netdev - Re: [PATCH net] ipv6: clear RA flags when adding a static route

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Message-ID: <17d7f19c-5f43-4cb9-a76a-a55dd5966c43@redhat.com>
Date: Thu, 13 Nov 2025 13:23:09 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Fernando Fernandez Mancera <fmancera@...e.de>, netdev@...r.kernel.org
Cc: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
 kuba@...nel.org, horms@...nel.org, Garri Djavadyan <g.djavadyan@...il.com>
Subject: Re: [PATCH net] ipv6: clear RA flags when adding a static route

On 11/13/25 1:15 PM, Fernando Fernandez Mancera wrote:
> On 11/13/25 1:03 PM, Paolo Abeni wrote:
>> On 11/11/25 12:04 AM, Fernando Fernandez Mancera wrote:
>>> When an IPv6 Router Advertisement (RA) is received for a prefix, the
>>> kernel creates the corresponding on-link route with flags RTF_ADDRCONF
>>> and RTF_PREFIX_RT configured and RTF_EXPIRES if lifetime is set.
>>>
>>> If later a user configures a static IPv6 address on the same prefix the
>>> kernel clears the RTF_EXPIRES flag but it doesn't clear the RTF_ADDRCONF
>>> and RTF_PREFIX_RT. When the next RA for that prefix is received, the
>>> kernel sees the route as RA-learned and wrongly configures back the
>>> lifetime. This is problematic because if the route expires, the static
>>> address won't have the corresponding on-link route.
>>>
>>> This fix clears the RTF_ADDRCONF and RTF_PREFIX_RT flags preventing that
>>> the lifetime is configured when the next RA arrives. If the static
>>> address is deleted, the route becomes RA-learned again.
>>>
>>> Fixes: 14ef37b6d00e ("ipv6: fix route lookup in addrconf_prefix_rcv()")
>>> Reported-by: Garri Djavadyan <g.djavadyan@...il.com>
>>> Closes: https://lore.kernel.org/netdev/ba807d39aca5b4dcf395cc11dca61a130a52cfd3.camel@gmail.com/
>>> Signed-off-by: Fernando Fernandez Mancera <fmancera@...e.de>
>>> ---
>>> Note: this has been broken probably since forever but I belive the
>>> commit in the fixes tag was aiming to fix this too. Anyway, any
>>> recommendation for a fixes tag is welcomed.
>>> ---
>>>   net/ipv6/ip6_fib.c | 4 ++++
>>>   1 file changed, 4 insertions(+)
>>>
>>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
>>> index 02c16909f618..2111af022d94 100644
>>> --- a/net/ipv6/ip6_fib.c
>>> +++ b/net/ipv6/ip6_fib.c
>>> @@ -1138,6 +1138,10 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
>>>   					fib6_set_expires(iter, rt->expires);
>>>   					fib6_add_gc_list(iter);
>>>   				}
>>> +				if (!(rt->fib6_flags & (RTF_ADDRCONF | RTF_PREFIX_RT))) {
>>> +					iter->fib6_flags &= ~RTF_ADDRCONF;
>>> +					iter->fib6_flags &= ~RTF_PREFIX_RT;
>>> +				}
>>>   
>>>   				if (rt->fib6_pmtu)
>>>   					fib6_metric_set(iter, RTAX_MTU,
>>
>> The patch makes sense to me, but I don't want to rush it in the net PR
>> I'm going to send soon. Also it would be great to have self-test
>> covering this case, could you have a reasonable shot at it?
>>
> 
> Sure, I am fine with aiming this for net-next instead if you consider it 
> safer.

Yep, net-next would be better, I think.

Thanks,

Paolo