netdev - Re: [RFC net-next] ipv6: Support for anonymous tunnel decapsulation

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Message-ID: <63ec6555-77f8-d241-80d5-a40bc4561d75@6wind.com>
Date:   Fri, 27 Aug 2021 10:20:36 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Justin Iurman <justin.iurman@...ege.be>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, tom@...bertland.com,
        edumazet@...gle.com
Subject: Re: [RFC net-next] ipv6: Support for anonymous tunnel decapsulation

Le 27/08/2021 à 09:51, Justin Iurman a écrit :
>> [snip]
>>
>>>>> Thoughts?
>>>> I'm not sure to understand why the current code isn't enough. The fallback
>>>> tunnels created by legacy IP tunnels drivers are able to receive and decapsulate
>>>> any encapsulated packets.
>>>
>>> Because, right now, you need to use the ip6_tunnel module and explicitly
>>> configure a tunnel, as you described below. The goal of this patch is to
>>> provide a way to apply an ip6ip6 decapsulation *without* having to configure a
>>> tunnel.
>>
>> What is the difference between setting a sysctl somewhere and putting an
>> interface up?
> 
> Well, correct me if I'm wrong but, it's more than just putting an interface up. You'd first need ip6_tunnel (and so tunnel6) module loaded, but you'd also need to configure a tunnel on the decap node.
No, you just need to have the module. The fallback device is automatically
created. And if the module is built-in, there is nothing to do.

 Indeed, the current ip6_tunnel fallback handler only works if a tunnel matches
the packet (i.e., ipxip6_rcv will return -1 since ip6_tnl_lookup will return
NULL, leading to *no* decapsulation from this handler).
No. ip6_tnl_lookup() won't return NULL if the fallback device exists and is up.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/ip6_tunnel.c#n168

The tunnels lookup algorithm has several steps:
 - try to match local and remote addr
 - try to match only local addr
 - try to match only dst addr
 - return the lwt tunnel if it exists
 - return the fallback device if it exists and is up

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/ip6_tunnel.c#n100
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/sit.c#n96
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/ip_tunnel.c#n72

> 
> So, again, think about the case where you have lots of ingresses and egresses that should be linked (= a tunnel for each pair) altogether in a domain. You'd need to configure N tunnels on the decap node, where N is the number of ingresses. Well, actually no, you could just configure one tunnel with "remote any", but you'd still depend on the ip6_tunnel module and play with tunnel configuration and its interface. This patch provides a way to avoid that by just enabling the ip6ip6 decapsulation through a per interface sysctl.
> 
I don't understand the problem of depending to the ip6_tunnel module.
Duplicating a subset of the existing code to avoid a dependency to an existing
module seems a bad idea for me, from a maintenance point of view.


Regards,
Nicolas