What is fbtftp?

fbtftp is Facebook's implementation of a dynamic TFTP server framework. It lets you create custom TFTP servers and wrap your own logic into it in a very simple manner. Facebook currently uses it in production, and it's deployed at global scale across all of our data centers.

Why did you do that?

We love to use existing open source software and to contribute upstream, but sometimes it's just not enough at our scale. We ended up writing our own tftp framework and decided to open source it.

fbtftp was born from the need of having an easy-to-configure and easy-to-expand TFTP server, that would work at large scale. The standard in.tftpd is a 20+ years old piece of software written in C that is very difficult to extend.

fbtftp is written in python3 and lets you plug your own logic to:

• publish per session and server wide statistics to your infrastructure
• define how response data is built:
  • can be a file from disk;
  • can be a file created dynamically;
  • you name it!

How do you use fbtftp at Facebook?

We created our own Facebook-specific server based on the framework to:

• stream static files (initrd and kernels) from our http repositories (no need to fill your tftp root directory with files);
• generate grub2 per-machine configuration dynamically (no need to copy grub2 configuration files on disk);
• publish per-server and per-connection statistics to our internal monitoring systems;
• deployment is easy and "container-ready", just copy the application somewhere, start it and you are done.

Is it better than the other TFTP servers?

It depends on your needs! fbtftp is written in Python 3 using a multiprocessing model; its primary focus is not speed, but flexibility and scalability. Yet it is fast enough at our datacenter scale :) It is well-suited for large installations where scalability and custom features are needed.

What does it support?

The framework implements the following RFCs:

• RFC 1350 (the main TFTP specification)
• RFC 2347 (Option Extension)
• RFC 2348 (Blocksize option)
• RFC 2349 (Timeout Interval and Transfer Size Options).

Note that the server framework only support RRQs (read only) operations. (Who uses WRQ TFTP requests in 2019? :P)

How does it work?

All you need to do is understanding three classes and two callback functions, and you are good to go:

• BaseServer: This class implements the process which deals with accepting new requests on the UDP port provided. Default TFTP parameters like timeout, port number and number of retries can be passed. This class doesn't have to be used directly, you must inherit from it and override get_handler() method to return an instance of BaseHandler. The class accepts a server_stats_callback, more about it below. the callback is not re-entrant, if you need this you have to implement your own locking logic. This callback is executed periodically and you can use it to publish server level stats to your monitoring infrastructure. A series of predefined counters are provided. Refer to the class documentation to find out more.

• BaseHandler: This class deals with talking to a single client. This class lives into its separate process, process which is spawned by the BaserServer class, which will make sure to reap the child properly when the session is over. Do not use this class as is, instead inherit from it and override the get_response_data() method. Such method must return an instance of a subclass of ResponseData.

• ResponseData: it's a file-like class that implements read(num_bytes), size() and close(). As the previous two classes you'll have to inherit from this and implement those methods. This class basically let you define how to return the actual data

• server_stats_callback: function that is called periodically (every 60 seconds by default). The callback is not re-entrant, if you need this you have to implement your own locking logic. This callback is executed periodically and you can use it to publish server level stats to your monitoring infrastructure. A series of predefined counters are provided. Refer to the class documentation to find out more.

• session_stats_callback: function that is called when a client session is over.

Requirements

• Linux (or any system that supports epoll)
• BSD (or any system that supports kqueue)
• Python 3.4+

Installation

fbtftp is distributed with the standard distutils package, so you can build it with:

python setup.py build

and install it with:

python setup.py install

Be sure to run as root if you want to install fbtftp system wide. You can also use a virtualenv, or install it as user by running:

python setup.py install --user

Example

Writing your own server is simple. Let's take a look at how to write a simple server that serves files from disk:

from fbtftp.base_handler import BaseHandler
from fbtftp.base_handler import ResponseData
from fbtftp.base_server import BaseServer

import os

class FileResponseData(ResponseData):
    def __init__(self, path):
        self._size = os.stat(path).st_size
        self._reader = open(path, 'rb')

    def read(self, n):
        return self._reader.read(n)

    def size(self):
        return self._size

    def close(self):
        self._reader.close()

def print_session_stats(stats):
    print(stats)

def print_server_stats(stats):
    counters = stats.get_and_reset_all_counters()
    print('Server stats - every {} seconds'.format(stats.interval))
    print(counters)

class StaticHandler(BaseHandler):
    def __init__(self, server_addr, peer, path, options, root, stats_callback):
        self._root = root
        super().__init__(server_addr, peer, path, options, stats_callback)

    def get_response_data(self):
        return FileResponseData(os.path.join(self._root, self._path))

class StaticServer(BaseServer):
    def __init__(self, address, port, retries, timeout, root,
                 handler_stats_callback, server_stats_callback=None):
        self._root = root
        self._handler_stats_callback = handler_stats_callback
        super().__init__(address, port, retries, timeout, server_stats_callback)

    def get_handler(self, server_addr, peer, path, options):
        return StaticHandler(
            server_addr, peer, path, options, self._root,
            self._handler_stats_callback)

def main():
    server = StaticServer(address='::', port=69, retries=3, timeout=5,
                          root='/var/tftproot', 
                          handler_stats_callback=print_session_stats,
                          server_stats_callback=print_server_stats)
    try:
        server.run()
    except KeyboardInterrupt:
        server.close()

if __name__ == '__main__':
    main()

Who wrote it?

fbtftp was created by Marcin Wyszynski (@marcinwyszynski) and Angelo Failla [email protected] at Facebook Ireland.

Other honorable contributors:

• Andrea Barberio [email protected]

License

MIT License