> I am using an NT 4 workstation with a dialup modem.
WinPcap doesn't support capturing on WAN/PPP links on NT:
http://www.ethereal.com/faq.html#q3.8
"Q 3.8: I'm running Ethereal on Windows; why doesn't my my (Token Ring,
PPP) network interface show up in the list of interfaces in the
"Interface" item in the "Capture Preferences" dialog box popped up by
the "Capture->Start" menu item?
A: 2.02 and earlier versions of the WinPcap driver and library that
Ethereal uses for packet capture didn't support Token Ring interfaces;
the current version, 2.2, does support Token Ring, and the current
version of Ethereal works with (and, in fact, requires) WinPcap 2.1 or
later.
If you are having problems capturing on Token Ring interfaces, and you
have WinPcap 2.02 or an earlier version of WinPcap installed, you should
uninstall WinPcap, download and install the current version of WinPcap,
and then install the latest version of Ethereal.
WinPcap doesn't support PPP WAN interfaces on Windows NT/2000, so
Ethereal cannot capture packets on those devices when running on Windows
NT/2000. Regular dial-up lines, ISDN lines, and various other lines
such as T1/E1 lines are all PPP interfaces. This may cause the
interface not to show up on the list of interfaces in the "Capture
Preferences" dialog.
For problems seen when installing the WinPcap driver or library, or seen
when capturing, check the WinPcap FAQ, or the Wiretapped.net mirror of
that FAQ, to see if your problem is mentioned there."