netdev - Re: [PATCH nf-next v3 3/3] netfilter: Introduce egress hook

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Date:   Sat, 5 Sep 2020 13:18:37 +0200
From:   Laura García Liébana <nevola@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Lukas Wunner <lukas@...ner.de>,
        John Fastabend <john.fastabend@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Netfilter Development Mailing list 
        <netfilter-devel@...r.kernel.org>, coreteam@...filter.org,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Thomas Graf <tgraf@...g.ch>, David Miller <davem@...emloft.net>
Subject: Re: [PATCH nf-next v3 3/3] netfilter: Introduce egress hook

Hi Daniel,

On Fri, Sep 4, 2020 at 11:14 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
[...]
>
> Its trivial to achieve with tc/BPF on the existing egress hook today. Probably
> takes less time than to write up this mail ...
>
> root@x:~/x# cat foo.c
>
> #include <linux/bpf.h>
> #include <linux/if_ether.h>
> #include <arpa/inet.h>
>
> #ifndef __section
> # define __section(NAME)                \
>     __attribute__((section(NAME), used))
> #endif
>
> #define ETH_P_KUNBUSGW  0x419C
>
> #define PASS    0
> #define DROP    2
>
> int foo(struct __sk_buff *skb)
> {
>         void *data_end = (void *)(long)skb->data_end;
>         void *data = (void *)(long)skb->data;
>         struct ethhdr *eth = data;
>
>         if (data + sizeof(*eth) > data_end)
>                 return DROP;
>
>         return eth->h_proto == htons(ETH_P_KUNBUSGW) ? PASS : DROP;
> }
>
> char __license[] __section("license") = "";
>
> root@x:~/x# clang -target bpf -Wall -O2 -c foo.c -o foo.o
> root@x:~/x# ip link add dev foo type dummy
> root@x:~/x# ip link set up dev foo
> root@x:~/x# tc qdisc add dev foo clsact
> root@x:~/x# tc filter add dev foo egress bpf da obj foo.o sec .text
>
> There we go, attached to the device on existing egress. Double checking it
> does what we want:
>
> root@x:~/x# cat foo.t
> {
>     0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa,
>     0xbb, 0xbb, 0xbb, 0xbb, 0xbb, 0xbb,
>     0x41, 0x9c
> }
> root@x:~/x# trafgen -i foo.t -o foo -n 1 -q
> root@x:~/x# tcpdump -i foo
> [...]
> 22:43:42.981112 bb:bb:bb:bb:bb:bb (oui Unknown) > aa:aa:aa:aa:aa:aa (oui Unknown), ethertype Unknown (0x419c), length 14:
>
> root@x:~/x# cat bar.t
> {
>     0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa,
>     0xbb, 0xbb, 0xbb, 0xbb, 0xbb, 0xbb,
>     0xee, 0xee
> }
> root@x:~/x# trafgen -i bar.t -o foo -n 1 -q
> root@x:~/x# tcpdump -i foo
> [... nothing/filtered ...]
>

Something like this seems more trivial to me:

table netdev mytable {
    chain mychain {
        type filter hook egress device "eth0" priority 100; policy drop;
        meta protocol != 0x419C accept
    }
}

Cheers.