Delay sending of code 226

An FTP client implementation has been observed to close the data connection as soon as it receives the 226 status code - even though it hasn't received all data, yet. To improve interoper with such buggy clients, sending of the 226 status code can now be delayed a bit. The optional delay is controlled by means of preprocessor define that can be set from the cmake command line through a cmake cache variable.
eclipse-ecal · Nov 8, 2023 · a5108c1 · a5108c1
1 parent 5a3c9fe
commit a5108c1
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Showing 3 changed files with 37 additions and 1 deletion.
diff --git a/fineftp-server/CMakeLists.txt b/fineftp-server/CMakeLists.txt
@@ -92,6 +92,14 @@ target_compile_options(${PROJECT_NAME} PRIVATE
                            $<$<CXX_COMPILER_ID:MSVC>:
                                 /W4>)
 
+# The below is used to configure an optional delay when sending the 226 response code when a file has been fetched.
+# The background for this option is that an FTP client implementation has been observed to close the data connection
+# as soon as it receives the 226 status code - even though it hasn't received/ all data, yet. To improve
+# interoperability with such buggy clients, sending of the 226 status code can be delayed a bit. The delay is specified
+# in milliseconds. If the delay is 0, no delay is introduced.
+set(FINEFTP_SERVER_DELAY_226_RESP_MS 0 CACHE STRING
+    "An optional delay (in ms) for the 226 response when a file has been fetched. Used to improve interoperability with buggy clients.")
+target_compile_definitions(${PROJECT_NAME} PRIVATE DELAY_226_RESP_MS=${FINEFTP_SERVER_DELAY_226_RESP_MS})
 
 # Add own public include directory
 target_include_directories(${PROJECT_NAME}

diff --git a/fineftp-server/src/ftp_session.cpp b/fineftp-server/src/ftp_session.cpp
@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <cassert> // assert
 #include <cctype>  // std::iscntrl, toupper
+#include <chrono>
 #include <cstddef>
 #include <cstdio>
 #include <fstream>
@@ -47,6 +48,7 @@ namespace fineftp
     , ftp_working_directory_("/")
     , data_acceptor_        (io_service)
     , data_socket_strand_   (io_service)
+    , timer_                (io_service)
   {
   }
 
@@ -1302,7 +1304,31 @@ namespace fineftp
                                                         }
                                                         else
                                                         {
-                                                          me->sendFtpMessage(FtpReplyCode::CLOSING_DATA_CONNECTION, "Done");
+                                                          // Ugly work-around:
+                                                          // An FTP client implementation has been observed to close the data connection
+                                                          // as soon as it receives the 226 status code - even though it hasn't received
+                                                          // all data, yet. To improve interoperability with such buggy clients, sending
+                                                          // of the 226 status code can be delayed a bit. The delay is defined through a
+                                                          // preprocessor definition. If the delay is 0, no delay is introduced at all.
+                                                          #if (0 == DELAY_226_RESP_MS)
+                                                            me->sendFtpMessage(FtpReplyCode::CLOSING_DATA_CONNECTION, "Done");
+                                                          #else
+                                                            // Close Data Socket properly
+                                                            {
+                                                              asio::error_code ec;
+                                                              data_socket->shutdown(asio::socket_base::shutdown_both, ec);
+                                                              data_socket->close(ec);
+                                                            }
+
+                                                            me->timer_.expires_after(std::chrono::milliseconds{DELAY_226_RESP_MS});
+                                                            me->timer_.async_wait(me->data_socket_strand_.wrap([me](const asio::error_code& ec)
+                                                                                  {
+                                                                                    if (ec != asio::error::operation_aborted)
+                                                                                    {
+                                                                                      me->sendFtpMessage(FtpReplyCode::CLOSING_DATA_CONNECTION, "Done");
+                                                                                    }
+                                                                                  }));
+                                                          #endif
                                                         }
                                                       });
                                   }

diff --git a/fineftp-server/src/ftp_session.h b/fineftp-server/src/ftp_session.h
@@ -207,5 +207,7 @@ namespace fineftp
     asio::io_service::strand                       data_socket_strand_;
     std::weak_ptr<asio::ip::tcp::socket>           data_socket_weakptr_;
     std::deque<std::shared_ptr<std::vector<char>>> data_buffer_;
+
+    asio::steady_timer                             timer_;
   };
 }