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



On Wed, 2020-09-09 at 12:39 +1000, Ian Wienand wrote:
> On Tue, Sep 01, 2020 at 08:27:47AM +0000, Cedric Bosdonnat wrote:
> > So the hypervisor has at least one (Router Advertised) RA route.
> > After defining a network like the following, the RA route is removed if
> > accept_ra isn't set to 2.
> > 
> > <network ipv6="yes">
> >   <name>test5</name>
> >   <forward mode="nat"/>
> >   <bridge name="708837c1d27-br0" stp="off"/>
> >   <mac address="52:54:00:45:5f:27"/>
> >   <ip
> >      family="ipv6"
> >      address="fc00:0000:0000:000f:0000:0000:0000:0001"
> >      prefix="64"/>
> > </network>
> > 
> > The RA route was removed in networkEnableIPForwarding() when
> > setting /proc/sys/net/ipv6/conf/all/forwarding to 1.
> > 
> > Me not being a network expert (and even less on ipv6) doesn't help.
> > 
> > I hope this explanation will help you better see the use case I had.
> 
> So it seems to be the intention of the kernel that when you enable
> forwarding your routes are flushed; changing the sysctl gets into
> addrconf_fixup_forwarding() [1] which then calls
> rt6_purge_dflt_routers() when the forwarding status is changed.  That
> then purges default routes, unless accept_ra == 2; that was introduced
> with [3].
> 
> I guess the idea is that a router should not accept
> auto-configuration?
> 
> HOWEVER ...
> 
>                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;
>                 }
> 
> I feel like this is checking the RTF_ADDRCONF flag before it flushes
> any routes.  Checking that flag ...
> 
>  #define RTF_ADDRCONF    0x00040000
> 
> which I do not have set at all, from :
> 
>  $ cat /proc/net/ipv6_route | awk  '{print $1 " " and(strtonum("0x"$9),strtonum("0x40000"))}'
> 
> Based on this, I'm concluding that the userspace tools do not set this
> flag on their routes, and so they are never flushed.  Empirically,
> fiddling forwarding on and off I don't see any routes flushed.
> 
> So, I do not think that enabling forwarding will remove routes on the
> most common "sitting in-front of the computer" cases where you're
> using NetworkManager/systemd userspace magic.
> 
> Given this, I'd propose we revert the check?

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.

--
Cedric


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