Re: Question for Corinna (was Re: Novice Question)

[Jon TURNEY <jon dot turney at dronecode dot org dot uk>]

On 01/07/2009 16:42, Corinna Vinschen wrote:
> On Jun 29 10:50, Christopher Faylor wrote:
> > On Mon, Jun 29, 2009 at 12:09:12PM +0100, Jon TURNEY wrote:
> > > Andy wrote:
> > > > /opt/wip/cygport-svn/xorg-server/xorg-server-1.6.0-10/src/xorg-server-1.6.0/
> > > > os/access.c
> > > > (gdb) p *ifr
> > > > $1 = {ifa_next = 0x1447798, ifa_name = 0x14474c4
> > > > "{B8B51884-C69A-4592-B65D-89ABB3DCF18D}", ifa_flags = 69635,
> > > >    ifa_addr = 0x14474f0, ifa_netmask = 0x14475f0, ifa_dstaddr = 0x0, ifa_data
> > > > = 0x0}
> > > :-)
> > >
> > > It seems that the (new for Cygwin 1.7) getifaddr() function can return
> > > interfaces with IFF_BROADCAST&  IFF_UP set, but no broadcast address, which
> > > the X server assumes never happens
> > I was going to say that this sounds wrong but I see interfaces on my
> > linux box which have no broadcast addresses but still have IFF_BROADCAST
> > set.
> >
> > Corinna, what do you think?
>
> Thanks for nudging me by PM.  I didn't notice this question, sorry.
>
> As for the broadcast address, you *have* to expect that an interface
> is broadcast capable and the ifa_broadaddr pointer is NULL.  This
> is at least true for all IPv6 entries.  For IPv4 that shouldn't occur
> under Cygwin.

The code in question (before I patched it) looks like this:

#if defined(IPv6) && defined(AF_INET6)
	    if (family == FamilyInternet6)
		/* IPv6 doesn't support broadcasting, so we drop out here */
		continue;
#endif
	    if ((ifr->ifa_flags & IFF_BROADCAST) &&
		(ifr->ifa_flags & IFF_UP))
		broad_addr = *ifr->ifa_broadaddr;
	    else
		continue;
	    XdmcpRegisterBroadcastAddress((struct sockaddr_in *)
					  &broad_addr);

Staring at the code a bit, I think this means we have either an IPv4 
interface, or an IPv6 interface with a mapped IPv4 address when the broadcast 
address is looked at.


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