How to reliably generate Ethernet frame errors in software?
Question:
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.
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.
Is there a way to do this in software?
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.
[EDIT]: I want to create cable faults - not detect them.
[EDIT]: 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
[EDIT]: See also a similar question in python.
Solution:
Well, after spending over a day fighting with C, this is the python solution.
First disable automatic checksumming of the ethernet card:
sudo ethtool -K eth1 tx off
Then, send your dodgy frame from python:
#!/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)
Who said it had to be complicated...
PS: I Love Python.
Answer
If you're working in C, you can send a raw Ethernet frame using socket(AF_PACKET, SOCK_RAW, ...), and passing it a pointer to a raw buffer that you've generated yourself (e.g. by following the frame layout at wikipedia ).
It's been a long time since I've done this, so I'm not going to attempt to write a representative code snippet...