Let's Encrypt with OpenBSD and Rex

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

Let's Encrypt with OpenBSD and Rex

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Published by Paul at 2022-07-30

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

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 renevals.

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 for. So it means you can also have certificates for abritary 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 which will be requested by Let's Encrypt in the next step for verification.
Let's Encrypt then will contact the hostname for the certificate through a special 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 also need to configure the HTTP daemon so that the "special" ACME requests made 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 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 is redirecting 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 actually 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 that 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 in order 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 of 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 also reloaded or restarted (smtpd doesn't support reload) whenever a certificate was 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 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 here 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 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 
+action receive mbox virtual 
+action outbound relay
+
+match from any for domain  action receive
+match from local for local action localmail
+match from local for any action outbound
+

All pieces together

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

+https://codeberg.org/snonux/rexfiles
+

Besides of ACME, also other things, such as DNS server, are rexified too. 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 task 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 reducing recurring manual maintenance work (creating and renewing certificates). Furthermore, all the certificates are free of costs! 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. 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. But I like underdogs.

I love the fact that a Rexfile is just a Perl DSL. Why re-inventing the wheel? 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 that I didn't have to install yet another shell as OpenBSD does not come with the Bash pre-installed.

E-Mail me your comments to paul at buetow dot org!

foo.zone feed

Let's Encrypt with OpenBSD and Rex

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