netdev - Re: "Forwarding" from TC classifier

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Date:   Mon, 18 May 2020 08:32:35 -0600
From:   David Ahern <dsahern@...il.com>
To:     Lorenz Bauer <lmb@...udflare.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Martynas Pumputis <m@...bda.lt>,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: "Forwarding" from TC classifier

On 5/18/20 3:38 AM, Lorenz Bauer wrote:
> On Fri, 15 May 2020 at 15:24, David Ahern <dsahern@...il.com> wrote:
>>
>> On 5/15/20 3:59 AM, Lorenz Bauer wrote:
>>>
>>> Yes, but that doesn't play well with changing the source address to
>>> the local machine's, since the upper part of the stack will drop the
>>> packet due to accept_local=0.
>>
>> Can you defer the source address swap to the Tx path? Let the packet go
>> up the stack and do the fib lookup again as an skb. neighbor entry does
>> not exist, so the packet is stashed, neighbor resolution done, once
>> resolved the packet goes out. tc program on the egress device can flip
>> the source address, and then subsequent packets take the XDP fast path.
> 
> Hm, that's an interesting idea! I guess this means I have to mark the packet
> somehow, to make sure I can identify it on the TX path. Plus, in theory
> the packet could exit via any interface, so I'd have to attach classifiers to
> a bunch of places if I want to be on the safe side.

Shared blocks might save you some overhead. Create a filter block that
is shared across devices.

> 
> Upside: this seems doable in current kernels. Downside: seems more fragile
> than I'd like.
> 
> Thanks for the thought, I'll play around with it :)
> 
>>
>> If the next host is on the same LAN I believe the stack will want to
>> generate an ICMP redirect, but that can be squashed.
>