https://forge.codesys.com/forge/talk/Engineering/thread/bba444aef6/2022-09-16T09:47:53.631000ZRecent posts to CodeSys UDP Broadcast mystery2022-09-16T09:47:53.631000Z2022-09-16T09:47:53.631000Zpaulpotathttps://forge.codesys.com/u/paulpotat/https://forge.codesys.com3f002568bd18c33cf098c8d04aad26e0342da2fb

<div class="markdown_content"><p>Okay I finally understood this ! <br/> The 9th byte is the last part of my IP address : for example if my ip is 192.168.1.42 then the 9th byte has to be equal to 42 (0x2A).</p> <p>So in case anybody needs this one day (who knows), to perform the same type of network scan CodeSys use you have to send a UDP broadcast frame containing the following data :</p> <div class="codehilite"><pre><span></span><span class="o">#</span> <span class="n">WIFI</span> <span class="n">c5</span> <span class="mi">74</span> <span class="mi">40</span> <span class="mi">03</span> <span class="mi">00</span> <span class="mi">20</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">03</span> <span class="n">xx</span> <span class="mi">83</span> <span class="mi">01</span> <span class="mi">02</span> <span class="n">c2</span> <span class="mi">00</span> <span class="mi">04</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">00</span> <span class="o">#</span> <span class="n">ETH</span> <span class="n">c5</span> <span class="mi">74</span> <span class="mi">40</span> <span class="mi">03</span> <span class="mi">00</span> <span class="mi">10</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">03</span> <span class="n">xx</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">02</span> <span class="n">c2</span> <span class="mi">00</span> <span class="mi">04</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">00</span> <span class="mi">00</span> </pre></div> <p>Where "xx" is the last byte of your ip address. </p> <p>I don't know how it works on networks where the mask is not 255.255.255.0 though...</p></div>

2022-09-14T07:32:39.995000Z2022-09-14T07:32:39.995000Zpaulpotathttps://forge.codesys.com/u/paulpotat/https://forge.codesys.com1a1f5804c9d6465c0fe9ce8bb741a5209ea16819

<div class="markdown_content"><p>After some investigation, it looks like the 9th byte changes depending on the network IP.<br/> For example on the same WiFI network :<br/> - if network ip is <code>192.168.100.xxx</code> then byte 9 is "05"<br/> - if network ip is <code>192.168.99.xxx</code> then byte 9 is "03"<br/> - if network ip is <code>192.168.43.xxx</code> then byte 9 is "8A"<br/> - if network ip is <code>192.168.1.xxx</code> then byte 9 is "34"</p> <p>I tried some math operations on the ip to get those values as results but I can't figure out how it's calculated...</p></div>

2022-09-12T09:29:05.713000Z2022-09-12T09:29:05.713000Zpaulpotathttps://forge.codesys.com/u/paulpotat/https://forge.codesys.com183a915ec937538ccdf935194b36c19333903515

<div class="markdown_content"><p>Ok so far I understood this :<br/> - <strong>Byte<span><span>[0:4]</span></span></strong> = Some sort of header (it never changes)<br/> - <strong>Byte 5</strong> = Change depending on the interface type (10 for ethernet and 20 for WiFi)<br/> - <strong>Byte<span><span>[6:7]</span></span></strong> = ? (can be set to 0 without issues)<br/> - <strong>Byte 8</strong> = Always equals to 03<br/> - <strong>Byte 9</strong> = ??? (can't be set to 0 otherwise devices won't answer the broadcast message)<br/> - <strong>Byte<span><span>[10:11]</span></span></strong> = Changes depending on the interface type (0000 for ethernet and 8301 for WiFi)<br/> - <strong>Byte<span><span>[12:15]</span></span></strong> = Always equals to 02c20004<br/> - <strong>Byte<span><span>[16:19]</span></span></strong> = Count of UDP broadcast messages sent (can be set to 0 without issues)</p> <p>So now my frame for WiFI interfaces looks like this :</p> <div class="codehilite"><pre><span></span><span class="err">c5 74 40 03 00 20 00 00 03 ?? 83 01 02 c2 00 04 00 00 00 00</span> </pre></div> <p>The only thing I really don't understand now is the 9th byte : what is it ? How is it computed ? <br/> It seems like it changes every time I restart my computer, but stays the same during the whole sessions afterwards...</p> <p>I hope someone can help me with this<br/> BR</p></div>

2022-09-09T14:58:39.322000Z2022-09-09T14:58:39.322000Zpaulpotathttps://forge.codesys.com/u/paulpotat/https://forge.codesys.com7a3a192f1cdf9342b786d4ed02e3878e359c7443

<div class="markdown_content"><p>Hello,<br/> I'm trying to understand how the "Scan Network" function of CodeSys operates.<br/> I looked at the packets using wireshark and it looks like it sends an UDP frame on the broadcast address of the network and wait for answers:<br/> <img alt="" rel="nofollow" src="https://i.imgur.com/5xfEkoQ.png"/></p> <p>This broadcasted packet contains 20 bytes of data and looks like this :</p> <div class="codehilite"><pre><span></span><span class="err">c5 74 40 03 00 20 b7 8b 03 8a 83 01 02 c2 00 04 01 dc 7d 06</span> </pre></div> <p>I tried to send this frame using a python script and I was able to get some answers from devices on the network ! However if the network switch (for example going from wifi to ethernet), it doesn't work anymore. </p> <p>It looks like these 20 bytes of data are changing based on the network / computer ?</p> <p>I was wondering if this was documented somewhere ? How is this frame built ?</p> <p>Thank you for your time,<br/> BR</p></div>