On Jan 6, 2009, at 8:49 AM, Peter Steele wrote:
Our efforts so far indicate the answer is no, which baffles us. We
want
to send a limited broadcast to 255.255.255.255 but the message never
arrives. The same code works fine under Linux. Is there a trick for
doing this kind of thing under FreeBSD?
What you're trying to do with sending to the all-ones broadcast
address is known as sending a "link-local" packet. On some systems,
sending a UDP packet to 255.255.255.255 will actually cause a packet
with that destination to be generated from all network interfaces
which are "UP". That seems to be the behavior you are expecting.
On FreeBSD, IIRC, the behavior you get is that it will send to the
local network broadcast address for each interface [1] using the
network broadcast address (ie, if an interface is configured for
10.1.1.1 with /16 netmask, the packet will have destination
10.1.255.255). If an interface is UP but not configured with an IP
+netmask, I don't believe a packet will be sent. (In fact, it might
depend upon whether the BROADCAST flag is enabled, which gets set when
an inet-enabled interface is setup with a netmask...)
Arguably, this is a bug in FreeBSD, but you can work around it by
using the BPF interface to send the traffic directly rather than using
the network stack via socket()+send()/write(). I believe the ISC DHCP
server software provides examples of how to do this, as dhclient is
commonly used to send DHCP requests to the all-ones broadcast addr,
without needing an interface being configured with an IP....
--
-Chuck
[1]: And I could be mis-remembering that part; it might do a routing
table lookup and use only the interface which matches the destination
IP, which is probably the default route.
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