[PATCH] network: allow accept_ra == 0 when enabling ipv6 forwarding

[Ian Wienand]

On Wed, Sep 09, 2020 at 06:38:04AM +0000, Cedric Bosdonnat wrote:
> The check didn't involve any NetworkManager at all, but a network with
> RA route for the default route. Completely removing the check is rather
> likely to introduce a regression on that side.

Thanks for putting up with my bumbling about here :)

So firstly; I think we can state the big picture original problem as
"the kernel was seen to flush existing ipv6 routes when ipv6
forwarding is enabled, unless accept_ra==2 is set, which
<may|probably|does> break peoples networking"?

If that premise is correct, then I also think I'm correct in saying
from ~[1] that the kernel will only flush routes with RTF_ADDRCONF
set?

  if (rt->fib6_flags & (RTF_DEFAULT | RTF_ADDRCONF) &&
                 (!idev || idev->cnf.accept_ra != 2) &&
                   fib6_info_hold_safe(rt)) {
                        rcu_read_unlock();
                        ip6_del_rt(net, rt);
                        goto restart;
  }

If that premise is correct, then I also think that userspace tools do
not set this flag on routes they setup when they handle RA's in
userspace.  I couldn't see it set on any of my routes on my laptop,
and enabling/disabling forwarding didn't seem to flush any routes.

If *that* premise is correct too, then as I understand the current
code it queries netlink for all the routes, checks if they have
RTPROT_RA set, and if accept_ra != 2 gives the error.

If **that** premise is correct, then I think that just checking
RTPROT_RA is too strict -- per the prior steps the route will only be
flushed by the kernel if it has RTF_ADDRCONF set on it, and for many
people using userspace tools their routing is not affected by enabling
forwarding at all.

If ***that*** premise is correct -- what to do about it?  I don't
think netlink exposes RTF_ADDRCONF?  It can be seen via the flags dump
in /proc/net/ipv6_route however (maybe that's a field in netlink?).
But there may be room for conversation on how much this warning helps
v hinders in 2020; it's not like it fixes the problem for you.

Happy to be pointed out at which step I've gone wrong :)

-i

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/route.c#n4282