chg: [doc] a sample usage of the D4 core client

README.md

@@ -5,7 +5,7 @@ to an existing sensor network using simple clients.
 
 ## D4 core client
 
-[D4 core client](https://github.com/D4-project/d4-core/tree/master/client) is a simple and minimal implementation of the [D4 encapsulation protocol](https://github.com/D4-project/architecture/tree/master/format).
+[D4 core client](https://github.com/D4-project/d4-core/tree/master/client) is a simple and minimal implementation of the [D4 encapsulation protocol](https://github.com/D4-project/architecture/tree/master/format). There is also a [portable D4 client](https://github.com/D4-project/d4-goclient) in Go including the support for the SSL/TLS connectivity.
 
 ### Requirements
 
@@ -13,6 +13,40 @@ to an existing sensor network using simple clients.
 - make
 - a recent C compiler
 
+### Usage
+
+The D4 client can be used to stream any byte stream towards a D4 server.
+
+As an example, you directly stream tcpdump output to a D4 server with the following
+script:
+
+````
+tcpdump -n -s0 -w - | ./d4 -c ./conf | socat - OPENSSL-CONNECT:$D4-SERVER-IP-ADDRESS:$PORT,verify=0
+````
+
+~~~~
+d4 - d4 client
+Read data from the configured <source> and send it to <destination>
+
+Usage: d4 -c  config_directory
+
+Configuration
+
+The configuration settings are stored in files in the configuration directory
+specified with the -c command line switch.
+
+Files in the configuration directory
+
+key         - is the private HMAC-SHA-256-128 key.
+              The HMAC is computed on the header with a HMAC value set to 0
+              which is updated later.
+snaplen     - the length of bytes that is read from the <source>
+version     - the version of the d4 client
+type        - the type of data that is send. pcap, netflow, ...
+source      - the source where the data is read from
+destination - the destination where the data is written to
+~~~~
+
 ### Installation
 
 ~~~~