Small-Time Email with Exim and Dovecot
If you want a super-simple SMTP / IMAP setup for a home server, this is the guide for you. This document covers the minimum steps to get email delivery up and running on a small home network. You're not going to want to use this for any serious enterprise stuff, but for a small home LAN it works well.
Why would anyone do this?
My personal motivation for creating this small-time email setup was to deliver alerts from Monit so I would know when my system needed attention. You can use it for this or similar minimalist email needs. Just don't do anything crazy like exposing it to the internet.
Why Exim and Dovecot?
For an email server, Exim is easy to configure. Dovecot is a little more complex, but not insurmountable. Both are well documented.
Installing the Packages
The first step is to install Exim, Dovecot, and Mailx. (Mailx is used for testing.)
apk add exim dovecot mailx
Configuring Exim
The next step is to get Exim working for delivering email to users on the system. This is a pretty simple configuration and there are only a few parameters to change in the delivered exim.conf file.
- Make a backup of /etc/exim/exim.conf
- Open /etc/exim/exim.conf in your favorite text editor.
- Make the changes stated below and save.
Find the lines that look like this:
# group = mail # mode = 0660
They'll be under the heading of local_delivery:
When you find them, remove the comment (hash symbol). The local_delivery section should now look like this:
local_delivery: driver = appendfile file = /var/mail/$local_part_data delivery_date_add envelope_to_add return_path_add group = mail mode = 0660
The only thing changed is the removal of the hash symbol from the last two lines.
Fixing Ownership and Permissions on /var/mail
As it stands, Exim will not be able to deliver messages to /var/mail, where the user mailboxes are stored. This is due to permissions.
To fix it, run these two commands:
chgrp mail /var/mail chmod 2775 /var/mail
When you're done, verify it with ls -ld /var/mail. It should look something like this:
$ ls -ld /var/mail/ drwxrwsr-x 3 root mail 4096 May 11 12:58 /var/mail/
Setting the group ownership to mail, lets exim write to users' mailboxes when new mail comes in.
Starting the Exim Service
Start Exim and configure it to start at boot time with the usual commands.
service exim start rc-update add exim
Testing the Exim Setup
Log in a a regular user and try sending a test email to yourself. You can do this with the mail command, like this:
mail -s Testing dave This is a test. .
This sends a test message to the user dave. (Obviously, you'll want to replace dave with your username.) The final . on the last line is important. It tells the mail command the message is done.
When the message is sent, check that you received it by running mail with no command-line parameters. If everything went well, it should look like the example below.
$ mail Mail version 8.1 6/6/93. Type ? for help. "/var/mail/dave": 1 messages > 1 dave@myserver.home Wed May 11 03:51 27/847 "Testing" &
You can type the message number (1) to display the contents of the mail and then type q to quit the mail program.
Troubleshooting Mail Delivery
If the mail test fails, look int the directory /var/spool/exim/msglog. If there are files in here, they are stuck messages. The files are plain text. Display the contents to show any error messages. In most cases, the problem will be related to permissions on the /var/mail directory or the mailbox files within the directory.
The directory permissions should look like this:
# ls -ld /var/mail drwxrwsr-x 3 root mail
The permissions on mailbox files inside should look like this:
# ls -l -rw-rw---- 1 dave mail
Configuring Dovecot
If everything is working with local delivery, it's time to set up IMAP using Dovecot.
The Dovecot package for Alpine comes with twenty configuration files in /etc/dovecot/conf.d. As a small-time email admin, you may feel overwhelmed. Don't worry, everything can be condensed down to a single config file of sixteen lines.
First, make a backup copy of /etc/dovecot/dovecot.conf.
Next, create a new dovecot.conf that looks like this:
listen = *
log_path = /var/log/dovecot.log
protocols = imap
disable_plaintext_auth = no
mail_privileged_group = mail
mail_location = mbox:~/mail:INBOX=/var/mail/%u
userdb {
driver = passwd
}
passdb {
driver = passwd-file
args = scheme=sha512-crypt username_format=%n /etc/dovecot/passwd
}
# These are self-signed certs generated when the dovecat apk was installed.
ssl=yes
ssl_cert=</etc/ssl/dovecot/server.pem
ssl_key=</etc/ssl/dovecot/server.key
This config does not have the !include conf.d/*.conf that was in the original dovecot.conf, so those twenty files in conf.d are going to be ignored. Everything is now in this single dovecot.conf.
Starting the Dovecot Service
Start Dovecot and configure it to start at boot time with the usual commands.
service dovecot start rc-update add dovecot
Creating Credentials for Dovecot Users
As it is configured, Dovecot will use /etc/passwd for looking up user information, but not authentication. Technically, /etc/passwd authentication can be done using Pluggable Authentication Modules (PAM), but PAM is not part of the base install of Alpine Linux. The next best thing is to use a separate password file for Dovecot credentials and to use the same SHA512-Crypt hashing algorithm used in /etc/passwd.
The Dovecot configuration above specifies a password file of /etc/dovecot/passwd. The Dovecot password file looks like this:
dave:{SHA512-CRYPT}$6$mQ1rxB0gZHqg8Tg9$nxZ8odJZ6xVpmOVpsnYfAo1i7SuoLDhsvoykieukWF9NyNBq.WwhDA7udcYxP1iEm/IzlBmnwz6/vOO3SX8gA.
There are two fields, username and password, separated by a colon. Notice the {SHA512-CRYPT} prefix to the password. This indicates the hashing algorithm.
You can create passwords with the doveadm command, like this:
# doveadm pw -s sha512-crypt Enter new password: Retype new password:
The command will output the hashed password. You'll need to edit Dovecot's password file with a text editor and create the username/password pair by hand.
The permissions on the Dovecot password file should be such that dovecot can read it, but not write to it. Only root should be able to write it.
ls -l /etc/dovecot/passwd -rw-r----- 1 root dovecot
Testing the Dovecot Setup
To test IMAP, you'll need an email client. Personally, I've used Thunderbird on Windows and K-9 Mail on Android. The trickiest part is getting the email client to trust the self-signed certificates. Configuring email clients is beyond the scope of this document.
From the server side, the Dovecot log file can help you diagnose errors. The dovecot.conf file specifies the location of the log file.
log_path = /var/log/dovecot.log
One of the common errors I've seen looks like this:
Disconnected: TLS initialization failed. Error: Failed to initialize SSL server context: Can't load SSL certificate
This was the result of a typographical error I made in the Dovecot config file.
You can further simplify things by commenting out the ssl lines in the dovecot.conf so it looks like this:
# These are self-signed certs generated when the dovecat apk was installed. #ssl=yes #ssl_cert=</etc/ssl/dovecot/server.pem #ssl_key=</etc/ssl/dovecot/server.key
Now TLS is out of the picture, letting you diagnose other potential problems. However, you may have to do some work to convince your mail client that sending login credentials in cleartext is okay. Only do this on a network where you trust your users!
Using and Enjoying Your Small-Time Email Setup
Now that everything is setup, you can start sending yourself cat pictures or you can configure other programs to use the email system to send notifications. For example, I use Monit to keep an eye on services and file system space. When Monit detects a problem, it sends me an email.
The setup presented in this guide uses port 25 for SMTP and port 143 for IMAP. There are no dedicated TLS ports. Encryption is done using STARTTLS.
A Word About Aliases
If you've ever used /etc/aliases for mail delivery, you should be aware that Exim puts this file in /etc/mail/aliases. The format is the same as Sendmail.
Scripted Installation and Configuration
If you like living dangerously (or if you have a test system you don't care about) you can do all of the server configuration presented above with a single script, as shown below:
chgrp mail /var/mail
chmod 2775 /var/mail
apk add exim mailx
sed -i~ \
-e 's/# group = mail/ group = mail/' \
-e 's/# mode = 0660/ mode = 0660/' \
/etc/exim/exim.conf
ln -s mail/aliases /etc/aliases
rc-update add exim
service exim start
apk add dovecot
mv /etc/dovecot/dovecot.conf /etc/dovecot/dovecot.conf~
cat <<EOF > /etc/dovecot/dovecot.conf
listen = *
log_path = /var/log/dovecot.log
protocols = imap
disable_plaintext_auth = no
mail_privileged_group = mail
mail_location = mbox:~/mail:INBOX=/var/mail/%u
userdb {
driver = passwd
}
passdb {
driver = passwd-file
args = scheme=sha512-crypt username_format=%n /etc/dovecot/passwd
}
# These are self-signed certs generated when the dovecat apk was installed.
ssl=yes
ssl_cert=</etc/ssl/dovecot/server.pem
ssl_key=</etc/ssl/dovecot/server.key
EOF
touch /etc/dovecot/passwd
chown root:dovecot /etc/dovecot/passwd
chmod 640 /etc/dovecot/passwd
service dovecot start
rc-update add dovecot
echo "Create dovecot user passwords with: doveadm pw -s sha512-crypt"