doc: More work on the tutorial.

doc/forwarding.dox

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+/**
+@page forwarding Chapter 7: Forwarding connections
+@section forwarding_connections Forwarding connections
+
+Port forwarding comes in SSH protocol in two different flavours:
+direct or reverse port forwarding. Direct port forwarding is also
+named local port forwardind, and reverse port forwarding is also called
+remote port forwarding.
+
+
+
+@subsection forwarding_direct Direct port forwarding
+
+Direct port forwarding is from client to server. The client opens a tunnel,
+and forwards whatever data to the server. Then, the server connects to an
+end point. The end point can reside on another machine or on the SSH
+server itself.
+
+Example of use of direct port forwarding:
+@verbatim
+Mail client application   Google Mail
+         |                    ^
+     5555 (arbitrary)         |
+         |                143 (IMAP2)
+         V                    |
+    SSH client   =====>   SSH server 
+
+Legend:
+--P-->: port connexion through port P
+=====>: SSH tunnel
+@endverbatim
+A mail client connects to port 5555 of a client. An encrypted tunnel is
+established to the server. The server connects to port 143 of Google Mail (the
+end point). Now the local mail client can retreive mail.
+
+
+@subsection forwarding_reverse Reverse port forwarding
+
+The reverse forwarding is slightly different. It goes from server to client,
+even though the client has the initiative of establishing the tunnel.
+Once the tunnel is established, the server will listen on a port. Whenever
+a connection to this port is made, the server forwards the data to the client.
+
+Example of use of reverse port forwarding:
+@verbatim
+ Local mail server    Mail client application
+         ^                     |
+         |               5555 (arbitrary)
+     143 (IMAP2)               |
+         |                     V
+    SSH client   <=====   SSH server
+
+Legend:
+--P-->: port connexion through port P
+=====>: SSH tunnel
+@endverbatim
+In this example, the SSH client establishes the tunnel,
+but it is used to forward the connections established at
+the server to the client.
+
+*** To be written ***
+
+*/