Email server using Postfix, OpenDKIM, and Dovecot

This article describe my current email server configuration using Postfix, OpenDKIM, and Dovecot.

For email server using OpenSMTPD, see the next article.

We will use domain name "kilabit.info" as an example in the script or configuration later. Make sure to replace them accordingly, if you copy-paste part of the script or commands from this article.

In this article, we use awwan script for configuration management, so if you see string like these {{.Val "section:sub:key"}} that means its an awwan variable and needs to be replaced according to your environment.

Email architecture

In email server, there are two incoming message flows. One from client, where they want to send message to other domain; and one from other domain (Mail Delivery Agent—MDA) where others want to send message to our domain.

    IMAP
    :993      +---------+
   +----------| Dovecot |
   |          +---------+
   |   SMTPS       ^
   |   :465        |
   v   :587   +---------+ :25
Client ------>| Postfix |-----> MDA
              +---------+
                   ^
                   |
                   v
              +----------+
              | OpenDKIM |
              +----------+

Client send message (submission) using SMTP to Postfix over secure line with TLS or STARTTLS, on port 465 or 587 with authentication. When Postfix receive the message it will forward it to OpenDKIM to sign the message. Once the message is signed, Postfix then forward a copy of it to Dovecot (so the message appear in Client "Sent" box) and transfer it to recipient in other server on port 25.

      SMTP
      :25  +---------+     +---------+
MDA <----->| Postfix |---->| Dovecot |
           +---------+     +---------+
               ^
               | :8891
               v
           +----------+
           | OpenDKIM |
           +----------+

The MDA send their message to our email server Postfix on port 25. Postfix validate the signature of message by forwarding it to OpenDKIM and the result is forwarded to Dovecot to be stored and later consumed by client from INBOX. This is the endpoint where spam can abuse our server. If we did not setup and secure it properly, email that is not from real domain may got into Client inbox or relay-ed to other server.

Setting up OpenDKIM

Is simple term, DomainKeys Identified Mail (DKIM) is a mechanism where email is signed using private key to prevent tampering during transit. The public key part is published inside DNS TXT record so others can verified the signature of message.

The following awwan script show how to setup OpenDKIM in Arch Linux (the line number is added for commentary),

 0: sudo pacman -Syu --noconfirm
 0: sudo pacman -S --noconfirm opendkim

 0: sudo systemctl enable opendkim.service

 1: sudo mkdir -p /etc/opendkim/keys/kilabit.info
 2: opendkim-genkey -r -s 20210411-1 -d kilabit.info
 3: sudo mv 20210411-1.* /etc/opendkim/keys/kilabit.info/
 4: sudo chmod 0600 /etc/opendkim/keys/kilabit.info/20210411-1.private

 5: sudo mkdir /var/lib/opendkim
 6: sudo chown opendkim:opendkim /var/lib/opendkim

 7: #put! {{.ScriptDir}}/etc/opendkim/KeyTable      /etc/opendkim/
 8: #put! {{.ScriptDir}}/etc/opendkim/SigningTable  /etc/opendkim/
 9: #put! {{.ScriptDir}}/etc/opendkim/TrustedHosts  /etc/opendkim/
10: #put! {{.ScriptDir}}/etc/opendkim/opendkim.conf /etc/opendkim/

11: sudo chown -R opendkim:mail /etc/opendkim

12: sudo systemctl restart opendkim
13: sudo systemctl status opendkim

In line number 2, we generate private key for selector "20210411-1". This selector must be unique, you should changes it according to your need. My recommendation is using current date as label.

The opendkim-genkey command will generated two files: 20210411-1.private and 20210411-1.txt. The 20210411-1.private file contains PEM encoded private key and the 20210411-1.txt file contains DNS record. Example of 20210411-1.txt,

20210411-1._domainkey  IN  TXT  ( "v=DKIM1; k=rsa; s=email; "
  "p=MIGfM...AQAB" ; ----- DKIM key 20210411-1 for kilabit.info
  )

In your DNS zone, create new subdomain "20210411-1._domainkey" with record type is TXT and its value is the content from "v=DKIM … p=MIGfM…AQAB" — including the double quote.

In line 7-10, we create and populate four file configurations, which is describe below.

/etc/opendkim/KeyTable:

20210411-1._domainkey.kilabit.info kilabit.info:20210411-1:/etc/opendkim/keys/kilabit.info/20210411-1.private

/etc/opendkim/SigningTable:

*@kilabit.info 20210411-1._domainkey.kilabit.info

/etc/opendkim/TrustedHosts:

127.0.0.1
::1
localhost
{{.Val "host::ip_external"}}
{{.Val "host::name"}}
kilabit.info

The "host::ip_external" and "host::name" is your server external IP address and local hostname, if set other than localhost.

/etc/opendkim/opendkim.conf:

BaseDirectory  /var/lib/opendkim

##  Select canonicalizations to use when signing.  If the "bodycanon" is
##  omitted, "simple" is used.  Valid values for each are "simple" and
##  "relaxed".

Canonicalization  relaxed/simple

##  Specify for which domain(s) signing should be done.  No default; must
##  be specified for signing.

Domain  kilabit.info

##  Names a file from which a list of externally-trusted hosts is read.
##  These are hosts which are allowed to send mail through you for signing.
##  Automatically contains 127.0.0.1.  See man page for file format.

ExternalIgnoreList  refile:/etc/opendkim/TrustedHosts

##  Names a file from which a list of internal hosts is read.  These are
##  hosts from which mail should be signed rather than verified.
##  Automatically contains 127.0.0.1.

InternalHosts  refile:/etc/opendkim/TrustedHosts

##  Defines a table that will be queried to convert key names to
##  sets of data of the form (signing domain, signing selector, private key).
##  The private key can either contain a PEM-formatted private key,
##  a base64-encoded DER format private key, or a path to a file containing
##  one of those.

KeyTable        refile:/etc/opendkim/KeyTable

SigningTable  refile:/etc/opendkim/SigningTable

##  Names the socket where this filter should listen for milter connections
##  from the MTA.  Required.  Should be in one of these forms:

Socket  inet:8891@127.0.0.1

##  Log informational and error activity to syslog?

Syslog			Yes

##  Specifies which directory will be used for creating temporary files
##  during message processing.

TemporaryDirectory	/run/opendkim

##  Change to user "userid" before starting normal operation?  May include
##  a group ID as well, separated from the userid by a colon.

UserID		opendkim

After the configurations has been populated, we can start the opendkim.service. Check the log if its fail and continue to next section if its started successfully.

Setting up Dovecot

Dovecot is the application that manage email in storage, usually in the format of mbox or Maildir (recommended). Client access the message from external using IMAP with authentication.

The following awwan script show how to setup Dovecot in Arch Linux (the line number added for commentary),

 0: sudo pacman -Syu --noconfirm
 0: sudo pacman -S --noconfirm dovecot

 0: sudo systemctl enable dovecot

 1: sudo groupadd {{.Val "email::group"}} -g {{.Val "email::gid"}}

 2: sudo useradd  {{.Val "email::user"}} \
	-r \
	-g {{.Val "email::gid"}} \
	-u {{.Val "email::uid"}} \
	-d {{.Val "email::dir"}} \
	-m -c "mail user"

 3: sudo mkdir -p /etc/dovecot

## Generate password for IMAP.

 4: #put: {{.ScriptDir}}/etc/dovecot/passwd.txt passwd.txt

 5: rm -f passwd

 6: while read -r email plain; do \
	hash=$(doveadm pw -s SHA1 -p "$plain"); \
	echo "$email:$hash:::" >> passwd; \
done < passwd.txt

 7: cat passwd

 8: #get: passwd {{.ScriptDir}}/etc/dovecot/passwd

 9: sudo mv passwd /etc/dovecot/passwd
10: rm -f passwd.txt

11: #put! {{.ScriptDir}}/etc/dovecot/dovecot.conf /etc/dovecot/

12: sudo chmod 0600 /etc/dovecot/{dovecot.conf,passwd}
13: sudo chown -R dovecot:dovecot /etc/dovecot

14: sudo systemctl restart dovecot
15: sudo systemctl status  dovecot

In line 1 and 2, we create separate user and group, even thought installing dovecot in Arch Linux create "dovecot" user in the system. This is the user that bridge between the Dovecot and Postfix. In case we want to replace Dovecot with other IMAP service in the future, we did not need to re-create or chown the emails that already stored by Dovecot.

In line 4-10, we generate the password hash for authentication with IMAP by reading the plain text password from "passwd.txt" and write the output back to "passwd" file.

Example content and format of "passwd.txt" file (not the actual password),

ms@kilabit.info          s333cr333t

Example of "passwd" output (not the actual hash),

ms@kilabit.info:9QJcSsuTQW1kz3AAl7N2OGWd7QE=:::

/etc/dovecot/dovecot.conf:

listen = 0.0.0.0
protocols = imap
disable_plaintext_auth = yes
auth_mechanisms = plain login
mail_access_groups = {{.Val "email::group"}}
default_login_user = {{.Val "email::user"}}
first_valid_uid = {{.Val "email::uid"}}
first_valid_gid = {{.Val "email::gid"}}
mail_location = maildir:{{.Val "email::dir"}}/%d/%n

passdb {
	driver = passwd-file
	args = scheme=SHA1 /etc/dovecot/passwd
}
userdb {
	driver = static
	args = uid={{.Val "email::uid"}} gid={{.Val "email::gid"}} home={{.Val "email::dir"}}/%d/%n allow_all_users=yes
}
service auth {
	unix_listener auth-client {
		group = postfix
		mode = 0660
		user = postfix
	}
	user = root
}
service imap-login {
	process_min_avail = 3
	user = {{.Val "email::user"}}

	inet_listener imap {
		port=0
	}
	inet_listener imaps {
		port = 993
		ssl = yes
	}
}
namespace inbox {
	inbox = yes

	mailbox Trash {
		auto = no
		special_use = \Trash
	}
	mailbox Drafts {
		auto = no
		special_use = \Drafts
	}
	mailbox Sent {
		auto = subscribe # autocreate and autosubscribe the Sent mailbox
		special_use = \Sent
	}
	mailbox Spam {
		auto = create # autocreate Spam, but don't autosubscribe
		special_use = \Junk
	}
}

##--- SSL/TLS

ssl = required
ssl_cipher_list = HIGH:!SSLv2:!aNULL@STRENGTH
ssl_prefer_server_ciphers = yes

ssl_cert = </etc/letsencrypt/live/kilabit.info/cert.pem
ssl_key = </etc/letsencrypt/live/kilabit.info/privkey.pem
ssl_dh = </etc/haproxy/dhparam

local_name kilabit.info {
  ssl_cert = </etc/letsencrypt/live/kilabit.info/fullchain.pem
  ssl_key  = </etc/letsencrypt/live/kilabit.info/privkey.pem
}

Setting up Postfix

The following awwan script show how to setup Postfix in Arch Linux,

sudo pacman -Syu --noconfirm
sudo pacman -S --noconfirm postfix

sudo systemctl enable  postfix

#put! {{.ScriptDir}}/etc/postfix/aliases /etc/postfix/aliases
sudo chown root:root /etc/postfix/aliases
sudo postalias /etc/postfix/aliases

#put! {{.ScriptDir}}/etc/postfix/vmail_aliases /etc/postfix/vmail_aliases
sudo postmap -o -p /etc/postfix/vmail_aliases

#put! {{.ScriptDir}}/etc/postfix/vmail_domains /etc/postfix/vmail_domains
sudo postmap -o -p /etc/postfix/vmail_domains

#put! {{.ScriptDir}}/etc/postfix/vmail_mailbox /etc/postfix/vmail_mailbox
sudo postmap -o -p /etc/postfix/vmail_mailbox

#put! {{.ScriptDir}}/etc/postfix/vmail_sni /etc/postfix/vmail_sni
sudo postmap -o -p -F hash:/etc/postfix/vmail_sni

#put! {{.ScriptDir}}/etc/postfix/master.cf /etc/postfix/
#put! {{.ScriptDir}}/etc/postfix/main.cf /etc/postfix/

sudo chown root:root /etc/postfix/*
sudo chmod 0644 /etc/postfix/*

sudo postfix check

sudo systemctl restart postfix
sudo systemctl status  postfix

/etc/postfix/aliases:

# Person who should get root's mail. Don't receive mail as root!
root:       {{.Val "email::user"}}

# Basic system aliases -- these MUST be present
MAILER-DAEMON:	postmaster
postmaster: root

# General redirections for pseudo accounts
bin:        root
daemon:     root
ftp:        root
ftp-bugs:   root
hostmaster: root
named:      root
news:       root
nobody:     root
postfix:    root
usenet:     root
uucp:       root
webmaster:  root
www:        root

# Put your local aliases here.

# Well-known aliases
manager:    root
dumper:     root
operator:   root
abuse:      postmaster

# trap decode to catch security attacks
decode:     root

The above aliases map where the local user delivery should go, in short we forward all local user email to vmail.

/etc/postfix/vmail_aliases:

ms@kilabit.info         ms@kilabit.info

The vmail_aliases contains mapping for virtual addresses.

/etc/postfix/vmail_domains:

kilabit.info  OK

The vmail_domains contains list of virtual domains that the Postfix will receives, can be define more than once domain, one per line.

/etc/postfix/vmail_mailbox:

ms@kilabit.info         kilabit.info/ms/

The vmail_mailbox define where the email message for virtual addresses will be located.

/etc/postfix/vmail_sni:

mail.kilabit.info
 /etc/letsencrypt/live/kilabit.info/privkey.pem
 /etc/letsencrypt/live/kilabit.info/fullchain.pem

The vmail_sni define the certificate for each virtual domain that we define in vmail_domains.

/etc/postfix/main.cf.

##
## COMPATIBILITY
##

compatibility_level = 3.6

##
## LOCAL PATHNAME INFORMATION
##

queue_directory   = /var/spool/postfix
command_directory = /usr/bin
daemon_directory  = /usr/lib/postfix/bin
data_directory    = /var/lib/postfix

##
## QUEUE AND PROCESS OWNERSHIP
##

mail_owner = postfix

##
## INTERNET HOST AND DOMAIN NAMES
##

##
## SENDING MAIL
##

#myorigin = $myhostname

##
## RECEIVING MAIL
##

inet_interfaces = all
inet_protocols  = ipv4
mydestination   = localhost.$mydomain, localhost

##
## REJECTING MAIL FOR UNKNOWN LOCAL USERS
##

unknown_local_recipient_reject_code = 550

##
## TRUST AND RELAY CONTROL
##

relay_domains           = $mydestination
virtual_alias_maps      = hash:/etc/postfix/vmail_aliases
virtual_mailbox_domains = hash:/etc/postfix/vmail_domains
virtual_mailbox_maps    = hash:/etc/postfix/vmail_mailbox

virtual_mailbox_base = {{.Val "email::dir"}}
virtual_minimum_uid  = {{.Val "email::uid"}}
virtual_transport    = virtual
virtual_uid_maps     = static:{{.Val "email::uid"}}
virtual_gid_maps     = static:{{.Val "email::uid"}}

##
## ALIAS DATABASE
##

alias_maps     = hash:/etc/postfix/aliases
alias_database = $alias_maps

##
## ADDRESS EXTENSIONS (e.g., user+foo)
##

recipient_delimiter = +

##
## DELIVERY TO MAILBOX
##

home_mailbox = Maildir/

##
## SHOW SOFTWARE VERSION OR NOT
##

#smtpd_banner = $myhostname ESMTP $mail_name
#smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)

##
## DEBUGGING CONTROL
##

debug_peer_level = 2
debugger_command =
	PATH=/bin:/usr/bin:/usr/local/bin; export PATH; (echo cont;
	echo where) | gdb $daemon_directory/$process_name $process_id 2>&1
	>$config_directory/$process_name.$process_id.log & sleep 5

##
## INSTALL-TIME CONFIGURATION INFORMATION
##

sendmail_path     = /usr/bin/sendmail
newaliases_path   = /usr/bin/newaliases
mailq_path        = /usr/bin/mailq
setgid_group      = postdrop
html_directory    = no
manpage_directory = /usr/share/man
readme_directory  = /usr/share/doc/postfix
inet_protocols    = ipv4
meta_directory    = /etc/postfix
shlib_directory   = /usr/lib/postfix

smtp_tls_security_level = encrypt

smtpd_sasl_auth_enable          = yes
smtpd_sasl_type                 = dovecot
smtpd_sasl_path                 = /var/run/dovecot/auth-client
smtpd_sasl_security_options     = noanonymous
smtpd_sasl_tls_security_options = $smtpd_sasl_security_options
smtpd_sasl_local_domain         = $mydomain
broken_sasl_auth_clients        = no

smtpd_tls_security_level = may
smtpd_use_tls = yes
smtpd_tls_cert_file = /etc/letsencrypt/live/kilabit.info/fullchain.pem
smtpd_tls_key_file  = /etc/letsencrypt/live/kilabit.info/privkey.pem
smtpd_tls_loglevel              = 0
smtpd_tls_received_header       = yes
smtpd_tls_session_cache_timeout = 3600s

smtpd_recipient_restrictions =
	permit_mynetworks
	permit_sasl_authenticated

smtpd_milters = inet:127.0.0.1:8891
non_smtpd_milters = $smtpd_milters
milter_default_action = accept

tls_server_sni_maps = hash:/etc/postfix/vmail_sni

/etc/postfix/master.cf.

#
# Postfix master process configuration file.  For details on the format
# of the file, see the master(5) manual page (command: "man 5 master" or
# on-line: http://www.postfix.org/master.5.html).
#
# Do not forget to execute "postfix reload" after editing this file.
#
# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (no)    (never) (100)
# ==========================================================================
smtp      inet  n       -       n       -       -       smtpd
  -o smtpd_milters=inet:127.0.0.1:8891
smtps     inet  n       -       n       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_recipient_restrictions=
  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING
submission inet  n       -       n       -       -       smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_reject_unlisted_recipient=no
  -o smtpd_recipient_restrictions=
  -o smtpd_relay_restrictions=permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination
  -o milter_macro_daemon_name=ORIGINATING
  -o smtpd_milters=inet:127.0.0.1:8891
pickup    unix  n       -       n       60      1       pickup
cleanup   unix  n       -       n       -       0       cleanup
qmgr      unix  n       -       n       300     1       qmgr
#qmgr     unix  n       -       n       300     1       oqmgr
tlsmgr    unix  -       -       n       1000?   1       tlsmgr
rewrite   unix  -       -       n       -       -       trivial-rewrite
bounce    unix  -       -       n       -       0       bounce
defer     unix  -       -       n       -       0       bounce
trace     unix  -       -       n       -       0       bounce
verify    unix  -       -       n       -       1       verify
flush     unix  n       -       n       1000?   0       flush
proxymap  unix  -       -       n       -       -       proxymap
proxywrite unix -       -       n       -       1       proxymap
smtp      unix  -       -       n       -       -       smtp
relay     unix  -       -       n       -       -       smtp
        -o syslog_name=postfix/$service_name
showq     unix  n       -       n       -       -       showq
error     unix  -       -       n       -       -       error
retry     unix  -       -       n       -       -       error
discard   unix  -       -       n       -       -       discard
local     unix  -       n       n       -       -       local
virtual   unix  -       n       n       -       -       virtual
lmtp      unix  -       -       n       -       -       lmtp
anvil     unix  -       -       n       -       1       anvil
scache    unix  -       -       n       -       1       scache
postlog   unix-dgram n  -       n       -       1       postlogd

One of the reason that the configurations are long like above is we did not have enough knowledges and times to check it one by one, we use the default and merge information here and there, some are by trial and errors.

Setting up fail2ban

Once your email server is up and working, you will see in the system log many unknown connections try to submit, relay, or login to your SMTP and IMAP services. Even if they fail, they will attempt several times probably with different authentication or IP addresses.

fail2ban is a service that read the failed login attempt from system log and block the origin IP addresses after N retry, for example three times.

The following awwan script show how to setup fail2ban in Arch Linux (the line number is added for brevity),

 1: sudo pacman -Sy --noconfirm fail2ban

 2: sudo mkdir -p /var/log/fail2ban/
 3: sudo mkdir -p /etc/systemd/system/fail2ban.service.d

 4: #put! {{.ScriptDir}}/etc/systemd/system/fail2ban.service.d/fail2ban.conf \
	/etc/systemd/system/fail2ban.service.d/

 5: sudo systemctl enable fail2ban

 8: #put! {{.ScriptDir}}/etc/fail2ban/fail2ban.local \
	/etc/fail2ban/fail2ban.local
 9: #put! {{.ScriptDir}}/etc/fail2ban/jail.local \
 	/etc/fail2ban/jail.local

10: sudo systemctl restart fail2ban
11: sudo systemctl status  fail2ban

12: sudo fail2ban-client status
13: sudo fail2ban-client banned

## unban my IP address

14: sudo fail2ban-client set postfix-sasl unbanip 182.253.127.130

In the line 8 and 9, we did not override default installation files, but provide our own, by prefixing it with ".local".

Line 12 is the command to show the status of fail2ban service. Line 13 is the command to show list of IP address banned by fail2ban service.

Line 14 is the command to unban your IP in case you get locked in the future.

/etc/systemd/system/fail2ban.service.d:

[Service]
PrivateDevices=yes
PrivateTmp=yes
ProtectHome=read-only
ProtectSystem=strict
ReadWritePaths=-/var/run/fail2ban
ReadWritePaths=-/var/lib/fail2ban
ReadWritePaths=-/var/log/fail2ban
ReadWritePaths=-/var/spool/postfix/maildrop
ReadWritePaths=-/run/xtables.lock
CapabilityBoundingSet=CAP_AUDIT_READ CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW

In this file we hardening the default fail2ban systemd service by make it not running as root. See Arch Linux Wiki on Fail2ban for more information.

/etc/fail2ban/fail2ban.local:

[Definition]
logtarget = /var/log/fail2ban/fail2ban.log

/etc/fail2ban/jail.local:

[DEFAULT]
ignoreip = 127.0.0.1/8 ::1
bantime  = 1w
banaction = nftables
banaction_allports = nftables[type=allports]

[dovecot]
enabled = true
port    = imaps

[postfix-sasl]
enabled  = true
bantime  = -1
maxretry = 1

In the jail.local we enable rules for dovecot and postfix-sasl. If the rules catch any failed login, the IP address will be banned for "bantime" (one week). Also, we use nftables for firewall backend, the "banaction" and "banaction_allports".