update docs: how mitmproxy works

1 files changed, 79 insertions, 78 deletions

diff --git a/docs/howmitmproxy.rst b/docs/howmitmproxy.rst
index 93602afe..133863e3 100644
--- a/docs/howmitmproxy.rst
+++ b/docs/howmitmproxy.rst
@@ -6,17 +6,17 @@ 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 SSL-protected traffic [#ssl]_ in the presence of `Server Name Indication`_.
+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:
+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
 
@@ -43,11 +43,11 @@ client connects to the proxy and makes a request that looks like this:
 
     CONNECT example.com:443 HTTP/1.1
 
-A conventional proxy can neither view nor manipulate an SSL-encrypted data
+A conventional proxy can neither view nor manipulate an 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 SSL connection happens over this pipe, and the subsequent
+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
@@ -58,17 +58,17 @@ 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 SSL 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 an SSL 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>`.
+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 an 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?
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -89,13 +89,12 @@ 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 SSL handshake
-with the server, and inspect the certificates it used. Now, we use the Common
-Name in the upstream SSL 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.
+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
@@ -103,31 +102,31 @@ 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 SSL 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 Common Name. 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.
+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 SSL is that each certificate requires its
+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 SSL and TLS protocols. This lets the client specify the remote server name
-at the start of the SSL handshake, which then lets the server select the right
-certificate to complete the process.
+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 SSL handshake to continue until just _after_ the SNI value has been
+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.
@@ -142,32 +141,31 @@ Lets put all of this together into the complete explicitly proxied HTTPS flow.
 
 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 SSL connection.
+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 an SSL connection using the SNI hostname
+4. Mitmproxy connects to the server, and establishes an TLS connection using the SNI hostname
    indicated by the client.
-5. The server responds with the matching SSL certificate, which contains the CN and SAN values
+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 SSL handshake paused in step 3.
-7. The client sends the request over the established SSL connection.
-8. Mitmproxy passes the request on to the server over the SSL connection initiated in step 4.
+   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 HTTP/S 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.
+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:
+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
 
@@ -175,32 +173,35 @@ which might look something like this:
 
 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.
+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.
+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. By default, we treat all
-traffic destined for ports 443 and 8443 as SSL.
+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
@@ -214,21 +215,21 @@ explicit HTTPS connections to establish the CN and SANs, and cope with SNI.
 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 SSL connection.
+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 an SSL connection using the SNI hostname
+4. Mitmproxy connects to the server, and establishes an TLS connection using the SNI hostname
    indicated by the client.
-5. The server responds with the matching SSL certificate, which contains the CN and SAN values
+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 SSL handshake paused in
+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 SSL connection.
-8. Mitmproxy passes the request on to the server over the SSL connection initiated in step 4.
+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
 
-.. [#ssl] I use "SSL" to refer to both SSL and TLS in the generic sense, unless otherwise
-          specified.
+.. [#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