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 lego is used under the hood. 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.

To use the ACME module, you must accept the provider's terms of service by setting to true. The Let's Encrypt ToS can be found here. 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 and/or on a per-cert basis with . This address is only used for registration and renewal reminders, and cannot be used to administer the certificates in any way. Alternatively, you can use a different ACME server by changing the option to a provider of your choosing, or just change the server for one cert with . You will need an HTTP server or DNS server for verification. For HTTP, the server must have a webroot defined that can serve .well-known/acme-challenge. 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.

NixOS supports fetching ACME certificates for you by setting enableACME = true; 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 foo.example.com the config would look like this: = true; = "admin+acme@example.com"; services.nginx = { enable = true; virtualHosts = { "foo.example.com" = { forceSSL = true; enableACME = true; # All serverAliases will be added as extra domain names on the certificate. serverAliases = [ "bar.example.com" ]; locations."/" = { root = "/var/www"; }; }; # We can also add a different vhost and reuse the same certificate # but we have to append extraDomainNames manually beforehand: # security.acme.certs."foo.example.com".extraDomainNames = [ "baz.example.com" ]; "baz.example.com" = { forceSSL = true; useACMEHost = "foo.example.com"; locations."/" = { root = "/var/www"; }; }; }; }

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.

First off you will need to set up a virtual host to serve the challenges. This example uses a vhost called certs.example.com, with the intent that you will generate certs for all your vhosts and redirect everyone to HTTPS. = true; = "admin+acme@example.com"; # /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 = [ "acme" ]; services.nginx = { enable = true; virtualHosts = { "acmechallenge.example.com" = { # Catchall vhost, will redirect users to HTTPS for all vhosts serverAliases = [ "*.example.com" ]; locations."/.well-known/acme-challenge" = { root = "/var/lib/acme/.challenges"; }; locations."/" = { return = "301 https://$host$request_uri"; }; }; }; } # Alternative config for Apache users.users.wwwrun.extraGroups = [ "acme" ]; services.httpd = { enable = true; virtualHosts = { "acmechallenge.example.com" = { # Catchall vhost, will redirect users to HTTPS for all vhosts serverAliases = [ "*.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. documentRoot = "/var/lib/acme/.challenges"; extraConfig = '' RewriteEngine On RewriteCond %{HTTPS} off RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge [NC] RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301] ''; }; }; } Now you need to configure ACME to generate a certificate. ."foo.example.com" = { webroot = "/var/lib/acme/.challenges"; email = "foo@example.com"; # 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 = "nginx"; # Since we have a wildcard vhost to handle port 80, # we can generate certs for anything! # Just make sure your DNS resolves them. extraDomainNames = [ "mail.example.com" ]; }; The private key key.pem and certificate fullchain.pem will be put into /var/lib/acme/foo.example.com. Refer to for all available configuration options for the security.acme module.

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 lego docs for provider/server specific configuration values. For the sake of these docs, we will provide a fully self-hosted example using bind. services.bind = { enable = true; extraConfig = '' include "/var/lib/secrets/dnskeys.conf"; ''; zones = [ rec { name = "example.com"; file = "/var/db/bind/${name}"; master = true; extraConfig = "allow-update { key rfc2136key.example.com.; };"; } ]; } # Now we can configure ACME = true; = "admin+acme@example.com"; ."example.com" = { domain = "*.example.com"; dnsProvider = "rfc2136"; credentialsFile = "/var/lib/secrets/certs.secret"; # We don't need to wait for propagation since this is a local DNS server dnsPropagationCheck = false; }; The dnskeys.conf and certs.secret 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: systemd.services.dns-rfc2136-conf = { requiredBy = ["acme-example.com.service", "bind.service"]; before = ["acme-example.com.service", "bind.service"]; unitConfig = { ConditionPathExists = "!/var/lib/secrets/dnskeys.conf"; }; serviceConfig = { Type = "oneshot"; UMask = 0077; }; path = [ pkgs.bind ]; script = '' 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 ''; }; Now you're all set to generate certs! You should monitor the first invocation by running systemctl start acme-example.com.service & journalctl -fu acme-example.com.service and watching its log output.

It is possible to use DNS-01 validation with all certificates, including those automatically configured via the Nginx/Apache enableACME option. This configuration pattern is fully supported and part of the module's test suite for Nginx + Apache. You must follow the guide above on configuring DNS-01 validation first, however instead of setting the options for one certificate (e.g. ) you will set them as defaults (e.g. ). # Configure ACME appropriately = true; = "admin+acme@example.com"; = { dnsProvider = "rfc2136"; credentialsFile = "/var/lib/secrets/certs.secret"; # 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 = { "foo.example.com" = { enableACME = true; acmeRoot = null; }; }; } And that's it! Next time your configuration is rebuilt, or when you add a new virtualHost, it will be DNS-01 validated.

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 acme), however you can use systemd's LoadCredential feature to resolve this elegantly. Below is an example configuration for OpenSMTPD, but this pattern can be applied to any service. # 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."mail.example.com".postRun = '' systemctl restart opensmtpd ''; # Now you must augment OpenSMTPD's systemd service to load # the certificate files. systemd.services.opensmtpd.requires = ["acme-finished-mail.example.com.target"]; systemd.services.opensmtpd.serviceConfig.LoadCredential = let certDir = config.security.acme.certs."mail.example.com".directory; in [ "cert.pem:${certDir}/cert.pem" "key.pem:${certDir}/key.pem" ]; # Finally, configure OpenSMTPD to use these certs. services.opensmtpd = let credsDir = "/run/credentials/opensmtpd.service"; in { enable = true; setSendmail = false; serverConfiguration = '' pki mail.example.com cert "${credsDir}/cert.pem" pki mail.example.com key "${credsDir}/key.pem" listen on localhost tls pki mail.example.com action act1 relay host smtp://127.0.0.1:10027 match for local action act1 ''; };

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 systemctl clean --what=state acme-example.com.service will remove all certificate files and the account data for the given domain, allowing you to then systemctl start acme-example.com.service to generate fresh ones.

It is possible that your account credentials file may become corrupt and need to be regenerated. In this scenario lego will produce the error JWS verification error. The solution is to simply delete the associated accounts file and re-run the affected service(s). # Find the accounts folder for the certificate systemctl cat acme-example.com.service | grep -Po 'accounts/[^:]*' export accountdir="$(!!)" # Move this folder to some place else mv /var/lib/acme/.lego/$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