Re: [mdnsext] Hierarchical (host) domain names in mDNS?
Kerry Lynn <kerlyn@ieee.org> Tue, 16 July 2013 14:49 UTC
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Date: Tue, 16 Jul 2013 10:49:11 -0400
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From: Kerry Lynn <kerlyn@ieee.org>
To: "Albrecht, Harald" <harald.albrecht@siemens.com>
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Subject: Re: [mdnsext] Hierarchical (host) domain names in mDNS?
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On Tue, Jul 16, 2013 at 7:32 AM, Albrecht, Harald < harald.albrecht@siemens.com> wrote: > Hello, > > something I couldn’t find a proper answer to I would like to ask the mDNS > experts on this list: is it possible to apply mDNS also to situations where > the decentralized, configuration-free operation of mDNS is required but > where the individual hosts within the same network need to make use of > hierarchical domain names? Such as “controller.machine2.local”? > > I think the answer is (or should be) "yes". First some guidance from RFC 6762: This document recommends a single flat namespace for dot-local host names, (i.e., the names of DNS "A" and "AAAA" records, which map names to IPv4 and IPv6 addresses), but other DNS record types (such as those used by DNS-Based Service Discovery [RFC6763 <http://tools.ietf.org/html/rfc6763>]) may contain as many labels as appropriate for the desired usage, up to a maximum of 255 bytes, plus a terminating zero byte at the end. Name length issues are discussed further in Appendix C <http://tools.ietf.org/html/rfc6762#appendix-C>. The meaning of "recommends" as a keyword is equivalent to SHOULD. Now why does RFC 6762 take this position? It has to do with automagic operation of an mDNS resolver: A malicious host could masquerade as "www.example.com." by answering the resulting Multicast DNS query for "www.example.com.local.". To avoid this, a host MUST NOT append the search suffix ".local.", if present, to any relative (partially qualified) host name containing two or more labels. Appending ".local." to single-label relative host names is acceptable, since the user should have no expectation that a single-label host name will resolve as is. So this places a restriction on resolvers, which is basically intended to keep the fox out of the hen house. One should still expect "controller.machine2.local." to resolve properly, if there is an authoritative responder for this host name. I assume it would be invisible to lookups in ".local.". Some months back, the main criticism of mDNS in homenet was that if I visited a friend's house then my browser might confuse my bookmarked "www.refrigerator.local." with my friend's. I mentioned at the time that most (all?) major printer vendors solve this problem by appending a collision-resistant substring to a human-readable substring to create the instance or host label, e.g., Officejet\032Pro\0328500\032A909g\032[4C0EA4]. A similar approach was proposed by Brian Carpenter, but he proposed creating "zones" in the .local. namespace, e.g., <ULA>.local. I suspect either approach can be made to work. I suggest you take a look at the Discovery section of the ZigBee Smart Energy Profile 2 (now IEEE 2030.5) specification: http://www.zigbee.org/Standards/ZigBeeSmartEnergy/ZigBeeSmartEnergy20Standard.aspx to see how your problem was solved using "fine-grained" discovery based on the former approach. Regards, -K-
- [mdnsext] Hierarchical (host) domain names in mDN… Albrecht, Harald
- Re: [mdnsext] Hierarchical (host) domain names in… Andrew Sullivan
- Re: [mdnsext] Hierarchical (host) domain names in… Kerry Lynn
- Re: [mdnsext] Hierarchical (host) domain names in… Andrew Sullivan
- Re: [mdnsext] Hierarchical (host) domain names in… Albrecht, Harald
- Re: [mdnsext] Hierarchical (host) domain names in… Erik Kline
- Re: [mdnsext] Hierarchical (host) domain names in… Albrecht, Harald
- Re: [mdnsext] Hierarchical (host) domain names in… Albrecht, Harald
- Re: [mdnsext] Hierarchical (host) domain names in… Kerry Lynn
- Re: [mdnsext] Hierarchical (host) domain names in… Albrecht, Harald