hi,  
 
it seems nmap is miscompiled / nonfunctional in stable. it happens when your 
route / default route is not via eth0, for example ath0:  
 
strace gives:  
 
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3 
ioctl(3, SIOCGIFCONF, {64, {{"lo", {AF_INET, inet_addr("127.0.0.1")}}, 
{"ath0", {AF_INET, inet_addr("192.168.1.22")}}}}) = 0 
ioctl(3, SIOCGIFNETMASK, {ifr_name="lo", ifr_netmask={AF_INET, 
inet_addr("255.0.0.0")}}) = 0 
ioctl(3, SIOCGIFFLAGS, {ifr_name="lo", ifr_flags=IFF_UP|IFF_LOOPBACK|
IFF_RUNNING}) = 0 
ioctl(3, SIOCGIFNETMASK, {ifr_name="ath0", ifr_netmask={AF_INET, 
inet_addr("255.255.255.0")}}) = 0 
ioctl(3, SIOCGIFFLAGS, {ifr_name="ath0", ifr_flags=IFF_UP|IFF_BROADCAST|
IFF_NOTRAILERS|IFF_RUNNING|IFF_MULTICAST}) = 0 
ioctl(3, SIOCGIFHWADDR, {ifr_name="ath0", ifr_hwaddr=00:05:4e:4d:05:6e}) = 0 
ioctl(3, SIOCGIFNETMASK, {ifr_name="", ???}) = -1 ENODEV (No such device) 
write(2, "Failed to determine the netmask "..., 36Failed to determine the 
netmask of !) = 36 
write(2, " : No such device\n", 18 : No such device 
)     = 18 
munmap(0x40017000, 4096)                = 0 
exit_group(1)                           = ? 
 
 
as you can see, ifr_name is suddenly the empty string