nixos/nginx: ensure TLS OCSP stapling works out of the box with LE

The recommended TLS configuration comes with `ssl_stapling on` and
`ssl_stapling_verify on`. However, this last directive also requires
the use of `ssl_trusted_certificate` to verify the received answer.
When using `enableACME` or similar, we can help the user by providing
the correct value for the directive.

The result can be tested with:

    openssl s_client -connect web.example.com:443 -status 2> /dev/null

Without OCSP stapling, we get:

    OCSP response: no response sent

After this change, we get:

    OCSP Response Data:
        OCSP Response Status: successful (0x0)
        Response Type: Basic OCSP Response
        Version: 1 (0x0)
        Responder Id: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
        Produced At: Aug 30 20:46:00 2018 GMT

2 files changed, 12 insertions, 0 deletions

diff --git a/nixos/modules/services/web-servers/nginx/default.nix b/nixos/modules/services/web-servers/nginx/default.nix
index 17b169f7c696..b231ee5a3f01 100644
--- a/nixos/modules/services/web-servers/nginx/default.nix
+++ b/nixos/modules/services/web-servers/nginx/default.nix
@@ -16,9 +16,11 @@ let
     } // (optionalAttrs vhostConfig.enableACME {
       sslCertificate = "${acmeDirectory}/${serverName}/fullchain.pem";
       sslCertificateKey = "${acmeDirectory}/${serverName}/key.pem";
+      sslTrustedCertificate = "${acmeDirectory}/${serverName}/full.pem";
     }) // (optionalAttrs (vhostConfig.useACMEHost != null) {
       sslCertificate = "${acmeDirectory}/${vhostConfig.useACMEHost}/fullchain.pem";
       sslCertificateKey = "${acmeDirectory}/${vhostConfig.useACMEHost}/key.pem";
+      sslTrustedCertificate = "${acmeDirectory}/${vhostConfig.useACMEHost}/full.pem";
     })
   ) cfg.virtualHosts;
   enableIPv6 = config.networking.enableIPv6;
@@ -228,6 +230,9 @@ let
             ssl_certificate ${vhost.sslCertificate};
             ssl_certificate_key ${vhost.sslCertificateKey};
           ''}
+          ${optionalString (hasSSL && vhost.sslTrustedCertificate != null) ''
+            ssl_trusted_certificate ${vhost.sslTrustedCertificate};
+          ''}
 
           ${optionalString (vhost.basicAuthFile != null || vhost.basicAuth != {}) ''
             auth_basic secured;
diff --git a/nixos/modules/services/web-servers/nginx/vhost-options.nix b/nixos/modules/services/web-servers/nginx/vhost-options.nix
index 1075b00768fd..6954d932eed4 100644
--- a/nixos/modules/services/web-servers/nginx/vhost-options.nix
+++ b/nixos/modules/services/web-servers/nginx/vhost-options.nix
@@ -129,6 +129,13 @@ with lib;
       description = "Path to server SSL certificate key.";
     };
 
+    sslTrustedCertificate = mkOption {
+      type = types.path;
+      default = null;
+      example = "/var/root.cert";
+      description = "Path to root SSL certificate for stapling and client certificates.";
+    };
+
     http2 = mkOption {
       type = types.bool;
       default = true;