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-How mitmproxy works
-===================
-
-Mitmproxy is an enormously flexible tool. Knowing exactly how the proxying
-process works will help you deploy it creatively, and take into account its
-fundamental assumptions and how to work around them. This document explains
-mitmproxy's proxy mechanism in detail, starting with the simplest unencrypted
-explicit proxying, and working up to the most complicated interaction -
-transparent proxying of TLS-protected traffic [#tls]_ in the presence of `Server
-Name Indication`_.
-
-Explicit HTTP
--------------
-
-Configuring the client to use mitmproxy as an explicit proxy is the simplest and
-most reliable way to intercept traffic. The proxy protocol is codified in the
-`HTTP RFC`_, so the behaviour of both the client and the server is well defined,
-and usually reliable. In the simplest possible interaction with mitmproxy, a
-client connects directly to the proxy, and makes a request that looks like this:
-
-.. code-block:: none
-
-    GET http://example.com/index.html HTTP/1.1
-
-This is a proxy GET request - an extended form of the vanilla HTTP GET request
-that includes a schema and host specification, and it includes all the
-information mitmproxy needs to proceed.
-
-.. image:: schematics/how-mitmproxy-works-explicit.png
-    :align: center
-
-1. The client connects to the proxy and makes a request.
-2. Mitmproxy connects to the upstream server and simply forwards the request on.
-
-
-Explicit HTTPS
---------------
-
-The process for an explicitly proxied HTTPS connection is quite different. The
-client connects to the proxy and makes a request that looks like this:
-
-.. code-block:: none
-
-    CONNECT example.com:443 HTTP/1.1
-
-A conventional proxy can neither view nor manipulate a TLS-encrypted data
-stream, so a CONNECT request simply asks the proxy to open a pipe between the
-client and server. The proxy here is just a facilitator - it blindly forwards
-data in both directions without knowing anything about the contents. The
-negotiation of the TLS connection happens over this pipe, and the subsequent
-flow of requests and responses are completely opaque to the proxy.
-
-The MITM in mitmproxy
-^^^^^^^^^^^^^^^^^^^^^
-
-This is where mitmproxy's fundamental trick comes into play. The MITM in its
-name stands for Man-In-The-Middle - a reference to the process we use to
-intercept and interfere with these theoretically opaque data streams. The basic
-idea is to pretend to be the server to the client, and pretend to be the client
-to the server, while we sit in the middle decoding traffic from both sides. The
-tricky part is that the `Certificate Authority`_ system is designed to prevent
-exactly this attack, by allowing a trusted third-party to cryptographically sign
-a server's certificates to verify that they are legit. If this signature doesn't
-match or is from a non-trusted party, a secure client will simply drop the
-connection and refuse to proceed. Despite the many shortcomings of the CA system
-as it exists today, this is usually fatal to attempts to MITM a TLS connection
-for analysis. Our answer to this conundrum is to become a trusted Certificate
-Authority ourselves. Mitmproxy includes a full CA implementation that generates
-interception certificates on the fly. To get the client to trust these
-certificates, we :ref:`register mitmproxy as a trusted CA with the device
-manually <certinstall>`.
-
-Complication 1: What's the remote hostname?
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-To proceed with this plan, we need to know the domain name to use in the
-interception certificate - the client will verify that the certificate is for
-the domain it's connecting to, and abort if this is not the case. At first
-blush, it seems that the CONNECT request above gives us all we need - in this
-example, both of these values are "example.com".  But what if the client had
-initiated the connection as follows:
-
-.. code-block:: none
-
-    CONNECT 10.1.1.1:443 HTTP/1.1
-
-Using the IP address is perfectly legitimate because it gives us enough
-information to initiate the pipe, even though it doesn't reveal the remote
-hostname.
-
-Mitmproxy has a cunning mechanism that smooths this over - :ref:`upstream
-certificate sniffing <upstreamcerts>`. As soon as we see the CONNECT request, we
-pause the client part of the conversation, and initiate a simultaneous
-connection to the server. We complete the TLS handshake with the server, and
-inspect the certificates it used. Now, we use the Common Name in the upstream
-certificates to generate the dummy certificate for the client. Voila, we have
-the correct hostname to present to the client, even if it was never specified.
-
-
-Complication 2: Subject Alternative Name
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-Enter the next complication. Sometimes, the certificate Common Name is not, in
-fact, the hostname that the client is connecting to. This is because of the
-optional `Subject Alternative Name`_ field in the certificate that allows an
-arbitrary number of alternative domains to be specified. If the expected domain
-matches any of these, the client will proceed, even though the domain doesn't
-match the certificate CN. The answer here is simple: when we extract the CN from
-the upstream cert, we also extract the SANs, and add them to the generated dummy
-certificate.
-
-
-Complication 3: Server Name Indication
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-One of the big limitations of vanilla TLS is that each certificate requires its
-own IP address. This means that you couldn't do virtual hosting where multiple
-domains with independent certificates share the same IP address. In a world with
-a rapidly shrinking IPv4 address pool this is a problem, and we have a solution
-in the form of the `Server Name Indication`_ extension to the TLS protocols.
-This lets the client specify the remote server name at the start of the TLS
-handshake, which then lets the server select the right certificate to complete
-the process.
-
-SNI breaks our upstream certificate sniffing process, because when we connect
-without using SNI, we get served a default certificate that may have nothing to
-do with the certificate expected by the client. The solution is another tricky
-complication to the client connection process. After the client connects, we
-allow the TLS handshake to continue until just **after** the SNI value has been
-passed to us. Now we can pause the conversation, and initiate an upstream
-connection using the correct SNI value, which then serves us the correct
-upstream certificate, from which we can extract the expected CN and SANs.
-
-Putting it all together
-^^^^^^^^^^^^^^^^^^^^^^^
-
-Lets put all of this together into the complete explicitly proxied HTTPS flow.
-
-.. image:: schematics/how-mitmproxy-works-explicit-https.png
-    :align: center
-
-1. The client makes a connection to mitmproxy, and issues an HTTP CONNECT request.
-2. Mitmproxy responds with a ``200 Connection Established``, as if it has set up the CONNECT pipe.
-3. The client believes it's talking to the remote server, and initiates the TLS connection.
-   It uses SNI to indicate the hostname it is connecting to.
-4. Mitmproxy connects to the server, and establishes a TLS connection using the SNI hostname
-   indicated by the client.
-5. The server responds with the matching certificate, which contains the CN and SAN values
-   needed to generate the interception certificate.
-6. Mitmproxy generates the interception cert, and continues the
-   client TLS handshake paused in step 3.
-7. The client sends the request over the established TLS connection.
-8. Mitmproxy passes the request on to the server over the TLS connection initiated in step 4.
-
-Transparent HTTP
-----------------
-
-When a transparent proxy is used, the connection 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 - proxy-oblivious Android applications being a common example.
-
-To achieve this, we need to introduce two extra components. The first is a
-redirection mechanism that transparently reroutes a TCP connection destined for
-a server on the Internet to a listening proxy server. This usually takes the
-form of a firewall on the same host as the proxy server - `iptables`_ on Linux
-or pf_ on OSX. Once the client has initiated the connection, it makes a vanilla
-HTTP request, which might look something like this:
-
-.. code-block:: none
-
-    GET /index.html HTTP/1.1
-
-Note that this request differs from the explicit proxy variation, in that it
-omits the scheme and hostname. How, then, do we know which upstream host to
-forward the request to? The routing mechanism that has performed the redirection
-keeps track of the original destination for us.  Each routing mechanism has a
-different way of exposing this data, so this introduces the second component
-required for working transparent proxying: a host module that knows how to
-retrieve the original destination address from the router. In mitmproxy, this
-takes the form of a built-in set of modules_ that know how to talk to each
-platform's redirection mechanism. Once we have this information, the process is
-fairly straight-forward.
-
-.. image:: schematics/how-mitmproxy-works-transparent.png
-    :align: center
-
-1. The client makes a connection to the server.
-2. The router redirects the connection to mitmproxy, which is typically
-   listening on a local port of the same host. Mitmproxy then consults the
-   routing mechanism to establish what the original destination was.
-3. Now, we simply read the client's request...
-4. ... and forward it upstream.
-
-Transparent HTTPS
------------------
-
-The first step is to determine whether we should treat an incoming connection as
-HTTPS. The mechanism for doing this is simple - we use the routing mechanism to
-find out what the original destination port is. All incoming connections pass
-through different layers which can determin the actual protocol to use.
-Automatic TLS detection works for SSLv3, TLS 1.0, TLS 1.1, and TLS 1.2 by
-looking for a *ClientHello* message at the beginning of each connection. This
-works independently of the used TCP port.
-
-From here, the process is a merger of the methods we've described for
-transparently proxying HTTP, and explicitly proxying HTTPS. We use the routing
-mechanism to establish the upstream server address, and then proceed as for
-explicit HTTPS connections to establish the CN and SANs, and cope with SNI.
-
-.. image:: schematics/how-mitmproxy-works-transparent-https.png
-    :align: center
-
-1. The client makes a connection to the server.
-2. The router redirects the connection to mitmproxy, which is typically listening on a local port
-   of the same host. Mitmproxy then consults the routing mechanism to establish what the original
-   destination was.
-3. The client believes it's talking to the remote server, and initiates the TLS connection.
-   It uses SNI to indicate the hostname it is connecting to.
-4. Mitmproxy connects to the server, and establishes a TLS connection using the SNI hostname
-   indicated by the client.
-5. The server responds with the matching certificate, which contains the CN and SAN values
-   needed to generate the interception certificate.
-6. Mitmproxy generates the interception cert, and continues the client TLS handshake paused in
-   step 3.
-7. The client sends the request over the established TLS connection.
-8. Mitmproxy passes the request on to the server over the TLS connection initiated in step 4.
-
-.. rubric:: Footnotes
-
-.. [#tls] The use of "TLS" refers to both SSL (outdated and insecure) and TLS
-          (1.0 and up) in the generic sense, unless otherwise specified.
-
-.. _Server Name Indication: https://en.wikipedia.org/wiki/Server_Name_Indication
-.. _HTTP RFC: https://tools.ietf.org/html/rfc7230
-.. _Certificate Authority: https://en.wikipedia.org/wiki/Certificate_authority
-.. _Subject Alternative Name: https://en.wikipedia.org/wiki/SubjectAltName
-.. _iptables: http://www.netfilter.org/
-.. _pf: https://en.wikipedia.org/wiki/PF_\(firewall\)
-.. _modules: https://github.com/mitmproxy/mitmproxy/tree/master/mitmproxy/platform