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SubjectRe: netlink backwards compatibility in userspace tools

> On Sep 29, 2017, at 3:22 AM, Jason A. Donenfeld <> wrote:
> Hi guys,
> One handy aspect of Netlink is that it's backwards compatible. This
> means that you can run old userspace utilities on new kernels, even if
> the new kernel supports new features and netlink attributes. The wire
> format is stable enough that the data marshaled can be extended
> without breaking compat. Neat.
> I was wondering, though, what you think the best stance is toward
> these old userspace utilities. What should they do if the kernel sends
> it netlink attributes that it does not recognize? At the moment, I'm
> doing something like this:
> static void warn_unrecognized(void)
> {
> static bool once = false;
> if (once)
> return;
> once = true;
> fprintf(stderr,
> "Warning: this program received from your kernel one or more\n"
> "attributes that it did not recognize. It is possible that\n"
> "this version of wg(8) is older than your kernel. You may\n"
> "want to update this program.\n");
> }
> This seems like a somewhat sensible warning, but then I wonder about
> distributions like Debian, which has a long stable life cycle, so it
> frequently has very old tools (ancient iproute2 for example). Then,
> VPS providers have these Debian images run on top of newer kernels.
> People in this situation would undoubtedly see the above warning a lot
> and not be able to do anything about it. Not horrible, but a bit
> annoying. Is this an okay annoyance? Or is it advised to just have no
> warning at all? One idea would be to put it behind an environment
> variable flag, but I don't like too many nobs.
> I'm generally wondering about attitudes toward this kind of userspace
> program behavior in response to newer kernels.
> Thanks,
> Jason

That seems like a bit much. Consider only emitting a message with the use of a verbose flag - or two. Even then the message should be shortened - the first sentence is entirely adequate even in verbose mode.

Mark Rustad, Networking Division, Intel Corporation

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