Monday, 5 February 2018

A small HTTP server in Python

The Python http.server module can be run from the commandline with

python3 -m http.server

to provide a simple http server that listens on a configurable port and basically serves any file from the directory it is started from.

That is a good start but I had some additional requirements to serve up files from my raspberry Pi:

  • The server must be able to run as a demonized process, i.e, in the background
  • It must server a limited set of files
  • from a configurable directory and
  • be able to run as a less privileged user
Now of course I could have opted for a light weight http server like nginx or lighttpd, but I decided to write my own so I wouldn't have to install a boatload of additional packages.

restrictedhttpserver.py

The server is based on the daemon module a wrote about in a previous article and can be started from the commandline in much the same way as http.server:


python3 restrictedhttpserver.py


It will run as the user that started it, will listen on port 8000 and start serving any file from the current directory. It will demonize itself as well.

It takes some additional options:

python3 restrictedhttpserver.py -h

usage: restrictedhttpserver.py [-h] [-p PID_FILE] [-l LOG_FILE] [-r ROOT_DIR]
                               [-n NAME] [-u USER] [-f] [-s] [-d]
                               [--bind ADDRESS] [-x] [-e EXT]
                               [port]

Example HTTP Server

positional arguments:
  port                  Specify alternate port [default: 8000]

optional arguments:
  -h, --help            show this help message and exit
  -p PID_FILE, --pid-file PID_FILE
  -l LOG_FILE, --log-file LOG_FILE
  -r ROOT_DIR, --root-dir ROOT_DIR
  -n NAME, --name NAME  name of the server used in log lines
  -u USER, --user USER  drop privileges of running server to those of user
  -f, --force           start a server even if pid file is present already
  -s, --stop            stop a running server
  -d, --debug           Run http server in foreground mode
  --bind ADDRESS, -b ADDRESS
                        Specify alternate bind address [default: all
                        interfaces]
  -x, --nodirlist       never show a directory listing
  -e EXT, --ext EXT     allowed file extensions (without .) May occur more
                        than once
For example if you wanted to run it as user www from the directory /public/www and just server html and css files, you could run it (as root) as follows:

python3 restrictedhttpserver.py -r /public/www -u www -e html -e css

Later you could stop this process by executing
python3 retrictedhttpserver.py -s


For more information have a look at the source code, it is pretty readable I think :-)

Availability

restrictedhttpserver.py and daemon.py are both available from this GitHub repository.

Wednesday, 31 January 2018

Demonizing a Python process

Properly demonizing a Python process is not easy, even on *nix, and although various sources of good material can be found on the internet (for example here and here) it didn't quite match my requirements and/or I didn't understand it completely.
I therefore decided to create both a daemon module and a http server to test it.

Code availability

Both modules are available on GitHub
The daemon module is called daemon.py and the httpserver is called restrictedhttpserver.py. Both have a fair amount of comments in their source code that are the result of me trying to fully understand what is required to get things working.

A small word of warning here: I only tested it with Python3 on Ubuntu and Raspbian and I do not expect it to work on any other operating system (although most *nix like systems probably will work) and in my world Python2 is no longer a valid option.

Also, the Daemon class makes use of an undocumented attribute in the logging.FileHandler class  (the stream.fileno attribute) which is necessary in order not to close this stream when forking a child process. Theoretically this might break in the future but I did want all logging to happen outside the chroot jail so that the process running as a daemon can log but does not have access to the logging. Whether this is a valid approach remains to be seen as this also prevents proper log rotation.

The Daemon class

The basic requirement are all implemented, an instance of a Daemon class can:
run in a chrooted jail
so it cannot access files outside a given directory
run with lowered privileges
so the risk of accessing files not owned by a specific user is lowered
can have a umask of your choice
which will ensure that newly created files by the daemon are not open to all
log outside its jail
so that we can log events without offering the process managed by daemon any way of altering the logging
and maintains a configurable pid file
so that we can prevent the daemon from starting more than once and providing a way the fond out the process id so that we can terminate the daemon
We also made the Daemon class follow the Singleton pattern, i.e. there can only be one instanced object of the class. This is sensible because a process can only be daemonized once. We also implemented to required methods to facilitate use as a context manager. A typical, minimal code snippet would look something like:
from daemon import Daemon

dm = Daemon()
with dm:
    ... do something forever ...
All configurable options have sensible defaults, but a call could look like this:
from daemon import Daemon

dm = Daemon(user='httpserver', rootdir='/var/www', umask=0o27,
             pidfile='/var/lockhttpserver.pid',
             logfile='/var/log/httpserver.log',
             name="Http Server")
with dm:
    ... do something forever ...
It is important to understand that when the Daemon object is created, the process is not immediately daemonized. This is handled by the context manager (or you could call the daemonize() method directly). There is also a stop() method provided that will check for a running daemon process and terminate it. This could look like this:
from daemon import Daemon
import argparse

parser = argparse.ArgumentParser(description="Example Daemon")
parser.add_argument('-s', '--stop', action='store_true', help='stop a running server')
args = parser.parse_args()

dm = Daemon()
if args.stop:
    dm.stop()
else:
    with dm:
        ... do something forever ...

More code

In a future article I will highlight the small http server that I implemented to test the daemon module.