If sniffit recognises the name, everything is okay. The ethernet headlength is the length in bytes of an ethernet packet hearder. So if you have to force a non-ethernet device, that is not recognised by sniffit, make sure you change that headlength correctly in the 'config.h' file. The -F option was added, because I noticed devicenames can differ from system to system, and because some ppl have multiple devices present. When having problems with this option, please think twice before you mail me.
Some people pretend that tcp wrappers and stuff can detect sniffers, well that's bullshit.