[DNSOP] Re: [EXTERNAL] Re: Advice sought: DNS record type for FedCM well-known file delegation

[Will Bartlett <[e-mail]>]

> Given that this problem is basically due to the limited capabilities of people running corporate web servers

This characterization of the problem space is incorrect, as I shared in my prior response to you.

Thank you for your guidance on other topics. Echoing:

  *
Use TXT.
  *
Register the label.
  *
Use a fixed prefix in the value to avoid confusion with wildcards.

I appreciate your guidance on these points.

Thanks,
Will

________________________________
From: John Levine <[e-mail]>
Sent: Wednesday, April 8, 2026 12:42 PM
To: [e-mail] <[e-mail]>
Cc: Will Bartlett <[e-mail]>
Subject: [EXTERNAL] Re: [DNSOP] Advice sought: DNS record type for FedCM well-known file delegation

Keeping in mind that I am unpersuaded there is anything for DNSOP to do here.

It appears that Will Bartlett  <[e-mail]> said:
>  1.
>Is SVCB appropriate here? We're not doing service binding in the traditional sense (ALPN negotiation, ECH, etc.) � we'd either be
>using TargetName purely for delegation (Option B) or embedding application-layer metadata in custom SvcParams (Option A). Is this a
>reasonable use of SVCB, or a misuse of the record type?

Given that this problem is basically due to the limited capabilities of people running corporate web servers,
it would not be a good idea to assume their DNS department can handle SVCB.

>TXT vs SVCB pragmatics. TXT at an underscore-prefixed name (� la DMARC _dmarc, MTA-STS _mta-sts) is universally supported by
>registrars today. SVCB support is still limited. Given that a goal is broad deployability (including small organizations managing
>DNS through commodity registrars), does the group have a view on whether new protocols should prefer SVCB over TXT for simple
>delegation, or is TXT still the practical choice?

An underscore prefixed TXT record is probably the least bad option here.

>  3.
>Naming convention. Is _web-identity.<domain> an appropriate underscore name? Any conflicts or conventions we should be aware of?
>Should we register in the Underscored and Globally Scoped DNS Node Names registry (RFC 8552)?

It takes five minutes and costs nothing once you have a reference you can point to, so sure.

>  4.
>Embedding data in DNS vs delegation. Option A puts application data (URL paths) directly in DNS records, avoiding an HTTP fetch. Is
>there precedent or guidance for/against this pattern? We're aware of the 65535-byte practical limit on DNS responses, but the data
>here is small (two short paths).

My inclination would be just to put the hostname into the record so you don't have to worry about encoding the funky
characters that might be in a URL.  You need a fixed known prefix on the record contents so lookups don't get confused
by domains that wildcard everything, e.g.

_web-identity.examp1e.com TXT "webident;idp.example.com"

R's,
John

PS:
>  *   Spec: https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffedidcg.github.io%2FFedCM%2F&data=05%7C02%7Cwibartle%40microsoft.com%7Cce5516f05d04400828ef08de95a6fc08%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C639112742198750060%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C80000%7C%7C%7C&sdata=MRYJ9quaE%2BHT0mEoXOYwrnS06ih1lxZo%2F3CUhYmVRZY%3D&reserved=0<https://fedidcg.github.io/FedCM/>

404 when I try to look at it