Set up 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. I haven’t had time to do the script so nevermind this, if I ever do it I’ll make a new entry regarding it.
This configuration works for local users (users that appear in /etc/passwd
), and does not use any type of SQL database. Do note that I’m not running Postfix in a chroot, which can be a problem if you’re following my steps as noted by Bojan; in the case that you want to run in chroot then add the steps chown in the Arch wiki: Postfix in a chroot jail; the issue faced if following my steps and using a chroot is that there will be issues resolving the hostname due to /etc/hosts
or /etc/hostname
not being available in the chroot.
All commands executed here are run with root privileges, unless stated otherwise.
Table of contents¶
Prerequisites¶
Basically the same as with the website with Nginx and Certbot, with the extras:
- You will need a CNAME for “mail” and (optionally) “www.mail”, or whatever you want to call the sub-domains (although the RFC 2181 states that it NEEDS to be an A record, fuck the police).
- An SSL certificate. You can use the SSL certificate obtained following my last post using
certbot
(just create amail.conf
and runcertbot --nginx
again). - Ports
25
,587
(SMTP),465
(SMTPS),143
(IMAP) and993
(IMAPS) open on the firewall (I useufw
).
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, for example it will end up being/home/david/Mail/Inbox
:
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. 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 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
OpenDKIM DNS TXT records¶
Add the following TXT records on your domain registrar (these examples are for Epik):
- 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).
-
DMARC entry: just
_dmarc.{yourdomain}
as the “Host” and"v=DMARC1; p=reject; rua=mailto:dmarc@{yourdomain}; fo=1"
as the “TXT Value”. -
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 because all the settings and steps detailed here just worked; I literally just finished doing everything on a new server as of the writing of this text, 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, 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):
- * server: subdomain.domain (mail.luevano.xyz in my case)
- SMTP port: 587
- SMTPS port: 465 (I use this one)
- IMAP port: 143
- IMAPS port: 993 (again, I use this one)
- Connection/security: SSL/TLS
- Authentication method: Normal password
- Username: just your
user
, not the whole email (david
in my case) - Password: your
user
password (as in the password you use to login to the server with that user)
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:
By David Luévano
Created: Sun, Mar 21, 2021 @ 04:05 UTC
Modified: Mon, Jun 19, 2023 @ 05:30 UTC