linux-kernel - Re: [PATCH] net/ipv6: allow device-only routes via the multipath API

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Message-ID: <c73cfca0-a43f-463a-a96d-7da3ede8fde0@6wind.com>
Date: Tue, 18 Nov 2025 17:41:14 +0100
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: David Ahern <dsahern@...nel.org>, azey <me@...y.net>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 netdev <netdev@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net/ipv6: allow device-only routes via the multipath API

Le 18/11/2025 à 17:04, David Ahern a écrit :
> On 11/18/25 4:00 AM, azey wrote:
>> On 2025-11-18 10:05:55, +0100 Nicolas Dichtel wrote:
>>> If I remember well, it was to avoid merging connected routes to ECMP routes.
>>> For example, fe80:: but also if two interfaces have an address in the same
>>> prefix. With the current code, the last route will always be used. With this
>>> patch, packets will be distributed across the two interfaces, right?
>>> If yes, it may cause regression on some setups.
>>
>> Thanks! Yes, with this patch routes with the same destination and metric automatically
>> become multipath. From my testing, for link-locals this shouldn't make a difference
>> as the interface must always be specified with % anyway.
>>
>> For non-LL addresses, this could indeed cause a regression in obscure setups. In my
Having an address in the same prefix on two interfaces is not an "obscure setups".

>> opinion though, I feel that it is very unlikely anyone who has two routes with the
>> same prefix and metric (which AFAIK, isn't really a supported configuration without
>> ECMP anyway) relies on this quirk. The most plausible setup relying on this I can
>> think of would be a server with two interfaces on the same L2 segment, and a
>> firewall somewhere that only allows the source address of one interface through.
>>
>> IMO, setups like that are more of a misconfiguration than a "practical use case"
>> that'd make this a real regression, but I'd completely understand if it'd be enough
>> to block this.
> 
> There is really no reason to take a risk of a regression. If someone
> wants ecmp with device only nexthops, then use the new nexthop infra to
> do it.
+1