An HTTP proxy server to automatically authenticate through an NTLM proxy
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What is Px?

Px is a HTTP(s) proxy server that allows applications to authenticate through an NTLM or Windows Kerberos authenticated proxy server, typically used in corporate deployments, without having to deal with the actual handshake. It is primarily designed to run on Windows systems and authenticates on behalf of the application using the currently logged in Windows user account.

Px is very similar to "NTLM Authorization Proxy Server" NTLMAPS and CNTLM in that it sits between the corporate proxy and applications and offloads the authentication. The primary difference in Px is to use the currently logged in user's credentials to log in automatically rather than requiring the user to provide the username, password (hash) and domain information. This is accomplished by using Microsoft SSPI to generate the tokens and signatures required to authenticate with the proxy. The other advantage is that Px supports Kerberos authentication as well, which NTLMAPS and CNTLM do not.

NTLMAPS and CNTLM were designed for non-Windows users stuck behind a corporate proxy. As a result, they require the user to provide the correct credentials to authenticate. On Windows, the user has already logged in with his credentials so Px is designed for Windows users who would like to use tools that aren't designed to deal with proxy authentication, without having to supply and maintain the credentials within Px.

The following link from Microsoft provides a good starting point to understand how NTLM authentication works:

And similarly for Kerberos (warning: long!)


Px can be obtained in multiple ways:-

Download the latest binary ZIP from the releases page:

Once downloaded, extract to a folder of choice and use the --save and --install commands as documented below.

If Python is already available, Px can be easily installed using the Python package manager pip:

  • This will download and install Px along with all dependencies.

    • Latest: pip install git+

    • Stable: pip install px-proxy

Px can also be run from source if Python is available:

  • Download a source ZIP of the latest release from above releases link

  • Clone the latest source:

    git clone

  • Download the latest source ZIP:

Running from source requires a few dependencies installed. Px along with all dependencies can be installed to the standard Python location using:

python install

After installation, Px can be run on the command line like an executable and the --save and --install commands can be used per usual.

px --save
px --install

NOTE: Command line parameters passed with --install are not saved for use on startup. The --save flag or manual editing of px.ini is required to provide configuration to Px on startup.

If installed, Px can be uninstalled as follows:

px --uninstall
pip uninstall px-proxy

Lastly, Px can be run as a standard Python script. Download the source as described above. Install all dependencies manually using pip and then run Px:

pip install netaddr psutil pywin32 winkerberos futures

python --help


Px requires only one piece of information in order to function - the server name and port of the proxy server. This needs to be configured in px.ini. If not specified, Px will check Internet Options for any proxy definitions and use them. Without this, Px will not work and exit immediately.

The noproxy capability allows Px to connect to hosts in the configured subnets directly, bypassing the proxy altogether. This allows clients to connect to hosts within the intranet without requiring additional configuration for each client or at the proxy. If noproxy is defined, the proxy is optional - this allows Px to run as a regular proxy full time if required.

There are a few other settings to tweak in the INI file but most are obvious. All settings can be specified on the command line for convenience. The INI file can also be created or updated from the command line using --save.

The binary distribution of Px runs in the background once started and can be quit by running px --quit. When run directly using Python, use CTRL-C to quit.

Px can also be setup to automatically run on startup with the --install flag. This is done by adding an entry into the Window registry which can be removed with --uninstall.


px [FLAGS]
python [FLAGS]

  Save configuration to px.ini or file specified with --config
    Allows setting up Px config directly from command line
    Values specified on CLI override any values in existing config file
    Values not specified on CLI or config file are set to defaults

  Add Px to the Windows registry to run on startup

  Remove Px from the Windows registry

  Quit a running instance of Px.exe

  Specify config file. Valid file path, default: px.ini in working directory

  --proxy=  --server=  proxy:server= in INI file
    Proxy server(s) to connect through. IP:port, hostname:port
    Multiple proxies can be specified comma separated. Px will iterate through
    and use the one that works. Required field unless --noproxy is defined. If
    remote server is not in noproxy list and proxy is undefined, Px will reject
    the request

  --listen=  proxy:listen=
  IP interface to listen on. Valid IP address, default:

  --port=  proxy:port=
  Port to run this proxy. Valid port number, default: 3128

  --gateway  proxy:gateway=
  Allow remote machines to use proxy. 0 or 1, default: 0
    Overrides 'listen' and binds to all interfaces

  --hostonly  proxy:hostonly=
  Allow only local interfaces to use proxy. 0 or 1, default: 0
    Px allows all IP addresses assigned to local interfaces to use the service.
    This allows local apps as well as VM or container apps to use Px when in a
    NAT config. Px does this by listening on all interfaces and overriding the
    allow list.

  --allow=  proxy:allow=
  Allow connection from specific subnets. Comma separated, default: *.*.*.*
    Whitelist which IPs can use the proxy. --hostonly overrides any definitions
    unless --gateway mode is also specified - specific ip
    192.168.0.* - wildcards - ranges - CIDR

  --noproxy=  proxy:noproxy=
  Direct connect to specific subnets like a regular proxy. Comma separated
    Skip the proxy for connections to these subnets - specific ip
    192.168.0.* - wildcards - ranges - CIDR

  --useragent=  proxy:useragent=
  Override or send User-Agent header on client's behalf

  --workers=  settings:workers=
  Number of parallel workers (processes). Valid integer, default: 2

  --threads=  settings:threads=
  Number of parallel threads per worker (process). Valid integer, default: 5

  --idle=  settings:idle=
  Idle timeout in seconds for HTTP connect sessions. Valid integer, default: 30

  --socktimeout=  settings:socktimeout=
  Timeout in seconds for connections before giving up. Valid float, default: 5

  --proxyreload=  settings:proxyreload=
  Time interval in seconds before reloading proxy info. Valid int, default: 60
    Proxy info is reloaded from a PAC file found via WPAD or AutoConfig URL, or
    manual proxy info defined in Internet Options

  --foreground  settings:foreground=
  Run in foreground when frozen or with pythonw.exe. 0 or 1, default: 0
    Px will attach to the console and write to it even though the prompt is
    available for further commands. CTRL-C in the console will exit Px

  --debug  settings:log=
  Enable debug logging. default: 0
    Logs are written to working directory and over-written on startup
    A log is automatically created if Px crashes for some reason

  Generate unique log file names
    Prevents logs from being overwritten on subsequent runs. Also useful if
    running multiple instances of Px


Use and allow requests from localhost only: px

Don't use any forward proxy at all, just log what's going on: px --noproxy= --debug

Allow requests from localhost and all locally assigned IP addresses. This is very useful for Docker for Windows and VMs in a NAT configuration because all requests originate from the host's IP: px --hostonly

Allow requests from localhost, locally assigned IP addresses and the IPs specified in the allow list outside the host: px --proxy=proxyserver:80 --hostonly --gateway --allow=172.*.*.*

Allow requests from everywhere. Be careful, every client will use your login: px --gateway

NOTE: In Docker for Windows you need to set your proxy to http://<your_ip>:3128 (or actual port Px is listening to) and be aware of

Workaround: docker build --build-arg http_proxy=http://<your ip>:3128 --build-arg https_proxy=http://<your ip>:3128 -t containername ../dir/with/Dockerfile


Px doesn't have any GUI and runs completely in the background. It is distributed using Python 3.x and PyInstaller to have a self-contained executable but can also be run using a Python distribution with the following additional packages.

netaddr, psutil, winkerberos

futures on Python 2.x

Px is tested with the latest releases of Python 2.7, 3.4, 3.5 and 3.6 using the Miniconda distribution.

In order to make Px a capable proxy server, it is designed to run in multiple processes. The number of parallel workers or processes is configurable. However, this only works on Python 3.3+ since that's when support was added to share sockets across processes in Windows. On older versions of Python, Px will run multi-threaded but in a single process. The number of threads per process is also configurable.


Px is definitely a work in progress and any feedback or suggestions are welcome. It is hosted on GitHub ( with an MIT license so issues, forks and PRs are most appreciated.


Px is based on code from all over the internet and especially acknowledges these sources:

Thank you to the following contributors for their PRs and all issue submitters.