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Full featured Slackware email server with sendmail and cyrus-imapd

Written by Solène, on 14 November 2020.
Tags: #slackware #email

Comments on Mastodon

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 laptop.

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 restart.

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. 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 /etc/letsencrypt/live/kongroo.eu/.

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 emails
  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 article explaining how to use opendkim with sendmail.

Opendkim isn’t part of slackware base packages, fortunately it is available in slackbuilds, 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
fi

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 there:

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 but it was really complicated, so I went to sendmail.

Installation

Download the two sendmail txz packages on a mirror in the “extra” directory: https://mirrors.slackware.com/slackware/slackware64-current/extra/sendmail/

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):

include(`../m4/cf.m4')
VERSIONID(`TLS supporting setup for Slackware Linux')dnl
OSTYPE(`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(`use_cw_file')dnl
FEATURE(`use_ct_file')dnl
FEATURE(`mailertable',`hash -o /etc/mail/mailertable.db')dnl
FEATURE(`virtusertable',`hash -o /etc/mail/virtusertable.db')dnl
FEATURE(`access_db', `hash -T<TMPF> /etc/mail/access')dnl
FEATURE(`blocklist_recipients')dnl
FEATURE(`local_procmail',`',`procmail -t -Y -a $h -d $u')dnl
FEATURE(`always_add_domain')dnl
FEATURE(`redirect')dnl
FEATURE(`no_default_msa')dnl
EXPOSED_USER(`root')dnl
LOCAL_DOMAIN(`localhost.localdomain')dnl
INPUT_MAIL_FILTER(`opendkim', `S=inet:8891@localhost')
MAILER(local)dnl
MAILER(smtp)dnl
MAILER(procmail)dnl
define(`confAUTH_OPTIONS', `A p y')dnl
define(`confAUTH_MECHANISMS', `LOGIN PLAIN DIGEST-MD5 CRAM-MD5')dnl
TRUST_AUTH_MECH(`LOGIN PLAIN DIGEST-MD5 CRAM-MD5')dnl
DAEMON_OPTIONS(`Port=smtp, Name=MTA')dnl
DAEMON_OPTIONS(`Port=smtps, Name=MSA-SSL, M=Esa')dnl
LOCAL_CONFIG
O CipherList=ALL:!ADH:!NULL:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:-LOW:+SSLv3:+TLSv1:-SSLv2:+EXP:+eNULL

Create the file /etc/sasl2/Sendmail.conf with this content:

pwcheck_method:saslauthd

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:

kongroo.eu

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 @@
 #  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 PRGNAM=cyrus-imapd
-VERSION=${VERSION:-2.5.11}
+VERSION=${VERSION:-2.5.16}
 BUILD=${BUILD:-1}
 TAG=${TAG:-_SBo}

@@ -107,6 +107,8 @@ CXXFLAGS="$SLKCFLAGS" \
   $DATABASE \
   --build=$ARCH-slackware-linux

+sed -i'' 's/gettid/_gettid/g' lib/cyrusdb_berkeley.c
+
 make PERL_MM_OPT='INSTALLDIRS=vendor'
 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 @@
 PRGNAM="cyrus-imapd"
 VERSION="2.5.11"
 HOMEPAGE="https://www.cyrusimap.org/"
-DOWNLOAD="ftp://ftp.cyrusimap.org/cyrus-imapd/cyrus-imapd-2.5.11.tar.gz"
-MD5SUM="674083444c36a786d9431b6612969224"
+DOWNLOAD="https://github.com/cyrusimap/cyrus-imapd/releases/download/cyrus-imapd-2.5.16/cyrus-imapd-2.5.16.tar.gz"
+MD5SUM="d5667e91d8e094ef24560a148e39c462"
 DOWNLOAD_x86_64=""
 MD5SUM_x86_64=""
 REQUIRES=""

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 boot:

if [ -x /etc/rc.d/rc.cyrus-imapd ]; then
  /etc/rc.d/rc.cyrus-imapd start
fi

And make the rc script executable:

# chmod +x /etc/rc.d/rc.cyrus-imapd

The official 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.

#!/bin/sh
DOMAIN=kongroo.eu
LIVEDIR=/etc/letsencrypt/live/$DOMAIN/
DESTDIR=/etc/letsencrypt/cyrus/

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.

MAILTO=""
PATH=/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin
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 $password).

# USER=foobar
# DOMAIN=kongroo.eu
# echo "cm INBOX" | rlwrap cyradm -u $USER $DOMAIN
Password:
IMAP Password:

Voila! The user should be able to connect using IMAP and receive emails.

Check your email setup

You can use the web service Mail tester 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 https://github.com/Ponce/slackbuilds/.