Re: sysctls below net.ipv[46].conf.all not working as expected

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello David,

On Thu, Dec 30, 2010 at 09:43:19PM +0100, Uwe Kleine-König wrote:
> On Thu, Dec 30, 2010 at 12:30:23PM -0800, David Miller wrote:
> > From: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> > Date: Wed, 29 Dec 2010 17:06:07 +0100
> > 
> > > I did the following:
> > > 
> > > 	cassiopeia:~# sysctl net.ipv6.conf.all.use_tempaddr
> > > 	net.ipv6.conf.all.use_tempaddr = 0
> > > 
> > > 	cassiopeia:~# sysctl net.ipv6.conf.eth0.use_tempaddr
> > > 	net.ipv6.conf.eth0.use_tempaddr = 0
> > > 
> > > 	cassiopeia:~# sysctl -w net.ipv6.conf.all.use_tempaddr=1
> > > 	net.ipv6.conf.all.use_tempaddr = 1
> > > 
> > > 	cassiopeia:~# sysctl net.ipv6.conf.all.use_tempaddr
> > > 	net.ipv6.conf.all.use_tempaddr = 1
> > > 
> > > 	cassiopeia:~# sysctl net.ipv6.conf.eth0.use_tempaddr
> > > 	net.ipv6.conf.eth0.use_tempaddr = 0
> > > 
> > > Here I would have expected that eth0's use_tempaddr is 1, too.  The
> > > problem is not that this entry isn't writeable:
> > 
> > The "all" value is propagated at the first moment that the ipv6 device
> > private is created, usually that is when the device is first brought up
> > which means it can happen as early as the exact moment the device is
> > registered.
> > 
> > Therefore, if you want "all" to apply to "eth0", you must make sure
> > the sysctl is set properly before the device is registered.
> I thought this is what "default" was used for?!
I took the time now to dig into the source for ipv4/{all,default}.

Unless I'm mistaken I found:

 - inetdev_init copies dev_net(dev)->ipv4.devconf_dflt to &in_dev->cnf
   That's called from inetdev_event if there is no in_dev.

 - devinet_copy_dflt_conf copies to all devices that have !test_bit(i,
   in_dev->cnf.state)
   devinet_copy_dflt_conf is called when /proc/sys/net/ipv4/conf/default
   is written to.  I didn't found where the in_dev->cnf.state bits are
   set.

 - net->ipv4.devconf_all is never propagated to the devices.

 - The data behind /proc/sys/net/ipv4/conf/all is used in registering
   net->ipv4.forw_hdr and seems only have to do with ip_forward

So I think that it's really "default" that propagates to a new device as
I expected and the documentation suggests.

So IMHO a better name for "all" would be "global" ...

> If you are really right, the documentation is (IMHO) misleading.  e.g.
> Documentation/networking/ip-sysctl.txt tells:
> 
> 	conf/default/*:
> 		Change the interface-specific default settings.
> 
> 	conf/all/*:
> 		Change all the interface-specific settings.
... and this is misleading.  I wonder if it would be considered OK, to
create a new config directory called "global" that acts like "all" now
and let "all" propagate all changes to all devices, "default" and
"global".  If not the documentation needs clearification.

I guess for ipv6 it's similar, though I didn't check that.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |