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

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.