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

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Message-ID: <effc10de-e7a9-4721-84ee-caafcf9aedb8@openvpn.net>
Date: Tue, 13 May 2025 09:51:55 +0200
From: Antonio Quartulli <antonio@...nvpn.net>
To: Paolo Abeni <pabeni@...hat.com>, 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 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.

Originally (in some old version of the original ovpn submission) I had 
skb->ignore_df=1 in the common path, but then it was removed when 
Sabrina highlighted the df param.

However, we overlooked that there is no such param/logic for IPv6, hence 
we need the explicit ignore_df=1 for IPv6.

Regards,

-- 
Antonio Quartulli
OpenVPN Inc.