wireguard: problem sending via libpcap's packet socket

[Willem de Bruijn]

On Sat, Jun 27, 2020 at 1:58 AM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
>
> Hi again Hans,
>
> A few remarks: although gre implements header_ops, it looks like
> various parts of the networking stack change behavior based on it. I'm
> still analyzing that to understand the extent of the effects.
> Something like <https://git.zx2c4.com/wireguard-linux/commit/?id=40c24fd379edc1668087111506ed3d0928052fe0>
> would work, but I'm not thrilled by it. Further research is needed.
>
> However, one thing I noticed is that other layer 3 tunnels don't seem
> to be a fan of libpcap. For example, try injecting a packet into an
> ipip interface. You'll hit exactly the same snag for skb->protocol==0.

Not setting skb protocol when sending over packet sockets causes many
headaches. Besides packet_parse_headers, virtio_net_hdr_to_skb also
tries to infer it.

Packet sockets give various options to configure it explicitly: by
choosing that protocol in socket(), bind() or, preferably, by passing
it as argument to sendmsg. The socket/bind argument also configures
the filter to receive packets, so for send-only sockets it is
especially useful to choose ETH_P_NONE (0) there. This is not an
"incorrect" option.

Libpcap does have a pcap_set_protocol function, but it is fairly
recent, so few processes will likely be using it. And again it is
still not ideal if a socket is opened only for transmit.

header_ops looks like the best approach to me, too. The protocol field
needs to reflect the protocol of the *outer* packet, of course, but if
I read wg_allowedips_lookup_dst correctly, wireguard maintains the
same outer protocol as the inner protocol, no sit (6-in-4) and such.