Let's Encrypt with OpenBSD and Rex

Published at 2022-07-30T12:14:31+01:00

I was amazed at how easy it is to automatically generate and update Let's Encrypt certificates with OpenBSD.

                                               /    _    \
  The Hebern Machine                            \ ." ". /
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                        _/  o     (O)     o   _            |
                      _/                    ." ".          |
                    I/    _________________/     \         |
                  _/I   ."                        |        |
          =====  /  I  /                         /         |
     =====  | | |   \ |       _________________."          |
=====  | |  | | |   /  \     /  _|_|__|_|_          __     |
  | |  | |  | | |   \   "._."  /  o    o  \       ."  ".   |
  |  --|  --|  -|   /          \         _/      /      \  |
   \____\____\__|   \  ______   |       /       |        | |
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              ( )        (O)          /          \      /  |
               -----------------------            ".__."   |
               _|__________________________________________|_
              /                                              \
             /________________________________________________\
                                 ASCII Art by John Savard

Let's Encrypt with OpenBSD and Rex
⇢ ⇢ What's Let's Encrypt?
⇢ Meet acme-client
⇢ Configuration
⇢ ⇢ acme-client.conf
⇢ ⇢ httpd.conf
⇢ ⇢ CRON job
⇢ relayd.conf and smtpd.conf
⇢ Rexification
⇢ ⇢ General ACME client configuration
⇢ ⇢ Service rexification
⇢ All pieces together
⇢ Conclusion

What's Let's Encrypt?

Let's Encrypt is a non-profit certificate authority run by Internet Security Research Group that provides X.509 certificates for Transport Layer Security (TLS) encryption at no charge. It is the world's largest certificate authority, used by more than 265 million websites, with the goal of all websites being secure and using HTTPS.

Source: Wikipedia

In short, it gives away TLS certificates for your website - for free! The catch is, that the certificates are only valid for three months. So it is better to automate certificate generation and renewals.

Meet acme-client

acme-client is the default Automatic Certifcate Management Environment (ACME) client on OpenBSD and part of the OpenBSD base system.

When invoked, the client first checks whether certificates actually require to be generated.

It first checks whether a certificate already exists; if not, it will attempt to generate a new one.
If the certificate already exists but expires within the next 30 days, it will renew it.
Otherwise, acme-client won't do anything.

Oversimplified, the following steps are undertaken by acme-client for generating a new certificate:

Reading its config file /etc/acme-client.conf for a list of hosts (and their alternative names) to generate certificates. So it means you can also have certificates for arbitrary subdomains!
Automatic generation of the private certificate part (the certificate key) and the certificate signing request (CSR) to /etc/ssl/....
Requesting Let's Encrypt to sign the certificate. This also includes providing a set of temporary files requested by Let's Encrypt in the next step for verification.
Let's Encrypt then will contact the hostname for the certificate through a particular URL (e.g. http://foo.zone/.well-known/acme-challenge/...) to verify that the requester is the valid owner of the host.
Let's Encrypt generates a certificate, which then is downloaded to /etc/ssl/....

Configuration

There is some (but easy) configuration required to make that all work on OpenBSD.

acme-client.conf

This is how my /etc/acme-client.conf looks like (I copied a template from /etc/examples/acme-client.conf to /etc/acme-client.conf and added my domains to the bottom:

#
# $OpenBSD: acme-client.conf,v 1.4 2020/09/17 09:13:06 florian Exp $
#
authority letsencrypt {
    api url "https://acme-v02.api.letsencrypt.org/directory"
    account key "/etc/acme/letsencrypt-privkey.pem"
}

authority letsencrypt-staging {
    api url "https://acme-staging-v02.api.letsencrypt.org/directory"
    account key "/etc/acme/letsencrypt-staging-privkey.pem"
}

authority buypass {
    api url "https://api.buypass.com/acme/directory"
    account key "/etc/acme/buypass-privkey.pem"
    contact "mailto:me@example.com"
}

authority buypass-test {
    api url "https://api.test4.buypass.no/acme/directory"
    account key "/etc/acme/buypass-test-privkey.pem"
    contact "mailto:me@example.com"
}

domain buetow.org {
    alternative names { www.buetow.org paul.buetow.org }
    domain key "/etc/ssl/private/buetow.org.key"
    domain full chain certificate "/etc/ssl/buetow.org.fullchain.pem"
    sign with letsencrypt
}

domain dtail.dev {
    alternative names { www.dtail.dev }
    domain key "/etc/ssl/private/dtail.dev.key"
    domain full chain certificate "/etc/ssl/dtail.dev.fullchain.pem"
    sign with letsencrypt
}

domain foo.zone {
    alternative names { www.foo.zone }
    domain key "/etc/ssl/private/foo.zone.key"
    domain full chain certificate "/etc/ssl/foo.zone.fullchain.pem"
    sign with letsencrypt
}

domain irregular.ninja {
    alternative names { www.irregular.ninja }
    domain key "/etc/ssl/private/irregular.ninja.key"
    domain full chain certificate "/etc/ssl/irregular.ninja.fullchain.pem"
    sign with letsencrypt
}

domain snonux.land {
    alternative names { www.snonux.land }
    domain key "/etc/ssl/private/snonux.land.key"
    domain full chain certificate "/etc/ssl/snonux.land.fullchain.pem"
    sign with letsencrypt
}

httpd.conf

For ACME to work, you will need to configure the HTTP daemon so that the "special" ACME requests from Let's Encrypt are served correctly. I am using the standard OpenBSD httpd here. These are the snippets I use for the foo.zone host in /etc/httpd.conf (of course, you need a similar setup for all other hosts as well):

server "foo.zone" {
  listen on * port 80
  location "/.well-known/acme-challenge/*" {
    root "/acme"
    request strip 2
  }
  location * {
    block return 302 "https://$HTTP_HOST$REQUEST_URI"
  }
}

server "foo.zone" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/foo.zone.fullchain.pem"
    key "/etc/ssl/private/foo.zone.key"
  }
  location * {
    root "/htdocs/gemtexter/foo.zone"
    directory auto index
  }
}

As you see, plain HTTP only serves the ACME challenge path. Otherwise, it redirects the requests to TLS. The TLS section then attempts to use the Let's Encrypt certificates.

It is worth noticing that httpd will start without the certificates being present. This will cause a certificate error when you try to reach the HTTPS endpoint, but it helps to bootstrap Let's Encrypt. As you saw in the config snippet above, Let's Encrypt only requests the plain HTTP endpoint for the verification process, so HTTPS doesn't need to be operational yet at this stage. But once the certificates are generated, you will have to reload or restart httpd to use any new certificate.

CRON job

You could now run doas acme-client foo.zone to generate the certificate or to renew it. Or you could automate it with CRON.

I have created a script /usr/local/bin/acme.sh for that for all of my domains:

#!/bin/sh

function handle_cert {
    host=$1
    # Create symlink, so that relayd also can read it.
    crt_path=/etc/ssl/$host
    if [ -e $crt_path.crt ]; then
        rm $crt_path.crt
    fi
    ln -s $crt_path.fullchain.pem $crt_path.crt
    # Requesting and renewing certificate.
    /usr/sbin/acme-client -v $host
}

has_update=no
handle_cert www.buetow.org
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.paul.buetow.org
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.tmp.buetow.org
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.dtail.dev
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.foo.zone
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.irregular.ninja
if [ $? -eq 0 ]; then
    has_update=yes
fi
handle_cert www.snonux.land
if [ $? -eq 0 ]; then
    has_update=yes
fi

# Pick up the new certs.
if [ $has_update = yes ]; then
    /usr/sbin/rcctl reload httpd
    /usr/sbin/rcctl reload relayd
    /usr/sbin/rcctl restart smtpd
fi

And added the following line to /etc/daily.local to run the script once daily so that certificates will be renewed fully automatically:

/usr/local/bin/acme.sh

I am receiving a daily output via E-Mail like this now:

Running daily.local:
acme-client: /etc/ssl/buetow.org.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/paul.buetow.org.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/tmp.buetow.org.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/dtail.dev.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/foo.zone.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/irregular.ninja.fullchain.pem: certificate valid: 80 days left
acme-client: /etc/ssl/snonux.land.fullchain.pem: certificate valid: 79 days left

relayd.conf and smtpd.conf

Besides httpd, relayd (mainly for Gemini) and smtpd (for mail, of course) also use TLS certificates. And as you can see in acme.sh, the services are reloaded or restarted (smtpd doesn't support reload) whenever a certificate is generated or updated.

Rexification

I didn't write all these configuration files by hand. As a matter of fact, everything is automated with the Rex configuration management system.

https://www.rexify.org

At the top of the Rexfile I define all my hosts:

our @acme_hosts = qw/buetow.org paul.buetow.org tmp.buetow.org dtail.dev foo.zone irregular.ninja snonux.land/;

General ACME client configuration

ACME will be installed into the frontend group of hosts. Here, blowfish is the primary, and twofish is the secondary OpenBSD box.

group frontends => 'blowfish.buetow.org', 'twofish.buetow.org';

This is my Rex task for the general ACME configuration:

desc 'Configure ACME client';
task 'acme', group => 'frontends',
  sub {
    file '/etc/acme-client.conf',
      content => template('./etc/acme-client.conf.tpl',
        acme_hosts => \@acme_hosts,
        is_primary => $is_primary),
      owner => 'root',
      group => 'wheel',
      mode => '644';

    file '/usr/local/bin/acme.sh',
      content => template('./scripts/acme.sh.tpl',
        acme_hosts => \@acme_hosts,
        is_primary => $is_primary),
      owner => 'root',
      group => 'wheel',
      mode => '744';

    file '/etc/daily.local',
      ensure => 'present',
      owner => 'root',
      group => 'wheel',
      mode => '644';

    append_if_no_such_line '/etc/daily.local', '/usr/local/bin/acme.sh';
  };

And there is also a Rex task just to run the ACME script remotely:

desc 'Invoke ACME client';
task 'acme_invoke', group => 'frontends',
  sub {
    say run '/usr/local/bin/acme.sh';
  };

Furthermore, this snippet (also at the top of the Rexfile) helps to determine whether the current server is the primary server (all hosts will be without the www. prefix) or the secondary server (all hosts will be with the www. prefix):

# Bootstrapping the FQDN based on the server IP as the hostname and domain
# facts aren't set yet due to the myname file in the first place.
our $fqdns = sub {
  my $ipv4 = shift;
  return 'blowfish.buetow.org' if $ipv4 eq '23.88.35.144';
  return 'twofish.buetow.org' if $ipv4 eq '108.160.134.135';
  Rex::Logger::info("Unable to determine hostname for $ipv4", 'error');
  return 'HOSTNAME-UNKNOWN.buetow.org';
};

# To determine whether the server is the primary or the secondary.
our $is_primary = sub {
  my $ipv4 = shift;
  $fqdns->($ipv4) eq 'blowfish.buetow.org';
};

The following is the acme-client.conf.tpl Rex template file used for the automation. You see that the www. prefix isn't sent for the primary server. E.g. foo.zone will be served by the primary server (in my case, a server located in Germany) and www.foo.zone by the secondary server (in my case, a server located in Japan):

#
# $OpenBSD: acme-client.conf,v 1.4 2020/09/17 09:13:06 florian Exp $
#
authority letsencrypt {
	api url "https://acme-v02.api.letsencrypt.org/directory"
	account key "/etc/acme/letsencrypt-privkey.pem"
}

authority letsencrypt-staging {
	api url "https://acme-staging-v02.api.letsencrypt.org/directory"
	account key "/etc/acme/letsencrypt-staging-privkey.pem"
}

authority buypass {
	api url "https://api.buypass.com/acme/directory"
	account key "/etc/acme/buypass-privkey.pem"
	contact "mailto:me@example.com"
}

authority buypass-test {
	api url "https://api.test4.buypass.no/acme/directory"
	account key "/etc/acme/buypass-test-privkey.pem"
	contact "mailto:me@example.com"
}

<%
  our $primary = $is_primary->($vio0_ip);
  our $prefix = $primary ? '' : 'www.';
%>

<% for my $host (@$acme_hosts) { %>
domain <%= $prefix.$host %> {
	domain key "/etc/ssl/private/<%= $prefix.$host %>.key"
	domain full chain certificate "/etc/ssl/<%= $prefix.$host %>.fullchain.pem"
	sign with letsencrypt
}
<% } %>

And this is the acme.sh.tpl:

#!/bin/sh

<%
  our $primary = $is_primary->($vio0_ip);
  our $prefix = $primary ? '' : 'www.';
-%>

function handle_cert {
    host=$1
    # Create symlink, so that relayd also can read it.
    crt_path=/etc/ssl/$host
    if [ -e $crt_path.crt ]; then
        rm $crt_path.crt
    fi
    ln -s $crt_path.fullchain.pem $crt_path.crt
    # Requesting and renewing certificate.
    /usr/sbin/acme-client -v $host
}

has_update=no
<% for my $host (@$acme_hosts) { -%>
handle_cert <%= $prefix.$host %>
if [ $? -eq 0 ]; then
    has_update=yes
fi
<% } -%>

# Pick up the new certs.
if [ $has_update = yes ]; then
    /usr/sbin/rcctl reload httpd
    /usr/sbin/rcctl reload relayd
    /usr/sbin/rcctl restart smtpd
fi

Service rexification

These are the Rex tasks setting up httpd, relayd and smtpd services:

desc 'Setup httpd';
task 'httpd', group => 'frontends',
  sub {
    append_if_no_such_line '/etc/rc.conf.local', 'httpd_flags=';

    file '/etc/httpd.conf',
      content => template('./etc/httpd.conf.tpl',
        acme_hosts => \@acme_hosts,
        is_primary => $is_primary),
      owner => 'root',
      group => 'wheel',
      mode => '644',
      on_change => sub { service 'httpd' => 'restart' };

    service 'httpd', ensure => 'started';
  };

desc 'Setup relayd';
task 'relayd', group => 'frontends',
  sub {
    append_if_no_such_line '/etc/rc.conf.local', 'relayd_flags=';

    file '/etc/relayd.conf',
      content => template('./etc/relayd.conf.tpl',
        ipv6address => $ipv6address,
        is_primary => $is_primary),
      owner => 'root',
      group => 'wheel',
      mode => '600',
      on_change => sub { service 'relayd' => 'restart' };

    service 'relayd', ensure => 'started';
  };

desc 'Setup OpenSMTPD';
task 'smtpd', group => 'frontends',
  sub {
    Rex::Logger::info('Dealing with mail aliases');
    file '/etc/mail/aliases',
      source => './etc/mail/aliases',
      owner => 'root',
      group => 'wheel',
      mode => '644',
      on_change => sub { say run 'newaliases' };

    Rex::Logger::info('Dealing with mail virtual domains');
    file '/etc/mail/virtualdomains',
      source => './etc/mail/virtualdomains',
      owner => 'root',
      group => 'wheel',
      mode => '644',
      on_change => sub { service 'smtpd' => 'restart' };

    Rex::Logger::info('Dealing with mail virtual users');
    file '/etc/mail/virtualusers',
      source => './etc/mail/virtualusers',
      owner => 'root',
      group => 'wheel',
      mode => '644',
      on_change => sub { service 'smtpd' => 'restart' };

    Rex::Logger::info('Dealing with smtpd.conf');
    file '/etc/mail/smtpd.conf',
      content => template('./etc/mail/smtpd.conf.tpl',
        is_primary => $is_primary),
      owner => 'root',
      group => 'wheel',
      mode => '644',
      on_change => sub { service 'smtpd' => 'restart' };

    service 'smtpd', ensure => 'started';
  };

This is the httpd.conf.tpl:

<%
  our $primary = $is_primary->($vio0_ip);
  our $prefix = $primary ? '' : 'www.';
%>

# Plain HTTP for ACME and HTTPS redirect
<% for my $host (@$acme_hosts) { %>
server "<%= $prefix.$host %>" {
  listen on * port 80
  location "/.well-known/acme-challenge/*" {
    root "/acme"
    request strip 2
  }
  location * {
    block return 302 "https://$HTTP_HOST$REQUEST_URI"
  }
}
<% } %>

# Gemtexter hosts
<% for my $host (qw/foo.zone snonux.land/) { %>
server "<%= $prefix.$host %>" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix.$host %>.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix.$host %>.key"
  }
  location * {
    root "/htdocs/gemtexter/<%= $host %>"
    directory auto index
  }
}
<% } %>

# DTail special host
server "<%= $prefix %>dtail.dev" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix %>dtail.dev.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix %>dtail.dev.key"
  }
  location * {
    block return 302 "https://github.dtail.dev$REQUEST_URI"
  }
}

# Irregular Ninja special host
server "<%= $prefix %>irregular.ninja" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix %>irregular.ninja.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix %>irregular.ninja.key"
  }
  location * {
    root "/htdocs/irregular.ninja"
    directory auto index
  }
}

# buetow.org special host.
server "<%= $prefix %>buetow.org" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix %>buetow.org.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix %>buetow.org.key"
  }
  block return 302 "https://paul.buetow.org"
}

server "<%= $prefix %>paul.buetow.org" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix %>paul.buetow.org.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix %>paul.buetow.org.key"
  }
  block return 302 "https://foo.zone/contact-information.html"
}

server "<%= $prefix %>tmp.buetow.org" {
  listen on * tls port 443
  tls {
    certificate "/etc/ssl/<%= $prefix %>tmp.buetow.org.fullchain.pem"
    key "/etc/ssl/private/<%= $prefix %>tmp.buetow.org.key"
  }
  root "/htdocs/buetow.org/tmp"
  directory auto index
}

and this the relayd.conf.tpl:

<%
  our $primary = $is_primary->($vio0_ip);
  our $prefix = $primary ? '' : 'www.';
%>

log connection

tcp protocol "gemini" {
    tls keypair <%= $prefix %>foo.zone
    tls keypair <%= $prefix %>buetow.org
}

relay "gemini4" {
    listen on <%= $vio0_ip %> port 1965 tls
    protocol "gemini"
    forward to 127.0.0.1 port 11965
}

relay "gemini6" {
    listen on <%= $ipv6address->($hostname) %> port 1965 tls
    protocol "gemini"
    forward to 127.0.0.1 port 11965
}

And last but not least, this is the smtpd.conf.tpl:

<%
  our $primary = $is_primary->($vio0_ip);
  our $prefix = $primary ? '' : 'www.';
%>

pki "buetow_org_tls" cert "/etc/ssl/<%= $prefix %>buetow.org.fullchain.pem"
pki "buetow_org_tls" key "/etc/ssl/private/<%= $prefix %>buetow.org.key"

table aliases file:/etc/mail/aliases
table virtualdomains file:/etc/mail/virtualdomains
table virtualusers file:/etc/mail/virtualusers

listen on socket
listen on all tls pki "buetow_org_tls" hostname "<%= $prefix %>buetow.org"
#listen on all

action localmail mbox alias <aliases>
action receive mbox virtual <virtualusers>
action outbound relay

match from any for domain <virtualdomains> action receive
match from local for local action localmail
match from local for any action outbound

All pieces together

For the complete Rexfile example and all the templates, please look at the Git repository:

https://codeberg.org/snonux/rexfiles

Besides ACME, other things, such as DNS servers, are also rexified. The following command will run all the Rex tasks and configure everything on my frontend machines automatically:

rex commons

The commons is a group of tasks I specified which combines a set of common tasks I always want to execute on all frontend machines. This also includes the ACME tasks mentioned in this article!

Conclusion

ACME and Let's Encrypt greatly help reduce recurring manual maintenance work (creating and renewing certificates). Furthermore, all the certificates are free of cost! I love to use OpenBSD and Rex to automate all of this.

OpenBSD suits perfectly here as all the tools are already part of the base installation. But I like underdogs. Rex is not as powerful and popular as other configuration management systems (e.g. Puppet, Chef, SALT or even Ansible). It is more of an underdog, and the community is small.

Why re-inventing the wheel? I love that a Rexfile is just a Perl DSL. Also, OpenBSD comes with Perl in the base system. So no new programming language had to be added to my mix for the configuration management system. Also, the acme.sh shell script is not a Bash but a standard Bourne shell script, so I didn't have to install an additional shell as OpenBSD does not come with the Bash pre-installed.

E-Mail your comments to paul@nospam.buetow.org :-)

Other *BSD related posts are:

2016-04-09 Jails and ZFS with Puppet on FreeBSD
2022-07-30 Let's Encrypt with OpenBSD and Rex (You are currently reading this)
2022-10-30 Installing DTail on OpenBSD
2024-01-13 One reason why I love OpenBSD
2024-04-01 KISS high-availability with OpenBSD
2024-11-17 f3s: Kubernetes with FreeBSD - Setting the stage - Part 1

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