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Date:   Mon, 18 May 2020 10:38:26 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     David Ahern <dsahern@...il.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 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.

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.

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com

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