Download package

wget https://www.tana.it/sw/zdkimfilter/zdkimfilter-3.8.tar.gz

Install necessary packages

sudo apt install pkg-config libbsd-dev libssl-dev zlib1g-dev libunistring-dev libidn2-dev libopendbx1-dev

Compile and install

tar xfz zdkimfilter-3.8.tar.gz
cd zdkimfilter-3.8
./configure
make
sudo make install

Configure

Get superpowers :

sudo -i

Generate keys

cd /etc/courier/filters
mkdir keys
chown courier:courier keys
cd keys
/usr/local/bin/zdkimgenkey -s mydomain -d thisdomain.org

Here, the selector mydomain is just a name that I chose different from the real domain thisdomain.org for the purpose of this tutorial. You would normally use something like thisdomain.

chown courier:courier *

Setup the DNS

You have to put the public key that you just generated (the mydomain.txt file) into your DNS. It is provided in a bind-compatible format. Since I use djbdns, I have to transform the entry. I use the site https://andersbrownworth.com/projects/sysadmin/djbdnsRecordBuilder/#domainKeys to do the dirty work :

Just enter what follows p= in the TXT.

Configure

cd ..
mv zdkimfilter.conf.dist zdkimfilter.conf

Add a default domain at the end of the config file. I use thisdomain.org but you can use any domain. It is used when zdkimfilter cannot guess the real domain, that is, when the sender is something like paul (without @somedomain.tld).

It is recommended to also add the following lines :

no_signlen = Y
header_canon_relaxed = Y
body_canon_relaxed = Y

Create the links and start the filter

cd keys
ln -s mydomain.private thisdomain.org
filterctl start zdkimfilter