Why can't I sendto() to 127.255.255.255

[Vince]

Abraham K. Mathen wrote:
> NOTE : A copy of this message was sent to freebsd-hackers also.
>           Apologies for the separate mails.
> 
> Hello freebsd-net,
>  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?
> ----------------------------------------------------------
> 
As far as I am aware the only RFC's that define the 127.0.0.0/8 block
are rfc3330 which states,

127.0.0.0/8 - This block is assigned for use as the Internet host
   loopback address.  A datagram sent by a higher level protocol to an
   address anywhere within this block should loop back inside the host.
   This is ordinarily implemented using only 127.0.0.1/32 for loopback,
   but no addresses within this block should ever appear on any network
   anywhere

and rfc1122 which says:
{ 127, <any> }

                 Internal host loopback address.  Addresses of this form
                 MUST NOT appear outside a host.

I would interpret this as 127.0.0.0/8 are all loopback host addresses,
but I'd be happy to be corrected.


Vince

>  Could you please help me understand this?
> 
> 
> sincerely
> 
> Mathen
> (Abraham K. Mathen)
> 
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