| DNSSD | A. Sullivan |
| Internet-Draft | Dyn |
| Intended status: Informational | March 4, 2015 |
| Expires: September 5, 2015 |
On Interoperation of Labels Between mDNS and DNS
draft-ietf-dnssd-mdns-dns-interop-00
Despite its name, DNS-Based Service Discovery can use naming systems other than the Domain Name System when looking for services. Different name systems use different conventions for the characters allowed in any name. In order for DNS-SD to be used effectively in environments where multiple different name systems are in use, it is important to attend to differences in the underlying technology. This memo presents an outline of the requirements for selection of labels for mDNS and DNS when they are expected to interoperate in this manner.
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DNS-Based Service Discovery (DNS-SD, [RFC6763]) specifies a mechanism for discovering services using queries both to the Domain Name System (DNS, [RFC1034], [RFC1035]) and to Multicast DNS (mDNS, [RFC6762]). Conventional use of the DNS generally follows the host name rules [RFC0952] for labels -- the so-called LDH rule. That convention is the reason behind the development of Internationalized Domain Names for Applications (IDNA2008, [RFC5890], [RFC5891], [RFC5892], [RFC5893], [RFC5894], [RFC5895]). It is worth noting that the LDH rule is a convention, and not a strict rule of the DNS. It is assumed to be true widely enough, however, that in many circumstances names cannot be used unless they cleave to the LDH rule.
At the same time, mDNS requires that labels be encoded in UTF-8, and permits a range of characters in labels that are not permitted by IDNA2008 or the LDH rule. For example, mDNS encourages the use of spaces and punctuation in mDNS names (see [RFC6763], section 4.1.3). It does not restrict which Unicode code points may be used in those labels, so long as the code points are UTF-8 in Net-Unicode [RFC5198] format.
Users of applications are, of course, frequently unconcerned with (not to say oblivious to) the name-resolution system(s) in service at any given moment, and are inclined simply to use the same names in different contexts. As a result, the same string might be tried as a name using different name resolution technologies. If DNS-SD is to be used in an environment where both mDNS and DNS are to be queried for services, then some parts of the names to be queried will need to be compatible with the rules and conventions for both DNS and mDNS.
One approach to interoperability under these circumstances is to use a single operational convention for names under the different naming systems. This memo assumes such a use profile, and outlines what is necessary to make it work.
It is worth noting that users of DNS-SD do not use the service discovery names in the same way that users of other domain names might. Most domain names might as easily be typed in as direct user input as any other method. But the service discovery context generally assumes users are picking a service from a list. As a result, the sorts of application considerations that are appropriate to the general-purpose DNS name, and that resulted in the A-label/U-label (see below) in IDNA2008, are not the right approach for DNS-SD.
Wherever appropriate, this memo uses the terminology defined in Section 2 of [RFC5890]. In particular, the reader is assumed to be familiar with the terms "U-label", "LDH label", and "A-label" from that document. Similarly, the reader is assumed to be familiar with the U+NNNN notation for Unicode code points used in [RFC5890] and other documents dealing with Unicode code points. In the interests of brevity and consistency, the definitions are not repeated here.
This memo refers to names in the DNS as though the LDH rule and IDNA2008 are strict requirements. They are not. DNS labels are, in principle, just collections of octets, and therefore in principle the LDH rule is not a constraint. In practice, applications often intercept labels that do not conform to the LDH rule and apply IDNA and other transformations.
The term "owner name" (common to the DNS vernacular) is used here to apply not just to the names to be looked up in the DNS, but to any name that might be looked up either in the DNS or using mDNS.
Any interoperability between mDNS and DNS will require interoperability across some of the portions of a DNS-SD Service Instance Name (see Section 3) that are implicated in regular mDNS and DNS lookups. Only some portions are implicated. In any case, if a given portion is implicated, the profile will need to apply to all labels in that portion.
In addition, because DNS-SD Service Instance Names can be used in a domain name slot, care must be taken by DNS-SD resolvers to undertake the special processing outlined here, so that DNS-SD portions that do not use IDNA2008 will not be treated as U-labels and will not undergo IDNA processing.
Because the profile will need to apply to names that might need to interoperate with names in the DNS, and because mDNS permits labels that IDNA does not, the profile might reduce the labels that could be used with mDNS. Consequently, some recommendations from [RFC6763] will not really be possible to implement using names subject to the profile. In particular, [RFC6763], section 4.1.3 recommends that labels always be stored and communicated as UTF-8, even in the DNS. Because IDNA2008 libraries will treat any Unicode-encoded labels as candidate U-labels and attempt to perform resolution in A-label form, the advice to store and transmit labels as UTF-8 in the DNS is likely to encounter problems. In particular, the <Domain> part of a Service Instance Name is unlikely to be found in its UTF-8 form in the public DNS tree for zones that are using IDNA2008. By contrast, mDNS normally uses UTF-8.
U-labels cannot contain upper case letters. That restriction extends to ASCII-range upper case letters that work fine in LDH-labels. It may be confusing that the character "A" works in the DNS when none of the characters in the label has a diacritic, but does not work when there is such a diacritic in the label. Labels in mDNS names may contain upper case characters, so the profile will need either to restrict the use of upper case or come up with a reliable and predictable (to users) convention for case folding even in the presence of diacritics.
DNS-SD specifies three portions of the owner name for a DNS-SD resource record. These are the <Instance> portion, the <Service> portion, and the <Domain>. The owner name made of these three parts is called the Service Instance Name. It is worth observing that a portion may be more than one label long. See [RFC6763], section 4.1.
[RFC6763] is clear that the <Instance> portion of the Service Instance Name is intended for presentation to users, and therefore virtually any character is permitted in it. There are two ways that a profile might address this portion.
The first way would be to treat this portion as likely to be intercepted by system-wide IDNA-aware resolvers. In this case, the portion needs to be made subject to the profile, thereby curtailing what characters may appear in this portion. This approach permits DNS-SD to use any standard system resolver but presents inconsistencies with the DNS-SD specification and with DNS-SD that is exclusively mDNS-based. Therefore, this strategy is rejected.
Instead, DNS-SD implementations can intercept the <Instance> portion of a Service Instance Name and ensure that those labels are never handed to IDNA-aware resolvers that might attempt to convert these labels into A-labels. Under this approach, the DNS-SD <Instance> portion works as it always does, but at the cost of using special resolution code built into the DNS-SD system.
DNS-SD includes a <Service> component in the Service Instance Name. This component is not really user-facing data, but is instead control data embedded in the Service Instance Name. This component includes so-called "underscore labels", which are labels prepended with U+005F (_). The underscore label convention was established by DNS SRV ([RFC2782]) for identifying metadata inside DNS names. A system-wide resolver (or DNS middlebox) that cannot handle underscore labels will not work with DNS-SD at all, so it is safe to suppose that such resolvers will not attempt to do special processing on these labels. Therefore, the <Service> portion of the Service Instance Name will not be subject to the profile.
The <Domain> portion of the Service Instance Name forms an integral part of the QNAME submitted for DNS resolution, and a system-wide resolver that is IDNA2008-aware is likely to interpret labels with UTF-8 in the QNAME as candidates for IDNA2008 processing. Operators of Internationalized Domain Names will frequently publish them in the DNS as A-labels. Therefore, these labels will need to be subject to the profile. DNS-SD implementations ought to identify the <Domain> portion of the Service Instance Name and treat it subject to IDNA2008 in case the domain is to be queried from the global DNS. In the event that the <Domain> portion of the Service Instance Name fails to resolve, it is acceptable to substitute labels with plain UTF-8, starting at the lowest label in the DNS tree and working toward the root. This approach different to the rule for resolution published in [RFC6763], because it privileges IDNA2008-compatible labels over UTF-8 labels.
One might argue against this restriction on either of two grounds: [RFC6763] was published, the bulk of IDNs were lower in the tree, but now that there are internationalized labels in the root zone, it seems reasonable to use only the single lookup strategy.
The first of these is rejected because it represents a potentially significant increase in DNS lookup traffic for no value. It is possible for a DNS-SD application to identify the <Domain> portion of the Service Instance Name. The standard way to publish IDNs on the Internet uses IDNA. Therefore, additional lookups should not be encouraged. When
The second reason depends on the idea that it is possible to maintain two names in sync with one another. This is not strictly speaking true, although in this case the domain operator could simply create a DNAME record [RFC6672] from the UTF-8 name to the IDNA2008 zone. This still, however, relies on being able to reach the (UTF-8) name in question, and it is unlikely that the UTF-8 version of the zone will be delegated from anywhere. Moreover, in many organizations the support for DNS-SD and the support for domain name delegations are not performed by the same department, and depending on a co-ordination between the two will make the system more fragile or slower or both.
The author gratefully acknowledges the insights of Stuart Cheshire, Kerry Lynn, and Dave Thaler.
This memo makes no requests of IANA.
This memo presents some requirements for future development, but does not specify anything. Therefore, it has no implications for security.
1st WG version
Add text to make clear that fallback from A-label lookup to UTF-8 label lookup ok, per WG comments at IETF 91
Initial version.