sftp: allow read-only anonymous FTP

hosts/by-name/servo/services/default.nix

@@ -7,7 +7,7 @@
     ./email
     ./ejabberd.nix
     ./freshrss.nix
-    ./ftp.nix
+    ./ftp
     ./gitea.nix
     ./goaccess.nix
     ./ipfs.nix

hosts/by-name/servo/services/ftp.nix

@@ -1,32 +0,0 @@
-# docs:
-# - <https://github.com/drakkan/sftpgo>
-# - config options: <https://github.com/drakkan/sftpgo/blob/main/docs/full-configuration.md>
-# - config defaults: <https://github.com/drakkan/sftpgo/blob/main/sftpgo.json>
-#
-# sftpgo is a FTP server that also supports WebDAV, SFTP, and web clients.
-
-{ ... }:
-{
-  sane.ports.ports."21" = {
-    protocol = [ "tcp" ];
-    visibleTo.lan = true;
-    description = "colin-FTP server";
-  };
-
-  services.sftpgo = {
-    enable = false;
-    # enable = true;
-    settings.ftpd.bindings = [{
-      address = "10.0.10.5";
-      port = 21;
-
-      banner = ''
-        Welcome, friends, to Colin's read-only FTP server! Also available via NFS on the same host.
-        Please let me know if anything's broken or not as it should be. Otherwise, browse and DL freely :)
-      '';
-      hash_support = true;
-
-      debug = true;
-    }];
-  };
-}

hosts/by-name/servo/services/ftp/default.nix

@@ -0,0 +1,70 @@
+# docs:
+# - <https://github.com/drakkan/sftpgo>
+# - config options: <https://github.com/drakkan/sftpgo/blob/main/docs/full-configuration.md>
+# - config defaults: <https://github.com/drakkan/sftpgo/blob/main/sftpgo.json>
+# - nixos options: <repo:nixos/nixpkgs:nixos/modules/services/web-apps/sftpgo.nix>
+#
+# sftpgo is a FTP server that also supports WebDAV, SFTP, and web clients.
+
+
+{ lib, pkgs, sane-lib, ... }:
+let
+  authProgram = pkgs.static-nix-shell.mkBash {
+    pname = "sftpgo_external_auth_hook";
+    src = ./.;
+  };
+in
+{
+  # Client initiates a FTP "control connection" on port 21.
+  # - this handles the client -> server commands, and the server -> client status, but not the actual data
+  # - file data, directory listings, etc need to be transferred on an ephemeral "data port".
+  # - 50000-50100 is a common port range for this.
+  sane.ports.ports = {
+    "21" = {
+      protocol = [ "tcp" ];
+      visibleTo.lan = true;
+      description = "colin-FTP server";
+    };
+  } // (sane-lib.mapToAttrs
+    (port: {
+      name = builtins.toString port;
+      value = {
+        protocol = [ "tcp" ];
+        visibleTo.lan = true;
+        description = "colin-FTP server data port range";
+      };
+    })
+    (lib.range 50000 50100)
+  );
+
+  services.sftpgo = {
+    enable = true;
+    settings = {
+      ftpd = {
+        bindings = [{
+          address = "10.0.10.5";
+          port = 21;
+          debug = true;
+        }];
+
+        # active mode is susceptible to "bounce attacks", without much benefit over passive mode
+        disable_active_mode = true;
+        hash_support = true;
+        passive_port_range = {
+          start = 50000;
+          end = 50100;
+        };
+
+        banner = ''
+          Welcome, friends, to Colin's read-only FTP server! Also available via NFS on the same host.
+          Please let me know if anything's broken or not as it should be. Otherwise, browse and DL freely :)
+        '';
+
+      };
+      data_provider = {
+        driver = "memory";
+        external_auth_hook = "${authProgram}/bin/sftpgo_external_auth_hook";
+      };
+    };
+  };
+}

hosts/by-name/servo/services/ftp/sftpgo_external_auth_hook

@@ -0,0 +1,55 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+# vim: set filetype=bash :
+#
+# available environment variables:
+# - SFTPGO_AUTHD_USERNAME
+# - SFTPGO_AUTHD_USER
+# - SFTPGO_AUTHD_IP
+# - SFTPGO_AUTHD_PROTOCOL  = { "DAV", "FTP", "HTTP", "SSH" }
+# - SFTPGO_AUTHD_PASSWORD
+# - SFTPGO_AUTHD_PUBLIC_KEY
+# - SFTPGO_AUTHD_KEYBOARD_INTERACTIVE
+# - SFTPGO_AUTHD_TLS_CERT
+#
+# user permissions:
+# - see <repo:drakkan/sftpgo:internal/dataprovider/user.go>
+# - "*" = grant all permissions
+# - read-only perms:
+#   - "list" = list files and directories
+#   - "download"
+# - rw perms:
+#   - "upload"
+#   - "overwrite" = allow uploads to replace existing files
+#   - "delete" = delete files and directories
+#     - "delete_files"
+#     - "delete_dirs"
+#   - "rename" = rename files and directories
+#     - "rename_files"
+#     - "rename_dirs"
+#   - "create_dirs"
+#   - "create_symlinks"
+#   - "chmod"
+#   - "chown"
+#   - "chtimes" = change atime/mtime (access and modification times)
+#
+# home_dir:
+# - it seems (empirically) that a user can't cd above their home directory.
+#   though i don't have a reference for that in the docs.
+# TODO: don't reuse /var/nfs/export here. formalize this some other way.
+
+
+if [ "$SFTPGO_AUTHD_USERNAME" = "anonymous" ]; then
+  echo '{'
+  echo '  "status":1,'
+  echo '  "username":"anonymous","expiration_date":0,'
+  echo '  "home_dir":"/var/nfs/export","uid":65534,"gid":65534,"max_sessions":0,"quota_size":0,"quota_files":100000,'
+  echo '  "permissions":{'
+  echo '    "/":["list", "download"]'
+  echo '  },'
+  echo '  "upload_bandwidth":0,"download_bandwidth":0,'
+  echo '  "filters":{"allowed_ip":[],"denied_ip":[]},"public_keys":[]'
+  echo '}'
+else
+  echo '{"username":""}'
+fi