add non-functional nsswitch module

nixos/modules/config/nsswitch.nix

@@ -0,0 +1,105 @@
+# Configuration for the Name Service Switch (/etc/nsswitch.conf).
+
+{ config, lib, ... }:
+
+with lib;
+
+let
+
+  # only with nscd up and running we can load NSS modules that are not integrated in NSS
+  canLoadExternalModules = config.services.nscd.enable;
+  myhostname = canLoadExternalModules;
+  mymachines = canLoadExternalModules;
+  nssmdns = canLoadExternalModules && config.services.avahi.nssmdns;
+  nsswins = canLoadExternalModules && config.services.samba.nsswins;
+  ldap = canLoadExternalModules && (config.users.ldap.enable && config.users.ldap.nsswitch);
+  sssd = canLoadExternalModules && config.services.sssd.enable;
+  resolved = canLoadExternalModules && config.services.resolved.enable;
+  gnunet = canLoadExternalModules && (config.services.gnunet.enable && config.services.gnunet.nsswitch);
+
+  hostArray = [ "files" ]
+    ++ optional mymachines "mymachines"
+    ++ optional nssmdns "mdns_minimal [NOTFOUND=return]"
+    ++ optional nsswins "wins"
+    ++ optional resolved "resolve [!UNAVAIL=return]"
+    ++ optional gnunet "gns [NOTFOUND=return]"
+    ++ [ "dns" ]
+    ++ optional nssmdns "mdns"
+    ++ optional myhostname "myhostname";
+
+  passwdArray = [ "files" ]
+    ++ optional sssd "sss"
+    ++ optional ldap "ldap"
+    ++ optional mymachines "mymachines"
+    ++ [ "systemd" ];
+
+  shadowArray = [ "files" ]
+    ++ optional sssd "sss"
+    ++ optional ldap "ldap";
+
+  servicesArray = [ "files" ]
+    ++ optional sssd "sss";
+
+in {
+  options = {
+
+    # NSS modules.  Hacky!
+    # Only works with nscd!
+    system.nssModules = mkOption {
+      type = types.listOf types.path;
+      internal = true;
+      default = [];
+      description = ''
+        Search path for NSS (Name Service Switch) modules.  This allows
+        several DNS resolution methods to be specified via
+        <filename>/etc/nsswitch.conf</filename>.
+      '';
+      apply = list:
+        {
+          inherit list;
+          path = makeLibraryPath list;
+        };
+    };
+
+  };
+
+  config = {
+    assertions = [
+      {
+        # generic catch if the NixOS module adding to nssModules does not prevent it with specific message.
+        assertion = config.system.nssModules.path != "" -> canLoadExternalModules;
+        message = "Loading NSS modules from path ${config.system.nssModules.path} requires nscd being enabled.";
+      }
+      {
+        # resolved does not need to add to nssModules, therefore needs an extra assertion
+        assertion = resolved -> canLoadExternalModules;
+        message = "Loading systemd-resolved's nss-resolve NSS module requires nscd being enabled.";
+      }
+    ];
+
+    # Name Service Switch configuration file.  Required by the C
+    # library.  !!! Factor out the mdns stuff.  The avahi module
+    # should define an option used by this module.
+    environment.etc."nsswitch.conf".text = ''
+      passwd:    ${concatStringsSep " " passwdArray}
+      group:     ${concatStringsSep " " passwdArray}
+      shadow:    ${concatStringsSep " " shadowArray}
+
+      hosts:     ${concatStringsSep " " hostArray}
+      networks:  files
+
+      ethers:    files
+      services:  ${concatStringsSep " " servicesArray}
+      protocols: files
+      rpc:       files
+    '';
+
+    # Systemd provides nss-myhostname to ensure that our hostname
+    # always resolves to a valid IP address.  It returns all locally
+    # configured IP addresses, or ::1 and 127.0.0.2 as
+    # fallbacks. Systemd also provides nss-mymachines to return IP
+    # addresses of local containers.
+    system.nssModules = optionals canLoadExternalModules [ config.systemd.package.out ];
+
+  };
+}