words without a password. A private key may
be stripped from its password by reading, importing and exporting the
key with OpenSSL. In the following example though, the
is used with OpenSSL during creation time of the key. This will prevent
OpenSSL from encrypting the private key in the first place.
1 2 3
The self-signed certificate and private key created with the one-liner above can be used for postfix. Postfix demands the private key file is owned by, and readable only by root. The certificate file may be world-readable. Because the example above uses the RSA algorithm, the following directives are used to refer to these files:
smtpd_tls smtp_tls When declaring these directives, take special care
to differentiate between
smtpd_tls_ directives and
directives. The latter are used for client authentication.
The certificate file must be in the PEM-format. This is the OpenSSL
standard export format, so no additional flags should be required. In
the example above, the RSA algorithm is used. Instead of RSA, DSA could
also be used as the encryption algorithm. When using the DSA algorithm,
the key needs to be generated using the
-t dsa option and the
directives for the certificate file and key slightly differ from their
ECDSA is another valid encryption algorithm for use with Postfix. But
because many mailservers still use OpenSSL 0.9.8 versions that lack the
proper ECDSA implementation, the use of ECDSA for Postfix with TLS is
currently not recommended. Currently RSA is the recommended encryption
algorithm to use with TLS for Postfix. However if you do want to use
ECDSA keys, the key must be generated using the
-t ecdsa option and
the certificate and key files have to be specified using the following
certificateself-signed CSR CA The OpenSSL example above creates a
self-signed certificate. This is a way of getting the Postfix STARTTLS
functionality working "quick and dirty". But in terms of authenticity,
self-signed certificates may not be validated by many public SMTP
servers. An alternative is to create a key and CSR, and sign the CSR
using your own CA. This is demonstrated in the Apache chapter. By using
certificates issued by your own CA, you could let your mailservers
validate each other. Yet another option is to create a key and CSR, and
have the CSR signed by a public CA. That way, public SMTP servers should
be able to validate the certificate offered by your Postfix server. When
going this route, it may be necessary to add additional root CA
references to complete the certificate chain. Additional root CA files
may be configured by using either the
smtpd_tls_CApath directives. The file directive should point to a (you
guessed it) file holding the necessary root CA information. When
concatenating several certificates in to one file, be aware that the
files are read "bottom to top". The
smtpd_tls_CApath directive should
point to a directory holding the necessary file(s).
Postfix uses opportunistic encryption by default. This means that a
connecting system will try to create an encrypted connection at first.
However, if this fails for any reason (say, due to an invalid
certificate) then the system will continue to perform all SMTP commands
without encryption. This behavior is determined by the value of the
smtpd_tls_security_level directive. By changing the value of this
encrypt, unencrypted sessions are prevented.
Be aware though! This directive may prevent the Postfix server from
performing any SMTP transactions anymore if TLS sessions cannot be
Certificate Authority Despite all the good intentions from various
public Certificate Authorities, the Certificate Authority system is kind
of flawed by design. After all, ANY Certificate Authority may issue
certificates for ANY service on ANY host. The chapter on DNS mentions
DANE TLSA records to combat this issue. When DNSSEC has been properly
configured and TLSA records are supported, it is recommended to create
301 or 311 TLSA records for a given TLS-capable Postfix server. By
shifting the trust from the CA trust store with over 1300 CA root
certificates to the authenticity and integrity of DNSSEC, it becomes
less of a problem to depend on self-signed certificates. If multiple
domains are served on the same Postfix server, it is recommended to
publish a digest for every seperate domain (and therefore certificate)
in DNS. The
smtpd_tls_support_level value can be set to either
dane-only to handle TLSA records. Refer to the
Directives and options Postfix has been written with many "what if..."
fall-back scenarios in mind. The result is a very resilient piece of
software. Being a result of good coding practices, Postfix uses a number
of mechanisms to determine if circumstances are favourable or not. If
system resources become scarse, the Postfix system will adapt as is
deemed suitable regarding the circumstances. When configuring Postfix to
use TLS encryption, there are some directives that deserve additional
attention. When configured correctly, directives should complement each
others functionality. When configured inconsistently or incorrectly,
directives may conflict with each others functionality. It is advised to
pay additional attention to directives that are involved when
configuring the allowed protocols and cipher suites. Modern Postfix
versions exclude the use of SSLv2. Because version upgrades may change
the default protocols and allowed cipher suites, there is no harm in
configuring these yourself. By declaring the following directives in
main.cf, these options will stay constant despite Postfix version
upgrades over time.
1 2 3 4 5
By disabling the
aNULL ciphers, use of anonymous ciphers is
prevented. The use of anonymous ciphers is one of the fall-back
scenarios that helps Postfix in providing availability, at the expense
As mentioned before, the TLS functionality in Postfix can be configured
smtpd_tls_security_level directive. According to RFC 2487,
this directive should be set to the value
may. This will result in
Postfix announcing the availability of
STARTTLS to connecting clients,
without demanding its use. If you want to enforce the use of
the value of
smtpd_tls_security_level should be set to
instead. Again, be aware that this may result in degraded server
compatibility towards other systems. By setting the directive value to
none, the Postfix server will refrain from announcing the
method at all. When a connecting system tries to initiate
the Postfix server will reply with a
502 5.5.1 Error: command not implemented message.
When enabling TLS for Postfix, various additional configuration
directives become available. Most directives starting with
smtp_tls_ are not significant for the working of Postfix unless TLS
is enabled. One of these directives is the
directive. This directive may be configured by setting a value from
0-4. It is not recommended to use a setting above
2, except for
debugging purposes. A value of
0 will disable logging of TLS events. A
1 will log a summary message on TLS handshake completion. A
2 will also log levels during TLS negotiation. A value of
will log all of the above plus hex and text dumps of the TLS
transaction. A value of
4 eventually will log all of the above but
will not stop dumping after the TLS transaction has been processed. It
will continue to dump the entire SMTP transmission after the STARTTLS
command. Use with caution!
On most Linux distributions, additional documentation regarding Postfix
can be found in the
/usr/share/doc/postfix directory. On Debian-based
systems, this requires installing the
postfix-doc package. Without the
addition of this package, the contents of that directory will be very
postfix-tls tlsmgr tlsproxy Postfix 3.x makes TLS easier by
postfix-tls. While reading up, you might also want
to check out what