Re: [DNSOP] SVCB and the specialness of _

Tommy Pauly <> Wed, 07 October 2020 13:23 UTC

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From: Tommy Pauly <>
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Date: Wed, 07 Oct 2020 06:23:34 -0700
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Subject: Re: [DNSOP] SVCB and the specialness of _
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This change seems reasonable, and makes “.” still more useful for the generic SVCB case.


> On Oct 6, 2020, at 4:51 PM, Ben Schwartz <> wrote:
> There was recently an interesting proposal [1] for a change to the SVCB draft that seems to merit input from the working group.
> Background:
> When using SVCB, clients append an underscore prefix ("Attrleaf") to the name, containing the "scheme".  When there is an actual URI involved this will literally be the URI scheme, e.g. <>.  Schemes can prepend additional underscore-prefix labels to encode scheme-specific service identifiers; the only anticipated use for this is a port number (e.g. <>).
> SVCB also contains a small optimization: In ServiceMode, if the TargetName is ".", this means "use the address records for the owner name".  This reduces packet size (since name compression is not possible in new RR types), but more importantly, it makes zone file editing easier and guides zone owners toward the "happy path", where there is no latency penalty because the resolver has already requested the corresponding address records.
> This construction works well for HTTPS on port 443, and for any lookup following an AliasMode record, because in those cases the SVCB/HTTPS, A, and AAAA queries all share the same QNAME.  However, it does not work when the initial query has underscore labels and returns a ServiceMode record, because the ServiceMode record's name is not the hostname (due to the underscore labels).
> Proposal:
> The proposal would change the meaning of "." from "Use the SVCB record's owner name" to "Use the SVCB record's owner name *minus any leading underscore labels*".
> Pro: TargetName = "." would now coincide with the "happy path" in all ordinary cases.
> * Simpler guidance to zone owners (always prefer ".")
> * Clearer design rationale ("." is where the address queries already went)
> * Shorter RDATA in the "happy path" case (for some non-HTTPS uses)
> Con:
> * Servers (recursive and authoritative) that perform Additional Section processing would have to implement this underscore-label stripping procedure.  This would be a rare violation of DNS's usual abstraction barrier (servers don't normally discriminate based on the format of labels).
> * There could be a change of operational control (e.g. zone cut or CNAME) between _port._scheme.$HOSTNAME and $HOSTNAME, in which case the ServiceMode record might actually prefer to use address records at its own name, despite the latency cost.
> * This gets very tricky if an AliasMode or CNAME target contains underscore prefixes.  It may be impossible to stay on the "happy path" in all these cases.
> [1] <>
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