services.openssh: add banner

Add the possibility to setup a banner.

Co-authored-by: Silvan Mosberger <github@infinisil.com>

nixos/modules/services/networking/ssh/sshd.nix

@@ -232,6 +232,14 @@ in
         '';
       };
 
+      banner = mkOption {
+        type = types.nullOr types.lines;
+        default = null;
+        description = ''
+          Message to display to the remote user before authentication is allowed.
+        '';
+      };
+
       authorizedKeysFiles = mkOption {
         type = types.listOf types.str;
         default = [];
@@ -474,6 +482,8 @@ in
       ''
         UsePAM yes
 
+        Banner ${if cfg.banner == null then "none" else pkgs.writeText "ssh_banner" cfg.banner}
+
         AddressFamily ${if config.networking.enableIPv6 then "any" else "inet"}
         ${concatMapStrings (port: ''
           Port ${toString port}