Re: ipv4 regression in 2.6.31 ?

Jarek Poplawski
2009-09-15T08:14:08Z

On 14-09-2009 18:31, Stephen Hemminger wrote:
> On Mon, 14 Sep 2009 17:55:05 +0200
> Stephan von Krawczynski <skraw@ithnet.com> wrote:
>
>> On Mon, 14 Sep 2009 15:57:03 +0200
>> Eric Dumazet <eric.dumazet@gmail.com> 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.