Summary: When pinging the ip address 255.255.255.255, the MAC address should be set to the broadcast address.



Description:

I narrowed this down to sys/netinet/ip_output.c where it looks like the decision about whether to set M_BCAST is happening on line 505 for this case:

isbroadcast = in_ifaddr_broadcast(gw->sin_addr, ia);

It looks like in_ifaddr_broadcast is only checking whether the address is the same as the interface broadcast (e.g. 192.168.122.255), but not 255.255.255.255.

If I ping 192.168.122.255, then I get the expected MAC broadcast address.


Actual results:
--------------

tcpdump -vvvv -xx -ni em0 icmp &
ping -nc 1 255.255.255.255

root@bsd-vbox:/home/jcaplan # tcpdump -vvvv -xx -ni em0 icmp &
[1] 5429
root@bsd-vbox:/home/jcaplan # tcpdump: listening on em0, link-type EN10MB (Ethernet), capture size 262144 bytes

root@bsd-vbox:/home/jcaplan # ping -nc 1 255.255.255.255
PING 255.255.255.255 (255.255.255.255): 56 data bytes
02:01:55.895257 IP (tos 0x0, ttl 64, id 46687, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.1.32 > 255.255.255.255: ICMP echo request, id 13845, seq 0, length 64
	0x0000:  3c37 862c 1f63 0800 27b8 57c0 0800 4500

Note the first six bytes are not 0xff.


Expected Results:
----------------


root@bsd-vbox:/home/jcaplan # tcpdump -vvvv -xx -ni em0 icmp &
[1] 5429
root@bsd-vbox:/home/jcaplan # tcpdump: listening on em0, link-type EN10MB (Ethernet), capture size 262144 bytes

root@bsd-vbox:/home/jcaplan # ping -nc 1 255.255.255.255
PING 255.255.255.255 (255.255.255.255): 56 data bytes
02:01:55.895257 IP (tos 0x0, ttl 64, id 46687, offset 0, flags [none], proto ICMP (1), length 84)
    192.168.1.32 > 255.255.255.255: ICMP echo request, id 13845, seq 0, length 64
	0x0000:  ffff ffff ffff 0800 27b8 57c0 0800 4500


Note the first 6 bytes are 0xff.