<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="module-security-acme">
<title>SSL/TLS Certificates with ACME</title>
<para>
NixOS supports automatic domain validation & certificate retrieval and
renewal using the ACME protocol. Any provider can be used, but by default
NixOS uses Let's Encrypt. The alternative ACME client <literal>lego</literal>
is used under the hood.
</para>
<para>
Automatic cert validation and configuration for Apache and Nginx virtual
hosts is included in NixOS, however if you would like to generate a wildcard
cert or you are not using a web server you will have to configure DNS
based validation.
</para>
<section xml:id="module-security-acme-prerequisites">
<title>Prerequisites</title>
<para>
To use the ACME module, you must accept the provider's terms of service
by setting <literal><xref linkend="opt-security.acme.acceptTerms" /></literal>
to <literal>true</literal>. The Let's Encrypt ToS can be found
<link xlink:href="https://letsencrypt.org/repository/">here</link>.
</para>
<para>
You must also set an email address to be used when creating accounts with
Let's Encrypt. You can set this for all certs with
<literal><xref linkend="opt-security.acme.email" /></literal>
and/or on a per-cert basis with
<literal><xref linkend="opt-security.acme.certs._name_.email" /></literal>.
This address is only used for registration and renewal reminders,
and cannot be used to administer the certificates in any way.
</para>
<para>
Alternatively, you can use a different ACME server by changing the
<literal><xref linkend="opt-security.acme.server" /></literal> option
to a provider of your choosing, or just change the server for one cert with
<literal><xref linkend="opt-security.acme.certs._name_.server" /></literal>.
</para>
<para>
You will need an HTTP server or DNS server for verification. For HTTP,
the server must have a webroot defined that can serve
<filename>.well-known/acme-challenge</filename>. This directory must be
writeable by the user that will run the ACME client. For DNS, you must
set up credentials with your provider/server for use with lego.
</para>
</section>
<section xml:id="module-security-acme-nginx">
<title>Using ACME certificates in Nginx</title>
<para>
NixOS supports fetching ACME certificates for you by setting
<literal><link linkend="opt-services.nginx.virtualHosts._name_.enableACME">enableACME</link>
= true;</literal> in a virtualHost config. We first create self-signed
placeholder certificates in place of the real ACME certs. The placeholder
certs are overwritten when the ACME certs arrive. For
<literal>foo.example.com</literal> the config would look like.
</para>
<programlisting>
<xref linkend="opt-security.acme.acceptTerms" /> = true;
<xref linkend="opt-security.acme.email" /> = "admin+acme@example.com";
services.nginx = {
<link linkend="opt-services.nginx.enable">enable</link> = true;
<link linkend="opt-services.nginx.virtualHosts">virtualHosts</link> = {
"foo.example.com" = {
<link linkend="opt-services.nginx.virtualHosts._name_.forceSSL">forceSSL</link> = true;
<link linkend="opt-services.nginx.virtualHosts._name_.enableACME">enableACME</link> = true;
# All serverAliases will be added as <link linkend="opt-security.acme.certs._name_.extraDomainNames">extra domain names</link> on the certificate.
<link linkend="opt-services.nginx.virtualHosts._name_.serverAliases">serverAliases</link> = [ "bar.example.com" ];
locations."/" = {
<link linkend="opt-services.nginx.virtualHosts._name_.locations._name_.root">root</link> = "/var/www";
};
};
# We can also add a different vhost and reuse the same certificate
# but we have to append extraDomainNames manually.
<link linkend="opt-security.acme.certs._name_.extraDomainNames">security.acme.certs."foo.example.com".extraDomainNames</link> = [ "baz.example.com" ];
"baz.example.com" = {
<link linkend="opt-services.nginx.virtualHosts._name_.forceSSL">forceSSL</link> = true;
<link linkend="opt-services.nginx.virtualHosts._name_.useACMEHost">useACMEHost</link> = "foo.example.com";
locations."/" = {
<link linkend="opt-services.nginx.virtualHosts._name_.locations._name_.root">root</link> = "/var/www";
};
};
};
}
</programlisting>
</section>
<section xml:id="module-security-acme-httpd">
<title>Using ACME certificates in Apache/httpd</title>
<para>
Using ACME certificates with Apache virtual hosts is identical
to using them with Nginx. The attribute names are all the same, just replace
"nginx" with "httpd" where appropriate.
</para>
</section>
<section xml:id="module-security-acme-configuring">
<title>Manual configuration of HTTP-01 validation</title>
<para>
First off you will need to set up a virtual host to serve the challenges.
This example uses a vhost called <literal>certs.example.com</literal>, with
the intent that you will generate certs for all your vhosts and redirect
everyone to HTTPS.
</para>
<programlisting>
<xref linkend="opt-security.acme.acceptTerms" /> = true;
<xref linkend="opt-security.acme.email" /> = "admin+acme@example.com";
services.nginx = {
<link linkend="opt-services.nginx.enable">enable</link> = true;
<link linkend="opt-services.nginx.virtualHosts">virtualHosts</link> = {
"acmechallenge.example.com" = {
# Catchall vhost, will redirect users to HTTPS for all vhosts
<link linkend="opt-services.nginx.virtualHosts._name_.serverAliases">serverAliases</link> = [ "*.example.com" ];
# /var/lib/acme/.challenges must be writable by the ACME user
# and readable by the Nginx user.
# By default, this is the case.
locations."/.well-known/acme-challenge" = {
<link linkend="opt-services.nginx.virtualHosts._name_.locations._name_.root">root</link> = "/var/lib/acme/.challenges";
};
locations."/" = {
<link linkend="opt-services.nginx.virtualHosts._name_.locations._name_.return">return</link> = "301 https://$host$request_uri";
};
};
};
}
# Alternative config for Apache
services.httpd = {
<link linkend="opt-services.httpd.enable">enable = true;</link>
<link linkend="opt-services.httpd.virtualHosts">virtualHosts</link> = {
"acmechallenge.example.com" = {
# Catchall vhost, will redirect users to HTTPS for all vhosts
<link linkend="opt-services.httpd.virtualHosts._name_.serverAliases">serverAliases</link> = [ "*.example.com" ];
# /var/lib/acme/.challenges must be writable by the ACME user and readable by the Apache user.
# By default, this is the case.
<link linkend="opt-services.httpd.virtualHosts._name_.documentRoot">documentRoot</link> = "/var/lib/acme/.challenges";
<link linkend="opt-services.httpd.virtualHosts._name_.extraConfig">extraConfig</link> = ''
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge [NC]
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301]
'';
};
};
}
</programlisting>
<para>
Now you need to configure ACME to generate a certificate.
</para>
<programlisting>
<xref linkend="opt-security.acme.certs"/>."foo.example.com" = {
<link linkend="opt-security.acme.certs._name_.webroot">webroot</link> = "/var/lib/acme/.challenges";
<link linkend="opt-security.acme.certs._name_.email">email</link> = "foo@example.com";
# Since we have a wildcard vhost to handle port 80,
# we can generate certs for anything!
# Just make sure your DNS resolves them.
<link linkend="opt-security.acme.certs._name_.extraDomainNames">extraDomainNames</link> = [ "mail.example.com" ];
};
</programlisting>
<para>
The private key <filename>key.pem</filename> and certificate
<filename>fullchain.pem</filename> will be put into
<filename>/var/lib/acme/foo.example.com</filename>.
</para>
<para>
Refer to <xref linkend="ch-options" /> for all available configuration
options for the <link linkend="opt-security.acme.certs">security.acme</link>
module.
</para>
</section>
<section xml:id="module-security-acme-config-dns">
<title>Configuring ACME for DNS validation</title>
<para>
This is useful if you want to generate a wildcard certificate, since
ACME servers will only hand out wildcard certs over DNS validation.
There a number of supported DNS providers and servers you can utilise,
see the <link xlink:href="https://go-acme.github.io/lego/dns/">lego docs</link>
for provider/server specific configuration values. For the sake of these
docs, we will provide a fully self-hosted example using bind.
</para>
<programlisting>
services.bind = {
<link linkend="opt-services.bind.enable">enable</link> = true;
<link linkend="opt-services.bind.extraConfig">extraConfig</link> = ''
include "/var/lib/secrets/dnskeys.conf";
'';
<link linkend="opt-services.bind.zones">zones</link> = [
rec {
name = "example.com";
file = "/var/db/bind/${name}";
master = true;
extraConfig = "allow-update { key rfc2136key.example.com.; };";
}
];
}
# Now we can configure ACME
<xref linkend="opt-security.acme.acceptTerms" /> = true;
<xref linkend="opt-security.acme.email" /> = "admin+acme@example.com";
<xref linkend="opt-security.acme.certs" />."example.com" = {
<link linkend="opt-security.acme.certs._name_.domain">domain</link> = "*.example.com";
<link linkend="opt-security.acme.certs._name_.dnsProvider">dnsProvider</link> = "rfc2136";
<link linkend="opt-security.acme.certs._name_.credentialsFile">credentialsFile</link> = "/var/lib/secrets/certs.secret";
# We don't need to wait for propagation since this is a local DNS server
<link linkend="opt-security.acme.certs._name_.dnsPropagationCheck">dnsPropagationCheck</link> = false;
};
</programlisting>
<para>
The <filename>dnskeys.conf</filename> and <filename>certs.secret</filename>
must be kept secure and thus you should not keep their contents in your
Nix config. Instead, generate them one time with these commands:
</para>
<programlisting>
mkdir -p /var/lib/secrets
tsig-keygen rfc2136key.example.com > /var/lib/secrets/dnskeys.conf
chown named:root /var/lib/secrets/dnskeys.conf
chmod 400 /var/lib/secrets/dnskeys.conf
# Copy the secret value from the dnskeys.conf, and put it in
# RFC2136_TSIG_SECRET below
cat > /var/lib/secrets/certs.secret << EOF
RFC2136_NAMESERVER='127.0.0.1:53'
RFC2136_TSIG_ALGORITHM='hmac-sha256.'
RFC2136_TSIG_KEY='rfc2136key.example.com'
RFC2136_TSIG_SECRET='your secret key'
EOF
chmod 400 /var/lib/secrets/certs.secret
</programlisting>
<para>
Now you're all set to generate certs! You should monitor the first invokation
by running <literal>systemctl start acme-example.com.service &
journalctl -fu acme-example.com.service</literal> and watching its log output.
</para>
</section>
<section xml:id="module-security-acme-regenerate">
<title>Regenerating certificates</title>
<para>
Should you need to regenerate a particular certificate in a hurry, such
as when a vulnerability is found in Let's Encrypt, there is now a convenient
mechanism for doing so. Running <literal>systemctl clean acme-example.com.service</literal>
will remove all certificate files for the given domain, allowing you to then
<literal>systemctl start acme-example.com.service</literal> to generate fresh
ones.
</para>
</section>
</chapter>
