Re: [PATCH] udp: create and send ICMPv4 to local peer when applicable

[Stefano Brivio <sbrivio@redhat.com>]

On Sun,  9 Feb 2025 12:00:56 -0500
Jon Maloy <jmaloy@redhat.com> wrote:

> When a local peer sends a UDP message to a non-existing port on an
> existing remote host, that host will return an ICMP message containing
> the error code ICMP_PORT_UNREACH, plus the header and the first eight
> bytes of the original message. If the sender socket has been connected,
> it uses this message to issue a "Connection Refused" event to the user.
> 
> Until now, we have only read such events from the externally facing
> socket, but we don't forward them back to the local sender because
> we cannot read the ICMP message directly to user space. Because of
> this, the local peer will hang and wait for a response that never
> arrives.

I haven't had a chance to really review this yet, in general it looks
great to me (I was afraid it would be more complicated).

I have a couple of preliminary questions though:

- referring to the paragraph above: what about TCP (which is the case
  where a peer might actually hang)? Do you plan to support errors for
  TCP's connect() in a separate patch?

> We now fix this for IPv4 by recreating and forwarding a correct ICMP
> message back to the internal sender. We synthesize the message based
> on the information in the extended error structure, plus the returned
> part of the original message body.

- ...and what about IPv6 and NDP? Also separate patch?

In that case, would it perhaps make sense to implement and submit that
as a series so that we have a consistent behaviour to begin with?

-- 
Stefano