[Guy Harris <gharris@xxxxxxxxx>]

Scott Lowrey wrote:
> Depending on the OS you are using, the kernel is probably smart enough 
> to route these packets directly between the two applications without 
> handing them down to the Ethernet device driver (and out onto the wire).
>
> However, you might trying capturing on the loopback device.

...if the OS you're using has one, and it supports packet capture.

On Linux, on various BSDs, and on Digital/Tru64 UNIX, it's possible to 
capture on the loopback device.

It's not possible to capture on the loopback device on Solaris and 
HP-UX, however, and there may be other OSes that don't support it.

Windows doesn't have a loopback device on which to capture.  (There's 
the "Microsoft Loopback Device", but

	1) it's an add-on - it's not what's used by default for traffic between 
two ports on the same machine;

	2) it apparently doesn't support packet capture.)

See

	http://www.ethereal.com/media.html

for information on what link-layer types are known to work, are known 
not to work, or haven't been reported on for various OSes.