In AliasMode, the SVCB record aliases a service to a TargetName. SVCB RRsets SHOULD only have a single RR in AliasMode. If multiple AliasMode RRs are present, clients or recursive resolvers SHOULD pick one at random.¶
The primary purpose of AliasMode is to allow aliasing at the zone apex, where CNAME is not allowed (see, for example, [RFC1912], Section 2.4). In AliasMode, the TargetName will be the name of a domain that resolves to SVCB, AAAA, and/or A records. (See Section 6 for aliasing of SVCB-compatible RR types.) Unlike CNAME, AliasMode records do not affect the resolution of other RR types and apply only to a specific service, not an entire domain name.¶
The AliasMode TargetName SHOULD NOT be equal to the owner name, as this would result in a loop. In AliasMode, recipients MUST ignore any SvcParams that are present. Zone-file parsers MAY emit a warning if an AliasMode record has SvcParams. The use of SvcParams in AliasMode records is currently not defined, but a future specification could extend AliasMode records to include SvcParams.¶
For example, the operator of "foo://example.com:8080" could point requests to a service operating at "foosvc.example.net" by publishing:¶
_8080._foo.example.com. 3600 IN SVCB 0 foosvc.example.net. ¶
Using AliasMode maintains a separation of concerns: the owner of "foosvc.example.net" can add or remove ServiceMode SVCB records without requiring a corresponding change to "example.com". Note that if "foosvc.example.net" promises to always publish a SVCB record, this AliasMode record can be replaced by a CNAME at the same owner name.¶
AliasMode is especially useful for SVCB-compatible RR types that do not require an underscore prefix, such as the HTTPS RR type. For example, the operator of "https://example.com" could point requests to a server at "svc.example.net" by publishing this record at the zone apex:¶
example.com. 3600 IN HTTPS 0 svc.example.net. ¶
Note that the SVCB record's owner name MAY be the canonical name of a CNAME record, and the TargetName MAY be the owner of a CNAME record. Clients and recursive resolvers MUST follow CNAMEs as normal.¶
To avoid unbounded alias chains, clients and recursive resolvers MUST impose a limit on the total number of SVCB aliases they will follow for each resolution request. This limit MUST NOT be zero, i.e., implementations MUST be able to follow at least one AliasMode record. The exact value of this limit is left to implementations.¶
... continue reading