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POHow to reliably generate Ethernet frame errors in software?
 

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  1. POHow to reliably generate Ethernet frame errors in software?
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    <h1>Question:</h1> <p>I'm testing a section of cable-fault finding software, and I'd like to reliably and reproducibly generate cable faults on a cat5 cable. </p> <p>At the moment I'm using a meter length of untwisted cable, and wriggle the cable manually next to a power supply, but I cannot detect any faults in the application (I'm reading the Ethernet fault counters off the Ethernet ASIC.) Whether this is because no faults are generated, or because the software/hardware detection is faulty, I cannot tell.</p> <p>Is there a way to do this in software? </p> <p>I'd settle for writing something in a higher level language, like Java or python, and as a last resort would be willing to put it together in C, but I'd really rather not re-write an Ethernet driver purely to fix a possible bug.</p> <p><strong>[EDIT]:</strong> I want to <strong>create</strong> cable faults - not detect them.</p> <p><strong>[EDIT]:</strong> I've transferred large files through FTP and SCP without problems with the doctored cable, and I see no errors coming up while inspecting the traffic with wireshark</p> <p><strong>[EDIT]:</strong> See also a <a href="https://stackoverflow.com/questions/723635/how-do-you-send-an-ethernet-frame-with-a-corrupt-fcs">similar question in python</a>.</p> <h1>Solution:</h1> <p>Well, after spending over a day fighting with C, this is the python solution.</p> <p>First disable automatic checksumming of the ethernet card:</p> <pre><code>sudo ethtool -K eth1 tx off </code></pre> <p>Then, send your dodgy frame from python:</p> <pre><code>#!/usr/bin/env python from socket import * # # Ethernet Frame: # [ # [ Destination address, 6 bytes ] # [ Source address, 6 bytes ] # [ Ethertype, 2 bytes ] # [ Payload, 40 to 1500 bytes ] # [ 32 bit CRC chcksum, 4 bytes ] # ] # s = socket(AF_PACKET, SOCK_RAW) s.bind(("eth1", 0)) src_addr = "\x01\x02\x03\x04\x05\x06" dst_addr = "\x01\x02\x03\x04\x05\x06" payload = ("["*30)+"PAYLOAD"+("]"*30) checksum = "\x00\x00\x00\x00" ethertype = "\x08\x01" s.send(dst_addr+src_addr+ethertype+payload+checksum) </code></pre> <p>Who said it had to be complicated...</p> <p>PS: <em>I Love Python.</em></p>
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      1. POHow to reliably generate Ethernet frame errors in software?
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    1. COAre you trying to detect how your software responds to incorrect packets? Or is your software not recognizing such malformed packets and the reason why they are malformed?
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      1. USVineet Reynolds
    2. COPresumably the "fault detector" is performing checksum-based checks. If you're prepared to generate your own Ethernet frames, then you could inject erroneous checksums?
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      1. POHow to reliably generate Ethernet frame errors in software?
      1. USOliver Charlesworth
    3. CO@Oli - Yes, you're right. It's just checksum based check. Do you of any software that would allow me to put together my own Ethernet frames?
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