Created attachment 599990 [details]
[Patch] find suitable default interface

Description of problem:

This issue is not interface specific, but depends on what libpcap sees as monitorable devices.

If no device is specified when arpwatch is executed, pcap_lookupdev() is called. That returns the first device in the list of libpcap monitorable devices.

For example, the list on my Fedora 17 box looks like this:
 virbr0
 bluetooth0
 nflog
 em1
 virbr1
 usbmon1
 usbmon2

In my case, "virbr0" is supplied to arpwatch as the first interface from the list. But on some machines, e.g. "usbmon1" can be first and then arpwatch fails to start.


Version-Release number of selected component (if applicable):
arpwatch-2.1a15-21

How reproducible:
Always (on the right machine)


Steps to Reproduce:
1. # arpwatch -d
2. # tail /var/log/messages
3. observe arpwatch error (see additional info)
  
Actual results:
arpwatch uses only the first device and fails, if it is not ethernet or fddi, but never tries additional devices in the list


Expected results:
arpwatch searches for suitable default device when no -i is specified


Additional info:
# cat /var/log/message:
...
Jun 21 10:28:16 myhost arpwatch: Link layer type 220 not ethernet or fddi
...