Subject: Re: ipv4 regression in 2.6.31 ?

From: Jarek Poplawski <[email-masked]>
Date: Tue, 15 Sep 2009 08:13:55 +0000

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.

Jarek P.