Re: [PATCH v1 2/2] ipv6: always accept routing headers with 0 segments left

From: Mathis Marion
Date: Wed Jun 26 2024 - 06:37:38 EST


On 26/06/2024 3:45 AM, Alexander Aring wrote:
Hi,

On Tue, Jun 25, 2024 at 5:39 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:

From: Mathis Marion <Mathis.Marion@xxxxxxxxxx>
Date: Mon, 24 Jun 2024 16:15:33 +0200
From: Mathis Marion <mathis.marion@xxxxxxxxxx>

Routing headers of type 3 and 4 would be rejected even if segments left
was 0, in the case that they were disabled through system configuration.

RFC 8200 section 4.4 specifies:

If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet, whose type
is identified by the Next Header field in the Routing header.

I think this part is only applied to an unrecognized Routing Type,
so only applied when the network stack does not know the type.

https://www.rfc-editor.org/rfc/rfc8200.html#section-4.4

If, while processing a received packet, a node encounters a Routing
header with an unrecognized Routing Type value, the required behavior
of the node depends on the value of the Segments Left field, as
follows:

If Segments Left is zero, the node must ignore the Routing header
and proceed to process the next header in the packet, whose type
is identified by the Next Header field in the Routing header.

That's why RPL with segment length 0 was accepted before 8610c7c6e3bd.

But now the kernel recognizes RPL and it's intentionally disabled
by default with net.ipv6.conf.$DEV.rpl_seg_enabled since introduced.

And SRv6 has been rejected since 1ababeba4a21f for the same reason.

so there might be a need to have an opt-in knob to actually tell the
kernel ipv6 stack to recognize or not recognize a next header field
for users wanting to bypass certain next header fields to the user
space?

- Alex


My point is that if a particular routing header support is disabled
through system configuration, it should be treated as any unrecognized
header. From my perspective, doing otherwise causes a regression every
time a new routing header is supported.