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Date:   Mon, 18 May 2020 10:38:26 +0100
From:   Lorenz Bauer <>
To:     David Ahern <>
Cc:     bpf <>, Networking <>,
        Martynas Pumputis <>,
        kernel-team <>
Subject: Re: "Forwarding" from TC classifier

On Fri, 15 May 2020 at 15:24, David Ahern <> 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

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