On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
> >> This will not be sufficient for my usecase: To stay compatible with older
> >> versions of fastd, I can't set the T flag in the first packet of the
> >> handshake, as it won't be known whether the peer has a new enough fastd
> >> version to understand packets that have this bit set. Luckily, the second
> >> handshake byte is always 0 in fastd's protocol, so these packets fail the
> >> tunnel version check and are passed to userspace regardless.
> >>
> >> I'm aware that this usecase is far outside of the original intentions of the
> >> code and can only be described as a hack, but I still consider this a
> >> regression in the kernel, as it was working fine in the past, without
> >> visible warnings.
> >>
> >
> > I'm sorry, but for the reasons stated above I disagree about it being
> > a regression.
>
> Hmm, is it common for protocol implementations in the kernel to warn about
> invalid packets they receive? While L2TP uses connected sockets and thus
> usually no unrelated packets end up in the socket, a simple UDP port scan
> originating from the configured remote address/port will trigger the "short
> packet" warning now (nmap uses a zero-length payload for UDP scans by
> default). Log spam caused by a malicous party might also be a concern.
Indeed, seems like appropriate counters would be a good fit here?
The prints are both potentially problematic for security and lossy.