33 KiB
Title: Setting up a mail server Date: 2015-04-24 Category: Tutorial
Introduction
In this first tutorial, I'll explain how I've configured my mail server using the following :
- A server running Linux Debian (jessie) ;
- Postfix ;
- Postfix-policyd-spf-python ;
- Dovecot ;
- Spamassassin ;
- OpenDKIM ;
- OpenDMARC ;
- Monit ;
- Rainloop.
I'm assuming you have some basic knowledge of Linux and DNS configuration.
You can host this server at home, but you might have issues with your ISP not allowing outbound traffic on TCP port 25, and your emails might be considered to be spam by other providers if your IP is dynamic and/or you can't configure a reverse DNS record on it.
The cheapest VMs from DigitalOcean or Vultr are powerful enough to have this configuration running smoothly.
We'll also need a SSL certificate for this configuration. You can create an auto-signed one or get a free valid one from StartSSL. For the purpose of this tutorial, I'll consider you've chosen the latter.
You'll also need a domain name. I've chosen Namecheap as a registrar. I won't go into details on how to configure it, but you'll need at the very least a A record on your server's IP as well as a MX record pointing to it.
I use the captainark.net domain as an example throughout this tutorial. You'll have to use your actual domain for your configuration to work !
Note: links in this section are sponsored.
Initial configuration
Installing the required packages
First thing first, we need to install the packages we'll need for this configuration :
apt update
apt install mysql-server mysql-client postfix postfix-mysql \
postfix-policyd-spf-python dovecot-core dovecot-imapd dovecot-lmtpd \
dovecot-mysql dovecot-sieve dovecot-managesieved dovecot-antispam \
opendkim opendkim-tools monit opendmarc spamassassin spamc
During its installation, Postfix will prompt you with configuration questions. Choose "Internet Site", and when asked about your System mail name, provide it with your server's FQDN (it should be the output of the hostname -f command on your server).
You'll also have to set-up a password for the MySQL root user.
Additional configuration
The PTR records on your server's IPv4 and/or IPv6 should match your server's FQDN (a dig -x on your server's IP should match a hostname -f on your server).
You'll have to open the following TCP ports on your server for this configuration to work : 25, 465, 587 and 993.
If you don't want to have to remember the root user MySQL password, you can create a .my.cnf file in your current user home directory containing the following lines :
[client]
host = localhost
user = root
password = myverysecurepassword
socket = /var/run/mysqld/mysqld.sock
Once it has been created, change the permissions on the file to make sure no other user can read it :
chmod 600 ~/.my.cnf
I also like to change the default MySQL shell to see what database I'm using at any given time. Since I use bash, I achieve this the following way :
echo 'export MYSQL_PS1="[\u@\h] (\d)> "' > ~/.bash_aliases
You'll have to logout from the current shell for the modification to be taken into account (if you're using SSH, log out and back into your server).
You should now be able to log into MySQL without specifying a password, and it should look like this :
:~$ mysql mysql
[...]
[root@localhost] (mysql)>
Configuring the MySQL database
Initial configuration
We now need to configure the MySQL database Postfix and Dovecot will be using. In this tutorial, we'll be calling it "mail", but you can name it whatever you want.
First, in a mysql shell, let's create the MySQL database :
CREATE DATABASE mail;
Now, we are going to create the user that Postfix and Dovecot will be using to access the database. We will only be granting this user select permission :
GRANT SELECT ON mail.* TO 'mail'@'localhost' IDENTIFIED BY 'mailpassword';
FLUSH PRIVILEGES;
We are now going to create the necessary tables for our needs. Let's first use the mail database :
USE mail;
The first table we are going to create will contain the domains we will be using with our mail server :
CREATE TABLE `virtual_domains` (
`id` INT NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Then, we are going to create the table that will contain our users and their password :
CREATE TABLE `virtual_users` (
`id` INT NOT NULL AUTO_INCREMENT,
`domain_id` INT NOT NULL,
`password` VARCHAR(106) NOT NULL,
`email` VARCHAR(120) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `email` (`email`),
FOREIGN KEY (domain_id) REFERENCES virtual_domains(id) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Finally, the last table we are going to create will contain our mail aliases :
CREATE TABLE `virtual_aliases` (
`id` INT NOT NULL AUTO_INCREMENT,
`domain_id` INT NOT NULL,
`source` varchar(100) NOT NULL,
`destination` varchar(100) NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (domain_id) REFERENCES virtual_domains(id) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Domains, users and aliases management
We are now going to add data to the tables we have created.
First, let's add a domain to the virtual_domains table :
INSERT INTO virtual_domains (`name`) VALUES ('captainark.net');
We can now create users associated with this domain in the virtual_users table :
INSERT INTO virtual_users (`domain_id`, `password` , `email`) VALUES
('1', ENCRYPT('notanactualpassword', CONCAT('$6$', SUBSTRING(SHA(RAND()), -16))),
'example@captainark.net');
This is not mandatory, but we can also create our first mail alias :
INSERT INTO virtual_aliases (`domain_id`, `source`, `destination`) VALUES
('1', 'alias@captainark.net', 'example@captainark.net');
Now, all messages sent to alias@captainark.net will be forwarded to example@captainark.net.
Use the same syntax to create additional domains, users and aliases. If you have more than one domains configured, be sure to associate your users and aliases with the correct domain_id.
Configuring Postfix
Next, we are going to configure Postfix.
Configuration backup
First, let's backup the original configuration files :
cp /etc/postfix/main.cf /etc/postfix/main.cf.orig
cp /etc/postfix/master.cf /etc/postfix/master.cf.orig
User and group creation
We are now going to create a user and group called vmail that will be used by both Postfix and Dovecot :
groupadd -g 5000 vmail
useradd -g vmail -u 5000 vmail -d /var/mail -m -s /bin/false
SSL certificates
Next, we are going to create the folder where we will store the SSL certificates :
mkdir /etc/postfix/ssl
chown root: /etc/postfix/ssl && chmod 600 /etc/postfix/ssl
Purists will probably want to store their certificates in /etc/ssl/private. If you choose to do so, you'll have to adapt the path of those files for the remainder of this tutorial.
If you've decided to create a certificate with StartSSL, you'll end up with two files, a .crt and a .key. I'll name those files server.crt and server-with-passphrase.key. Put both these files in the folder we've just created.
Now, let's remove the passphrase from the key :
cd /etc/postfix/ssl
openssl rsa -in server-with-passphrase.key -out server.key
You'll be prompted for the passphrase you chose during the certificate generation.
Next, we have to download the appropriate intermediate certificate :
wget -O /etc/postfix/ssl/sub.class1.server.ca.pem \
http://www.startssl.com/certs/sub.class1.server.ca.pem
We now have to make sure that the permissions on those files are correct :
chown root: /etc/postfix/ssl/* && chmod 600 /etc/postfix/ssl/*
The last thing we have to do here is to generate Diffie-Hellman keys for Perfect Forward Secrecy (PFS) :
openssl gendh -out /etc/postfix/dh_512.pem -2 512
openssl gendh -out /etc/postfix/dh_1024.pem -2 1024
Postifx configuration
First, let's edit the /etc/postfix/main.cf file. It should end up looking something like that :
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no
broken_sasl_auth_clients = yes
config_directory = /etc/postfix
disable_vrfy_command = yes
smtpd_data_restrictions = reject_unauth_pipelining, permit
smtpd_helo_required = yes
queue_directory = /var/spool/postfix
append_dot_mydomain = no
readme_directory = no
smtpd_use_tls=yes
smtpd_tls_auth_only = yes
smtpd_tls_cert_file=/etc/postfix/ssl/server.crt
smtpd_tls_key_file=/etc/postfix/ssl/server.key
smtpd_tls_CAfile=/etc/postfix/ssl/sub.class1.server.ca.pem
smtpd_tls_mandatory_protocols=!SSLv2,!SSLv3
smtpd_tls_protocols=!SSLv2,!SSLv3
smtpd_tls_mandatory_ciphers=high
smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem
smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
smtpd_tls_eecdh_grade = strong
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
tls_preempt_cipherlist = yes
tls_random_source = dev:/dev/urandom
smtpd_data_restrictions = reject_unauth_pipelining, permit
smtpd_helo_required = yes
smtp_tls_CAfile = $smtpd_tls_CAfile
smtp_tls_mandatory_protocols=!SSLv2,!SSLv3
smtp_tls_protocols=!SSLv2,!SSLv3
smtp_tls_security_level = may
smtp_tls_loglevel = 1
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_milters =
non_smtpd_milters = $smtpd_milters
milter_protocol = 2
milter_default_action = accept
smtpd_recipient_restrictions =
reject_invalid_hostname,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_sender_domain,
reject_unknown_recipient_domain,
permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destination,
permit
smtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain = $myhostname
smtpd_sasl_security_options = noanonymous
smtpd_sasl_tls_security_options = $smtpd_sasl_security_options
smtpd_tls_auth_only = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
myhostname = myserver.captainark.net ### CHANGE THIS
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = localhost, myserver.captainark.net ### CHANGE THIS
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
default_transport = smtp
relay_transport = smtp
inet_interfaces = all
inet_protocols = all
virtual_transport = lmtp:unix:private/dovecot-lmtp
virtual_mailbox_domains = mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf
virtual_mailbox_maps = mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf
virtual_alias_maps = mysql:/etc/postfix/mysql-virtual-alias-maps.cf
The variable "myhostname" has to be defined to you server's FQDN. The file /etc/mailname should contain your server's FQDN as well.
Next, we need to edit the /etc/postfix/master.cf file. You need to uncomment the following lines :
submission inet n - - - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o tls_preempt_cipherlist=yes
-o smtpd_sasl_auth_enable=yes
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
smtps inet n - - - - smtpd
-o syslog_name=postfix/smtps
-o smtpd_tls_wrappermode=yes
-o smtpd_sasl_auth_enable=yes
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
You also have to add the following lines at the end of the file :
dovecot unix - n n - - pipe
flags=DRhu user=vmail:vmail argv=/usr/lib/dovecot/deliver -f ${sender} -d ${recipient}
MySQL access for Postfix
We now need to allow Postfix to connect to the MySQL database we have created earlier. To that end, we must create three files.
/etc/postfix/mysql-virtual-mailbox-domains.cf should contain the following lines :
user = mail
password = mailpassword
hosts = 127.0.0.1
dbname = mail
query = SELECT 1 FROM virtual_domains WHERE name='%s'
/etc/postfix/mysql-virtual-mailbox-maps.cf should contain the following lines :
user = mail
password = mailpassword
hosts = 127.0.0.1
dbname = mail
query = SELECT 1 FROM virtual_users WHERE email='%s'
/etc/postfix/mysql-virtual-alias-maps.cf should contain the following lines :
user = mail
password = mailpassword
hosts = 127.0.0.1
dbname = mail
query = SELECT destination FROM virtual_aliases WHERE source='%s'
Since these files contain a password, let's make sure they are not world-readable :
chown root: /etc/postfix/mysql* && chmod 600 /etc/postfix/mysql*
You can use the command postmap to confirm that everything is working properly :
postmap -q captainark.net mysql:/etc/postfix/mysql-virtual-mailbox-domains.cf
postmap -q example@captainark.net mysql:/etc/postfix/mysql-virtual-mailbox-maps.cf
postmap -q alias@captainark.net mysql:/etc/postfix/mysql-virtual-alias-maps.cf
Let's restart postfix for our modifications to be taken into account :
systemctl restart postfix
That's it for Postfix, for now ; Dovecot is next !
Configuring Dovecot
Dovecot global configuration
By default, on Debian, Dovecot uses multiple configuration files in /etc/dovecot/conf.d. I found it annoying to maintain, and I ended up only using the /etc/doveconf.conf file.
As always, let's start by backing up the original configuration file :
mv /etc/dovecot/dovecot.conf /etc/dovecot/dovecot.conf.orig
Next, we are going to create a new /etc/dovecot/dovecot.conf file. It should contain the following lines :
!include_try /usr/share/dovecot/protocols.d/*.protocol
protocols = imap lmtp sieve
mail_location = maildir:/var/mail/%d/%n
mail_privileged_group = vmail
mail_plugin_dir = /usr/lib/dovecot/modules
mail_plugins =
disable_plaintext_auth = yes
auth_mechanisms = plain login
service director {
unix_listener login/director {
}
fifo_listener login/proxy-notify {
}
unix_listener director-userdb {
}
inet_listener {
}
}
namespace inbox {
inbox = yes
type = private
mailbox Drafts {
auto = subscribe
special_use = \Drafts
}
mailbox Junk {
auto = subscribe
special_use = \Junk
}
mailbox Sent {
auto = subscribe
special_use = \Sent
}
mailbox Trash {
auto = subscribe
special_use = \Trash
}
}
service imap-login {
inet_listener imap {
port = 0
}
inet_listener imaps {
port = 993
ssl = yes
}
}
service pop3-login {
inet_listener pop3 {
port = 0
}
inet_listener pop3s {
port = 0
}
}
service lmtp {
unix_listener /var/spool/postfix/private/dovecot-lmtp {
mode = 0600
user = postfix
group = postfix
}
}
service imap {
}
service pop3 {
}
service auth {
unix_listener /var/spool/postfix/private/auth {
mode = 0666
user = postfix
group = postfix
}
unix_listener auth-userdb {
mode = 0600
user = vmail
}
user = dovecot
}
service auth-worker {
user = vmail
}
service dict {
unix_listener dict {
}
}
ssl = required
ssl_cert = </etc/postfix/ssl/server.crt
ssl_key = </etc/postfix/ssl/server.key
ssl_ca = </etc/postfix/ssl/sub.class1.server.ca.pem
ssl_protocols = !SSLv2 !SSLv3
ssl_cipher_list = AES128+EECDH:AES128+EDH:!aNULL;
protocol lda {
mail_plugins = $mail_plugins sieve
}
protocol imap {
mail_plugins = $mail_plugins
}
protocol lmtp {
mail_plugins = $mail_plugins sieve
}
plugin {
sieve = /var/mail/sieve/users/%u.sieve
sieve_after = /var/mail/sieve/after
sieve_before = /var/mail/sieve/before
sieve_global_dir = /var/lib/dovecot/sieve/
sieve_dir = ~/sieve
}
passdb {
driver = sql
args = /etc/dovecot/sql.conf
}
userdb {
driver = static
args = uid=vmail gid=vmail home=/var/mail/%d/%n
}
Dovecot will use the same SSL certificate as Postfix.
Using this configuration, your virtual users' emails will be stored in /var/mail/$domain/$user/ and will be owned by the vmail user.
For this to work, we have to create the domain folder :
mkdir -p /var/mail/captainark.net
chown vmail: /var/mail/captainark.net && chmod 770 /var/mail/captainark.net
Dovecot will create the virtual users' folders automatically.
Dovecot access to the MySQL database
We now need to allow Dovecot to connect to the mail database we have populated earlier. To do so, we are going to create a /etc/dovecot/sql.conf file with the following content :
driver = mysql
connect = host=localhost dbname=mail user=mail password=mailpassword
default_pass_scheme = SHA512-CRYPT
password_query = SELECT email as user, password FROM virtual_users WHERE email='%u';
You'll have to change the password to the one you have defined earlier. Since this file contains a password, let's make sure it's not world-readable :
chown root: /etc/dovecot/sql.conf && chmod 600 /etc/dovecot/sql.conf
Configuring Sieve
The last thing we need to configure here is sieve. The idea is to have all messages flagged as spam automatically moved to the mailbox Junk folder.
To do so, let's first create the required folders :
mkdir -p /var/mail/sieve/before
mkdir /var/mail/sieve/after
mkdir /var/mail/sieve/users
chown -R vmail: /var/mail/sieve && chmod -R 770 /var/mail/sieve
If you want to have sieve rules for a specific user, simply create $user@$domain.sieve file in the users folder (example@captainark.net in my case).
All .sieve files in the before folder will be used for all your virtual users, before their individual configuration ; the .sieve files in the after folder will be used, well, you guessed it, after.
Let's create a filter.sieve file in the /var/mail/sieve/before folder with the following content :
require ["envelope", "fileinto", "imap4flags", "regex"];
if not header :regex "message-id" ".*@.*\." {
fileinto "Junk";
}
if header :contains "X-Spam-Level" "*****" {
fileinto "Junk";
}
Last thing we have to do is to change the permissions on the newly created file :
chown vmail: /var/mail/sieve/before/filter.sieve && \
chmod 660 /var/mail/sieve/before/filter.sieve
That's all ; now, all email we receive that is flagged as spam by SpamAssassin will be moved to the Junk folder.
Let's restart dovecot :
systemctl restart dovecot
We now have a working mail server !
To connect to it and access your mailbox, configure your email client as follow :
- Username: example@captainark.net ;
- Password: the password you chose for your virtual user ;
- IMAP: your server's FQDN, port 993 (SSL/TLS with normal password) ;
- SMTP: your server's FQDN, port 465 (SSL/TLS with normal password).
Configuring SpamAssassin
The alternatives
Next thing we have to do is to configure the actual anti-spam. I tried a few, but I ended up sticking with SpamAssassin. Here's why :
- DSPAM is no longer maintained and has been removed from Debian Jessie ;
- Rspamd is interesting, has been integrated in Debian Jessie, but is poorly documented at this time ;
- Bogofilter does not seem to have the greatest server integration.
The actual configuration
SpamAssassin's configuration is pretty straightforward. First, let's edit the /etc/default/spamassassin file :
ENABLED=1
[...]
CRON=1
Before the cron runs for the first time, we have to manually update SpamAssassin's ruleset :
sa-learn
Next, as usual, let's back up the original configuration file :
mv /etc/spamassassin/local.cf /etc/spamassassin/local.cf.orig
Let's create a new /etc/spamassassin/local.cf file with the following content :
rewrite_header Subject [SPAM]
report_safe 0
required_score 5.0
use_bayes 1
bayes_auto_learn 1
whitelist_from *@captainark.net
Next, to have Postfix send incoming emails through SpamAssassin, we have to edit the /etc/postfix/master.cf file. At the very beginning, we have to add a line under the smtp definition :
smtp inet n - - - - smtpd
-o content_filter=spamassassin
At the very end of the same file, we have to add the following lines :
spamassassin unix - n n - - pipe
user=debian-spamd argv=/usr/bin/spamc -f -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}
Let's restart SpamAssassin and Postfix :
systemctl restart postfix
systemctl restart spamassassin
That's all for SpamAssassin ! To check if it is working, send yourself an email from another provider. You should see the following headers in it :
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
myserver.captainark.net
X-Spam-Level:
Configuring SPF
Allowing your server to send emails for your domain
SPF (Sender Policy Framework) is a mechanism that confirms that your server's IP is allowed to send emails for your domain. Technically, it is a TXT DNS record which looks something like this :
captainark.net IN TXT "v=spf1 mx ~all"
This DNS record lets other mail servers know that hosts that have a MX record for my domain are also allowed to send emails for it.
For more information on SPF syntax, you can consult the official documentation.
Without a properly configured SPF record, other mail servers might flag your emails as spam or outright drop them.
Checking SPF record for inbound mail
Now that we have set up our own SPF record, let's configure Postfix to check that other mail servers communicating with us have done the same.
First, let's add the two following lines at the end of /etc/postfix-policyd-spf-python/policyd-spf.conf :
Header_Type = AR
Authserv_Id = "<server's FQDN>"
Then, let's edit the /etc/postfix/master.cf file and add the following lines at the end :
policy-spf unix - n n - - spawn
user=nobody argv=/usr/bin/policyd-spf
Let's now edit the /etc/postfix/main.cf. In the "smtpd_recipient_restrictions" section, add the "check_policy_service" line as seen below :
smtpd_recipient_restrictions =
[...]
reject_unauth_destination,
check_policy_service unix:private/policy-spf,
permit
We now have to restart postfix :
systemctl restart postfix
Our server is now checking other mail server's SPF records.
To make sure that it is working, send yourself an email from another provider. You should see the following header in it :
Authentication-Results: myserver.captainark.net; spf=pass (sender SPF authorized)
[...] receiver=example@captainark.net)
Configuring OpenDKIM
DKIM (DomainKeys Identified Mail) is a mechanism that validates a domain name identity for an email through cryptographic authentication.
While not mandatory, setting up DKIM improves the odds of emails sent from your server not being flagged as spam by other providers.
With this configuration, OpenDKIM will also check the key for inbound emails.
Software side
First, let's backup the original configuration file and create a folder for the configuration files :
mv /etc/opendkim.conf /etc/opendkim.conf.orig
mkdir /etc/opendkim.d
We now have to create a /etc/opendkim.conf file with the following content :
AutoRestart Yes
AutoRestartRate 10/1h
UMask 002
Syslog yes
SyslogSuccess Yes
LogWhy Yes
OversignHeaders From
AlwaysAddARHeader yes
Canonicalization relaxed/simple
ExternalIgnoreList refile:/etc/opendkim.d/TrustedHosts
InternalHosts refile:/etc/opendkim.d/dkim/TrustedHosts
KeyTable refile:/etc/opendkim.d/dkim/KeyTable
SigningTable refile:/etc/opendkim.d/dkim/SigningTable
Mode sv
PidFile /run/opendkim/opendkim.pid
SignatureAlgorithm rsa-sha256
UserID opendkim:opendkim
Socket local:/var/spool/postfix/opendkim/opendkim.sock
Let's then create the necessary folders :
mkdir -p /etc/opendkim.d/keys/captainark.net/
Now, we are going to create the /etc/opendkim.d/TrustedHosts file with the following content :
localhost
127.0.0.1
::1
captainark.net
This file contains the hosts and domains for which OpenDKIM should sign emails.
Next, let's create the /etc/opendkim.d/KeyTable :
mail._domainkey.captainark.net captainark.net:mail:/etc/opendkim.d/keys/captainark.net/mail.private
This file tells OpenDKIM which key it should use for each selector.
Finally, let's create the /etc/opendkim.d/SigningTable file :
*@captainark.net mail._domainkey.captainark.net
This file tells OpenDKIM which selector it should use for each domain.
We now have to generate the private/public key pair for our domain :
cd /etc/opendkim.d/keys/captainark.net/
opendkim-genkey -s mail -d captainark.net
This creates two files ; mail.private contains our private key, mail.txt contains our public key.
Let's change the permissions on those files :
chown -R opendkim: /etc/opendkim.d/keys
chmod -R 700 /etc/opendkim.d/keys
chmod 600 /etc/opendkim.d/captainark.net/*
Postfix integration
The last thing we have to do is to configure Postfix to communicate with OpenDKIM.
First, let's create the necessary folders :
mkdir /var/spool/postfix/opendkim
chown opendkim: /var/spool/postfix/opendkim
We also have to add the postfix user to the opendkim group :
useradd -G opendkim postfix
Now, let's edit the /etc/postfix/master.cf file, like so :
smtpd_milters = unix:/opendkim/opendkim.sock
We now have to restart OpenDKIM and Postfix :
systemctl restart opendkim
systemctl restart postfix
DNS side
For DKIM to work, you have to configure a DNS TXT record in your zone. This record was automatically generated by OpenDKIM in the mail.txt file mentioned earlier :
mail._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCkJq0CW3tl2XHZ1CN5XdbqRDU7KfXOJ70nlwI09bHmDU63/Yz3J5rl863S0t2ncVHfIudZANj0OaiJe5HRR7WCsjuNIhQFfPFGIWLNClpxqdQVQURI38sAGeyn7Ed/Cor1AiWABzFWzel0kvXILw8K/NTzxaAPeSa9ttwQEgSmowIDAQAB" ; ----- DKIM key mail for captainark.net
All you have to do is to copy and paste this record in your DNS zone file.
To make sure that OpenDKIM is working, you can send an empty email to check-auth@verifier.port25.com. You should receive a response with the following content :
==========================================================
Summary of Results
==========================================================
SPF check: pass
DomainKeys check: neutral
DKIM check: pass
Sender-ID check: pass
SpamAssassin check: ham
Configuring OpenDMARC
DMARC (Domain-based Message Authentication, Reporting & Conformance) standardizes SPF and DKIM authentication mechanisms.
It lets the owner of a domain name indicate that his email is protected by SPF and/or DKIM and what other providers should do with emails that do not pass those checks.
Software side
Once again, let's backup the original configuration file :
mv /etc/opendmarc.conf /etc/opendmarc.conf.orig
We now have to create a /etc/opendmarc.conf file with the following content :
AutoRestart Yes
AutoRestartRate 10/1h
UMask 0002
Syslog true
AuthservID "<your server's FQDN>"
TrustedAuthservIDs "<your server's FQDN>"
IgnoreHosts /etc/opendkim.d/TrustedHosts
RejectFailures false
UserID opendmarc:opendmarc
PidFile /run/opendmarc.pid
Socket local:/var/spool/postfix/opendmarc/opendmarc.sock
Postfix integration
The last thing we have to do is to configure Postfix to communicate with OpenDMARC.
First, let's create the necessary folders :
mkdir /var/spool/postfix/opendmarc
chown opendmarc: /var/spool/postfix/opendmarc
We also have to add the postfix user to the opendmarc group :
useradd -G opendmarc postfix
Now, let's edit the /etc/postfix/master.cf file, like so :
smtpd_milters = unix:/opendkim/opendkim.sock, unix:/opendmarc/opendmarc.sock
We now have to restart OpenDMARC and Postfix :
systemctl restart opendmarc
systemctl restart postfix
You should now see the following headers in your incoming emails :
Authentication-Results: myserver.captainark.net; dmarc=pass header.from=gmail.com
DNS side
DMARC, like SPF and DKIM, is based on DNS TXT records.
Here is how I configured it for the captainark.net domain :
_dmarc IN TXT "v=DMARC1; p=none; rua=mailto:postmaster@captainark.net; ruf=mailto:postmaster@captainark.net"
This tells other providers to not reject or quarantine emails should a SPF or DKIM check fail, but to send a daily report of those checks to postmaster@captainark.net.
For more information on the DMARC syntax, here is an article from Google.
Configuring Monit
Monit is a daemon that makes sure that other daemons are running. If they crash, it restarts them automatically. Is is not directly related to a mail server per say, but it's pretty easy to set up.
First, as always, let's backup the original configuration file :
mv /etc/monit/monitrc /etc/monit/monitrc.orig
We now have to create a new /etc/monit/monitrc file with the following content :
set daemon 30
set logfile syslog facility log_daemon
set httpd port 2812 and
use address localhost
allow localhost
set mailserver localhost
with timeout 30 seconds
using hostname myserver.captainark.net
set mail-format { from: monit@captainark.net }
include /etc/monit/conf.d/*
Then, we are going to create a /etc/monit/conf.d/mail file with the following content :
check process postfix
with pidfile "/var/spool/postfix/pid/master.pid"
start program = "/bin/systemctl start postfix"
stop program = "/bin/systemctl stop postfix"
alert monit@captainark.net
group mail
check process dovecot
with pidfile "/run/dovecot/master.pid"
start program = "/bin/systemctl start dovecot"
stop program = "/bin/systemctl stop dovecot"
alert monit@captainark.net
group mail
depends on postfix
check process spamassassin
with pidfile "/run/spamassassin.pid"
start program = "/bin/systemctl start spamassassin"
stop program = "/bin/systemctl stop spamassassin"
alert monit@captainark.net
group mail
depends on postfix, dovecot
check process opendkim
with pidfile "/run/opendkim/opendkim.pid"
start program = "/bin/systemctl start opendkim"
stop program = "/bin/systemctl stop opendkim"
alert monit@captainark.net
group mail
depends on postfix, dovecot
check process opendmarc
with pidfile "/run/opendmarc/opendmarc.pid"
start program = "/bin/systemctl start opendmarc"
stop program = "/bin/systemctl stop opendmarc"
alert monit@captainark.net
group mail
depends on postfix, dovecot
Let's make sure that permissions on the file are correct :
chown root: /etc/monit/conf.d/mail && chmod 600 /etc/monit/conf.d/mail
Then, we have to reload the monit daemon :
monit reload
Now, the monit summary command should have the following output :
The Monit daemon 5.4 uptime: 3d 0h 41m
Process 'postfix' Running
Process 'dovecot' Running
Process 'spamassassin' Running
Process 'opendkim' Running
Process 'opendmarc' Running
Configuring Rainloop
Rainloop is a web-based email client. I won't go into details on how to configure it in this tutorial ; here's a link to the official documentation.
You'll need a web server with PHP 5.3+ to run Rainloop. You do not have to run Rainloop on the same host as your mail server. No database is required.
Conclusion
We now have a mail server that should be running pretty smoothly. It could still be improved by setting up things such as greylisting or virus detection.
If you have found this tutorial useful, if you've found an error in it or if you have any question, please feel free to leave a comment below or to contact me on Twitter.
References
Here are the tutorials I used to set up my own mail server :