Title : Full featured Slackware email server with sendmail and cyrus-imapd
Author: Solène
Date : 14 November 2020
Tags : slackware email
This article is about making your own mail server using Slackware
linux distribution, sendmail and cyrus-imap. This choice is because I
really love Slackware and I also enjoy non-mainstream stacks. While
everyone would recommend postfix/dovecot, I prefer using
sendmail/cyrus-imap. Please not this article contain ironical
statements, I will try to write them with some *emphasis*.
*While some people use fossil fuel cars, some people use Slackware.*
If you are used to clean, reproducible and automated deployments, the
present how-to is the totally opposite. *This is the /Slackware/ way*.
## Slackware
Slackware is one of the oldest (maybe the oldest with debian) linux
distribution out there and it's still usable. The last release (14.2)
is 4 years old but there are still security updates. I choose to use
the development branch slackware-current for this article.
I discovered an alternative to Windows in the early 2000' with a
friend showing me a « Linux » magazine, featuring Slackware
installation CDs and the instructions to install. It was my very first
contact with Linux and open source ever. I used Slackware multiple
times over time, and it was always a great system for me on my main
The Slackware specifics could be said as: "not changing much" and
"quite limited". Slackware never change much between releases, from
2010 to 2020, it's pretty much the same system when you use it. I say
it's rather limited, package wise, the default Slackware installation
requires like 15 GB on your disk because it bundles KDE and all the
kde apps, a bunch of editors (emacs,vim,vs,elvis), lot of
compilers/interpreter (gcc, llvm, ada, scheme, python, ruby
etc..). While it provides a LOT of things out of the box, you really
get all Slackware can offer. If something isn't in the packages, you
need to install it yourself.
## Full Disk Encryption or nothing
I recommend to EVERYONE the practice of having a full disk encryption
(phone, laptop, workstation, servers). If your system get stolen, you
will only lose hardware when you use full disk encryption.
Without encryption, the thief can access all your data forever.
Slackware provides a file `README_CRYPT.txt` explaining how to install
on an encrypted partition. Don't forget to tell the bootloader *LILO*
about the initrd, and keep in mind the initrd must be recreated after
kernel upgrade
## Use ntpd
It's important to have a correct time on your server.
# chmod +x /etc/rc.d/rc.ntpd
# /etc/rc.d/rc.ntpd start
## Disable ssh password authentication
In `/etc/ssh/sshd_config` there are two changes to do:
Turn `UsePam yes` into `UsePam no` and add `PasswordAuthentication`.
Changes can be applied by restarting ssh with `/etc/rc.d/rc.sshd
Before enabling this, don't forget to deploy your public key to an
user who is able to become to root.
## Get a SSL certificate
We need a SSL certificate for the infrastructure, so we will install
[certbot](https://certbot.eff.org/). Unfortunately, certbot-auto
doesn't work on Slackware because the system is unsupported. So we
will use pip and call certbot in standalone mode so we don't need a
web server.
# pip3 install certbot
# certbot certonly --standalone -d mydomain.foobar -m usernam@example
My domain being `kongroo.eu` the files are generated under
## Configure the DNS
Three DNS entries have to be added for a working email server.
1. SPF to tell the world which addresses have the right send your
2. MX to tell the world which addresses will receive the emails and in
which order
3. DKIM (a public key) to allow recipients to check your emails really
comes from your servers (signed used a private key)
4. DMARC to tell recipient what to do with mails not respecting SPF
### SPF
Simple, add an entry with `v=spf1 mx` if you want to allow your MX
servers to send emails. Basically, for simple setups, the same server
receive and send emails.
@ 1800 IN SPF "v=spf1 mx"
### MX
My server with the address `kongroo.eu` will receive the emails.
@ 10800 IN MX 50 kongroo.eu.
### DKIM
This part will be a bit more complicated. We have to generate a pair
of public and private keys and run a daemon that will sign outgoing
emails with the private key, so recipients can verify the emails
signature using the public key available in the DNS. We will use
opendkim, I found this
[very good](https://philio.me/setting-up-dkim-with-sendmail-on-ubuntu-14-04/)
article explaining how to use opendkim with sendmail.
Opendkim isn't part of slackware base packages, fortunately it is
available in [slackbuilds](https://slackbuilds.org/), you can check my
previous article explaining how to setup slackbuilds.
# groupadd -g 305 opendkim
# useradd -r -u 305 -g opendkim -d /var/run/opendkim/ -s /sbin/nologin \
-c "OpenDKIM Milter" opendkim
# sboinstall opendkim
We want to enable opendkim at boot, as it's not a service from the
base system, so we need to "register" it in rc.local and enable both.
Add the following to `/etc/rc.d/rc.local`:
if [ -x /etc/rc.d/rc.opendkim ]; then
/etc/rc.d/rc.opendkim start
Make the scripts executable so they will be run at boot:
# chmod +x /etc/rc.d/rc.local
# chmod +x /etc/rc.d/rc.opendkim
Create the key pair:
# mkdir /etc/opendkim
# cd /etc/opendkim
# opendkim-genkey -t -s default -d kongroo.eu
Get the content of `default.txt`, we will use it as a content for a
TXT entry in the DNS, select only the content between parenthesis
without double quotes: your DNS tool (like on Gandi) may take
everything without warning which would produce an invalid DKIM
signature. *Been there, done that.*
The file should looks like:
default._domainkey IN TXT ( "v=DKIM1; k=rsa; t=y; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5iBUyQ02H5sfS54hg155eQBxtMuhcwB4b896S7o97pPGZEiteby/RtCOz9VV2TOgGckz8eOEeYHnONdlnYWGv8HqVwngPWJmiU7xbyoH489ZkG397ouEJI4mBrU9ZTjULbweT2sVXpiMFCalNraKHMVjqgZWxzqoE3ETGpMNNSwIDAQAB" )
But the content I used for my entry at gandi is:
v=DKIM1; k=rsa; t=y; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5iBUyQ02H5sfS54hg155eQBxtMuhcwB4b896S7o97pPGZEiteby/RtCOz9VV2TOgGckz8eOEeYHnONdlnYWGv8HqVwngPWJmiU7xbyoH489ZkG397ouEJI4mBrU9ZTjULbweT2sVXpiMFCalNraKHMVjqgZWxzqoE3ETGpMNNSwIDAQAB
Now we need to configure opendkim to use our keys. Edit
`/etc/opendkim.conf` to changes the following lines already
Domain kongroo.eu
KeyFile /etc/opendkim/default.private
ReportAddress postmaster@kongroo.eu
### Dmarc
We have to tell DMARC, this may help being accepted by big corporate
mail servers.
_dmarc.kongroo.eu. IN TXT "v=DMARC1;p=none;pct=100;rua=mailto:postmaster@kongroo.eu;"
This will tell the recipient that we don't give specific instruction
to what to do with suspicious mails from our domain and tell
postmaster@kongroo.eu about the reports. **Expect daily mail from
every mail server reached in the day to arrive on that address.**
## Install Sendmail
*Unfortunately Slackware team dropped sendmail in favor to postfix* in
the default install, this may be a good thing but I want
sendmail. *Good news: sendmail is still in the extra directory*.
I wanted to use [citadel](https://citadel.org/) but *it was really
complicated, so I went to sendmail.*
### Installation
Download the two sendmail txz packages on a mirror in the "extra"
Run `/sbin/installpkg` on both packages.
### Configuration
We will disable postfix.
# sh /etc/rc.d/rc.postfix stop
# chmod -x /etc/rc.d/rc.postfix
Enable sendmail and saslauthd
# chmod +x /etc/rc.d/rc.sendmail
# chmod +x /etc/rc.d/rc.saslauthd
All the configuration will be done in `/usr/share/sendmail/cf/cf`, we
will use a default template from the package. As explained in the cf
files, we need to use a template and rebuild from this directory
containing all the macros.
# cp sendmail-slackware-tls-sasl.mc /usr/share/sendmail/cf/cf/config.mc
Every time we want to rebuild the configuration file, we need to apply
the m4 macros to have the real configuration file.
# sh Build config.mc
# cp config.cf /etc/mail/sendmail.cf
My `config.mc` file looks like this (I stripped the comments):
VERSIONID(`TLS supporting setup for Slackware Linux')dnl
define(`confCACERT_PATH', `/etc/letsencrypt/live/kongroo.eu/')
define(`confCACERT', `/etc/letsencrypt/live/kongroo.eu/cert.pem')
define(`confSERVER_CERT', `/etc/letsencrypt/live/kongroo.eu/fullchain.pem')
define(`confSERVER_KEY', `/etc/letsencrypt/live/kongroo.eu/privkey.pem')
define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl
define(`confTO_IDENT', `0')dnl
FEATURE(`mailertable',`hash -o /etc/mail/mailertable.db')dnl
FEATURE(`virtusertable',`hash -o /etc/mail/virtusertable.db')dnl
FEATURE(`access_db', `hash -T /etc/mail/access')dnl
FEATURE(`local_procmail',`',`procmail -t -Y -a $h -d $u')dnl
INPUT_MAIL_FILTER(`opendkim', `S=inet:8891@localhost')
define(`confAUTH_OPTIONS', `A p y')dnl
DAEMON_OPTIONS(`Port=smtp, Name=MTA')dnl
DAEMON_OPTIONS(`Port=smtps, Name=MSA-SSL, M=Esa')dnl
Create the file `/etc/sasl2/Sendmail.conf` with this content:
This will tell sendmail to use saslauthd for PLAIN and LOGIN
connections. Any SMTP client will have to use either PLAIN or LOGIN.
If you start sendmail and saslauthd, you should be able to send
e-mails with authentication.
We need to edit `/etc/mail/local-host-names` to tell sendmail for
which domain it should accept local deliveries.
Simply add your email domain:
The mail logs are located under `/var/log/maillog`, every mail sent
well signed with DKIM should appear under a line like this:
[time] [host] sm-mta[2520]: 0AECKet1002520: Milter (opendkim) insert (1): header: DKIM-Signature: [whole signature]
## Configure DKIM
This has been explained in a subsection of sendmail configuration. If
you didn't read this step because you don't want to setup dkim, you
missed information required for the next steps.
## Install cyrus-imap
**Slackware** ships with dovecot in the default installation, but
cyrus-imapd is available in slackbuilds.
The bad news is that the slackbuild is outdated, so here it a simple
patch to apply in `/usr/sbo/repo/network/cyrus-imapd`. This patch also
fixes a compilation issue.
diff --git a/network/cyrus-imapd/cyrus-imapd.SlackBuild b/network/cyrus-imapd/cyrus-imapd.SlackBuild
index 48e2c54e55..251ca5f207 100644
--- a/network/cyrus-imapd/cyrus-imapd.SlackBuild
+++ b/network/cyrus-imapd/cyrus-imapd.SlackBuild
@@ -23,7 +23,7 @@
@@ -107,6 +107,8 @@ CXXFLAGS="$SLKCFLAGS" \
+sed -i'' 's/gettid/_gettid/g' lib/cyrusdb_berkeley.c
make install DESTDIR=$PKG
diff --git a/network/cyrus-imapd/cyrus-imapd.info b/network/cyrus-imapd/cyrus-imapd.info
index 99b2c68075..6ae26365dc 100644
--- a/network/cyrus-imapd/cyrus-imapd.info
+++ b/network/cyrus-imapd/cyrus-imapd.info
@@ -1,8 +1,8 @@
You can apply it by carefully copying the content in a file and use
the command `patch`.
We can now proceed with cyrus-imapd compilation and installation.
# env DATABASE=sqlite sboinstall cyrus-imapd
As explained in the README file shown during installation, we need to
do a few instructions.
# mkdir -m 750 -p /var/imap /var/spool/imap /var/sieve
# chown cyrus:cyrus /var/imap /var/spool/imap /var/sieve
# su - cyrus
# /usr/doc/cyrus-imapd-2.5.16/tools/mkimap
# logout
Add the following to `/etc/rc.d/rc.local` to enable cyrus-imapd at
if [ -x /etc/rc.d/rc.cyrus-imapd ]; then
/etc/rc.d/rc.cyrus-imapd start
And make the rc script executable:
# chmod +x /etc/rc.d/rc.cyrus-imapd
[The official](https://www.cyrusimap.org/imap/installing.html) cyrus
documentation is very well done and was very helpful while writing this.
The configuration file is `/etc/imapd.conf`:
configdirectory: /var/imap
partition-default: /var/spool/imap
sievedir: /var/sieve
admins: cyrus
sasl_pwcheck_method: saslauthd
allowplaintext: yes
tls_server_cert: /etc/letsencrypt/cyrus/fullchain.pem
tls_server_key: /etc/letsencrypt/cyrus/privkey.pem
tls_client_ca_dir: /etc/ssl/certs
There is another file `/etc/cyrusd.conf` used but we don't need to
make changes in it.
We will have to copy the certificates into a separate place and allow
cyrus user to read them. This will have to be done every time the
certificate are renewed. Let's add the certbot command so we can use
this script as a cron.
certbot certonly --standalone -d $DOMAIN -m usernam@example
mkdir -p $DESTDIR
install -o cyrus -g cyrus -m 400 $LIVEDIR/fullchain.pem $DESTDIR
install -o cyrus -g cyrus -m 400 $LIVEDIR/privkey.pem $DESTDIR
/etc/rc.d/rc.sendmail restart
/etc/rc.d/rc.cyrus-imapd restart
Add a crontab entry to run this script once a day, using `crontab -e`
to change root crontab.
0 5 * * * sh /root/renew_certs.sh
## Starting the mail server
We prepared the mail server to be working on reboot, but the services
aren't started yet.
# /etc/rc.d/rc.saslauthd start
# /etc/rc.d/rc.sendmail start
# /etc/rc.d/rc.cyrus-imapd start
# /etc/rc.d/rc.opendkim start
## Adding a new user
Add a new user to your system.
# useradd $username
# passwd $username
For some reasons the user mailboxes must be initialized. The same
password must be typed twice (or passed as parameter using `-w
# USER=foobar
# DOMAIN=kongroo.eu
# echo "cm INBOX" | rlwrap cyradm -u $USER $DOMAIN
IMAP Password:
Voila! The user should be able to connect using IMAP and receive
## Check your email setup
You can use the web service [Mail
tester](https://www.mail-tester.com/) by sending an email. You could
copy/paste a real email to avoid having a bad mark due to spam
recognition (which happens if you send a mail with a few words). The
bad spam core isn't relevant anyway as long as it's due to the content
of your email.
## Conclusion
I had real fun writing this article, digging hard in Slackware and
playing with unusual programs like sendmail and cyrus-imapd. I hope
you will enjoy too as much as I enjoyed writing it!
If you find mistakes or bad configuration settings, please contact me
so, I will be happy to discuss about the change and fix this how-to.
Nota Bene: Slackbuilds aren't mean to be used on the current version,
but really on the last release. There is a github repository carrying
the -current changes on a github repository