Subject: Re: ipv4 regression in 2.6.31 ?

From: Jarek Poplawski <[email-masked]>
Date: Wed, 16 Sep 2009 05:23:04 +0000

On Tue, Sep 15, 2009 at 03:57:19PM -0700, Stephen Hemminger wrote:
> On Tue, 15 Sep 2009 08:13:55 +0000
> Jarek Poplawski <[email-masked]> wrote:
> 
> > On 14-09-2009 18:31, Stephen Hemminger wrote:
> > > On Mon, 14 Sep 2009 17:55:05 +0200
> > > Stephan von Krawczynski <[email-masked]> wrote:
> > > 
> > >> On Mon, 14 Sep 2009 15:57:03 +0200
> > >> Eric Dumazet <[email-masked]> wrote:
> > >>
> > >>> Stephan von Krawczynski a A~(c)crit :
> > >>>> Hello all,
> > ...
> > >>> rp_filter - INTEGER
> > >>>         0 - No source validation.
> > >>>         1 - Strict mode as defined in RFC3704 Strict Reverse Path
> > >>>             Each incoming packet is tested against the FIB and if the interface
> > >>>             is not the best reverse path the packet check will fail.
> > >>>             By default failed packets are discarded.
> > >>>         2 - Loose mode as defined in RFC3704 Loose Reverse Path
> > >>>             Each incoming packet's source address is also tested against the FIB
> > >>>             and if the source address is not reachable via any interface
> > >>>             the packet check will fail.
> > ...
> > > RP filter did not work correctly in 2.6.30. The code added to to the loose
> > > mode caused a bug; the rp_filter value was being computed as:
> > >   rp_filter = interface_value & all_value;
> > > So in order to get reverse path filter both would have to be set.
> > > 
> > > In 2.6.31 this was change to:
> > >    rp_filter = max(interface_value, all_value);
> > > 
> > > This was the intended behaviour, if user asks all interfaces to have rp
> > > filtering turned on, then set /proc/sys/net/ipv4/conf/all/rp_filter = 1
> > > or to turn on just one interface, set it for just that interface.
> > 
> > Alas this max() formula handles also cases where both values are set
> > and it doesn't look very natural/"user friendly" to me. Especially
> > with something like this: all_value = 2; interface_value = 1
> > Why would anybody care to bother with interface_value in such a case?
> > 
> > "All" suggests "default" in this context, so I'd rather expect
> > something like:
> >     rp_filter = interface_value ? : all_value;
> > which gives "the inteded behaviour" too, plus more...
> > 
> > We'd only need to add e.g.:
> >  0 - Default ("all") validation. (No source validation if "all" is 0).
> >  3 - No source validation on this interface.
> 
> More values == more confusion.
> I chose the maxconf() method to make rp_filter consistent with other
> multi valued variables (arp_announce and arp_ignore).

This additional value is not necessary (it'd give as superpowers).
Max seems logical to me only when values are sorted (especially if
max is the strictest).

Jarek P.

> 
> --------
> Subject: [PATCH] Document rp_filter behaviour
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> 
> --- a/Documentation/networking/ip-sysctl.txt	2009-09-15 15:54:25.844934373 -0700
> +++ b/Documentation/networking/ip-sysctl.txt	2009-09-15 15:55:40.709205883 -0700
> @@ -744,6 +744,8 @@ rp_filter - INTEGER
>  	Default value is 0. Note that some distributions enable it
>  	in startup scripts.
>  
> +	The max value from conf/{all,interface}/rp_filter is used.
> +
>  arp_filter - BOOLEAN
>  	1 - Allows you to have multiple network interfaces on the same
>  	subnet, and have the ARPs for each interface be answered
> 
> 
> 
> 
> 
> 
> 
> --