uftpd: small FTP server for ESP8266, ESP32 and Pyboard D

Intro

Based on the work of chrisgp - Christopher Popp and pfalcon - Paul Sokolovsky Christopher made a first uftp server script, which runs in foreground. Paul made webrepl with the framework for background operations, which then was used also by Christopher to implement his utelnetsever code. My task was to put all these pieces together and assemble this uftpd.py script, which runs in background and acts as ftp server. Due to its size, for ESP8266 it either has to be integrated into the flash image as frozen bytecode, by placing it into the esp8266/modules folder and performing a rebuild, or it must be compiled into bytecode using mpy-cross and loaded as an .mpy file. The frozen bytecode variant is preferred.

Limitations

The server has some limitations:

Binary mode only
Limited multi-session support. The server accepts multiple sessions, but only one session command at a time is served while the other sessions receive a 'busy' response, which still allows interleaved actions.
No user authentication. Any user may log in without a password. User authentication may be added easily, if required.
Not all ftp commands are implemented.
ESP8266 is NOT a multitasking platform and the system calls are NOT re-entrant. Even when the ftp server sits in background and can serve requests, no foreground tasks should run at that time, especially if they execute system calls. The effects is hardly to predict, although most of the time the device simply crashes. Also in turn, when using the SITE command, the code in the payload MUST NOT be blocking, because that will block the device.
ESP32 The server is supported from version='v1.9.3-575 on. That is the version which introduced webrepl.

Start-up

You'll start the server with:

import uftpd

The service will immediately be started at port 21 in silent mode. You may stop the service then with:

uftpd.stop()

When stopped or not started yet, start it manually with:

uftpd.start([port = 21][, verbose = level]) or uftpd.restart([port = 21][, verbose = level])

port is the port number (default 21) verbose controls the level of printed activity messages, values 0 .. 2

You may use uftd.restart([port = 21][, verbose = level]) as a shortcut for uftp.stop() and uftpd.start().

Coverage

The server works well with most dedicated ftp clients, and most browsers and file managers. These are test results with an arbitrary selected set:

Linux

ftp: works for file & directory operations including support for the m* commands
filezilla, fireftp: work fine, including loading into the editor & saving back. Take care to limit the number of data session to 1.
Nautilus: works mostly, including loading into the editor & saving back. Copying multiple files at once to the esp8266 fails, because nautilus tries to open multiple sessions for that purpose. Configure Nautilus with dconf-editor to show directory count for local dirs only. Once mounted, you can even open a terminal at that spot. The path is something like: /run/user/1000/gvfs/ftp:host=x.y.y.z.
Thunar: works fine, including loading & saving of files. directly into e.g. an editor & saving back.
Dolphin, Konqueror: work fine most of the time, including loading directly into e.g. an editor & saving back. But no obvious disconnect.
Chrome, Firefox: view/navigate directories & and view files

Mac OS X, various Versions

ftp: works like on Linux
Chrome, Firefox: view/navigate directories & and view files
FileZilla, FireFtp, Cyberduck: Full operation, once proper configured (see above). Configure Cyberduck to transfer data in the command session.
Finder: Fails. It connects, but then locks in the attempt to display the top level directory repeating attempts to open new sessions. Finder needs full multi-session support, and never closes sessions properly.
Mountainduck: Works well, including proper disconnect when closing.

Windows 10 (and Windows XP)

ftp: supported. Be aware that the Windows variant of ftp differs slightly from the Linux variant, but the most used commands are the same.
File explorer: view/navigate directories & and copy files. For editing files you have to copy them to your PC and back. Windows explorer does not always release the connection when it is closed, which just results in a silent connection, which is closed latest when Windows is shut down.
FileZilla, FireFtp, Cyberduck: Full operation, once proper configured (see above). Configure Cyberduck to transfer data in the command session.
WinSCP: works fine
NppFTP - FTP extension to Notepad++: Works fine and is very convenient.
Mountainduck: Works to some extent, but sometimes stumbles and takes a long time to open a file.

Android

ftp inside the terminal emulator termux: full operation.
ftp-express
Chrome: view/navigate directories & and view files

IOS 9.1

FTP Client lite: works flawless

Windows 10 mobile

Metro file manager: Works with file/directory view & navigate, file download, file upload, file delete, file rename. Slow and chaotic sequence of FTP commands. Many unneeded re-login attempts.

Conclusion: All dedicated ftp clients work fine, and most of the file managers too.

Trouble shooting

The only trouble observed so far was clients not releasing the connections. You may tell by the value of uftp.client_list, which should be empty if no client is connected, or by issuing the command rstat in ftp, which shows the number of connected clients. In that case you may restart the server with uftpd.restart(). If uftd.client_busy is True when no client is connected, then restart the server with with uftpd.restart(). If you want to see what happens at the server, you may set verbose to 2. Just restart it with uftpd.restart(verbose = 1), or set uftpd.verbose_l = 1, and uftpd.verbose_l = 0 to stop control messages again.

Files