netdev - Re: [PATCH net-next] ipv6: do not merge differe type and protocol routes

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Message-ID: <db56de33-2112-5a4c-af94-6c8d26a8bfc1@6wind.com>
Date: Thu, 31 Aug 2023 10:17:19 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: David Ahern <dsahern@...nel.org>, Hangbin Liu <liuhangbin@...il.com>,
 netdev@...r.kernel.org
Cc: "David S . Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Ido Schimmel <idosch@...sch.org>,
 Thomas Haller <thaller@...hat.com>
Subject: Re: [PATCH net-next] ipv6: do not merge differe type and protocol
 routes

Le 30/08/2023 à 20:57, David Ahern a écrit :
> On 8/30/23 9:29 AM, Nicolas Dichtel wrote:
>> Le 30/08/2023 à 08:15, Hangbin Liu a écrit :
>>> Different with IPv4, IPv6 will auto merge the same metric routes into
>>> multipath routes. But the different type and protocol routes are also
>>> merged, which will lost user's configure info. e.g.
>>>
>>> + ip route add local 2001:db8:103::/64 via 2001:db8:101::10 dev dummy1 table 100
>>> + ip route append unicast 2001:db8:103::/64 via 2001:db8:101::10 dev dummy2 table 100
>>> + ip -6 route show table 100
>>> local 2001:db8:103::/64 metric 1024 pref medium
>>>         nexthop via 2001:db8:101::10 dev dummy1 weight 1
>>>         nexthop via 2001:db8:101::10 dev dummy2 weight 1
>>>
>>> + ip route add 2001:db8:104::/64 via 2001:db8:101::10 dev dummy1 proto kernel table 200
>>> + ip route append 2001:db8:104::/64 via 2001:db8:101::10 dev dummy2 proto bgp table 200
>>> + ip -6 route show table 200
>>> 2001:db8:104::/64 proto kernel metric 1024 pref medium
>>>         nexthop via 2001:db8:101::10 dev dummy1 weight 1
>>>         nexthop via 2001:db8:101::10 dev dummy2 weight 1
>>>
>>> So let's skip counting the different type and protocol routes as siblings.
>>> After update, the different type/protocol routes will not be merged.
>>>
>>> + ip -6 route show table 100
>>> local 2001:db8:103::/64 via 2001:db8:101::10 dev dummy1 metric 1024 pref medium
>>> 2001:db8:103::/64 via 2001:db8:101::10 dev dummy2 metric 1024 pref medium
>>>
>>> + ip -6 route show table 200
>>> 2001:db8:104::/64 via 2001:db8:101::10 dev dummy1 proto kernel metric 1024 pref medium
>>> 2001:db8:104::/64 via 2001:db8:101::10 dev dummy2 proto bgp metric 1024 pref medium
>>
>> This seems wrong. The goal of 'ip route append' is to add a next hop, not to
>> create a new route. Ok, it adds a new route if no route exists, but it seems
>> wrong to me to use it by default, instead of 'add', to make things work magically.
> 
> Legacy API; nothing can be done about that (ie., that append makes a new
> route when none exists).
> 
>>
>> It seems more correct to return an error in these cases, but this will change
>> the uapi and it may break existing setups.
>>
>> Before this patch, both next hops could be used by the kernel. After it, one
>> route will be ignored (the former or the last one?). This is confusing and also
>> seems wrong.
> 
> Append should match all details of a route to add to an existing entry
> and make it multipath. If there is a difference (especially the type -
> protocol difference is arguable) in attributes, then they are different
> routes.
> 

As you said, the protocol difference is arguable. It's not a property of the
route, just a hint.
I think the 'append' should match a route whatever the protocol is.
'ip route change' for example does not use the protocol to find the existing
route, it will update it:

$ ip -6 route add 2003:1:2:3::/64 via 2001::2 dev eth1
$ ip -6 route
2003:1:2:3::/64 via 2001::2 dev eth1 metric 1024 pref medium
$ ip -6 route change 2003:1:2:3::/64 via 2001::2 dev eth1 protocol bgp
$ ip -6 route
2003:1:2:3::/64 via 2001::2 dev eth1 proto bgp metric 1024 pref medium
$ ip -6 route change 2003:1:2:3::/64 via 2001::2 dev eth1 protocol kernel
$ ip -6 route
2003:1:2:3::/64 via 2001::2 dev eth1 proto kernel metric 1024 pref medium

Why would 'append' selects route differently?

This patch breaks the legacy API.