netdev - Re: [PATCH net-next 03/10] ovpn: set skb->ignore_df = 1 before sending IPv6 packets out

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Message-ID: <7b5f7e09-7df0-43cb-9acd-c31720002860@redhat.com>
Date: Tue, 13 May 2025 10:51:49 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Antonio Quartulli <antonio@...nvpn.net>, netdev@...r.kernel.org
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Sabrina Dubroca <sd@...asysnail.net>, Gert Doering <gert@...enie.muc.de>
Subject: Re: [PATCH net-next 03/10] ovpn: set skb->ignore_df = 1 before
 sending IPv6 packets out

On 5/13/25 9:51 AM, Antonio Quartulli wrote:
> On 13/05/2025 09:37, Paolo Abeni wrote:
>> On 5/9/25 4:26 PM, Antonio Quartulli wrote:
>>> IPv6 user packets (sent over the tunnel) may be larger than
>>> the outgoing interface MTU after encapsulation.
>>> When this happens ovpn should allow the kernel to fragment
>>> them because they are "locally generated".
>>>
>>> To achieve the above, we must set skb->ignore_df = 1
>>> so that ip6_fragment() can be made aware of this decision.
>>
>> Why the above applies only to IPv6? AFAICS the same could happen even
>> for IPv4.
> 
> For IPv4 we have the 'df=0' param that is passed to 
> udp_tunnel_xmit_skb(), which basically leads to the same result.

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