Add Support for the PLAIN authMechanism

README.md

@@ -705,6 +705,11 @@ separated in sub folders and module namespaces.
 ## Auth Mechanisms
 
 For versions of Mongo 3.0 and greater, the auth mechanism defaults to SCRAM.
+
+If connecting to MongoDB Enterprise Edition or MongoDB Atlas, the [PLAIN](https://www.mongodb.com/docs/manual/tutorial/authenticate-nativeldap-activedirectory/)
+auth mechanism is supported for LDAP authentication. The GSSAPI auth mechanism used for Kerberos authentication
+is not currently supported.
+
 If you'd like to use [MONGODB-X509](https://www.mongodb.com/docs/v6.0/tutorial/configure-x509-client-authentication/)
 authentication, you can specify that as a `start_link` option. 
 

lib/mongo/auth.ex

@@ -39,6 +39,10 @@ defmodule Mongo.Auth do
     Mongo.Auth.X509
   end
 
+  defp mechanism(%{wire_version: version, auth_mechanism: :plain}) when version >= 3 do
+    Mongo.Auth.PLAIN
+  end
+
   defp mechanism(%{wire_version: version}) when version >= 3 do
     Mongo.Auth.SCRAM
   end

lib/mongo/auth/plain.ex

@@ -0,0 +1,31 @@
+defmodule Mongo.Auth.PLAIN do
+  @moduledoc false
+  alias Mongo.MongoDBConnection.Utils
+
+  def auth({nil, nil}, _db, _s) do
+    :ok
+  end
+
+  def auth({username, password}, _db, s) do
+    auth_payload = build_auth_payload(username, password)
+    message = [saslStart: 1, mechanism: "PLAIN", payload: auth_payload]
+
+    case Utils.command(-3, message, s) do
+      {:ok, _flags, %{"ok" => ok, "done" => true}} when ok == 1 ->
+        :ok
+
+      {:ok, _flags, %{"ok" => ok, "errmsg" => reason, "code" => code}} when ok == 0 ->
+        {:error, Mongo.Error.exception(message: "auth failed for user #{username}: #{reason}", code: code)}
+
+      error ->
+        error
+    end
+  end
+
+  defp build_auth_payload(username, password) do
+    # https://www.ietf.org/rfc/rfc4616.txt
+    # Null separate listed of authorization ID (blank), username, password. These are sent as raw UTF-8.
+    payload = "\0#{username}\0#{password}"
+    %BSON.Binary{binary: payload}
+  end
+end

lib/mongo/url_parser.ex

@@ -113,6 +113,7 @@ defmodule Mongo.UrlParser do
 
   defp decode_percent(:username, value), do: URI.decode_www_form(value)
   defp decode_percent(:password, value), do: URI.decode_www_form(value)
+  defp decode_percent(:auth_source, value), do: URI.decode_www_form(value)
   defp decode_percent(_other, value), do: value
 
   defp parse_query_options(opts, %{"options" => options}) when is_binary(options) do

test/mongo/url_parser_test.exs

@@ -199,5 +199,25 @@ defmodule Mongo.UrlParserTest do
       username = Keyword.get(opts, :username)
       assert username == real_username
     end
+
+    test "external auth source " do
+      encoded_external_auth_source = URI.encode_www_form("$external")
+      url = "mongodb://user:[email protected]:27017,seed2.domain.com:27017,seed3.domain.com:27017/db_name?replicaSet=set-name&authMechanism=PLAIN&authSource=#{encoded_external_auth_source}&tls=true"
+
+      assert UrlParser.parse_url(url: url) |> Keyword.drop([:pw_safe]) == [
+               password: "*****",
+               username: "user",
+               database: "db_name",
+               tls: true,
+               auth_source: "$external",
+               auth_mechanism: :plain,
+               set_name: "set-name",
+               seeds: [
+                 "seed1.domain.com:27017",
+                 "seed2.domain.com:27017",
+                 "seed3.domain.com:27017"
+               ]
+             ]
+    end
   end
 end