VPS Setup

Email server

There are several Open Source alternatives available. I settled on the Postfix/ Dovecot combination which is known to be fairly efficient, quite flexible and reasonably easy to configure.

Ubuntu 12.04 comes with an outdated version of Dovecot which was a problem for me because I wanted to use the latest version of the Pigeonhole plugin to script my email server. Thankfully, somebody created a PPA with a suitably fresh version.

NB: The truly paranoid should not trust a random PPA but build the package from source instead.

add-apt-repository ppa:kokelnet/dovecot22
apt-get update
apt-get install postfix dovecot dovecot-pigeonhole

The postfix package will ask you a few questions during setup via an ugly ncurses interface. You can safely ignore them as we’ll edit the configuration manually anyway.

Certificates

First, let’s take care of the certificates we’re going to use to encrypt SMTP and IMAP connections (oh, by the way, I’m not going to bother with POP3 but it’s supported by dovecot and I hear it ‘s not hard to setup).

./gencert.sh /etc/ssl/private/smtpd.key /etc/ssl/certs/smtpd.crt
./gencert.sh /etc/ssl/private/dovecot.pem /etc/ssl/certs/dovecot.pem

The script will prompt you for some fields of the Distinguished Name of the certs. Answer carefully, at the very least for the CN field, or your certs may be rejected by some servers/clients. In particular the CN of the SMTP cert MUST match your mail domain.

Dovecot

Dovecot provides a couple of different components of interest to us:

  • SASL authentication with pluggable backends
  • IMAP server to access your emails from a remote client (mobile or desktop)
  • LMTP server that sits between Postfix and local maildirs
  • Sieve interpreter to customize the behavior of said LMTP server

First, we need to enable IMAP and LMTP in /etc/dovecot/dovecot.conf :

protocols = imap lmtp

The second most important config file is /etc/dovecot/conf.d/10-master.conf, where you should adjust LMTP and SASL settings as follows:

service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
  }
}
service auth {
  unix_listener auth-userdb {
    mode = 0660
  }

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

Then, each component can be configured via it’s own config file in /etc/dovecot/conf.d. This document only address the handful I tweaked but it’s acceptable (in fact even recommended) to poke around and adapt the system to your particular needs.

SASL is configured via /etc/dovecot/10-auth.conf. Mine looks roughly like this:

disable_plaintext_auth = yes
auth_mechanisms = plain
!include auth-passwdfile.conf.ext

The apparent contradiction between the first two lines lies in the fact that plaintext auth is perfectly acceptable over a TLS connection.

As already mentioned, Dovecot authentication is pretty versatile. I opted for a simple passwdfile but another option may be better for you.

Make sure /etc/dovecot/10-ssl.conf points to the key and certificate you generated:

ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem

The location of maildirs is controlled by /etc/dovecot/10-mail.conf, in which the important fields are:

mail_home = /srv/data01/mail/%n/home
mail_location = maildir:/srv/data01/mail/%n

Make sure you understand their meaning and pick your values with care, taking full advantage of the available variables.

To enable Sieve, change /etc/dovecot/20-lmtp.conf:

protocol lmtp {
  postmaster_address = hugues@bruant.info
  mail_plugins = $mail_plugins sieve
}

NOTE Beware of typos in the configuration, in some cases it will simply prevent Dovecot from starting and the reason will not be immediately apparent buntil you look into /var/log/upstart/dovecot.log.

Postfix

Postifx is our SMTP server, the crucial component that ensures that both incoming and outgoing emails get routed correctly.

The main postfix configuration is stored in /etc/postfix/main.cf. Mine uses virtual mailboxes, pipes all emails to Dovecot LMTP, enables TLS and proxies SASL auth through Dovecot:

myhostname = bruant.info
myorigin = $myhostname
mydestination =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128

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

relayhost =
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all

virtual_mailbox_domains = $myhostname
virtual_alias_maps = hash:/etc/postfix/virtual
virtual_transport = lmtp:unix:private/dovecot-lmtp

smtp_tls_security_level = may
smtp_tls_note_starttls_offer = yes

smtpd_use_tls=yes
smtpd_tls_loglevel = 1
smtpd_tls_auth_only = no
smtpd_tls_security_level = may
smtpd_tls_received_header = yes
smtpd_tls_cert_file = /etc/ssl/certs/smtpd.crt
smtpd_tls_key_file = /etc/ssl/private/smtpd.key
smtpd_tls_CAfile = /etc/ssl/certs/cacert.pem
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_local_domain = $myhostname
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination

In /etc/postfix/master.cf make sure the smtp and submission protocols are handled correctly. It is particularly important to get the “chroot” option right.

smtp      inet  n       -       n       -       -       smtpd
submission inet n       -       n       -       -       smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_sasl_path=private/auth
  -o smtpd_sasl_security_options=noanonymous
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o smtpd_sender_restrictions=permit_sasl_authenticated,reject
  -o smtpd_recipient_restrictions=reject_non_fqdn_recipient,permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING

Testing

Restart all services to ensure they pick up configuration changes:

service dovecot restart && service postfix restart

Use telnet or netcat to check that all services are up and running. Try on the server first and then on a local machine. The relevant ports are 25 and 587 for SMTP and 143 and 993 for IMAP. For instance:

$ netcat bruant.info 143
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE STARTTLS LOGINDISABLED] Dovecot ready.
^C
$ $ netcat bruant.info 587
220 bruant.info ESMTP Postfix (Ubuntu)
^C

It is recommended to watch /var/log/mail.log while testing to detect any anomaly:

tail -f /var/log/mail.log

Client configuration

Provided you did not diverge too much from the above instructions, your server should be reachable by any email client if you feed it the following parameters:

  • SMTP: port 587, with PLAIN auth and TLS

  • IMAP: either of

    • port 143, with PLAIN auth and STARTTLS
    • port 993, with PLAIN auth and TLS

I opted to exclude the domain name from the login used for SMTP/IMAP auth but both Dovecot will accept it (as long as it matches the expectation of the authentication backend you picked).

Try sending an email to your freshly created address, and see if it arrives to your email client (and keep watching /var/log/mail.log to look for problems).

Then try sending an email from your freshly created address to any of your old ones and see if it arrives.