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.

Solution

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...