testing ethernet device with external loopback

Hi,

for hardware testing purposes, I would like to perform an external loopback on my ethernet interface. For this, I wrote 2 little executables, one sending a raw ethernet packet (using 'sendto') on a socket opened with the parameters PF_PACKET, SOCK_RAW, htons(ETH_P_ALL), the other one doing a recvfrom from a raw socket. The problem is that on the reception side, I always receive what I send, even when
the external RJ45 stub is not plugged !
Is there a way to tell to the kernel on the reception side that I want to get only packets coming from the external world ?

Thanks,

Sebastien.

Presumably you are trying to sendto() a local IP address. When you do this, packets are looped back before they get to the ethernet driver. Pull out all eth cables and do a ping to one of your local IP addresses and you'll see ping responses. :)

Try using pktgen to generate packets from the kernel to the IP address of your interface. The packets are generated inside the kernel, below the IP stack, so they don't get looped back in software. With pktgen, you set MAC addresses, IP addresses, protocol etc using a /proc interface. There is documentation in the kernel source tree, or google pktgen.

James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development

Sebastien,

check out the snull network driver from the "Linux Device Drivers" book. it has a short explanation on how to "trick" the kernel into sending the packet even though the destination is on the local machine:

http://www.linuxdriver.co.il/ldd3/chp-17-sect-1.shtml

dave aka prpplague