Question for Corinna (was Re: Novice Question)

Wed Jul 1 18:46:00 GMT 2009

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 */
	    if ((ifr->ifa_flags & IFF_BROADCAST) &&
		(ifr->ifa_flags & IFF_UP))
		broad_addr = *ifr->ifa_broadaddr;
	    XdmcpRegisterBroadcastAddress((struct sockaddr_in *)

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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