An OpenBSD E-Mail Server Using OpenSMTPD, Dovecot, Rspamd, and RainLoop

Introduction

This tutorial demonstrates a full-featured email server running on OpenBSD using OpenSMTPD, Dovecot, Rspamd, and RainLoop. OpenSMTPD is the default mail server for OpenBSD. Choose a Vultr Compute Cloud instance with plenty of storage for the expected number of users.

Preliminary Steps

Verify the server's outbound port status.

Set up your user account to perform tasks as root.

su -
usermod -G wheel <username>
echo "permit nopass keepenv :wheel" > /etc/doas.conf
exit

Set up the package repository for OpenBSD.

doas su
echo "https://cdn.openbsd.org/pub/OpenBSD" > /etc/installurl
exit

Add the required packages.

doas pkg_add opensmtpd-table-passwd opensmtpd-filter-rspamd dovecot dovecot-pigeonhole rspamd redis

Configure OpenSMTPD

By default, OpenSMTPD only listens on localhost. It must be explicitly configured to listen on external interfaces. It should be configured to use virtual users instead of system users for security.

Backup the default smtpd.conf file and create a new one from scratch.

cd /etc/mail
mv smtpd.conf smtpd.conf.default

Create a new smtpd.conf as shown below. Replace example.com with your domain. This initial configuration does not activate the rspamd filter while testing OpenSMTP. The spam filter will be activated later.

pki "mail" cert "/etc/ssl/mail.crt"
pki "mail" key "/etc/ssl/private/mail.key"

table aliases file:/etc/mail/aliases
table credentials passwd:/etc/mail/credentials
table virtuals file:/etc/mail/virtuals

filter "rspamd" proc-exec "/usr/local/libexec/smtpd/filter-rspamd"

# To accept external mail, replace with: listen on all

# listen on all tls pki "mail" hostname "mail.example.com"
listen on egress port submission tls-require pki "mail" hostname "mail.example.com" auth <credentials>

action "local_mail" mbox alias <aliases>
action "domain_mail" maildir "/var/vmail/example.com/%{dest.user:lowercase}" virtual <virtuals>
action "outbound" relay

# Uncomment the following to accept external mail for domain "example.org"

# match from any for domain "example.com" action "domain_mail"
match from local for local action "local_mail"

match from local for any action "outbound"
match auth from any for any action "outbound"

Create the Credentials File

OpenSMTPD and Dovecot can share an authentication database. This database resembles the system password file in format, with two extra fields for Dovecot. The two special fields define the virtual home directory and the mail location. Passwords are in blowfish format. This tutorial creates three example users.

Generate the passwords and concatenate them to the /etc/mail/credentials file.

doas su
smtpctl encrypt example_password1 >> /etc/mail/credentials
smtpctl encrypt example_password2 >> /etc/mail/credentials
smtpctl encrypt example_password3 >> /etc/mail/credentials
exit

The output looks similar to this:

$2b$10$_EXAMPLE_PASSWORD1_3JbO4Ns2jJNZQfTS45MAnKi.IPrkKITyTa6
$2b$10$_EXAMPLE_PASSWORD2_YKD.K0kQ2oylOmQ9SBUb0hIopBsmNxYPb4e
$2b$10$_EXAMPLE_PASSWORD3_IvRu4xbeOqOJJXlgEAKuS5sIrBvfdPvEzeq

Edit /etc/mail/credentials to add the required fields. Each line maps to a system account, vmail, with UID and GID of 2000. Replace example.com with your domain. Replace the example passwords with the passwords you generated in the previous step. The virtual user name is the complete email address.

john@example.com:$2b$10$_EXAMPLE_PASSWORD1_C3JbO4Ns2jJNZQfTS45MAnKi.IPrkKITyTa6:vmail:2000:2000:/var/vmail/example.com/john::userdb_mail=maildir:/var/vmail/example.com/john
adam@example.com:$2b$10$_EXAMPLE_PASSWORD2_YKD.K0kQ2oylOmQ9SBUb0hIopBsmNxYPb4e:vmail:2000:2000:/var/vmail/example.com/adam::userdb_mail=maildir:/var/vmail/example.com/adam
natalie@example.com:$2b$10$_EXAMPLE_PASSWORD3_IvRu4xbeOqOJJXlgEAKuS5sIrBvfdPvEzeq:vmail:2000:2000:/var/vmail/example.com/natalie::userdb_mail=maildir:/var/vmail/example.com/natalie

Create Virtual Mail Account and Set Security

• Set /etc/mail/credentials permissions to read-only for _smtpd and _dovecot system users.

• Create the vmail system user, group, and home directory.

• When you create the vmail system user, you will receive the following warning: useradd: Warning: home directory '/var/vmail' doesn't exist, and -m was not specified. This is expected. This avoids cluttering the directory with dot files from /etc/skel. They are not required because the vmail account does not allow login.

    doas chmod 0440 /etc/mail/credentials
    doas chown _smtpd:_dovecot /etc/mail/credentials
    doas useradd -c "Virtual Mail Account" -d /var/vmail -s /sbin/nologin -u 2000 -g =uid -L staff vmail
    doas mkdir /var/vmail
    doas chown vmail:vmail /var/vmail

Create the Virtual User Mapping

Create /etc/mail/virtuals to define the valid email addresses.

• The first four lines assign john@example.com aliases for abuse, hostmaster, postmaster, and webmaster.

• The last three lines map the email addresses to the vmail account. OpenSMTPD will deliver the messages to /var/vmail/example.com/<user>.

• Mail delivery attempted for addresses not defined in this file will be bounced with a Delivery Status Notification.

    abuse@example.com: john@example.com
    hostmaster@example.com: john@example.com
    postmaster@example.com: john@example.com
    webmaster@example.com: john@example.com
    john@example.com: vmail
    adam@example.com: vmail
    natalie@example.com: vmail

Create Public/Private Keys for OpenSMTPD

This example uses a self-signed certificate. Use a valid signed certificate if you have one. When prompted for the common name, be sure it matches the FQDN of the server. This example uses mail.example.com.

doas su
cd /etc/ssl
openssl genrsa -out private/mail.key 4096
openssl req -x509 -new -nodes -key private/mail.key -out mail.crt -days 3650 -sha256
chmod 0400 /etc/ssl/private/mail.key
exit

Test the Server

Use the OpenSMTPD configuration syntax checker. If no problems are found, restart the smtpd daemon.

doas smtpd -n
doas rcctl restart smtpd

From an outside mail account, send a test email to one of the users.

• OpenSMTPD will create the maildir folder structure below /var/vmail and deliver the mail to /var/vmail/example.com/<username>/new.

• As the root user, browse to this location and verifiy you have a file named similar to this: 1576339842.4d64757b.example.com:2,.

• Review the contents of the file, including all of the mail headers, to verify the email delivery works properly.

    Return-Path: <n0244e80da3-54b1ed125c5342fc-adam===example.org@bounce.example.org>
    Delivered-To: adam@example.com
    Received: from spruce-goose-ba.twitter.com (spruce-goose-ba.twitter.com [199.59.150.96])
        by mail.example.com (OpenSMTPD) with ESMTPS id 75b514d3 (TLSv1.2:ECDHE-RSA-AES256-GCM-    SHA384:256:NO)
        for <adam@example.com>;
        Sat, 14 Dec 2019 11:10:40 -0500 (EST)
    DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=twitter.com;
        s=dkim-201406; t=1576339839;
        bh=jhKB5/w9v87GaXSuizT576ntJ/72gvLRDhsqmGQQrCE=;
        h=Date:From:To:Subject:MIME-Version:Content-Type:List-Unsubscribe:
        Message-ID;
        b=TWn/QVUJ1VDlWiweWoanwHLABCL1nqmm0+TBzh3PjmYNm0quRMXB7QL2ykzHGME5A
        DTz/JFHa0cOvQsrdhxxbjOLFKimK0nF+Ou5kI+2HzTzfVNZS0qGnTVP/tZyaIsWjjl
        an5EiR6HFOHG4iClOOEOJW4oLDEZfPTefrlW+378bmHGIRUNDvVKrbXKunL9fJFAb3
        JSrhWQNwbrF/aARFzw4nKfb1I7vTRSrN1eXE5JxzGwI2XAjqDIWdR5ExwUNbJH5ZPs
        wQ85j8KLZEEgQkbH9CypgeUMJWsVK95FqOCCaqKMS10M7intGMb3aeiiFcB7yDHi9t
        u7rVESm4eGp/g==
    X-MSFBL: DM7pSZns+YDRgNEmGNre9aPjZTtc1tDlN97w5rQDBts=|eyJ1IjoibWF0dEBnb2J
        sYWNrY2F0LmNvbUBpaWQjIzU0YjFlZDEyNWM1MzQyZmNiNThiMzVmNzI0NDZlMGF
        mQHVzYiMjNkAyNDRAMTA4MjgwNTAxMDYzNzk1MDk3NkAwQDA4MjY5ZWI4OTI3YzR
        kNTFiNTZkMjY3YzY2OGRmN2IwY2Y4M2ExZGIiLCJyIjoibWF0dEBnb2JsYWNrY2F
        0LmNvbSIsImciOiJCdWxrIiwiYiI6InNtZjEtYmd4LTM0LXNyMS1CdWxrLjE4NiJ
        9
    Date: Sat, 14 Dec 2019 16:10:39 +0000
    ...

Configure Dovecot IMAP

Set the Login Class

Dovecot requires the ability to have a larger number of files open for reading and writing than the default class allows. Failing to do this will cause errors that are difficult to troubleshoot.

Define a login class for the Dovecot daemon. At the bottom of /etc/login.conf add the following lines.

dovecot:\
    :openfiles-cur=1024:\
    :openfiles-max=2048:\
    :tc=daemon:    

Create the Dovecot Configuration File

Create /etc/dovecot/local.conf.

auth_mechanisms = plain
first_valid_uid = 2000
first_valid_gid = 2000
mail_location = maildir:/var/vmail/%d/%n
mail_plugin_dir = /usr/local/lib/dovecot
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex  imap4flags copy include variables body enotify environment mailbox date index ihave duplicate mime foreverypart extracttext imapsieve vnd.dovecot.imapsieve
mbox_write_locks = fcntl
mmap_disable = yes
namespace inbox {
  inbox = yes
  location =
  mailbox Archive {
  auto = subscribe
  special_use = \Archive
  }
  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
  }
  prefix =
}
passdb {
  args = scheme=CRYPT username_format=%u /etc/mail/credentials
  driver = passwd-file
  name =
}
plugin {
  imapsieve_mailbox1_before = file:/usr/local/lib/dovecot/sieve/report-spam.sieve
  imapsieve_mailbox1_causes = COPY
  imapsieve_mailbox1_name = Junk
  imapsieve_mailbox2_before = file:/usr/local/lib/dovecot/sieve/report-ham.sieve
  imapsieve_mailbox2_causes = COPY
  imapsieve_mailbox2_from = Junk
  imapsieve_mailbox2_name = *
  sieve = file:~/sieve;active=~/.dovecot.sieve
  sieve_global_extensions = +vnd.dovecot.pipe +vnd.dovecot.environment
  sieve_pipe_bin_dir = /usr/local/lib/dovecot/sieve
  sieve_plugins = sieve_imapsieve sieve_extprograms
}
protocols = imap sieve
service imap-login {
  inet_listener imap {
  port = 0
  }
}
service managesieve-login {
  inet_listener sieve {
  port = 4190
  }
  inet_listener sieve_deprecated {
  port = 2000
  }
}
ssl_cert = </etc/ssl/mail.crt
ssl_key = </etc/ssl/private/mail.key
userdb {
  args = username_format=%u /etc/mail/credentials
  driver = passwd-file
  name =
}
protocol imap {
  mail_plugins = " imap_sieve"
}

Dovecot Bug Fix

There is a bug in Dovecot where the ssl_cert and ssl_key settings do not get overridden in the local.conf file so we have to comment them out. If you miss this step, Dovecot will fail to start correctly.

Edit /etc/dovecot/conf.d/10-ssl.conf as shown.

...
# PEM encoded X.509 SSL/TLS certificate and private key. They're opened before
# dropping root privileges, so keep the key file unreadable by anyone but
# root. Included doc/mkcert.sh can be used to easily generate self-signed
# certificate, just make sure to update the domains in dovecot-openssl.cnf
#ssl_cert = </etc/ssl/dovecotcert.pem
#ssl_key = </etc/ssl/private/dovecot.pem
...

Create the Sieve Scripts

Sieve scripts train Rspamd on spam and ham. Moving email into and out of the junk folder triggers an event to train Rspamd.

These files are located at /usr/local/lib/dovecot/sieve.

Create the report-ham.sieve file.

require ["vnd.dovecot.pipe", "copy", "imapsieve", "environment", "variables"];

if environment :matches "imap.mailbox" "*" {
  set "mailbox" "${1}";
}

if string "${mailbox}" "Trash" {
  stop;
}

if environment :matches "imap.user" "*" {
  set "username" "${1}";
}

pipe :copy "sa-learn-ham.sh" [ "${username}" ];

Create the report-spam.sieve file.

require ["vnd.dovecot.pipe", "copy", "imapsieve", "environment", "variables"];

if environment :matches "imap.user" "*" {
  set "username" "${1}";
}

pipe :copy "sa-learn-spam.sh" [ "${username}" ];

Compile the files.

sievec report-ham.sieve
sievec report-spam.sieve 

Create the following two shell scripts in /usr/local/lib/dovecot/sieve

Add the following to sa-learn-ham.sh

#!/bin/sh
exec /usr/local/bin/rspamc -d "${1}" learn_ham

Add the following to sa-learn-spam.sh

#!/bin/sh
exec /usr/local/bin/rspamc -d "${1}" learn_spam

Make the files executable.

chmod 0755 sa-learn-ham.sh
chmod 0755 sa-learn-spam.sh

Enable and start Dovecot.

rcctl enable dovecot
rcctl start dovecot

Check that Dovecot started properly.

ps ax | grep dovecot

88005 ??  I        0:00.11 /usr/local/sbin/dovecot
69640 ??  I        0:00.03 dovecot/anvil
91207 ??  I        0:00.03 dovecot/log
98178 ??  I        0:00.19 dovecot/config
34712 ??  I        0:00.06 dovecot/stats
96674 ??  I        0:00.03 dovecot/imap-login
 8891 ??  S        0:00.02 dovecot/imap

Verify Dovecot can correctly read /etc/mail/credentials

doveadm user john@example.com

field    value
uid      2000
gid      2000
home /var/vmail/example.com/john
mail maildir:/var/vmail/example.com/john

Verify a mail user can log in.

doveadm auth login john@example.com

Password: ********
passdb: john@example.com auth succeeded
extra fields:
  user=john@example.com

userdb extra fields:
  john@example.com
  mail=maildir:/var/vmail/example.com/john
  uid=2000
  gid=2000
  home=/var/vmail/example.com/john
  auth_mech=PLAIN

Set up Rspamd

This is a basic Rspamd configuration, refer to the offical documentation for more details. This example creates a definition for our domain to enable DKIM signing.

Create a public/private keypair in /etc/mail/dkim and set the correct permissions.

doas su
mkdir /etc/mail/dkim
cd /etc/mail/dkim
openssl genrsa -out example.com.key 2048
openssl rsa -in example.com.key -pubout -out public.key
chmod 0440 example.com.key
chown root:_rspamd example.com.key

Create a DNS record for DKIM containing the public key. Refer to your DNS provider for details of how to create a DKIM record. Copy the contents from /etc/mail/dkim/public.key and paste it after the p= part of the DKIM record as shown below. Note this example also creates an SPF record.

default._domainkey.example.com. IN TXT "v=DKIM1;k=rsa;p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQClcuK3FV3Ug64li8iFsuJ2ykgb7FMZsujk9uG79ppPUp57vCfjzO7F+HBfx5qIwvlGxv2IJXK86FZwhpUX+HFCDUtfB2z0ZNGerWcZfNzM1w1Bru/fdMd2tCYkiHEa5RWIkLfs/Fm+neXxRZfAG2UDWmAghNbgzYv7xViwgufDIQIDAQAB"
example.com. IN TXT "v=spf1 a ip4:192.0.2.1 mx ~all"

Create a DMARC record.

_dmarc.example.com. IN TXT "v=DMARC1;p=none;pct=100;rua=mailto:postmaster@example.com"

Create the /etc/rspamd/local.d/dkim_signing.conf configuration file.

• The selector="default"; line is derived from the first part of the DKIM DNS record (default._domainkey....) created above.

    domain {
        example.com {
            path = "/etc/mail/dkim/example.com.key";
            selector = "default";
        }
    }

Enable and start Rspamd.

doas rcctl enable redis rspamd
doas rcctl start redis rspamd

Change the lines below in /etc/mail/smtpd.conf and restart OpenSMTPD to enable Rspamd.

...
listen on all tls pki "mail" hostname "mail.example.com" filter "rspamd"
listen on egress port submission tls-require pki "mail" hostname "mail.example.com" \
  auth <credentials> filter "rspamd"
...
rcctl restart smtpd

Test the mail server with POP3 or IMAP email client. If you do not require webmail, stop here.

(Optional) Configure RainLoop Webmail

Install Prerequisite Packages

When prompted, please choose the most current version of PHP.

pkg_add php php-curl php-pdo_sqlite php-zip pecl73-mcrypt zip unzip wget curl

Fetch the RainLoop webmail tarball and extract it to /var/www/htdocs/.

Use the Standard Edition which includes an automatic updater.

cd /tmp
wget https://www.rainloop.net/repository/webmail/rainloop-latest.zip
unzip rainloop-latest.zip -d /var/www/htdocs/rainloop
chown -R www:www /var/www/htdocs

Create Let's Encrypt SSL Certificate

• Copy /etc/examples/acme-client.conf to /etc

• Add the following lines at the bottom of the file:

    domain webmail.example.com {
    domain key "/etc/ssl/private/webmail.example.com.key"
    domain full chain certificate "/etc/ssl/webmail.example.com.crt"
    sign with letsencrypt
    }

Configure httpd

• Create a DNS entry (either CNAME or A record) for subdomain webmail.example.com

• Edit /etc/httpd.conf following the example below.

    prefork 3
    types { include "/usr/share/misc/mime.types" }
  
    server "default" {
        listen on egress port 80 
        root "/htdocs"
        directory index index.html
  
        location "/.well-known/acme-challenge/*" {
                root "/acme"
                request strip 2
        }
    }

Run the httpd syntax check.

httpd -n

Enable and start httpd.

rcctl enable httpd
rcctl start httpd

Request the Let's Encrypt certificate.

acme-client -v webmail.example.com

Add the server definitions for RainLoop to /etc/httpd.conf

server "webmail.example.com" {
    listen on egress port 80
    block return 302 "https://$SERVER_NAME$REQUEST_URI"
}

server "webmail.example.com" {
    listen on egress tls port 443
    root "/htdocs/rainloop"
    directory index "index.php"

    tcp { nodelay, backlog 10 }
    
    tls {
            certificate "/etc/ssl/webmail.example.com.crt"
            key "/etc/ssl/private/webmail.example.com.key"
    }

    hsts {
            max-age 31556952
            preload
    }

    # Value below is 25MB in bytes. 1MB = 1048576 bytes
    connection max request body 26214400

    location "/data*" {
            block return 403
    }

    location "*.php*" {
            fastcgi socket "/run/php-fpm.sock"
    }
}

Configure PHP to allow attachments up to 25 megabytes. Make the following changes in /etc/php-7.3.ini:

; Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
upload_max_filesize = 25M
...
; Maximum size of POST data that PHP will accept.
; Its value may be 0 to disable the limit. It is ignored if POST data reading
; is disabled through enable_post_data_reading.
; http://php.net/post-max-size
post_max_size = 29M

Enable the necessary PHP modules by copying their configuration files to /etc/php-7.3/

cp /etc/php-7.3.sample/* /etc/php-7.3/.

Check the syntax of /etc/httpd.conf

httpd -n

Enable and start httpd and php-fpm.

rcctl reload httpd
rcctl enable php73_fpm
rcctl start php73_fpm

Final Test

Browse to https://webmail.example.com/?admin.

The default username is: admin
The default password is: 12345.

Browse to each of the configuration sections and make your desired changes. Once you have finished with the configuration, log into webmail at https://webmail.example.com