TLS is used to secure web services. This was expensive until Let's Encrypt became first Free SSL certificate service for providing TLS security for network services.

For some time certbot-auto was the standard method for updating certificates for Let's Encrypt but this is now obsolete so here we look at acme.sh which by default uses ZeroSSL.

Install prerequisites and then clone acme.sh and create a working account with your valid email address.

sudo apt install git wget socat
git clone https://github.com/acmesh-official/acme.sh.git
cd acme.sh
./acme.sh --install -m user@example.com

This process creates an install within your home directory in a subdir named .acme.sh. It will also create a daily cron job.

Disable the following cron job which will be replaced later by a different script.

crontab -l
47 0 * * * "/home/user/.acme.sh"/acme.sh --cron --home "/home/user/.acme.sh" > /dev/null

Additionally, a line is added to your shell profile, eg. for TCSH:

source "/home/user/.acme.sh/acme.sh.csh"

The source file included contains the following

setenv LE_WORKING_DIR "/home/user/.acme.sh"
alias acme.sh "/home/user/.acme.sh/acme.sh"

Issue a certificate as a logged in user in the htdocs directory. This has to be only done once and likely before certificates are configured in the web server configuration.

#! /bin/bash

# issue.sh
ID=`id -u`
if test "$ID" -eq 0; then
        echo "run as user"
        exit 1
fi
if test $# -ne 1; then
        echo "missing vhost"
        exit 1
fi
VHOST=$1

sudo mkdir -p /var/www/htdocs/$VHOST/.well-known/acme-challenge
sudo chown -R $USER:$GROUP /var/www/htdocs/$VHOST/.well-known

export LE_WORKING_DIR="$HOME/.acme.sh"
$HOME/.acme.sh/acme.sh --force --issue -d $VHOST -w /var/www/htdocs/$VHOST -k 2048

sudo mkdir -p /var/www/ssl
sudo chown 0:0 /var/www/ssl

sudo mkdir -p /var/www/ssl/$VHOST
sudo chown -R $USER:$GROUP /var/www/ssl/$VHOST

$HOME/.acme.sh/acme.sh --install-cert --domain $VHOST --cert-file /var/www/ssl/$VHOST/cert.pem --key-file /var/www/ssl/$VHOST/key.pem --ca-file /var/www/ssl/$VHOST/ca.pem --fullchain-file /var/www/ssl/$VHOST/fullchain.pem

exit 0

This process should be run once per month or every two months to update all certificates. It can also be used to update single certificate if the virtual host is specified. The server process needs a signal to reload the files.

#! /bin/bash

# renew.sh
ID=`id -u`
if test "$ID" -eq 0; then
        echo "run as user"
        exit 1
fi

DIRS=`find /var/www/htdocs/* -type d -prune`
for DIR in $DIRS; do
        ACME="$DIR/.well-known/acme-challenge"
        sudo mkdir -p "$ACME"
        sudo chown -R $USER:$USER "$ACME"
done

export LE_WORKING_DIR="$HOME/.acme.sh"
if test $# -ne 1; then
        sleep $[($RANDOM % 40) + 10]
        $HOME/.acme.sh/acme.sh --renew-all --force
else
        VHOST=$1
        $HOME/.acme.sh/acme.sh --renew -d $VHOST --force
fi

#sudo kill -HUP  `ps auxw | egrep '^root.*nginx: master' | grep -v grep | awk '{print $2}'`
#sudo kill -USR1 `ps auxw | egrep '^root.*apache2' | grep -v grep | awk '{print $2}'`
sudo /etc/init.d/nginx reload

exit 0

Eg.

0 0 2 * * /home/user/bin/renew.sh 1>/dev/null 2>/dev/null

#! /bin/bash

# remove.sh
ID=`id -u`
if test "$ID" -eq 0; then
        echo "run as user"
        exit 1
fi
if test $# -ne 1; then
        echo "missing vhost"
        exit 1
fi
VHOST=$1

export LE_WORKING_DIR="$HOME/.acme.sh"
$HOME/.acme.sh/acme.sh --remove -d $VHOST

rm -Rf $HOME/.acme.sh/$VHOST*
sudo rm -Rf /var/www/ssl/$VHOST

exit 0

# m h dom mon dow command
0 0 2 * * /home/user/bin/renew.sh 1>/dev/null 2>/dev/null

You may wish to change the default issuer since zerossl doesn't support IPv6 but Let's Encrypt does.

acme.sh --set-default-ca --server letsencrypt

acme.sh --set-default-ca --server zerossl

After issuing and later renewing certificates fullchain.pem and key.pem may be copied and utilised by both exim and dovecot. Exim will need read permission for the exim user.

For example, fullchain and key are copied into /etc/exim4/ssl

tls_certificate = /etc/exim4/ssl/certificate.pem
tls_privatekey  = /etc/exim4/ssl/privatekey.pem

ssl_cert = </etc/exim4/ssl/certificate.pem
ssl_key = </etc/exim4/ssl/privatekey.pem

acme.sh on github