mitmproxy/doc-src/modes.html

Mitmproxy comes with several modes of operation, which allow you to use mitmproxy in a variety of scenarios.
This documents briefly explains each mode and possible setups.
<hr>
Mitmproxy has four modes of operation:
<ul>
    <li>Regular Mode (this is what you get by default)</li>
    <li>Transparent Mode</li>
    <li>Reverse Proxy Mode</li>
    <li>Upstream Proxy Mode</li>
</ul>

<p>Now, which one should you pick? Use this flow chart:
</p>

<img src="@!urlTo('schematics/proxy-modes-flowchart.png')!@"><br><br>

<div class="page-header">
    <h1>Regular Proxy</h1>
</div>

Mitmproxy's regular mode it the most simple one and the easiest to set up.

<ol>
    <li>Start mitmproxy.</li>
    <li>Configure your client to use mitmproxy. This means that you either adjust the proxy setting of your local browser
    or point an external device to your proxy (which should look like
        <a href="@!urlTo('screenshots/ios-manual.png')!@">this</a>).</li>
    <li>Quick Check: You can already visit an unencrypted HTTP site over the proxy.</li>
    <li>Open the magic domain <strong>mitm.it</strong> and install the certificate for your device.</li>
</ol>

<div class="well">
    <strong>Heads Up:</strong> Unfortunately, some applications prefer to bypass the HTTP proxy settings of the system -
    Android applications are a common example. In these cases, you need to use mitmproxy's transparent mode.
</div>

<p>If you are proxying an external device, your network will probably look like this:</p>
<img src="@!urlTo('schematics/proxy-modes-regular.png')!@">
<br><br>
<p>The square brackets signify the source and destination IP addresses. Your client explicitly connects
    to mitmproxy and mitmproxy explicitly connects to the target server.
</p>

<div class="page-header">
    <h1>Transparent Proxy</h1>
</div>

When a transparent proxy is used, traffic is redirected into a proxy at the network layer, without any client
configuration being required. This makes transparent proxying ideal for those situations where you can't change client
behaviour. The basic principle is that mitmproxy sits somewhere on the line from the client to the internet and
transparently intercepts the request. In the graphic below, a machine running mitmproxy has been inserted between
the router and the internet:

<a href="@!urlTo('schematics/proxy-modes-transparent-1.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-transparent-1.png')!@"></a>
<p>The square brackets signify the source and destination IP addresses. Round brackets mark the next
    hop on the <strong>Ethernet</strong>/data link layer. This distinction is important to make: When the packet arrives
    at the mitmproxy machine, it must still be addressed to the target server. In other words: A simple IP redirect on
    the router does not work - this would remove the target information, leaving mitmproxy unable to
    determine the real destination.
</p>
<a href="@!urlTo('schematics/proxy-modes-transparent-wrong.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-transparent-wrong.png')!@"></a>

<h2>Common Configurations</h2>

The first graphic is a little bit idealistic: Usually, you'll have your local wireless lan network and no
machines between your router and the internet. Fortunately, there are other ways to configure your network:
(a) Configuring the client to use a custom gateway/router/"next hop", (b) Implementing custom routing on the router
or (c) setting up a separate wireless network router which gets proxied.
There are of course other options, but we'll look at these three. In most cases, setting (a) is recommended due to its
ease of use.

<h3>(a) Custom Gateway</h3>

<p>Looking at your local home network, it's clear what happens if you enter "example.com" into your address bar: After you
press enter, your OS sends a packet to your router, which then sends this to your ISP, which then sends it to some
Tier-1 carrier, which then sends it... I think you get the idea. The important part for us is the first step here:
Your machine is configured to use your router as the next hop. Your router certainly doesn't host example.com, but your
machine knows that your router will forward it upstream. On the technical level, your router probably provides a DHCP
server, which instructs all clients to use his address as the <em>Default Gateway</em> for connections that leave the
current subnet (your local network).</p>
<p>
How does this help us? Here comes our trick: By configuring the client to use our machine as its Gateway, all traffic
will be sent to our machine, which then forwards it to the router. This provides us with the scenario we'd like to have,
namely packets on our doorstep that are addressed for someone else:
</p>
<a href="@!urlTo('schematics/proxy-modes-transparent-2.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-transparent-2.png')!@"></a>

Given this concept, we can set up mitmproxy:
<ol>
    <li>Configure your proxy machine for transparent mode.<br>You can find instructions
    in the <em>Transparent Proxying</em> section of the mitmproxy docs.</li>
    <li>Configure your client to use your proxy machine's IP as the default gateway. This setting is usually called
        <em>Standard Gateway, Router</em> or something along these lines
        (<a href="@!urlTo('screenshots/ios-gateway.png')!@">iOS screenshot</a>).</li>
    <li>Quick Check: You can already visit an unencrypted HTTP site over the proxy.</li>
    <li>Open the magic domain <strong>mitm.it</strong> and install the certificate for your device.</li>
</ol>

<div class="well">
    <strong style="text-align: center; display: block">Troubleshooting Transparent Mode</strong>
    <p>Wrong transparent mode configurations are a frequent source of
    error. If it doesn't work for you, try the following things:</p>
    <ul>
        <li>Open mitmproxy's event log (press `e`) - can you spot clientconnect messages?
        If not, the packets are not arriving at the proxy. A common source is the occurence of ICMP redirects,
            which means that your machine is telling the client that there's a faster way to the internet by contacting
            your router directly (see the <em>Transparent Proxying</em> section on how to disable them). If in doubt,
            <a href="https://wireshark.org/">Wireshark</a> may help you to see whether something arrives at your machine
            or not.
        </li>
        <li>
            Have you explicitly configured an HTTP proxy on your device? You do not need mitmproxy's transparent mode
            then, just start mitmproxy normally. Explicitly setting a proxy and transparent mode contradict each other,
            settle for one. Do not explicitly redirect traffic to mitmproxy anywhere except for the Gateway setting.
        </li>
        <li>
            Re-check the instructions in the <em>Transparent Proxying</em> section. Anything you missed?
        </li>
    </ul>
    If you encounter any other pitfalls that should be listed here, please let us know!
</div>

<h3>(b) Custom Routing</h3>

Custom routing is a fairly advanced setup which we'll only document briefly here.
First and foremost, it usually requires root on your router. The basic idea is to teach your router a custom routing
table that says "for requests from ip X, the proxy machine is the next gateway".

<a href="@!urlTo('schematics/proxy-modes-transparent-3.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-transparent-3.png')!@"></a>

For this setup, we expect you to have a basic understanding of networking in general. In short, you should get started
with <a href="@!urlTo('custom-routing.txt')!@">these routing commands</a>. The Troubleshooting part directly above this
section might be helpful for you as well.

<h3>(c) Separate Network</h3>

Setting up a separate network using a cheap router might be a viable option, too. Such a configuration mostly resembles
the idealistic graphic from the beginning (Variant 1). Take a look at the
<a href="@!urlTo('tutorials/transparent-dhcp.html')!@">Transparently proxify virtual machines</a> tutorial to see how
such a network could be implemented. The troubleshooting section for custom gateways may be helpful for you, too.


<div class="page-header">
    <h1>Reverse Proxy</h1>
</div>

Mitmproxy is usually used with a client that uses the proxy to access the Internet. Using reverse proxy mode, you can
use mitmproxy to represent a server:

<a href="@!urlTo('schematics/proxy-modes-reverse.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-reverse.png')!@"></a>

There are various use-cases:
<ul>
<li>
    Say you have an internal API running at http://example.local/. You could now setup mitmproxy in
    reverse proxy mode at http://debug.example.local/ and dynamically point clients to this new API endpoint,
    which provides clients with the same data and you with debug information. Similarly, you could move your real server
    to a different ip/port and setup mitmproxy at the original place to debug all sessions.
</li>
<li>
    Say you're a web developer working on example.com (with a development version running on localhost:8000).
    You can modify your hosts file so that example.com points to 127.0.0.1 and then run mitmproxy in reverse proxy
    mode on port 80. You can test your app on the example.com domain and get all requests recorded in mitmproxy.
</li>
<li>
    Say you have some toy project that should get SSL support. Simply setup mitmproxy with SSL termination and you're
    done (<code>mitmdump -p 443 -R https2http://localhost:80/</code>). There are better tools for this specific task (we don't
    have C performance obviously), but it's definitely a nice and very quick way to setup an SSL-speaking server.
</li>
<li>
    Want to add a non-SSL-capable compression proxy in front of your server? You could even spawn a mitmproxy instance
    that terminates SSL (https2http://...), point it to the compression proxy and let the compression proxy point
    to a SSL-initiating mitmproxy (http2https://...), which then points to the real server. As you see, it's a fairly
    flexible thing.
</li>
</ul>

<p>
Please note that cloning Google by using <code>mitmproxy -R http://google.com/</code> does <em>not</em> really work
(as in <a href="@!urlTo('screenshots/ios-reverse.png')!@">this screenshot</a>).
This may work for the first request, but the HTML remains unchanged: As soon as the user clicks on an non-relative URL
(or downloads a non-relative image resource), they speak with Google directly again.
</p>
<p>
    On another note, mitmproxy either supports an HTTP or an HTTPS upstream server, not both at the same time. You can
    simply work around this by spawning a second mitmproxy instance. Each instance listens to one port and talks to one
    port.
</p>

<div class="page-header">
    <h1>Upstream Proxy</h1>
</div>

<p>
If you want to add mitmproxy in front of a different proxy appliance, you can use mitmproxy's upstream mode.
In upstream mode, all requests are unconditionally transferred to an upstream proxy or your choice.
</p>

<a href="@!urlTo('schematics/proxy-modes-upstream.png')!@">
    <img src="@!urlTo('schematics/proxy-modes-upstream.png')!@"></a>

<p>
mitmproxy supports both explicit HTTP and explicit HTTPS in upstream proxy mode. You could in theory chain multiple
mitmproxy instances in a row, but that doesn't make any sense in practice (i.e. outside of our tests).
</p>