Create a mail server with Postfix, Dovecot, SpamAssassin and OpenDKIM

The entry is going to be long because it’s a tedious process. This is also based on Luke Smith’s script, but adapted to Arch Linux (his script works on debian-based distributions). This entry is mostly so I can record all the notes required while I’m in the process of installing/configuring the mail server on a new VPS of mine; also I’m going to be writing a script that does everything in one go (for Arch Linux), that will be hosted here.

This configuration works for local users (users that appear in /etc/passwd), and does not use any type of SQL Database. And note that most if not all commands executed here are run with root privileges.

Prerequisites

Basically the same as with the website with Nginx and Certbot, with the extras:

Postfix

Postfix is a “mail transfer agent” which is the component of the mail server that receives and sends emails via SMTP.

Install the postfix package:

pacman -S postfix

We have two main files to configure (inside /etc/postfix): master.cf (master(5)) and main.cf (postconf(5)). We’re going to edit main.cf first either by using the command postconf -e 'setting' or by editing the file itself (I prefer to edit the file).

Note that the default file itself has a lot of comments with description on what each thing does (or you can look up the manual, linked above), I used what Luke’s script did plus some other settings that worked for me.

Now, first locate where your website cert is, mine is at the default location /etc/letsencrypt/live/, so my certdir is /etc/letsencrypt/live/luevano.xyz. Given this information, change {yourcertdir} on the corresponding lines. The configuration described below has to be appended in the main.cf configuration file.

Certificates and ciphers to use for authentication and security:

smtpd_tls_key_file = {yourcertdir}/privkey.pem
smtpd_tls_cert_file = {yourcertdir}/fullchain.pem
smtpd_use_tls = yes
smtpd_tls_auth_only = yes
smtp_tls_security_level = may
smtp_tls_loglevel = 1
smtp_tls_CAfile = {yourcertdir}/cert.pem
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
tls_preempt_cipherlist = yes
smtpd_tls_exclude_ciphers = aNULL, LOW, EXP, MEDIUM, ADH, AECDH, MD5,
                DSS, ECDSA, CAMELLIA128, 3DES, CAMELLIA256,
                RSA+AES, eNULL

smtp_tls_CApath = /etc/ssl/certs
smtpd_tls_CApath = /etc/ssl/certs

smtpd_relay_restrictions = permit_sasl_authenticated, permit_mynetworks, defer_unauth_destination

Also, for the connection with dovecot, append the next few lines (telling postfix that dovecot will use user/password for authentication):

smtpd_sasl_auth_enable = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_tls_security_options = noanonymous

Specify the mailbox home (this is going to be a directory inside your user’s home containing the actual mail files):

home_mailbox = Mail/Inbox/

Pre-configuration to work seamlessly with dovecot and opendkim:

myhostname = {yourdomainname}
mydomain = localdomain
mydestination = $myhostname, localhost.$mydomain, localhost

milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:127.0.0.1:8891
non_smtpd_milters = inet:127.0.0.1:8891
mailbox_command = /usr/lib/dovecot/deliver

Where {yourdomainname} is luevano.xyz in my case, or if you have localhost configured to your domain, then use localhost for myhostname (myhostname = localhost).

Lastly, if you don’t want the sender’s IP and user agent (application used to send the mail), add the following line:

smtp_header_checks = regexp:/etc/postfix/smtp_header_checks

And create the /etc/postfix/smtp_header_checks file with the following content:

/^Received: .*/     IGNORE
/^User-Agent: .*/   IGNORE

That’s it for main.cf, now we have to configure master.cf. This one is a bit more tricky.

First look up lines (they’re uncommented) smtp inet n - n - - smtpd, smtp unix - - n - - smtp and -o syslog_name=postfix/$service_name and either delete or uncomment them… or just run sed -i "/^\s*-o/d;/^\s*submission/d;/\s*smtp/d" /etc/postfix/master.cf as stated in Luke’s script.

Lastly, append the following lines to complete postfix setup and pre-configure for spamassassin.

smtp unix - - n - - smtp
smtp inet n - y - - smtpd
    -o content_filter=spamassassin
submission inet n - y - - smtpd
    -o syslog_name=postfix/submission
    -o smtpd_tls_security_level=encrypt
    -o smtpd_sasl_auth_enable=yes
    -o smtpd_tls_auth_only=yes
smtps inet n - y - - smtpd
    -o syslog_name=postfix/smtps
    -o smtpd_tls_wrappermode=yes
    -o smtpd_sasl_auth_enable=yes
spamassassin unix - n n - - pipe
    user=spamd argv=/usr/bin/vendor_perl/spamc -f -e /usr/sbin/sendmail -oi -f \${sender} \${recipient}

Now, I ran into some problems with postfix, one being smtps: Servname not supported for ai_socktype, to fix it, as Till posted in that site, edit /etc/services and add:

smtps 465/tcp
smtps 465/udp

Before starting the postfix service, you need to run newaliases first, but you can do a bit of configuration beforehand editing the file /etc/postfix/aliases. I only change the root: you line (where you is the account that will be receiving “root” mail). After you’re done, run:

postalias /etc/postfix/aliases
newaliases

At this point you’re done configuring postfix and you can already start/enable the postfix service:

systemctl start postfix.service
systemctl enable postfix.service

Dovecot

Dovecot is an IMAP and POP3 server, which is what lets an email application retrieve the mail.

Install the dovecot and pigeonhole (sieve for dovecot) packages:

pacman -S dovecot pigeonhole

On arch, by default, there is no /etc/dovecot directory with default configurations set in place, but the package does provide the example configuration files. Create the dovecot directory under /etc and, optionally, copy the dovecot.conf file and conf.d directory under the just created dovecot directory:

mkdir /etc/dovecot
cp /usr/share/doc/dovecot/example-config/dovecot.conf /etc/dovecot/dovecot.conf
cp -r /usr/share/doc/dovecot/example-config/conf.d /etc/dovecot

As Luke stated, dovecot comes with a lot of “modules” (under /etc/dovecot/conf.d/ if you copied that folder) for all sorts of configurations that you can include, but I do as he does and just edit/create the whole dovecot.conf file; although, I would like to check each of the separate configuration files dovecot provides I think the options Luke provides are more than good enough.

I’m working with an empty dovecot.conf file. Add the following lines for SSL and login configuration (also replace {yourcertdir} with the same certificate directory described in the Postfix section above, note that the < is required):

ssl = required
ssl_cert = <{yourcertdir}/fullchain.pem
ssl_key = <{yourcertdir}/privkey.pem
ssl_min_protocol = TLSv1.2
ssl_cipher_list = ALL:!RSA:!CAMELLIA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SHA1:!SHA256:!SHA384:!LOW@STRENGTH
ssl_prefer_server_ciphers = yes
ssl_dh = </etc/dovecot/dh.pem

auth_mechanisms = plain login
auth_username_format = %n
protocols = $protocols imap

You may notice we specify a file we don’t have under /etc/dovecot: dh.pem. We need to create it with openssl (you should already have it installed if you’ve been following this entry and the one for nginx). Just run (might take a few minutes):

openssl dhparam -out /etc/dovecot/dh.pem 4096

After that, the next lines define what a “valid user is” (really just sets the database for users and passwords to be the local users with their password):

userdb {
    driver = passwd
}

passdb {
    driver = pam
}

Next, comes the mail directory structure (has to match the one described in the Postfix section). Here, the LAYOUT option is important so the boxes are .Sent instead of Sent. Add the next lines (plus any you like):

mail_location = maildir:~/Mail:INBOX=~/Mail/Inbox:LAYOUT=fs
namespace inbox {
    inbox = yes

    mailbox Drafts {
        special_use = \Drafts
        auto = subscribe
        }

    mailbox Junk {
        special_use = \Junk
        auto = subscribe
        autoexpunge = 30d
        }

    mailbox Sent {
        special_use = \Sent
        auto = subscribe
        }

    mailbox Trash {
        special_use = \Trash
        }

    mailbox Archive {
        special_use = \Archive
        }
}

Also include this so Postfix can use Dovecot’s authentication system:

service auth {
    unix_listener /var/spool/postfix/private/auth {
        mode = 0660
        user = postfix
        group = postfix
        }
}

Lastly (for Dovecot at least), the plugin configuration for sieve (pigeonhole):

protocol lda {
    mail_plugins = $mail_plugins sieve
}

protocol lmtp {
    mail_plugins = $mail_plugins sieve
}

plugin {
    sieve = ~/.dovecot.sieve
    sieve_default = /var/lib/dovecot/sieve/default.sieve
    sieve_dir = ~/.sieve
    sieve_global_dir = /var/lib/dovecot/sieve/

Where /var/lib/dovecot/sieve/default.sieve doesn’t exist yet. Create the folders:

mkdir -p /var/lib/dovecot/sieve

And create the file default.sieve inside that just created folder with the content:

require ["fileinto", "mailbox"];
if header :contains "X-Spam-Flag" "YES" {
    fileinto "Junk";
}

Now, if you don’t have a vmail (virtual mail) user, create one and change the ownership of the /var/lib/dovecot directory to this user:

grep -q "^vmail:" /etc/passwd || useradd -m vmail -s /usr/bin/nologin
chown -R vmail:vmail /var/lib/dovecot

Note that I also changed the shell for vmail to be /usr/bin/nologin. After that, to compile the configuration file run:

sievec /var/lib/dovecot/sieve/default.sieve

A default.svbin file will be created next to default.sieve.

Next, add the following lines to /etc/pam.d/dovecot if not already present (shouldn’t be there if you’ve been following these notes):

auth required pam_unix.so nullok
account required pam_unix.so

That’s it for Dovecot, at this point you can start/enable the dovecot service:

systemctl start dovecot.service
systemctl enable dovecot.service

OpenDKIM

OpenDKIM is needed so services like G**gle (we don’t mention that name here [[[this is a meme]]]) don’t throw the mail to the trash. DKIM stands for “DomainKeys Identified Mail”.

Install the opendkim package:

pacman -S opendkim

Generate the keys for your domain:

opendkim-genkey -D /etc/opendkim -d {yourdomain} -s {yoursubdomain} -r -b 2048

Where you need to change {yourdomain} and {yoursubdomain} (doesn’t really need to be the sub-domain, could be anything that describes your key) accordingly, for me it’s luevano.xyz and mail, respectively. After that, we need to create some files inside the /etc/opendkim directory. First, create the file KeyTable with the content:

{yoursubdomain}._domainkey.{yourdomain} {yourdomain}:{yoursubdomain}:/etc/opendkim/{yoursubdomain}.private

So, for me it would be:

mail._domainkey.luevano.xyz luevano.xyz:mail:/etc/opendkim/mail.private

Next, create the file SigningTable with the content:

*@{yourdomain} {yoursubdomain}._domainkey.{yourdomain}

Again, for me it would be:

*@luevano.xyz mail._domainkey.luevano.xyz

And, lastly create the file TrustedHosts with the content:

127.0.0.1
::1
10.1.0.0/16
1.2.3.4/24
localhost
{yourserverip}
...

And more, make sure to include your server IP and something like subdomain.domainname.

Next, edit /etc/opendkim/opendkim.conf to reflect the changes (or rather, addition) of these files, as well as some other configuration. You can look up the example configuration file located at /usr/share/doc/opendkim/opendkim.conf.sample, but I’m creating a blank one with the contents:

Domain {yourdomain}
Selector {yoursubdomain}

Syslog Yes
UserID opendkim

KeyFile /etc/opendkim/{yoursubdomain}.private
Socket inet:8891@localhost

Now, change the permissions for all the files inside /etc/opendkim:

chown -R root:opendkim /etc/opendkim
chmod g+r /etc/postfix/dkim/*

I’m using root:opendkim so opendkim doesn’t complain about the {yoursubdomani}.private being insecure (you can change that by using the option RequireSafeKeys False in the opendkim.conf file, as stated here).

That’s it for the general configuration, but you could go more in depth and be more secure with some extra configuration.

Now, just start/enable the opendkim service:

systemctl start opendkim.service
systemctl enable opendkim.service

And don’t forget to add the following TXT records on your domain registrar (these examples are for Epik):

  1. DKIM entry: look up your {yoursubdomain}.txt file, it should look something like:
{yoursubdomain}._domainkey IN TXT ( "v=DKIM1; k=rsa; s=email; "
    "p=..."
    "..." )  ; ----- DKIM key mail for {yourdomain}

In the TXT record you will place {yoursubdomain}._domainkey as the “Host” and "v=DKIM1; k=rsa; s=email; " "p=..." "..." in the “TXT Value” (replace the dots with the actual value you see in your file).

  1. DMARC entry: just _dmarc.{yourdomain} as the “Host” and "v=DMARC1; p=reject; rua=mailto:dmarc@{yourdomain}; fo=1" as the “TXT Value”.

  2. SPF entry: just @ as the “Host” and "v=spf1 mx a:{yoursubdomain}.{yourdomain} - all" as the “TXT Value”.

And at this point you could test your mail for spoofing and more.

SpamAssassin

SpamAssassin is just a mail filter to identify spam.

Install the spamassassin package (which will install a bunch of ugly perl packages…):

pacman -S spamassassin

For some reason, the permissions on all spamassassin stuff are all over the place. First, change owner of the executables, and directories:

chown spamd:spamd /usr/bin/vendor_perl/sa-*
chown spamd:spamd /usr/bin/vendor_perl/spam*
chwown -R spamd:spamd /etc/mail/spamassassin

Then, you can edit local.cf (located in /etc/mail/spamassassin) to fit your needs (I only uncommented the rewrite_header Subject ... line). And then you can run the following command to update the patterns and compile them:

sudo -u spamd sa-update
sudo -u spamd sa-compile

And since this should be run periodically, create the service spamassassin-update.service under /etc/systemd/system with the following content:

[Unit]
Description=SpamAssassin housekeeping
After=network.target

[Service]
User=spamd
Group=spamd
Type=oneshot

ExecStart=/usr/bin/vendor_perl/sa-update --allowplugins
SuccessExitStatus=1
ExecStart=/usr/bin/vendor_perl/sa-compile
ExecStart=/usr/bin/systemctl -q --no-block try-restart spamassassin.service

And you could also execute sa-learn to train spamassassin‘s bayes filter, but this works for me. Then create the timer spamassassin-update.timer under the same directory, with the content:

[Unit]
Description=SpamAssassin housekeeping

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

You can now start/enable the spamassassin-update timer:

systemctl start spamassassin-update.timer
systemctl enable spamassassin-update.timer

Next, you may want to edit the spamassassin service before starting and enabling it, because by default, it could spawn a lot of “childs” eating a lot of resources and you really only need one child. Append --max-children=1 to the line ExecStart=... in /usr/bin/systemd/system/spamassassin.service:

...
ExecStart=/usr/bin/vendor_perl/spamd -x -u spamd -g spamd --listen=/run/spamd/spamd.sock --listen=localhost --max-children=1
...

Finally, start and enable the spamassassin service:

systemctl start spamassassin.service
systemctl enable spamassassin.service

Wrapping up

We should have a working mail server by now. Before continuing check your journal logs (journalctl -xe --unit={unit}, where {unit} could be spamassassin.service for example) to see if there was any error whatsoever and try to debug it, it should be a typo somewhere (the logs are generally really descriptive) because all the settings and steps detailed here just (literally just finished doing everything on a new server as of the writing of this text) worked (((it just werks on my machine))).

Now, to actually use the mail service: first of all, you need a normal account (don’t use root) that belongs to the mail group (gpasswd -a user group to add a user user to group group) and that has a password.

Next, to actually login into a mail app/program/whateveryouwanttocallit, you will use the following settings, at least for thunderdbird(I tested in windows default mail app and you don’t need a lot of settings):

All that’s left to do is test your mail server for spoofing, and to see if everything is setup correctly. Go to DKIM Test and follow the instructions (basically click next, and send an email with whatever content to the email that they provide). After you send the email, you should see something like:

DKIM Test successful
DKIM Test successful

Finally, that’s actually it for this entry, if you have any problem whatsoever you can contact me.

By David Luévano

Created: Sun, Mar 21, 2021 @ 04:05 UTC

Modified: Tue, Jun 08, 2021 @ 06:27 UTC

Tags: english, server, tools, tutorial