Nmap - "WARNING: Unable to find appropriate interface"

Dica rápida:

O nmap  me aprontou um pequeno problema no FreeBSD.

"WARNING: Unable to find appropriate interface"

# nmap -sP 192.168.1.20       

Starting Nmap 5.61TEST2 ( http://nmap.org ) at 2012-10-30 05:59 BRST
WARNING: Unable to find appropriate interface for system route to 192.168.1.0/24 gw 192.168.1.1
Note: Host seems down. If it is really up, but blocking our ping probes, try -Pn
Nmap done: 1 IP address (0 hosts up) scanned in 3.02 seconds


Isso ocorre pois ele 'se perde' nas rotas, então devo indicar um caminho para a saida, abaixo usando a opção "-e" ele executa corretamente o que eu quero - apenas o MAC Address.

# nmap -sP 192.168.1.20 -e alc0
Host is up (0.00018s latency).
MAC Address: C8:DC:DC:C0:ED:03 (Elitegroup Computer System CO.)
Nmap done: 1 IP address (1 host up) scanned in 0.26 seconds


# man nmap
-e interface (Use specified interface) .
           Tells Nmap what interface to send and receive packets on. Nmap
           should be able to detect this automatically, but it will tell you
           if it cannot.

e um abraço.