Original text:
Therefore it needs to respond to the request without waiting for a packet from the target.  However, if the target_host is a DNS name, the UDP proxy MUST perform DNS resolution before replying to the HTTP <del> request.  If errors occur during this process (for example, a DNS resolution failure), the UDP proxy MUST fail the request and SHOULD send details using an appropriate "Proxy-Status" header field PROXY- STATUS. </del>  UDP proxies can use connected UDP sockets if their operating system supports them, as that allows the UDP proxy to rely on the kernel to

Reviewer comments:
I also want to point an issue due to this stance. The fact that we are not explicit and discuss the expectations and realities of different IP versions we have interesting failure cases due to this text from Section 3.1: “However, if the target_host is a DNS name, the UDP proxy MUST perform DNS resolution before replying to the HTTP request.” No where in the follow paragraphs in this section do it discuss the slight issue that a DNS resolution can return only an AAAA record and the MASQUE server may only support IPv4. What is the error response? Should this be a proxy-status response. I would not there are currently no response for “IP version not supported”.
These concerns apply equally to TCP proxying via HTTP. We should strive to provide parity to how CONNECT treats these matters. HTTP semantics don't say anything
+1 to Lucas' meta-comment about striving for parity with CONNECT There is no such thing as a "unsupported IP version" DNS failure. Let's take an example where the client wants to CONNECT-UDP via the UDP proxy to the target . This particular host is configured such that querying AAAA will get a valid address, and querying A will yield a "no answer no error" DNS result. Let's assume that in this example the UDP proxy is IPv4-only. In that scenario, the UDP proxy will only query A (because it doesn't know what AAAA is) and it will get "no result no answer" so it will send
NAME in light of the previous comments, can you clarify what you think is missing from the document please?
First of all, I think that just answering lets do like Connect is a poor answer. There are benefits to alignment, but also one should ask if one are preventing useful functionality. In this case I think there are a relevant difference between what could be answered depending on how one implement stuff. Your suggest road says that for a IPv4 only proxy with a DNS name that has AAAA record but no A record the answer is: a) There is no answer For a node that would query both the answer could actually be: b) There is host, but I can't reach it as I don't support the right IP version, try another proxy. Sure, limiting ourselves to a) is easy, but is it the most useful for the protocol?
Disagree it's a poor answer. An HTTP proxy that inplements CONNECT for UDP is likely going to implement CONNECT for TCP. How to treat name resolution for both of these has to be consistent. The Proxy-Status header field is the appropriate way to signal such failures. Any text more complicated than what we have now, should be pursued in a separate document IMO. Not added at this late stage. It would be straightforward to form up a document like "DNS considerations for HTTP proxying" that is additive to the specifications we have.
I agree: Proxy-Status is the right place for this discussion, and CONNECT-UDP already recommends Proxy-Status. Also, as a technical matter, this is not how dual-stack connectivity works. As noted above, an IPv4-only UDP client will never issue a AAAA query for a destination hostname.
I agree with what Lucas and Ben said above. Let's move this discussion to an extension draft.
I agree that this discussion probably deserves to be in an extension draft. As is stands today, a DNS failure as described in this thread can just as easily result in a destination-unreachable / connection-closed error for the CONNECT-UDP stream, and an extension to provide detailed information as to why seems like the natural path forward.
Proxy status is absolutely the right thing to do here. The proxy-status is what we are already doing in production for CONNECT and CONNECT-UDP. The current text seems entirely appropriate for this: The only thing I could see saying further is to mention the error parameter, like this: I do not think we need any extension draft for this. This is very straightforward, and doesn't need to say much.
Thanks Tommy, I think that's a useful clarification. I wrote up PR based on your proposal to make this clearer
Ok, I am fine with this resolution.
Thank you for confirming Magnus!

Revised text:
Therefore it needs to respond to the request without waiting for a packet from the target.  However, if the target_host is a DNS name, the UDP proxy MUST perform DNS resolution before replying to the HTTP  <ins> request.  If errors occur during this process, the UDP proxy MUST fail the request and SHOULD send details using an appropriate "Proxy- Status" header field PROXY-STATUS (for example, if DNS resolution returns an error, the proxy can use the dns_error Proxy Error Type from PROXY-STATUS). </ins> UDP proxies can use connected UDP sockets if their operating system supports them, as that allows the UDP proxy to rely on the kernel to