netdev - Re: [PATCH net-next] net: bridge: do not send arp replies if src and target hw addr is the same

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Message-ID: <73f2854c-f532-462f-addc-f275fbd3e5d9@blackwall.org>
Date: Tue, 9 Jan 2024 14:18:13 +0200
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Paolo Abeni <pabeni@...hat.com>, Felix Fietkau <nbd@....name>,
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: bridge: do not send arp replies if src and
 target hw addr is the same

On 09/01/2024 14:14, Paolo Abeni wrote:
> On Tue, 2024-01-09 at 12:58 +0100, Felix Fietkau wrote:
>> On 09.01.24 12:36, Paolo Abeni wrote:
>>> On Thu, 2024-01-04 at 15:25 +0100, Felix Fietkau wrote:
>>>> There are broken devices in the wild that handle duplicate IP address
>>>> detection by sending out ARP requests for the IP that they received from a
>>>> DHCP server and refuse the address if they get a reply.
>>>> When proxyarp is enabled, they would go into a loop of requesting an address
>>>> and then NAKing it again.
>>>
>>> Can you instead provide the same functionality with some nft/tc
>>> ingress/ebpf filter?
>>>
>>> I feel uneasy to hard code this kind of policy, even if it looks
>>> sensible. I suspect it could break some other currently working weird
>>> device behavior.
>>>
>>> Otherwise it could be nice provide some arpfilter flag to
>>> enable/disable this kind filtering.
>>
>> I don't see how it could break anything, 
> 
> FTR, I don't either. But I've been surprised too much times from
> extremely weird expectations from random devices, broken by "obviously
> correct" behaviors change.
> 
>> because it wouldn't suppress 
>> non-proxied responses. nft/arpfilter is just too expensive, and I don't 
>> think it makes sense to force the use of tc filters to suppress 
>> nonsensical responses generated by the bridge layer.
> 
> Then what about adding a flag to enable/disable this new behavior?
> 

If you decide to go down this path consider using bridge's boolopts. Personally
I still prefer to avoid adding such policies in the bridge, but instead to implement
them via other means, it's not a hard "no" so an option with default to current
behaviour would be acceptable.

> Cheers,
> 
> Paolo
>