RFC 919 compliance (broadcasts to 255.255.255.255)

[Benjamin Lutz]

On Saturday 05 August 2006 07:18, you wrote:
> On Fri, 4 Aug 2006, Benjamin Lutz wrote:
> > Hello,
> >
> > I've noticed that FreeBSD does not by default comply to RFC 919, Chapter
> > 7 (http://tools.ietf.org/html/rfc919). Specifically, it does not handle
> > IP packets with a destination address of 255.255.255.255 properly.
> >
> > 255.255.255.255 is a "limited broadcast address" (the term is not
> > mentioned in the RFC, but seems to be in use everywhere else). An IP
> > packet send to that address should be broadcast to the whole IP subnet of
> > the broadcasting device. However, in FreeBSD (4.11, 5.5, 6.1) this does
> > not work, as is evident by this tcpdump output:
> >
> > 17:00:59.125132 00:12:17:5a:b3:b6 > 00:40:63:d9:a9:28, ethertype IPv4
> > (0x0800), length 98: 10.0.0.1 > 255.255.255.255: ICMP echo request, id
> > 33319, seq 0, length 64
> >
> > The destination MAC address is that of my gateway, but it should be
> > ff:ff:ff:ff:ff:ff. You can reproduce this by running "tcpdump -en ip
> > proto 1" and "ping 255.255.255.255".
> >
> > I found a discussion from 2003 about this, but it seems to have trailed
> > off without coming to a conclusion:
> > http://lists.freebsd.org/pipermail/freebsd-net/2003-July/000921.html
> >
> > I've noticed that the ip(7) manpage lists a SO_ONESBCAST option. The
> > intention seems to be to enable packets to 255.255.255.255. However a
> > test with a small program showed that this option seems to have no
> > effect, outgoing packets still carried the gateway's MAC address as
> > destination.
> >
> > Btw, Linux and NetBSD both handle packets to 255.255.255.255 as
> > broadcasts.
> >
> > Now, is this behaviour intentional? Is there a way to turn on RFC 919
> > compliance? If not, would someone be willing to add this to the kernel?
> > Should I submit a PR?
>
> Does this feature really necessary? For waht purpose?

I discovered it when playing with Wake-on-LAN packets. It is possible to send 
those with the real subnet's broadcast address, but then you first have to 
figure that one out. If you have a network device that doesn't have an IP 
configured yet, it would not know the subnet broadcast address.

> I believe this feature is more harmful than useful.

Please elaborate. Remember that routers do not pass on packets to 
255.255.255.255. The TTL is effectively 1.

Cheers
Benjamin
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-hackers/attachments/20060805/ed5b0704/attachment.pgp