docs: add chapter "Capturing and analysing tunneldigger packets"

Add tcpdump pcap filter examples and describe
how to use the wireshark dissector.

docs/capture_analyse_packets.rst

@@ -0,0 +1,52 @@
+Capturing and analysing tunneldigger packets
+############################################
+
+Example pcap filter using tcpdump
+*********************************
+
+See :ref:`PDU types`
+
+capture control traffic
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: sh
+
+   # capture all tunneldigger control traffic
+   tcpdump -i eth0 -w /tmp/output.pcap 'udp port 8942 and udp[8] == 0x80'
+
+capture only USAGE packets
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: sh
+
+   # capture only USAGE packets
+   tcpdump -i eth0 -w /tmp/output.pcap 'udp port 8942 and udp[8] == 0x80 and udp[12] == 0x0a'
+
+capture control packets except KEEPALIVE, PMTUD, PMTUD_ACK, PMTUD_NTFY
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code:: sh
+
+   # capture control packets except KEEPALIVE, PMTUD, PMTUD_ACK, PMTUD_NTFY
+   tcpdump -i eth0 -w /tmp/output.pcap 'udp port 8942 and udp[8] == 0x80 and (udp[12] != 5 && udp[12] != 6 && udp[12] != 7 && udp[12] != 9)'
+
+
+Using wireshark
+***************
+
+There is a `custom dissector <https://github.com/wlanslovenija/tunneldigger/blob/master/docs/wireshark-tunneldigger.lua>`_ for tunneldigger written in lua. The dissector is registered as **TD**.
+
+To use the wireshark dissector call wireshark with:
+
+.. code:: sh
+
+   cd tunneldigger/docs/
+   wireshark -Xlua_script:wireshark-tunneldigger.lua
+
+Wireshark might decode the user data as a different protocol (e.g. Cisco HDLC). This can be changed by:
+
+* Click on "wrong" protocol in "Packet Details" pane (usually the pane in the middle).
+* Right mouse click, select **Decode As** (Ctrl-Shift-U).
+* A new window with decodes should open
+* A new row should be already created. The field should be called **L2TPv3 payload type**.
+* Select **Ethernet** in the **Current** column.

docs/index.rst

@@ -25,6 +25,7 @@ Contents
    server
    client
    protocol
+   capture_analyse_packets
 
 Source Code and Issue Tracker
 -----------------------------

docs/protocol.rst

@@ -38,6 +38,8 @@ All fields are encoding with network byte order.
 * Type of the PDU
 * Length of the value
 
+.. _PDU types:
+
 PDU types
 ^^^^^^^^^