netdev - Re: [Patch net] ipv4: fib: check forwarding before checking send

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Date:	Tue, 8 Apr 2014 14:17:08 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Julian Anastasov <ja@....bg>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [Patch net] ipv4: fib: check forwarding before checking send_redirects

On Tue, Apr 8, 2014 at 1:43 PM, David Miller <davem@...emloft.net> wrote:
> From: Cong Wang <xiyou.wangcong@...il.com>
> Date: Tue,  8 Apr 2014 12:31:22 -0700
>
>> From: Cong Wang <cwang@...pensource.com>
>>
>> We have seen in a weird case we had to disable send_redirects in order
>> to pass rp filter check even though we don't set forwarding at all.
>> This looks wrong, at least according to ip-sysctl.txt send_redirects should
>> only make sense when we enable forwarding.
>>
>> Cc: Eric Biederman <ebiederm@...ssion.com>
>> Cc: Julian Anastasov <ja@....bg>
>> Cc: David S. Miller <davem@...emloft.net>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> Signed-off-by: Cong Wang <cwang@...pensource.com>
>
> I'm not so sure about this.
>
> This test here is just an optimization, which bypasses the long path
> processing of FIB source address validation if certain strict
> conditions are met.
>
> __fib_validate_source() should do the right thing if it is executed,
> it is just the slow path, and you should determine why it is rejecting
> your traffic instead.
>
> Your change is a valid optimization perhaps, but not a bug fix.

In our case, we have the following setting:

1) rp_filter = 0 (set manually)
2) forwarding = 0 (default)
3) send_redirects = 1 (default)

We are not supposed even to execute __fib_validate_source() in such case,
are we? :)
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