From 1945a75292daac6ae01b90d0e3d50d8be92d13a7 Mon Sep 17 00:00:00 2001
From: Harald Welte <laforge@gnumonks.org>
Date: Mon, 26 Mar 2018 19:33:09 +0200
Subject: add README.md

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+udtrace - Unix Domain socket tracing
+====================================
+
+This is a LD_PRELOAD wrapper library which can be used to trace the data
+sent and/or received via unix domain sockets.
+
+Unlike IP based communication that can be captured/traced with pcap
+programs like tcpdump or wireshark, there is no similar mechanism
+available for unix domain sockets.
+
+This LD_PRELOAD library intercepts the C library function calls of
+dynamically linked programs.  It will detect all file descriptors
+representing unix domain sockets and will then print traces of all
+data sent/received via the socket.
+
+Usage
+-----
+
+Simply build **libudtrace.so** using the **make** command, and then
+start your to-be-traced program with
+
+  LD_PRELOAD=libudtrace.os
+
+e.g.
+
+  LD_PRELOAD=libudtrace.so systemctl status
+
+which will produce output like this:
+
+  >>> UDTRACE: Unix Domain Socket Trace initialized (TITAN support DISABLED)
+  >>> UDTRACE: Adding FD 4
+  >>> UDTRACE: connect(4, "/run/dbus/system_bus_socket")
+  4 sendmsg W 00415554482045585445524e414c20
+  4 sendmsg W 3331333033303330
+  4 sendmsg W 0d0a4e45474f54494154455f554e49585f46440d0a424547494e0d0a
+  [...]
+
+Output Format
+-------------
+
+Currently, **udtrace** will produc the following output:
+
+
+At time a FD for a unix domain socket is created:
+
+  >>> UDTRACE: Adding FD 8
+
+At time a FD for a unix domain socket is closed:
+
+  >>> UDTRACE: Removing FD 8
+
+At time a FD for a unix domain socket is bound or connected:
+
+  >>> UDTRACE: connect(9, "/tmp/mncc")
+
+When data is read from the socket:
+
+  9 read R 00040000050000004403000008000000680000001c0300002c03000000000000
+
+When data is written to the socket:
+
+  9 write W 00040000050000004403000008000000680000001c0300002c03000000000000
+
+Where
+ * *9* is the file dsecriptor on which the event happened
+ * *read/write* is the name of the syscall, could e.g. also be sendmsg / readv / etc.
+ * *R|W* is Read / Write (from the process point of view)
+ * followed by a hex-dump of the raw data.  Only data successfully
+   written (or read) will be printed, not the entire buffer passed to
+   the syscall.  The rationale is to only print data  that was actually
+   sent to or received from the socket.
+
+TITAN decoder support
+---------------------
+
+Getting hex-dumps is nice and fine, but normally one wants to have a
+more detailed decode of the data that is being passed on the socket.
+
+For TCP based protocols, there is wireshark.  But most protocols on unix
+domain sockets don't follow inter-operable / public standards, so even
+if one was to pass the traces into wireshark somehow, there would be no
+decoder.
+
+In the [Osmocom project](https://osmocom.org/), we already had some type
+definitions and decoders for our protocols written in the TTCN-3
+programming language, using [Eclipse TITAN](https://projects.eclipse.org/projects/tools.titan).
+In order to build those decoders fro MNCC and PCUIF, please use
+
+  make ENABLE_TITAN=1
+
+when building the code.
+
+Please note that this introduces a run-time dependency to
+libttcn3-dynamic.so, which is (at least on Debian GNU/Linux) not
+installed in a default library search path, so you will have to use
+something like:
+
+  LD_LIBRARY_PATH=/usr/lib/titan LD_PRELOAD=libudtrace.so systemctl status
+
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