[v6ops] Re: Carrying large DNS packets over UDP in IPv6 networks

"C. M. Heard" <heard@pobox.com> Sun, 23 June 2024 21:07 UTC

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From: "C. M. Heard" <heard@pobox.com>
Date: Sun, 23 Jun 2024 14:06:55 -0700
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Subject: [v6ops] Re: Carrying large DNS packets over UDP in IPv6 networks
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On Sun, Jun 23, 2024 at 1:32 PM Paul Vixie wrote:
> C. M. Heard wrote on 2024-06-23 13:06:
> > ...
> >>> If I remember the discussion correctly, quite a few open source DNS servers
> >>> do not set the DF flag for IPv4 and have no plans to do so.
> >
> > It would be more correct to say "do not plan to do so for the time being." The
> > issue seems to be that many or most versions of Linux systems do not provide a
> > means to set the DF bit without turning on IPv4 PMTU discovery, which is seen
> > as problematic owing to the ease with which ICMP messages can be forged. See
> > https://mailarchive.ietf.org/arch/msg/dnsop/Z0LeUQRBKqRwgTA12tq5Z7g8pWA/
>
> i think the Linux syscall API has been and could still be revised from
> time to time, but that even if it could not be revised, that would not
> be an excuse to freeze the protocol at the API's current capability level.

Indeed not, and what I was implicitly suggesting there (and explicitly
elsewhere)
that INTAREA and V6OPS could profitably write documents urging implementors of
APIs to "do the right thing" and make it possible for application maintainers to
implement the recommendations now in draft-ietf-dnsop-avoid-fragmentation.

> > It is granted that the DF issue that Linux has does not affect IPv6.
> > But security issues with forged ICMP PTB messages certainly do.
>
> if we're going to change the ICMP specification to address security
> risks, i'll join that discussion. until we do that, i don't expect any
> existing DF=1 sender (such as some TCP stacks) to stop trusting PTB. in
> other words this is a distraction.

Your point is understood, though I'm willing to bet that others (including some
operators and many in the security community) strongly disagree that the issue
is a distraction. Resolving such disagreements is part of making progress.

Mike Heard