netdev - Re: "Forwarding" from TC classifier

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Date:   Thu, 14 May 2020 16:41:33 +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 Wed, 13 May 2020 at 22:23, David Ahern <dsahern@...il.com> wrote:
>
> On 5/13/20 10:40 AM, Lorenz Bauer wrote:
> > Really, I'd like to get rid of step 1, and instead rely on the network
> > stack to switch or route
> > the packet for me. The bpf_fib_lookup helper is very close to what I need. I've
> > hacked around a bit, and come up with the following replacement for step 1:
> >
> >     switch (bpf_fib_lookup(skb, &fib, sizeof(fib), 0)) {
> >     case BPF_FIB_LKUP_RET_SUCCESS:
> >         /* There is a cached neighbour, bpf_redirect without going
> > through the stack. */
> >         return bpf_redirect(...);
>
> BTW, as shown in samples/bpf/xdp_fwd_kern.c, you have a bit more work to
> do for proper L3 forwarding:
>
>         if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
>                 ...
>                 if (h_proto == htons(ETH_P_IP))
>                         ip_decrease_ttl(iph);
>                 else if (h_proto == htons(ETH_P_IPV6))
>                         ip6h->hop_limit--;
>
>                 memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
>                 memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
>                 return bpf_redirect_map(&xdp_tx_ports,
> fib_params.ifindex, 0);
>
> The ttl / hoplimit decrements assumed you checked it earlier to be > 1

Thanks for the pointer :)


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

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