Herbert Xu wrote:
> David Miller <davem@davemloft.net> wrote:
>> The user is pretty much screwed in one way or the other.
>> For example:
>>
>> 1) If 'default' propagates to all devices, any specific
>>   setting for a device is lost.
>>
>> 2) If 'default' does not propagate, there is no way to
>>   have 'default' influence devices which have already
>>   been loaded.
> 
> Well the way it works on IPv4 currently (for most options) is
> that we'll propagate default settings to a device until either:
> 
> 1) the user modifies the setting for that device;
> 2) or that an IPv4 address has been added to the device.

BTW, this is not 100% true. Look, in rtm_to_ifaddr()
I see the following code flow:

        ipv4_devconf_setall(in_dev);

        ifa = inet_alloc_ifa();
        if (ifa == NULL) {
                /*
                 * A potential indev allocation can be left alive, it stays
                 * assigned to its device and is destroy with it.
                 */
                err = -ENOBUFS;
                goto errout;
        }

if we fail to allocate the ifa (hard to happen, but), we will
make this device not to accept the default propagation.

If this is a relevant note, I can prepare the patch.
 
> 2) was done to preserve backwards compatibility as the controls
> were previously only available after address addition and we did
> not propagate default settings in that case..
> 
> We could easily extend this so that the default propagation
> worked until the user modified the setting, with an ioctl to
> revert to the current behaviour for compatibility.
> 
> Cheers,