Why can't I sendto() to 127.255.255.255

[Abraham K. Mathen]

Hello freebsd-hackers,
I wrote a short program (on FreeBSD 6.0), that attempts
to call sendto() on a UDP socket, with 127.255.255.255 as
the destination address. It failed - with errno 49 (EADDRNOTAVAIL).
Setting SO_BROADCAST and IP_ONESBCAST did not help.

After examining
- various RFC's,
- source code under /usr/src/sys/netinet/ and
- archives of freebsd-hackers & freebsd-net,
I have not been able to determine the reason.

Is it possible to successfully sendto() on a UDP socket
with 127.255.255.255 as the destination address? If yes,
how can that be done.

Specific questions :
----------------------------------------------------------
- Is it correct to understand that 127.255.255.255 is the
directed network broadcast address for net 127? If yes,
which RFC specifies that?

- If not, is 127.255.255.255 considered to be a host
address? If yes, which RFC specifies that?
----------------------------------------------------------

Could you please help me understand this?


sincerely

Mathen
(Abraham K. Mathen)

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[David S. Madole]

From: Abraham K. Mathen on Monday, April 30, 2007 5:54 AM
I wrote a short program (on FreeBSD 6.0), that attempts
to call sendto() on a UDP socket, with 127.255.255.255 as
the destination address. It failed - with errno 49 (EADDRNOTAVAIL).
Setting SO_BROADCAST and IP_ONESBCAST did not help.

Perhaps because the matched route for that address points to lo0 on your system and lo0 is not a broadcast-capable interface?

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000

Note that there is no "BROADCAST" flag. Just a guess.

David