nixpkgs/nixos/modules/security/acme/default.xml

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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="module-security-acme">
  <title>SSL/TLS Certificates with ACME</title>
  <para>
    NixOS supports automatic domain validation &amp; 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
    <link xlink:href="https://go-acme.github.io/lego/">lego</link> 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
      <xref linkend="opt-security.acme.acceptTerms" /> 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
      <xref linkend="opt-security.acme.defaults.email" /> and/or on a
      per-cert basis with
      <xref linkend="opt-security.acme.certs._name_.email" />. 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
      <xref linkend="opt-security.acme.defaults.server" /> option to a
      provider of your choosing, or just change the server for one cert
      with <xref linkend="opt-security.acme.certs._name_.server" />.
    </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>enableACME = 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 this:
    </para>
    <programlisting>
security.acme.acceptTerms = true;
security.acme.defaults.email = &quot;admin+acme@example.com&quot;;
services.nginx = {
  enable = true;
  virtualHosts = {
    &quot;foo.example.com&quot; = {
      forceSSL = true;
      enableACME = true;
      # All serverAliases will be added as extra domain names on the certificate.
      serverAliases = [ &quot;bar.example.com&quot; ];
      locations.&quot;/&quot; = {
        root = &quot;/var/www&quot;;
      };
    };

    # We can also add a different vhost and reuse the same certificate
    # but we have to append extraDomainNames manually beforehand:
    # security.acme.certs.&quot;foo.example.com&quot;.extraDomainNames = [ &quot;baz.example.com&quot; ];
    &quot;baz.example.com&quot; = {
      forceSSL = true;
      useACMEHost = &quot;foo.example.com&quot;;
      locations.&quot;/&quot; = {
        root = &quot;/var/www&quot;;
      };
    };
  };
}
</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 <quote>nginx</quote> with <quote>httpd</quote> 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>
security.acme.acceptTerms = true;
security.acme.defaults.email = &quot;admin+acme@example.com&quot;;

# /var/lib/acme/.challenges must be writable by the ACME user
# and readable by the Nginx user. The easiest way to achieve
# this is to add the Nginx user to the ACME group.
users.users.nginx.extraGroups = [ &quot;acme&quot; ];

services.nginx = {
  enable = true;
  virtualHosts = {
    &quot;acmechallenge.example.com&quot; = {
      # Catchall vhost, will redirect users to HTTPS for all vhosts
      serverAliases = [ &quot;*.example.com&quot; ];
      locations.&quot;/.well-known/acme-challenge&quot; = {
        root = &quot;/var/lib/acme/.challenges&quot;;
      };
      locations.&quot;/&quot; = {
        return = &quot;301 https://$host$request_uri&quot;;
      };
    };
  };
}
# Alternative config for Apache
users.users.wwwrun.extraGroups = [ &quot;acme&quot; ];
services.httpd = {
  enable = true;
  virtualHosts = {
    &quot;acmechallenge.example.com&quot; = {
      # Catchall vhost, will redirect users to HTTPS for all vhosts
      serverAliases = [ &quot;*.example.com&quot; ];
      # /var/lib/acme/.challenges must be writable by the ACME user and readable by the Apache user.
      # By default, this is the case.
      documentRoot = &quot;/var/lib/acme/.challenges&quot;;
      extraConfig = ''
        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>
security.acme.certs.&quot;foo.example.com&quot; = {
  webroot = &quot;/var/lib/acme/.challenges&quot;;
  email = &quot;foo@example.com&quot;;
  # Ensure that the web server you use can read the generated certs
  # Take a look at the group option for the web server you choose.
  group = &quot;nginx&quot;;
  # Since we have a wildcard vhost to handle port 80,
  # we can generate certs for anything!
  # Just make sure your DNS resolves them.
  extraDomainNames = [ &quot;mail.example.com&quot; ];
};
</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 are 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 = {
  enable = true;
  extraConfig = ''
    include &quot;/var/lib/secrets/dnskeys.conf&quot;;
  '';
  zones = [
    rec {
      name = &quot;example.com&quot;;
      file = &quot;/var/db/bind/${name}&quot;;
      master = true;
      extraConfig = &quot;allow-update { key rfc2136key.example.com.; };&quot;;
    }
  ];
}

# Now we can configure ACME
security.acme.acceptTerms = true;
security.acme.defaults.email = &quot;admin+acme@example.com&quot;;
security.acme.certs.&quot;example.com&quot; = {
  domain = &quot;*.example.com&quot;;
  dnsProvider = &quot;rfc2136&quot;;
  credentialsFile = &quot;/var/lib/secrets/certs.secret&quot;;
  # We don't need to wait for propagation since this is a local DNS server
  dnsPropagationCheck = 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 a systemd service:
    </para>
    <programlisting>
systemd.services.dns-rfc2136-conf = {
  requiredBy = [&quot;acme-example.com.service&quot; &quot;bind.service&quot;];
  before = [&quot;acme-example.com.service&quot; &quot;bind.service&quot;];
  unitConfig = {
    ConditionPathExists = &quot;!/var/lib/secrets/dnskeys.conf&quot;;
  };
  serviceConfig = {
    Type = &quot;oneshot&quot;;
    UMask = 0077;
  };
  path = [ pkgs.bind ];
  script = ''
    mkdir -p /var/lib/secrets
    chmod 755 /var/lib/secrets
    tsig-keygen rfc2136key.example.com &gt; /var/lib/secrets/dnskeys.conf
    chown named:root /var/lib/secrets/dnskeys.conf
    chmod 400 /var/lib/secrets/dnskeys.conf

    # extract secret value from the dnskeys.conf
    while read x y; do if [ &quot;$x&quot; = &quot;secret&quot; ]; then secret=&quot;''${y:1:''${#y}-3}&quot;; fi; done &lt; /var/lib/secrets/dnskeys.conf

    cat &gt; /var/lib/secrets/certs.secret &lt;&lt; EOF
    RFC2136_NAMESERVER='127.0.0.1:53'
    RFC2136_TSIG_ALGORITHM='hmac-sha256.'
    RFC2136_TSIG_KEY='rfc2136key.example.com'
    RFC2136_TSIG_SECRET='$secret'
    EOF
    chmod 400 /var/lib/secrets/certs.secret
  '';
};
</programlisting>
    <para>
      Now you’re all set to generate certs! You should monitor the first
      invocation by running
      <literal>systemctl start acme-example.com.service &amp; journalctl -fu acme-example.com.service</literal>
      and watching its log output.
    </para>
  </section>
  <section xml:id="module-security-acme-config-dns-with-vhosts">
    <title>Using DNS validation with web server virtual hosts</title>
    <para>
      It is possible to use DNS-01 validation with all certificates,
      including those automatically configured via the Nginx/Apache
      <link linkend="opt-services.nginx.virtualHosts._name_.enableACME"><literal>enableACME</literal></link>
      option. This configuration pattern is fully supported and part of
      the module’s test suite for Nginx + Apache.
    </para>
    <para>
      You must follow the guide above on configuring DNS-01 validation
      first, however instead of setting the options for one certificate
      (e.g.
      <xref linkend="opt-security.acme.certs._name_.dnsProvider" />) you
      will set them as defaults (e.g.
      <xref linkend="opt-security.acme.defaults.dnsProvider" />).
    </para>
    <programlisting>
# Configure ACME appropriately
security.acme.acceptTerms = true;
security.acme.defaults.email = &quot;admin+acme@example.com&quot;;
security.acme.defaults = {
  dnsProvider = &quot;rfc2136&quot;;
  credentialsFile = &quot;/var/lib/secrets/certs.secret&quot;;
  # We don't need to wait for propagation since this is a local DNS server
  dnsPropagationCheck = false;
};

# For each virtual host you would like to use DNS-01 validation with,
# set acmeRoot = null
services.nginx = {
  enable = true;
  virtualHosts = {
    &quot;foo.example.com&quot; = {
      enableACME = true;
      acmeRoot = null;
    };
  };
}
</programlisting>
    <para>
      And that’s it! Next time your configuration is rebuilt, or when
      you add a new virtualHost, it will be DNS-01 validated.
    </para>
  </section>
  <section xml:id="module-security-acme-root-owned">
    <title>Using ACME with services demanding root owned
    certificates</title>
    <para>
      Some services refuse to start if the configured certificate files
      are not owned by root. PostgreSQL and OpenSMTPD are examples of
      these. There is no way to change the user the ACME module uses (it
      will always be <literal>acme</literal>), however you can use
      systemd’s <literal>LoadCredential</literal> feature to resolve
      this elegantly. Below is an example configuration for OpenSMTPD,
      but this pattern can be applied to any service.
    </para>
    <programlisting>
# Configure ACME however you like (DNS or HTTP validation), adding
# the following configuration for the relevant certificate.
# Note: You cannot use `systemctl reload` here as that would mean
# the LoadCredential configuration below would be skipped and
# the service would continue to use old certificates.
security.acme.certs.&quot;mail.example.com&quot;.postRun = ''
  systemctl restart opensmtpd
'';

# Now you must augment OpenSMTPD's systemd service to load
# the certificate files.
systemd.services.opensmtpd.requires = [&quot;acme-finished-mail.example.com.target&quot;];
systemd.services.opensmtpd.serviceConfig.LoadCredential = let
  certDir = config.security.acme.certs.&quot;mail.example.com&quot;.directory;
in [
  &quot;cert.pem:${certDir}/cert.pem&quot;
  &quot;key.pem:${certDir}/key.pem&quot;
];

# Finally, configure OpenSMTPD to use these certs.
services.opensmtpd = let
  credsDir = &quot;/run/credentials/opensmtpd.service&quot;;
in {
  enable = true;
  setSendmail = false;
  serverConfiguration = ''
    pki mail.example.com cert &quot;${credsDir}/cert.pem&quot;
    pki mail.example.com key &quot;${credsDir}/key.pem&quot;
    listen on localhost tls pki mail.example.com
    action act1 relay host smtp://127.0.0.1:10027
    match for local action act1
  '';
};
</programlisting>
  </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 --what=state acme-example.com.service</literal>
      will remove all certificate files and the account data for the
      given domain, allowing you to then
      <literal>systemctl start acme-example.com.service</literal> to
      generate fresh ones.
    </para>
  </section>
  <section xml:id="module-security-acme-fix-jws">
    <title>Fixing JWS Verification error</title>
    <para>
      It is possible that your account credentials file may become
      corrupt and need to be regenerated. In this scenario lego will
      produce the error <literal>JWS verification error</literal>. The
      solution is to simply delete the associated accounts file and
      re-run the affected service(s).
    </para>
    <programlisting>
# Find the accounts folder for the certificate
systemctl cat acme-example.com.service | grep -Po 'accounts/[^:]*'
export accountdir=&quot;$(!!)&quot;
# Move this folder to some place else
mv /var/lib/acme/.lego/$accountdir{,.bak}
# Recreate the folder using systemd-tmpfiles
systemd-tmpfiles --create
# Get a new account and reissue certificates
# Note: Do this for all certs that share the same account email address
systemctl start acme-example.com.service
</programlisting>
  </section>
</chapter>