– A lightweight, single file HTTP proxy server in python


To facilitate end-to-end testing for such scenarios, I architected a proxy infrastructure; A stripped-down version of which was a - lightweight HTTP proxy server in Python.

Blog post:

Github page:


  • Fast & Scalable
    • Scales by using all available cores on the system
    • Threadless executions using coroutine
    • Made to handle tens-of-thousands connections / sec
      # On Macbook Pro 2015 / 2.8 GHz Intel Core i7
      ❯ hey -n 10000 -c 100 http://localhost:8899/
        Total:	0.6157 secs
        Slowest:	0.1049 secs
        Fastest:	0.0007 secs
        Average:	0.0055 secs
        Requests/sec:	16240.5444
        Total data:	800000 bytes
        Size/request:	80 bytes
      Response time histogram:
        0.001 [1]     |
        0.011 [9565]	|■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■
        0.022 [332]	|
  • Lightweight
    • Uses only ~5-20MB RAM
    • No external dependency other than standard Python library
  • Programmable
    • Optionally enable builtin Web Server
    • Customize proxy and http routing via plugins
    • Enable plugin using command line option e.g. --plugins proxy.plugin.CacheResponsesPlugin
    • Plugin API is currently in development phase, expect breaking changes.
  • Realtime Dashboard
    • Optionally enable bundled dashboard.
      • Available at http://localhost:8899/dashboard.
    • Inspect, Monitor, Control and Configure at runtime.
    • Extend dashboard using plugins.
    • Dashboard is currently in development phase, expect breaking changes.
  • Secure
  • Man-In-The-Middle
    • Can decrypt TLS traffic between clients and upstream servers
    • See TLS Interception
  • Supported proxy protocols
    • http(s)
      • http1
      • http1.1 pipeline
    • http2
    • websockets
  • Optimized for large file uploads and downloads
  • IPv4 and IPv6 support
  • Basic authentication support
  • Can serve a PAC (Proxy Auto-configuration) file
    • See --pac-file and --pac-file-url-path flags


Using PIP

Stable Version with PIP

Install from PyPi

❯ pip install --upgrade

or from GitHub master branch

❯ pip install git+

Development Version with PIP

❯ pip install git+[email protected]

Using Docker

Stable Version from Docker Hub

❯ docker run -it -p 8899:8899 --rm abhinavsingh/

Build Development Version Locally

❯ git clone
❯ cd
❯ make container
❯ docker run -it -p 8899:8899 --rm abhinavsingh/

WARNING docker image is currently broken on macOS due to incompatibility with vpnkit.

Using HomeBrew

Stable Version with HomeBrew

❯ brew install

Development Version with HomeBrew

❯ brew install


From command line when installed using PIP

When is installed using pip, an executable named proxy is placed under your $PATH.

Run it

Simply type proxy on command line to start it with default configuration.

❯ proxy
...[redacted]... - Loaded plugin proxy.http_proxy.HttpProxyPlugin
...[redacted]... - Starting 8 workers
...[redacted]... - Started server on ::1:8899

Understanding logs

Things to notice from the above logs:

  • Loaded plugin - will load proxy.http.proxy.HttpProxyPlugin by default. As the name suggests, this core plugin adds http(s) proxy server capabilities to
  • Started N workers - Use --num-workers flag to customize the number of worker processes. By default, will start as many workers as there are CPU cores on the machine.
  • Started server on ::1:8899 - By default, listens on IPv6 ::1, which is equivalent of IPv4 If you want to access externally, use --hostname :: or --hostname or bind to any other interface available on your machine.
  • Port 8899 - Use --port flag to customize the default TCP port.

Enable DEBUG logging

All the logs above are INFO level logs, default --log-level for

Let's start with DEBUG level logging:

❯ proxy --log-level d
...[redacted]... - Open file descriptor soft limit set to 1024
...[redacted]... - Loaded plugin proxy.http_proxy.HttpProxyPlugin
...[redacted]... - Started 8 workers
...[redacted]... - Started server on ::1:8899

As we can see, before starting up:

  • also tried to set open file limit ulimit on the system.
  • Default value for --open-file-limit used is 1024.
  • --open-file-limit flag is a no-op on Windows operating systems.

See flags for full list of available configuration options.

From command line using repo source

If you are trying to run from source code, there is no binary file named proxy in the source code.

To start from source code follow these instructions:

  • Clone repo
    ❯ git clone
    ❯ cd
  • Create a Python 3 virtual env
    ❯ python3 -m venv venv
    ❯ source venv/bin/activate
  • Install deps
    ❯ pip install -r requirements.txt
    ❯ pip install -r requirements-testing.txt
  • Run tests
    ❯ make
  • Run
    ❯ python -m proxy

Also see Plugin Developer and Contributor Guide if you plan to work with source code.

Docker image

Customize startup flags

By default docker binary is started with IPv4 networking flags:

--hostname --port 8899

To override input flags, start docker image as follows. For example, to check the version within Docker image:

❯ docker run -it \
    -p 8899:8899 \
    --rm abhinavsingh/ \

Plugin Examples

  • See plugin module for full code.
  • All the bundled plugin examples also works with https traffic
  • Plugin examples are also bundled with Docker image.

HTTP Proxy Plugins


Add support for short links in your favorite browsers / applications.

Start as:

❯ proxy \
    --plugins proxy.plugin.ShortLinkPlugin

Now you can speed up your daily browsing experience by visiting your favorite website using single character domain names :). This works across all browsers.

Following short links are enabled by default:

Short Link Destination URL
proxy/ localhost:8899


Modifies POST request body before sending request to upstream server.

Start as:

❯ proxy \
    --plugins proxy.plugin.ModifyPostDataPlugin

By default plugin replaces POST body content with hardcoded b'{"key": "modified"}' and enforced Content-Type: application/json.

Verify the same using curl -x localhost:8899 -d '{"key": "value"}'

  "args": {},
  "data": "{\"key\": \"modified\"}",
  "files": {},
  "form": {},
  "headers": {
    "Accept": "*/*",
    "Content-Length": "19",
    "Content-Type": "application/json",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "json": {
    "key": "modified"
  "origin": ",",
  "url": ""

Note following from the response above:

  1. POST data was modified "data": "{\"key\": \"modified\"}". Original curl command data was {"key": "value"}.
  2. Our curl command did not add any Content-Type header, but our plugin did add one "Content-Type": "application/json". Same can also be verified by looking at json field in the output above:
    "json": {
     "key": "modified"
  3. Our plugin also added a Content-Length header to match length of modified body.


Mock responses for your server REST API. Use to test and develop client side applications without need of an actual upstream REST API server.

Start as:

❯ proxy \
    --plugins proxy.plugin.ProposedRestApiPlugin

Verify mock API response using curl -x localhost:8899

{"count": 2, "next": null, "previous": null, "results": [{"email": "[email protected]", "groups": [], "url": "", "username": "admin"}, {"email": "[email protected]", "groups": [], "url": "", "username": "admin"}]}

Verify the same by inspecting logs:

2019-09-27 12:44:02,212 - INFO - pid:7077 - access_log:1210 - ::1:64792 - GET None:None/v1/users/ - None None - 0 byte

Access log shows None:None as server ip:port. None simply means that the server connection was never made, since response was returned by our plugin.

Now modify ProposedRestApiPlugin to returns REST API mock responses as expected by your clients.


Redirects all incoming http requests to custom web server. By default, it redirects client requests to inbuilt web server, also running on 8899 port.

Start and enable inbuilt web server:

❯ proxy \
    --enable-web-server \
    --plugins proxy.plugin.RedirectToCustomServerPlugin

Verify using curl -v -x localhost:8899

... [redacted] ...
< HTTP/1.1 404 NOT FOUND
< Server: v1.0.0
< Connection: Close
* Closing connection 0

Above 404 response was returned from web server.

Verify the same by inspecting the logs for Along with the proxy request log, you must also see a http web server request log.

2019-09-24 19:09:33,602 - INFO - pid:49996 - access_log:1241 - ::1:49525 - GET /
2019-09-24 19:09:33,603 - INFO - pid:49995 - access_log:1157 - ::1:49524 - GET localhost:8899/ - 404 NOT FOUND - 70 bytes


Drops traffic by inspecting upstream host. By default, plugin drops traffic for and

Start as:

❯ proxy \
    --plugins proxy.plugin.FilterByUpstreamHostPlugin

Verify using curl -v -x localhost:8899

... [redacted] ...
< HTTP/1.1 418 I'm a tea pot
< Proxy-agent: v1.0.0
* no chunk, no close, no size. Assume close to signal end
* Closing connection 0

Above 418 I'm a tea pot is sent by our plugin.

Verify the same by inspecting logs for

2019-09-24 19:21:37,893 - ERROR - pid:50074 - handle_readables:1347 - HttpProtocolException type raised
Traceback (most recent call last):
... [redacted] ...
2019-09-24 19:21:37,897 - INFO - pid:50074 - access_log:1157 - ::1:49911 - GET None:None/ - None None - 0 bytes


Caches Upstream Server Responses.

Start as:

❯ proxy \
    --plugins proxy.plugin.CacheResponsesPlugin

Verify using curl -v -x localhost:8899

... [redacted] ...
< HTTP/1.1 200 OK
< Access-Control-Allow-Credentials: true
< Access-Control-Allow-Origin: *
< Content-Type: application/json
< Date: Wed, 25 Sep 2019 02:24:25 GMT
< Referrer-Policy: no-referrer-when-downgrade
< Server: nginx
< X-Content-Type-Options: nosniff
< X-Frame-Options: DENY
< X-XSS-Protection: 1; mode=block
< Content-Length: 202
< Connection: keep-alive
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": ",",
  "url": ""
* Connection #0 to host localhost left intact

Get path to the cache file from logs:

... [redacted] ... - GET - 200 OK - 556 bytes
... [redacted] ... - Cached response at /var/folders/k9/x93q0_xn1ls9zy76m2mf2k_00000gn/T/

Verify contents of the cache file cat /path/to/your/cache/

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Wed, 25 Sep 2019 02:24:25 GMT
Referrer-Policy: no-referrer-when-downgrade
Server: nginx
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Length: 202
Connection: keep-alive

  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": ",",
  "url": ""


Modifies upstream server responses.

Start as:

❯ proxy \
    --plugins proxy.plugin.ManInTheMiddlePlugin

Verify using curl -v -x localhost:8899

... [redacted] ...
< HTTP/1.1 200 OK
< Content-Length: 28
* Connection #0 to host localhost left intact
Hello from man in the middle

Response body Hello from man in the middle is sent by our plugin.


Forward incoming proxy requests to a set of upstream proxy servers.

By default, ProxyPoolPlugin is hard-coded to use localhost:9000 and localhost:9001 as upstream proxy server.

Let's start upstream proxies first.

Start on port 9000 and 9001

❯ proxy --port 9000
❯ proxy --port 9001

Now, start with ProxyPoolPlugin (on default 8899 port):

❯ proxy \
    --plugins proxy.plugin.ProxyPoolPlugin

Make a curl request via 8899 proxy:

curl -v -x localhost:8899

Verify that 8899 proxy forwards requests to upstream proxies by checking respective logs.

HTTP Web Server Plugins

Reverse Proxy

Extend in-built Web Server to add Reverse Proxy capabilities.

Start as:

❯ proxy --enable-web-server \
    --plugins proxy.plugin.ReverseProxyPlugin

With default configuration, ReverseProxyPlugin plugin is equivalent to following Nginx config:

location /get {

Verify using curl -v localhost:8899/get:

  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "localhost",
    "User-Agent": "curl/7.64.1"
  "origin": ",",
  "url": "https://localhost/get"

Web Server Route

Demonstrates inbuilt web server routing using plugin.

Start as:

❯ proxy --enable-web-server \
    --plugins proxy.plugin.WebServerPlugin

Verify using curl -v localhost:8899/http-route-example, should return:

HTTP route response

Plugin Ordering

When using multiple plugins, depending upon plugin functionality, it might be worth considering the order in which plugins are passed on the command line.

Plugins are called in the same order as they are passed. Example, say we are using both FilterByUpstreamHostPlugin andRedirectToCustomServerPlugin. Idea is to drop all incoming http requests for and and redirect other http requests to our inbuilt web server.

Hence, in this scenario it is important to use FilterByUpstreamHostPlugin before RedirectToCustomServerPlugin. If we enable RedirectToCustomServerPlugin before FilterByUpstreamHostPlugin, google requests will also get redirected to inbuilt web server, instead of being dropped.

End-to-End Encryption

By default, uses http protocol for communication with clients e.g. curl, browser. For enabling end-to-end encrypting using tls / https first generate certificates:

make https-certificates

Start as:

❯ proxy \
    --cert-file https-cert.pem \
    --key-file https-key.pem

Verify using curl -x https://localhost:8899 --proxy-cacert https-cert.pem

  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": ",",
  "url": ""

TLS Interception

By default, will not decrypt https traffic between client and server. To enable TLS interception first generate root CA certificates:

❯ make ca-certificates

Lets also enable CacheResponsePlugin so that we can verify decrypted response from the server. Start as:

❯ proxy \
    --plugins proxy.plugin.CacheResponsesPlugin \
    --ca-key-file ca-key.pem \
    --ca-cert-file ca-cert.pem \
    --ca-signing-key-file ca-signing-key.pem

:note: MacOS users also need to pass explicit CA file path needed for validation of peer certificates. See --ca-file flag.

Verify TLS interception using curl

❯ curl -v -x localhost:8899 --cacert ca-cert.pem
*  issuer: C=US; ST=CA; L=SanFrancisco;; OU=CA; CN=Proxy PY CA; [email protected]
*  SSL certificate verify ok.
> GET /get HTTP/1.1
... [redacted] ...
< Connection: keep-alive
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": ",",
  "url": ""

The issuer line confirms that response was intercepted.

Also verify the contents of cached response file. Get path to the cache file from logs.

❯ cat /path/to/your/tmp/directory/

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Wed, 25 Sep 2019 23:07:05 GMT
Referrer-Policy: no-referrer-when-downgrade
Server: nginx
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Length: 202
Connection: keep-alive

  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": ",",
  "url": ""

Viola!!! If you remove CA flags, encrypted data will be found in the cached file instead of plain text.

Now use CA flags with other plugin examples to see them work with https traffic.

Proxy Over SSH Tunnel

Requires paramiko to work. See requirements-tunnel.txt

Proxy Remote Requests Locally

+------------+          |            +----------+
|   LOCAL    |          |            |  REMOTE  |
|   HOST     | <== SSH ==== :8900 == |  SERVER  |
+------------+          |            +----------+
:8899          |
                  (allow tcp/22)


Proxy HTTP(s) requests made on a remote server through server running on localhost.


  • Requested remote port is forwarded over the SSH connection.
  • running on the localhost handles and responds to remote proxy requests.


  1. localhost MUST have SSH access to the remote server
  2. remote server MUST be configured to proxy HTTP(s) requests through the forwarded port number e.g. :8900.
    • remote and localhost ports CAN be same e.g. :8899.
    • :8900 is chosen in ascii art for differentiation purposes.

Try it

Start as:

# On localhost
❯ proxy --enable-tunnel \
    --tunnel-username username \
    --tunnel-hostname \
    --tunnel-port 22 \
    --tunnel-remote-port 8899

Make a HTTP proxy request on remote server and verify that response contains public IP address of localhost as origin:

# On remote
❯ curl -x
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Host": "",
    "User-Agent": "curl/7.54.0"
  "origin": "x.x.x.x, y.y.y.y",
  "url": ""

Also, verify that logs on localhost contains remote IP as client IP.

access_log:328 - remote:52067 - GET

Proxy Local Requests Remotely

+------------+          |     +----------+
|   LOCAL    |          |     |  REMOTE  |
|   HOST     | === SSH =====> |  SERVER  |
+------------+          |     +----------+
                        |     :8899
                 (allow tcp/22)


Blocking Mode

Start in embedded mode with default configuration by using proxy.main method. Example:

import proxy

if __name__ == '__main__':

Customize startup flags by passing list of input arguments:

import proxy

if __name__ == '__main__':
    '--hostname', '::1',
    '--port', '8899'

or, customize startup flags by passing them as kwargs:

import ipaddress
import proxy

if __name__ == '__main__':

Note that:

  1. Calling main is simply equivalent to starting from command line.
  2. main will block until shuts down

Non-blocking Mode

Start in non-blocking embedded mode with default configuration by using start method: Example:

import proxy

if __name__ == '__main__':
  with proxy.start([]):
    # ... your logic here ...

Note that:

  1. start is similar to main, except start won't block.
  2. start is a context manager. It will start when called and will shut it down once scope ends.
  3. Just like main, startup flags with start method can be customized by either passing flags as list of input arguments e.g. start(['--port', '8899']) or by using passing flags as kwargs e.g. start(port=8899).

Unit testing with


To setup and teardown for your Python unittest classes, simply use proxy.TestCase instead of unittest.TestCase. Example:

import proxy

class TestProxyPyEmbedded(proxy.TestCase):

    def test_my_application_with_proxy(self) -> None:

Note that:

  1. proxy.TestCase overrides method to setup and teardown
  2. server will listen on a random available port on the system. This random port is available as self.PROXY_PORTwithin your test cases.
  3. Only a single worker is started by default (--num-workers 1) for faster setup and teardown.
  4. Most importantly, proxy.TestCase also ensures server is up and running before proceeding with execution of tests. By default, proxy.TestCase will wait for 10 seconds for server to start, upon failure a TimeoutErrorexception will be raised.

Override startup flags

To override default startup flags, define a PROXY_PY_STARTUP_FLAGS variable in your test class. Example:

class TestProxyPyEmbedded(TestCase):

        '--num-workers', '1',

    def test_my_application_with_proxy(self) -> None:

See for full working example.

With unittest.TestCase

If for some reasons you are unable to directly use proxy.TestCase, then simply override yourself to setup and teardown Example:

import unittest
import proxy

class TestProxyPyEmbedded(unittest.TestCase):

    def test_my_application_with_proxy(self) -> None:

    def run(self, result: Optional[unittest.TestResult] = None) -> Any:
        with proxy.start([
                '--num-workers', '1',
                '--port', '... random port ...']):

or simply setup / teardown within setUpClass and teardownClass class methods.

Plugin Developer and Contributor Guide

Everything is a plugin

As you might have guessed by now, in everything is a plugin.

  • We enabled proxy server plugins using --plugins flag. All the plugin examples were implementingHttpProxyBasePlugin. See documentation of HttpProxyBasePlugin for available lifecycle hooks. Use HttpProxyBasePlugin to modify behavior of http(s) proxy protocol between client and upstream server. Example, FilterByUpstreamHostPlugin.
  • We also enabled inbuilt web server using --enable-web-server. Inbuilt web server implements HttpProtocolHandlerPlugin plugin. See documentation of HttpProtocolHandlerPlugin for available lifecycle hooks. Use HttpProtocolHandlerPlugin to add new features for http(s) clients. Example, HttpWebServerPlugin.
  • There also is a --disable-http-proxy flag. It disables inbuilt proxy server. Use this flag with --enable-web-serverflag to run as a programmable http(s) server. HttpProxyPlugin also implements HttpProtocolHandlerPlugin.

Internal Architecture

  • HttpProtocolHandler thread is started with the accepted TcpClientConnection. HttpProtocolHandler is responsible for parsing incoming client request and invoking HttpProtocolHandlerPlugin lifecycle hooks.
  • HttpProxyPlugin which implements HttpProtocolHandlerPlugin also has its own plugin mechanism. Its responsibility is to establish connection between client and upstream TcpServerConnection and invoke HttpProxyBasePlugin lifecycle hooks.
  • HttpProtocolHandler threads are started by Acceptor processes.
  • --num-workers Acceptor processes are started by AcceptorPool on start-up.
  • AcceptorPool listens on server socket and pass the handler to Acceptor processes. Workers are responsible for accepting new client connections and starting HttpProtocolHandler thread.

Development Guide

Setup Local Environment

Contributors must start from source to verify and develop new features / fixes.

See Run from command line using repo source for details.

Setup pre-commit hook

Pre-commit hook ensures lint checking and tests execution.

  1. cd /path/to/
  2. ln -s $(PWD)/git-pre-commit .git/hooks/pre-commit

Sending a Pull Request

Every pull request is tested using GitHub actions.

See GitHub workflow for list of tests.


TCP Sockets


Attempts to create an IPv4 connection, then IPv6 and finally a dual stack connection to provided address.

>>> conn = new_socket_connection(('', 80))
>>> ...[ use connection ]...
>>> conn.close()


socket_connection is a convenient decorator + context manager around new_socket_connection which ensures conn.close is implicit.

As a context manager:

>>> with socket_connection(('', 80)) as conn:
>>>   ... [ use connection ] ...

As a decorator:

>>> @socket_connection(('', 80))
>>> def my_api_call(conn, *args, **kwargs):
>>>   ... [ use connection ] ...

Http Client


Generate HTTP GET request

>>> build_http_request(b'GET', b'/')
b'GET / HTTP/1.1\r\n\r\n'

Generate HTTP GET request with headers

>>> build_http_request(b'GET', b'/',
        headers={b'Connection': b'close'})
b'GET / HTTP/1.1\r\nConnection: close\r\n\r\n'

Generate HTTP POST request with headers and body

>>> import json
>>> build_http_request(b'POST', b'/form',
        headers={b'Content-type': b'application/json'},
        body=proxy.bytes_(json.dumps({'email': '[email protected]'})))
    b'POST /form HTTP/1.1\r\nContent-type: application/json\r\n\r\n{"email": "[email protected]"}'


    status_code: int,
    protocol_version: bytes = HTTP_1_1,
    reason: Optional[bytes] = None,
    headers: Optional[Dict[bytes, bytes]] = None,
    body: Optional[bytes] = None) -> bytes


API Usage


    key_path: str,
    password: str,
    bits: int = 2048,
    timeout: int = 10) -> bool


    public_key_path: str,
    private_key_path: str,
    private_key_password: str,
    subject: str,
    alt_subj_names: Optional[List[str]] = None,
    extended_key_usage: Optional[str] = None,
    validity_in_days: int = 365,
    timeout: int = 10) -> bool


    key_in_path: str,
    password: str,
    key_out_path: str,
    timeout: int = 10) -> bool


    csr_path: str,
    key_path: str,
    password: str,
    crt_path: str,
    timeout: int = 10) -> bool


    csr_path: str,
    crt_path: str,
    ca_key_path: str,
    ca_key_password: str,
    ca_crt_path: str,
    serial: str,
    alt_subj_names: Optional[List[str]] = None,
    extended_key_usage: Optional[str] = None,
    validity_in_days: int = 365,
    timeout: int = 10) -> bool

See and for usage examples

CLI Usage

Use proxy.common.pki module for:

  1. Generation of public and private keys
  2. Generating CSR requests
  3. Signing CSR requests using custom CA.
python -m proxy.common.pki -h
usage: [-h] [--password PASSWORD] [--private-key-path PRIVATE_KEY_PATH]
              [--public-key-path PUBLIC_KEY_PATH] [--subject SUBJECT]
              action v2.1.2 : PKI Utility

positional arguments:
  action                Valid actions: remove_passphrase, gen_private_key,
                        gen_public_key, gen_csr, sign_csr

optional arguments:
  -h, --help            show this help message and exit
  --password PASSWORD   Password to use for encryption. Default:
  --private-key-path PRIVATE_KEY_PATH
                        Private key path
  --public-key-path PUBLIC_KEY_PATH
                        Public key path
  --subject SUBJECT     Subject to use for public key generation. Default:

Internal Documentation

Browse through internal class hierarchy and documentation using pydoc3. Example:

❯ pydoc3 proxy

    common (package)
    core (package)
    http (package)


Frequently Asked Questions

Threads vs Threadless

Pre v2.x, used to spawn new threads for handling client requests.

Starting v2.x, added support for threadless execution of client requests using asyncio.

In future, threadless execution will be the default mode.

Till then if you are interested in trying it out, start with --threadless flag.

SyntaxError: invalid syntax is strictly typed and uses Python typing annotations. Example:

>>> my_strings : List[str] = []
>>> #############^^^^^^^^^#####

Hence a Python version that understands typing annotations is required. Make sure you are using Python 3.6+.

Verify the version before running

❯ python --version

All typing annotations can be replaced with comment-only annotations. Example:

>>> my_strings = [] # List[str]
>>> ################^^^^^^^^^^^

It will enable to run on Python pre-3.6, even on 2.7. However, as all future versions of Python will support typing annotations, this has not been considered.

Unable to load plugins

Make sure plugin modules are discoverable by adding them to PYTHONPATH. Example:

PYTHONPATH=/path/to/my/app proxy --plugins my_app.proxyPlugin

...[redacted]... - Loaded plugin proxy.HttpProxyPlugin
...[redacted]... - Loaded plugin my_app.proxyPlugin

OR, simply pass fully-qualified path as parameter, e.g.

proxy --plugins /path/to/my/app/my_app.proxyPlugin

Unable to connect with from remote host

Make sure is listening on correct network interface. Try following flags:

  • For IPv6 --hostname ::
  • For IPv4 --hostname

Basic auth not working with a browser

Most likely it's a browser integration issue with system keychain.

  • First verify that basic auth is working using curlcurl -v -x username:password@localhost:8899
  • See this thread for further details.

Docker image not working on macOS

It's a compatibility issue with vpnkit.

See moby/vpnkit exhausts docker resources and Connection refused: The proxy could not connect for some background.

GCE log viewer integration for

A starter fluentd.conf template is available.

  1. Copy this configuration file as under /etc/google-fluentd/config.d/
  2. Update path field to log file path as used with --log-file flag. By default /tmp/proxy.log path is tailed.
  3. Reload google-fluentd:sudo service google-fluentd restart

Now logs can be browsed using GCE log viewer.

ValueError: filedescriptor out of range in select is made to handle thousands of connections per second without any socket leaks.

  1. Make use of --open-file-limit flag to customize ulimit -n.
  2. Make sure to adjust --backlog flag for higher concurrency.

If nothing helps, open an issue with requests per second sent and output of following debug script:

❯ ./helper/ <proxy-py-pid>

None: None in access logs

Sometimes you may see None:None in access logs. It simply means that an upstream server connection was never established i.e. upstream_host=None, upstream_port=None.

There can be several reasons for no upstream connection, few obvious ones include:

  1. Client established a connection but never completed the request.
  2. A plugin returned a response prematurely, avoiding connection to upstream server.


❯ proxy -h
usage: proxy [-h] [--backlog BACKLOG] [--basic-auth BASIC_AUTH]
             [--ca-key-file CA_KEY_FILE] [--ca-cert-dir CA_CERT_DIR]
             [--ca-cert-file CA_CERT_FILE]
             [--ca-signing-key-file CA_SIGNING_KEY_FILE]
             [--cert-file CERT_FILE]
             [--client-recvbuf-size CLIENT_RECVBUF_SIZE]
             [--devtools-ws-path DEVTOOLS_WS_PATH]
             [--disable-headers DISABLE_HEADERS] [--disable-http-proxy]
             [--enable-dashboard] [--enable-devtools] [--enable-events]
             [--enable-static-server] [--enable-web-server]
             [--hostname HOSTNAME] [--key-file KEY_FILE]
             [--log-level LOG_LEVEL] [--log-file LOG_FILE]
             [--log-format LOG_FORMAT] [--num-workers NUM_WORKERS]
             [--open-file-limit OPEN_FILE_LIMIT] [--pac-file PAC_FILE]
             [--pac-file-url-path PAC_FILE_URL_PATH]
             [--pid-file PID_FILE] [--plugins PLUGINS] [--port PORT]
             [--server-recvbuf-size SERVER_RECVBUF_SIZE]
             [--static-server-dir STATIC_SERVER_DIR] [--threadless]
             [--timeout TIMEOUT] [--version] v2.1.2

optional arguments:
  -h, --help            show this help message and exit
  --backlog BACKLOG     Default: 100. Maximum number of pending connections to
                        proxy server
  --basic-auth BASIC_AUTH
                        Default: No authentication. Specify colon separated
                        user:password to enable basic authentication.
  --ca-key-file CA_KEY_FILE
                        Default: None. CA key to use for signing dynamically
                        generated HTTPS certificates. If used, must also pass
                        --ca-cert-file and --ca-signing-key-file
  --ca-cert-dir CA_CERT_DIR
                        Default: ~/ Directory to store dynamically
                        generated certificates. Also see --ca-key-file, --ca-
                        cert-file and --ca-signing-key-file
  --ca-cert-file CA_CERT_FILE
                        Default: None. Signing certificate to use for signing
                        dynamically generated HTTPS certificates. If used,
                        must also pass --ca-key-file and --ca-signing-key-file
  --ca-file CA_FILE     Default: None. Provide path to custom CA file for peer
                        certificate validation. Specially useful on MacOS.
  --ca-signing-key-file CA_SIGNING_KEY_FILE
                        Default: None. CA signing key to use for dynamic
                        generation of HTTPS certificates. If used, must also
                        pass --ca-key-file and --ca-cert-file
  --cert-file CERT_FILE
                        Default: None. Server certificate to enable end-to-end
                        TLS encryption with clients. If used, must also pass
  --client-recvbuf-size CLIENT_RECVBUF_SIZE
                        Default: 1 MB. Maximum amount of data received from
                        the client in a single recv() operation. Bump this
                        value for faster uploads at the expense of increased
  --devtools-ws-path DEVTOOLS_WS_PATH
                        Default: /devtools. Only applicable if --enable-
                        devtools is used.
  --disable-headers DISABLE_HEADERS
                        Default: None. Comma separated list of headers to
                        remove before dispatching client request to upstream
  --disable-http-proxy  Default: False. Whether to disable
  --enable-dashboard    Default: False. Enables dashboard.
  --enable-devtools     Default: False. Enables integration with Chrome
                        Devtool Frontend. Also see --devtools-ws-path.
  --enable-events       Default: False. Enables core to dispatch lifecycle
                        events. Plugins can be used to subscribe for core
                        Default: False. Enable inbuilt static file server.
                        Optionally, also use --static-server-dir to serve
                        static content from custom directory. By default,
                        static file server serves out of installed
                        python module folder.
  --enable-web-server   Default: False. Whether to enable
  --hostname HOSTNAME   Default: ::1. Server IP address.
  --key-file KEY_FILE   Default: None. Server key file to enable end-to-end
                        TLS encryption with clients. If used, must also pass
  --log-level LOG_LEVEL
                        Valid options: DEBUG, INFO (default), WARNING, ERROR,
                        CRITICAL. Both upper and lowercase values are allowed.
                        You may also simply use the leading character e.g.
                        --log-level d
  --log-file LOG_FILE   Default: sys.stdout. Log file destination.
  --log-format LOG_FORMAT
                        Log format for Python logger.
  --num-workers NUM_WORKERS
                        Defaults to number of CPU cores.
  --open-file-limit OPEN_FILE_LIMIT
                        Default: 1024. Maximum number of files (TCP
                        connections) that can open concurrently.
  --pac-file PAC_FILE   A file (Proxy Auto Configuration) or string to serve
                        when the server receives a direct file request. Using
                        this option enables proxy.HttpWebServerPlugin.
  --pac-file-url-path PAC_FILE_URL_PATH
                        Default: /. Web server path to serve the PAC file.
  --pid-file PID_FILE   Default: None. Save parent process ID to a file.
  --plugins PLUGINS     Comma separated plugins
  --port PORT           Default: 8899. Server port.
  --server-recvbuf-size SERVER_RECVBUF_SIZE
                        Default: 1 MB. Maximum amount of data received from
                        the server in a single recv() operation. Bump this
                        value for faster downloads at the expense of increased
  --static-server-dir STATIC_SERVER_DIR
                        Default: "public" folder in directory where
                        is placed. This option is only applicable when static
                        server is also enabled. See --enable-static-server.
  --threadless          Default: False. When disabled a new thread is spawned
                        to handle each client connection.
  --timeout TIMEOUT     Default: 10. Number of seconds after which an inactive
                        connection must be dropped. Inactivity is defined by
                        no data sent or received by the client.
  --version, -v         Prints version. not working? Report at:



  • No longer a single file module.
  • Added support for threadless execution.
  • Added dashboard app.
  • Added support for unit testing.


  • Python3 only.
    • Deprecated support for Python 2.x.
  • Added support multi core accept.
  • Added plugin support.


  • Single file.
  • Single threaded server.

For detailed changelog refer to release PRs or commit history.

Blog post:

Github page:

May 21, 2020


Hakin9 TEAM
Hakin9 is a monthly magazine dedicated to hacking and cybersecurity. In every edition, we try to focus on different approaches to show various techniques - defensive and offensive. This knowledge will help you understand how most popular attacks are performed and how to protect your data from them. Our tutorials, case studies and online courses will prepare you for the upcoming, potential threats in the cyber security world. We collaborate with many individuals and universities and public institutions, but also with companies such as Xento Systems, CATO Networks, EY, CIPHER Intelligence LAB, redBorder, TSG, and others.
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