Using Let’s encrypt for non-web servers

I think I don’t need to explain Let’s encrypt anymore. But what many people are struggling with is to use Let’s encrypt certificates for other services like SMTP, IMAP, IRC, etc.

Using certbot this is quiet easy (See https://certbot.eff.org for installation instructions).

When certbot is installed you can use it in standalone mode. This means it starts a built-in webserver which is used for the authentication process and gets stopped again a few seconds later.

To make it short here is an example command to create a new certificate for your mail server:

./certbot-auto certonly --standalone -d smtp.yourdomain.com -d imap.yourdomain.com

Of course the standalone webserver must be reachable from the internet, so ensure that no firewall is blocking port 443 (https). In my case I have a firewall running, so I need to temporary enable https. Certbot also supports this by using the options pre-hook and post-hook.

./certbot-auto certonly --standalone --pre-hook /root/enable_https --post-hook /root/disable_https -d smtp.domain.com -d imap.domain.com

The example hook scripts insert a firewall rule for https and remove it again. This again are just examples that you need to adapt to your needs.

enable_https:

#!/bin/bash
IPTABLES=/sbin/iptables
$IPTABLES -I INPUT 8 -i eth0 -p tcp -m conntrack --ctstate NEW -m tcp --dport 443 -j ACCEPT

disable_https:

#!/bin/bash
IPTABLES=/sbin/iptables
$IPTABLES -D INPUT -i eth0 -p tcp -m conntrack --ctstate NEW -m tcp --dport 443 -j ACCEPT

See man(8) iptables for more information on this.

Renewing is also easy. By default the “certbot-auto renew” command will renew all certificates with the same options. Only the hooks must be give again at the command line.

./certbot-auto renew --pre-hook /root/enable_https --post-hook /root/disable_https

It is recommended to call this twice a day. Certbot will only really renew it when the certificate is about to expire. To automate this process you can create a cronjob.

I used the template which gets created when installing certbot using Debian Jessie.

/etc/cron.d/certbot:

# /etc/cron.d/certbot: crontab entries for the certbot package
#
# Upstream recommends attempting renewal twice a day
#
# Eventually, this will be an opportunity to validate certificates
# haven't been revoked, etc.  Renewal will only occur if expiration
# is within 30 days.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# twice a day
0 */12 * * * root test -x /root/certbot-auto && perl -e 'sleep int(rand(3600))' && /root/certbot-auto -q renew --pre-hook /root/enable_https --post-hook /root/disable_https

Finally you need to update your mail server configuration to use the new certificates. Let’s encrypt stores the currently active certificate in /etc/letsencrypt/live/<your domain>/. This folder contains only symlinks to /etc/letsencrypt/archive/<your domain>/ with the real certificate files and keys, chains, etc.

In my case I simply edited /etc/postfix/main.cf and /etc/imapd.conf to use these new files.
Cyrus was no able to access the files by default, because the default file permissions prevented it to access the folders of “Let’s Encrypt”.
I fixed this by giving cyrus readonly access using the ssl-cert group.

chgrp -R ssl-cert /etc/letsencrypt/live /etc/letsencrypt/archive
chmod 750 /etc/letsencrypt/live /etc/letsencrypt/archive
usermod -a -G ssl-cert cyrus
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