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Versions: 00

Network Working Group                                        M. Stenberg
Internet-Draft
Intended status: Standards Track                           June 26, 2013
Expires: December 28, 2013


Hybrid Unicast/Multicast DNS-Based Service Discovery Auto-Configuration
                              Using OSPFv3
          draft-stenberg-homenet-dnssdext-hybrid-proxy-ospf-00

Abstract

   This document describes how a proxy functioning between Unicast DNS-
   Based Service Discovery and Multicast DNS can be automatically
   configured using automatically configured routing protocol or some
   other network-level state sharing mechanism.  Zero-configuration
   OSPFv3 is used to describe one concrete way to implement this scheme.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 28, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.












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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements language . . . . . . . . . . . . . . . . . . . .   3
   3.  Hybrid proxy - what to configure  . . . . . . . . . . . . . .   3
     3.1.  Conflict resolution with OSPFv3 . . . . . . . . . . . . .   4
     3.2.  Per-link DNS-SD forward zone names  . . . . . . . . . . .   4
     3.3.  Reasonable defaults . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Network-wide unique link name (scheme 1)  . . . . . .   5
       3.3.2.  Router name (scheme 2)  . . . . . . . . . . . . . . .   5
       3.3.3.  Link name (scheme 2)  . . . . . . . . . . . . . . . .   5
   4.  OSPFv3 auto-configuration TLVs  . . . . . . . . . . . . . . .   6
     4.1.  DNS Delegated Zone TLV  . . . . . . . . . . . . . . . . .   6
     4.2.  Domain Name TLV . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Router Name TLV . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  DNS Server TLV  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Desirable router behavior . . . . . . . . . . . . . . . . . .   9
     5.1.  DNS search path . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Hybrid proxy  . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  OSPFv3 daemon . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative references  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative references  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Example configuration  . . . . . . . . . . . . . . .  12
     A.1.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  OSPFv3-DNS interaction  . . . . . . . . . . . . . . . . .  12
     A.3.  OSPFv3 state  . . . . . . . . . . . . . . . . . . . . . .  13
     A.4.  DNS zone  . . . . . . . . . . . . . . . . . . . . . . . .  14
     A.5.  Interaction with hosts  . . . . . . . . . . . . . . . . .  14
   Appendix B.  Implementation . . . . . . . . . . . . . . . . . . .  15
   Appendix C.  Why not just proxy Multicast DNS?  . . . . . . . . .  15
     C.1.  General problems  . . . . . . . . . . . . . . . . . . . .  15
     C.2.  Stateless proxying problems . . . . . . . . . . . . . . .  16
     C.3.  Stateful proxying problems  . . . . . . . . . . . . . . .  16
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17



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1.  Introduction

   Section 3 ("Hybrid Proxy Operation") of [I-D.cheshire-mdnsext-hybrid]
   describes how to translate queries from Unicast DNS-Based Service
   Discovery described in [RFC6763] to Multicast DNS described in
   [RFC6762], and how to filter the responses and translate them back to
   unicast DNS.

   This document describes what sort of configuration the participating
   DNS servers require, as well as how it can be provided using auto-
   configured OSPFv3 described in [I-D.ietf-ospf-ospfv3-autoconfig] and
   a naming scheme which does not even need to be same across the whole
   covered network.  The scheme can be used to provision both forward
   and reverse DNS zones which employ hybrid proxy for heavy lifting.

   While this document describes the data to be transferred in auto-
   configured OSPFv3 TLVs, in principle same scheme could work across
   other routing protocols, or even some non-routing protocol, as long
   as the consistent state for it is available across the whole covered
   network (by for example site-scoped multicast, or some other flooding
   scheme).

   We go through the mandatory specification of the language used in
   Section 2, then describe what needs to be configured in hybrid
   proxies and participating DNS servers across the network in
   Section 3.  How the data is exchanged in OSPFv3 is described in
   Section 4.  Finally, some overall notes on desired behavior of
   different router components is mentioned in Section 5.

2.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
   "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [RFC2119].

3.  Hybrid proxy - what to configure

   Beyond the low-level translation mechanism between unicast and
   multicast service discovery, the hybrid proxy draft
   [I-D.cheshire-mdnsext-hybrid] describes just that there have to be NS
   records pointing to hybrid proxy responsible for each link within the
   covered network.

   The links to be covered is also non-trivial choice; we can use the
   border discovery functionality (if available) to determine internal
   and external links.  Or we can use OSPFv3 presence (or lack of it) on
   a link to determine internal links within the covered network, and
   some other signs (depending on the deployment) such as DHCPv6 Prefix



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   Delegation (as described in [RFC3633] to determine external links
   that should not be covered.

   For each covered link we want forward DNS zone delegation to an
   appropriate router which is connected to a link, and running hybrid
   proxy.  We also want to populate reverse DNS zone similarly per each
   prefix in use.  Links' forward DNS zone names should be unique.

   There should be DNS-SD domain search list provided for the network's
   domain which contains domain for each physical link only once,
   regardless of how many routers and hybrid proxy implementations are
   connected to it.

   Yet another case to consider is the list of DNS-SD domains that we
   want hosts to enumerate for domain lists.  Typically, it contains
   only that the network's domain, but there may be also other networks
   we may want to pretend to be local but are in different scope, or
   controlled by different organization.  For example, a home user might
   see both home domain's services (TBD-TLD), as well as ISP's services
   under isp.example.com.

3.1.  Conflict resolution with OSPFv3

   Any naming-related choice on a router may have conflicts in the
   network.

   We use similar conflict resolution scheme as described in the prefix
   assignment draft[I-D.arkko-homenet-prefix-assignment].  That is, if a
   conflict is encountered, the router with highest router ID MUST keep
   the name they have chosen.  The one(s) with lower router ID MUST
   either try different one (that is not in use at all according to the
   current link state information), or choose not to publish the name
   altogether.

   If router needs to pick a different name, any algorithm works,
   although simple algorithm choice is just like the one described in
   Multicast DNS[RFC6762]: append -2, -3, and so forth, until there are
   no conflicts in the network for the given name.

3.2.  Per-link DNS-SD forward zone names

   How to name the links of a whole network in automated fashion?  Two
   different approaches seem obvious:

   1.  Unique link name based - (unique-link).(domain).

   2.  Router and link name - (link).(router).(domain).




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   The first choice is appealing as it can be much more friendly
   (especially given manual configuration).  For example, it could mean
   just lan.example.com and wlan.example.com for a simple home network.
   The second choice, on the other hand, has a nice property of being
   local choice as long as router name can be made unique.

   The type of naming scheme to use can be left as implementation
   option.  And the actual names themselves SHOULD be also overridable,
   if the end-user wants to customize them in some way.

3.3.  Reasonable defaults

   Note that any manual configuration, which SHOULD be possible, MUST
   override the defaults provided here or chosen by the creator of the
   implementation.

3.3.1.  Network-wide unique link name (scheme 1)

   It is not obvious how to produce network-wide unique link names for
   the (unique-link).(domain) scheme.  One option would be to base it on
   type of physical network layer, and then hope that the number of the
   networks won't be significant enough to confuse (e.g. "lan", or
   "wlan").

   In general network-wide unique link names should be only used in
   small networks.  Given larger network, after conflict resolution,
   finding which network is 'lan-42.example.com' may be challenging.

3.3.2.  Router name (scheme 2)

   Recommendation is to use some short form which indicates the type of
   router it is, for example, "openwrt.example.com".  As the name is
   visible to users, it should be kept as short as possible.  If theory
   even more exact model could be helpful, for example, "openwrt-
   buffalo-wzr-600-dhr.example.com".  In practise, though, providing
   some other records indicating exact router information (and access to
   management UI) might be more sensible.

   If scheme 2 is used, and there is no desire to implement conflict
   resolution related TLV described in Section 4.3, a safe default might
   be to default to router ID; that is, use as router name value such as
   r-(router ID as single 32-bit number).  It is guaranteed to be unique
   across the network, but not as user-friendly as the descriptive
   router name promoted here.

3.3.3.  Link name (scheme 2)





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   Recommendation for (link) portion of (link).(router).(domain) is to
   use either physical network layer type as base, possibly even just
   interface name on the router, if it's descriptive enough, for
   example, eth0.router1.example.com and wlan0.router1.example.com may
   be good enough.

4.  OSPFv3 auto-configuration TLVs

   To implement this specification fully, support for following three
   different new OSPFv3 auto-configuration TLVs is needed.  However,
   only the DNS Delegated Zone TLVs MUST be supported, and the other two
   SHOULD be supported.

4.1.  DNS Delegated Zone TLV

   This TLV is effectively a combined NS and A/AAAA record for a zone.
   It MUST be supported by implementations conforming to this
   specification.  Implementations SHOULD provide forward zone per link
   (or optimizing a bit, zone per link with Multicast DNS traffic).
   Implementations MAY provide reverse zone per prefix using this same
   mechanism.  If multiple routers advertise same reverse zone, it
   should be assumed that they all have access to the link with that
   prefix.  However, as noted in Section 5.3, mainly only the router
   with highest router ID on the link should publish this TLV.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      TBD-BY-IANA-1            |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           Address                             |
   |                          (16 bytes)                           |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Reserved   |S|B| Zone (DNS label sequence - variable length)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         DNS Delegated Zone TLV


   Address  field is IPv6 address (e.g. 2001:db8::3) or IPv4 address
      mapped to IPv6 address (e.g. ::FFFF:192.0.2.1) where the
      authoritative DNS server for Zone can be found.  If the address
      field is all zeros, the Zone is under global DNS hierarchy and can
      be found using normal recursive name lookup starting at the
      authoritative root servers (This is mostly relevant with the S bit
      below).




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   S  indicates that this delegated zone consists of a full DNS-SD
      domain, which should be used as base for DNS-SD domain enumeration
      (that is, (field)._dns-sd._udp.(zone) exists).  Forward zones MAY
      have this set.  Reverse zones MUST NOT have this set.  This can be
      used to provision DNS search path to hosts for non-local services
      (such as those provided by ISP, or other manually configured
      service providers).

   B  indicates that this delegated zone should be included in network's
      DNS-SD list of domains recommended for browsing at b._dns-
      sd._udp.(domain).  Local forward zones SHOULD have this set.
      Reverse zones SHOULD NOT have this set.

   Zone  is the label sequence of the zone, encoded according to section
      3.1.  ("Name space definitions") of [RFC1035].  Note that name
      compression is not required here (and would not have any point in
      any case), as we encode the zones one by one.  The zone MUST end
      with empty label.

4.2.  Domain Name TLV

   This TLV is used to indicate the base (domain) to be used for the
   network.  If multiple routers advertise different ones, the conflict
   resolution rules in Section 3.1 should result in only the one with
   highest router ID advertising one, eventually.  In case of such
   conflict, user SHOULD be notified somehow about this, if possible,
   using the configuration interface or some other notification
   mechanism for the routers.

   This TLV SHOULD be supported if at all possible.  It may be derived
   using some future DHCPv6 option, or be set by manual configuration.
   Even on routers without manual configuration options, being able to
   read the domain name provided by a different router could make the
   user experience better due to consistent naming of zones across the
   network.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      TBD-BY-IANA-2            |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Domain (DNS label sequence - variable length)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            Domain Name TLV


   Like the Zone field in Section 4.1, the Domain Name TLV's contents
   are encoded as label sequence.



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   By default, if no router advertises domain name TLV, hard-coded
   default (TBD) should be used.

4.3.  Router Name TLV

   This TLV is used to advertise a router's name.  After the conflict
   resolution procedure described in Section 3.1 finishes, there should
   be exactly zero to one routers publishing each router name.

   This TLV SHOULD be supported if at all possible.  If not supported,
   and another router chooses to use the (link).(router) naming scheme
   with this router's name, the contents of the network's domain may
   look misleading (but due to conflict resolution of per-link zones,
   still functional).

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      TBD-BY-IANA-3            |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Name (not even null terminated - variable length)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            Router Name TLV


   If the router name has been configured manually, and there is a
   conflict, user SHOULD be notified somehow about this, if possible,
   using the configuration interface or some other notification
   mechanism for the routers.

4.4.  DNS Server TLV

   This TLV is used to announce address of a fallback recursive DNS
   server (provided by e.g. ISP).  If the DNS server implementations
   used in the network are not full recursive resolver implementations,
   they may respond to network-specific queries within network, and
   forward the rest to the provided DNS servers.  Even recursive
   resolver implementations may want to use these servers, if available,
   for better caching and therefore more responsive user experience.

   Typically, these addresses are gleaned from (for example) a DHCPv4/
   DHCPv6 exchange, or from Router Advertisements.

   Any router on the home network can publish 0-N of these TLVs, and the
   order in which they are used is not defined (we assume that the DNS
   view of the world is consistent; this may not be true in all cases).





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   This TLV SHOULD be supported by routers, but the routers (and DNS
   servers in the network) MUST be able to cope even in the absence of
   the TLV.  This can be handled by (for example) DNS servers providing
   recursive resolving fallback functionality, or defaulting to some
   known global recursive resolver.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      TBD-BY-IANA-4            |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                           Address                             |
   |                          (16 bytes)                           |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            DNS Server TLV


   The address may be again either IPv4 or IPv6 address, with the IPv4
   address encoded under the ::FFFF:/96 prefix.

   It is important to note that if the network's domain forward or
   reverse resolution will not work globally, using network-external DNS
   server directly is not good.  Therefore the network's local DNS
   servers should be announced to hosts in e.g. DHCPv4/DHCPv6/RA, and
   then only those DNS servers can use the contents of this TLV as fall-
   back for non-local resolution if so desired.  How these local DNS
   server addresses are propagated within home network is outside the
   scope of this document

5.  Desirable router behavior

5.1.  DNS search path

   The routers following this specification SHOULD provide the used
   (domain) as one item in the search path to it's hosts, so that DNS-SD
   browsing will work correctly.  They also SHOULD include any DNS
   Delegated Zone TLVs' zones, that have S bit set.

5.2.  Hybrid proxy










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   The hybrid proxy implementation SHOULD support both forward zones,
   and IPv4 and IPv6 reverse zones.  It SHOULD also detect whether or
   not there are any Multicast DNS entities on a link, and make that
   information available to OSPFv3 daemon.  This can be done by (for
   example) passively monitoring traffic on all covered links, and doing
   infrequent service enumerations on links that seem to be up, but
   without any Multicast DNS traffic (if so desired).

   Hybrid proxy SHOULD also publish it's own name via Multicast DNS
   (both forward A/AAAA records, as well as reverse PTR records) to
   facilitate applications that trace network topology.

5.3.  OSPFv3 daemon

   OSPFv3 daemon should avoid publishing TLVs about links that have no
   Multicast DNS traffic to keep the DNS-SD browse domain list as
   concise as possible.  It also SHOULD NOT publish delegated zones for
   links that it does not have highest router ID that supports this
   specification.  (Support for this specification can be deduced by
   e.g. presence of any TLVs from this draft advertised by a router.)

   OSPFv3 daemon (or other entity with access to the TLVs) SHOULD
   generate zone information for DNS implementation that will be used to
   serve the (domain) zone to hosts.  Domain Name TLV described in
   Section 4.2 should be used as base for the zone, and then all DNS
   Delegated Zones described in Section 4.1 should be used to produce
   the rest of the entries in zone (see Appendix A.4 for example
   interpretation of the TLVs in Appendix A.3.

6.  Security Considerations

   There is a trade-off between security and zero-configuration in
   general; if used routing protocol is not authenticated (and in zero-
   configuration case, it most likely is not), it is vulnerable to local
   spoofing attacks.  We assume that this scheme is used either within
   (lower layer) secured networks, or with not-quite-zero-configuration
   routing protocol set-up which has authentication.

   If some sort of dynamic inclusion of links to be covered using border
   discovery or such is used, then effectively service discovery will
   share fate with border discovery (and also security issues if any).

7.  IANA Considerations

   This document makes two allocations out of the OSPFv3 Auto-
   Configuration (AC) LSA TLV namespace
   [I-D.ietf-ospf-ospfv3-autoconfig]:




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   o  The DNS Delegated Zone TLV in Section 4.1 takes the value TBD-BY-
      IANA-1 (suggested value is 4).

   o  The Domain Name TLV in Section 4.2 takes the value TBD-BY-IANA-2
      (suggested value is 5).

   o  The Router Name TLV in Section 4.3 takes the value TBD-BY-IANA-3
      (suggested value is 6).

   o  The DNS Server TLV in Section 4.4 takes the value TBD-BY-IANA-4
      (suggested value is 7).

8.  References

8.1.  Normative references

   [I-D.cheshire-mdnsext-hybrid]
              Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
              Discovery", draft-cheshire-mdnsext-hybrid-01 (work in
              progress), January 2013.

   [I-D.ietf-ospf-ospfv3-autoconfig]
              Lindem, A. and J. Arkko, "OSPFv3 Auto-Configuration",
              draft-ietf-ospf-ospfv3-autoconfig-02 (work in progress),
              April 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              February 2013.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

8.2.  Informative references

   [I-D.arkko-homenet-prefix-assignment]
              Arkko, J. and A. Lindem, "Prefix Assignment in a Home
              Network", draft-arkko-homenet-prefix-assignment-01 (work
              in progress), October 2011.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.



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   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

Appendix A.  Example configuration

A.1.  Topology

   Let's assume home network that looks like this:

          |[0]
       +-----+
       | CER |
       +-----+
    [1]/    \[2]
      /      \
   +-----+ +-----+
   | IR1 |-| IR2 |
   +-----+ +-----+
    |[3]|   |[4]|


   We're not really interested about links [0], [1] and [2], or the
   links between IRs.  Given the optimization described in Section 4.1,
   they should not produce anything to OSPF state (and therefore to DNS
   either) as there isn't any Multicast DNS traffic there.

   The user-visible set of links are [3] and [4]; each consisting of a
   LAN and WLAN link.  We assume that ISP provides 2001:db8::/48 prefix
   to be delegated in the home via [0].

A.2.  OSPFv3-DNS interaction

   Given implementation that chooses to use the second naming scheme
   (link).(router).(domain), and no configuration whatsoever, here's
   what happens (the steps are interleaved in practise but illustrated
   here in order):

   1.  OSPFv3 auto-configuration takes place, routers get their router
       IDs.  For ease of illustration, CER winds up with 2, IR1 with 3,
       and IR2 with 1.

   2.  Prefix delegation takes place.  IR1 winds up with 2001:db8:1:1::/
       64 for LAN and 2001:db8:1:2::/64 for WLAN.  IR2 winds up with
       2001:db8:2:1::/64 for LAN and 2001:db8:2:2::/64 for WLAN.

   3.  IR1 is assumed to be reachable at 2001:db8:1:1::1 and IR2 at
       2001:db8:2:1::1.



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   4.  Each router wants to be called 'router' due to lack of branding
       in drafts.  They announce that using the router name TLV defined
       in Section 4.3.  They also advertise their local zones, but as
       that information may change, it's omitted here.

   5.  Conflict resolution ensues.  As IR1 has highest router ID, it
       becomes "router".  CER and IR2 have to rename, and (depending on
       timing) one of them becomes "router-2" and other one "router-3".
       Let us assume IR2 is "router-2".  During conflict resolution,
       each router publishes TLVs for it's own set of delegated zones.

   6.  CER learns ISP-provided domain "isp.example.com" using DHCPv6
       domain list option defined in [RFC3646].  The information is
       passed along as S-bit enabled delegated zone TLV.

A.3.  OSPFv3 state

   Once there is no longer any conflict in the system, we wind up with
   following TLVs within OSPFv3 (RN is used as abbreviation for Router
   Name, and DZ for Delegated Zone TLVs):

   (from CER)
   DZ {s=1,zone="isp.example.com"}

   (from IR1)
   RN {name="router"}

   DZ {address=2001:db8:1:1::1, b=1,
       zone="lan.router.example.com."}
   DZ {address=2001:db8:1:1::1,
       zone="1.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa."}

   DZ {address=2001:db8:1:1::1, b=1,
       zone="wlan.router.example.com."}
   DZ {address=2001:db8:1:1::1,
       zone="2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa."}

   (from IR2)
   RN {name="router-2"}

   DZ {address=2001:db8:2:1::1, b=1,
       zone="lan.router-2.example.com."}
   DZ {address=2001:db8:2:1::1,
       zone="1.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa."}

   DZ {address=2001:db8:2:1::1, b=1,
       zone="wlan.router-2.example.com."}
   DZ {address=2001:db8:2:1::1,



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       zone="2.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa."}



A.4.  DNS zone

   In the end, we should wind up with following zone for (domain) which
   is example.com in this case, available at all routers, just based on
   dumping the delegated zone TLVs as NS+AAAA records, and optionally
   domain list browse entry for DNS-SD:

   b._dns_sd._udp PTR lan.router
   b._dns_sd._udp PTR wlan.router

   b._dns_sd._udp PTR lan.router-2
   b._dns_sd._udp PTR wlan.router-2

   router AAAA 2001:db8:1:1::1
   router-2 AAAA 2001:db8:2:1::1

   router NS router
   router-2 NS router-2

   1.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. NS router.example.com.
   2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. NS router.example.com.
   1.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. NS router-2.example.com.
   2.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. NS router-2.example.com.


   Internally, the router may interpret the TLVs as it chooses to, as
   long as externally defined behavior follows semantics of what's given
   in the above.

A.5.  Interaction with hosts

   So, what do the hosts receive from the routers?  Using e.g. DHCPv6
   DNS options defined in [RFC3646], DNS server address should be one
   (or multiple) that point at DNS server that has the zone information
   described in Appendix A.4.  Domain list provided to hosts should
   contain both "example.com" (the hybrid-enabled domain), as well as
   the externally learned domain "isp.example.com".

   When hosts start using DNS-SD, they should check both b._dns-
   sd._udp.example.com, as well as b._dns-sd._udp.isp.example.com for
   list of concrete domains to browse, and as a result services from two
   different domains will seem to be available.





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Appendix B.  Implementation

   There is an prototype implementation of this draft (and transitively
   also of [I-D.cheshire-mdnsext-hybrid]) at hnet-core github repository
   [1] which contains variety of other homenet WG-related things'
   implementation too.

   hp.lua binary can be used to start hybrid proxy either as one-router
   stand-alone implementation (that can be used to e.g. use statically
   configured DNS zones), or as part of zeroconf OSPFv3 configured set
   of proxies.

   Sample usage case:

   # sudo lua hp.lua eth0 eth1
   .. no output ..


   Given the command, hybrid proxy is started for interfaces eth0 and
   eth1, and it will publish DNS zones l-eth0.r-router.home,
   l-eth1.r-router.home (and home zone with relevant DNS-SD sub-zone,
   and default forward behavior) in DNS port.  It has -h option for
   seeing various options that can be set, notable one being --ospf (use
   OSPFv3 autoconfigured hnet infrastructure).

   Disclaimer: The set-up of third-party libraries etc to get the
   implementation running may be painful and is omitted here.  Use of
   ready UML NetKit-based test environment or building image for a real
   router using hnet github repository [2] is recommended.

Appendix C.  Why not just proxy Multicast DNS?

   Over the time number of people have asked me about how, why, and if
   we should proxy (originally) link-local Multicast DNS over multiple
   links.

   At some point I meant to write a draft about this, but I think I'm
   too lazy; so some notes left here for general amusement of people
   (and to be removed if this ever moves beyond discussion piece).

C.1.  General problems

   There are two main reasons why Multicast DNS is not proxyable in the
   general case.

   First reason is the conflict resolution depends on ordering which
   depends on the RRsets staying constant.  That is not possible across
   multiple links (due to e.g. link-local addresses having to be



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   filtered).  Therefore, conflict resolution breaks, or at least
   requires ugly hacks to work around.

   A workaround for this is to make sure that in conflict resolution,
   propagated resources always loses.  Due to conflict handling ordering
   logic, and the arbitrary order in which the original records may be
   in, this is non-trivial.

   Second reason is timing, which is relatively tight in the conflict
   resolution phase, especially given lossy and/or high latency
   networks.

C.2.  Stateless proxying problems

   In general, typical stateless proxy has to involve flooding, as
   Multicast DNS assumes that most messages are received by every host.
   And it won't scale very well, as a result.

   The conflict resolution is also harder without state.  It may result
   in Multicast DNS responder being in constant probe-announce loop,
   when it receives altered records, notes that it's the one that should
   own the record.  Given stateful proxying, this would be just a
   transient problem but designing stateless proxy that won't cause this
   is non-trivial exercise.

C.3.  Stateful proxying problems

   One option is to write proxy that learns state from one link, and
   propagates it in some way to other links in the network.

   A big problem with this case lies in the fact that due to conflict
   resolution concerns above, it is easy to accidentally send packets
   that will (possibly due to host mobility) wind up at the originator
   of the service, who will then perform renaming.  That can be
   alleviated, though, given clever hacks with conflict resolution
   order.

   The stateful proxying may be also too slow to occur within the
   timeframe allocated for announcing, leading to excessive later
   renamings based on delayed finding of duplicate services with same
   name

   A work-around exists for this though; if the game doesn't work for
   you, don't play it.  One option would be simply not to propagate ANY
   records for which conflict has seen even once.  This would work, but
   result in rather fragile, lossy service discovery infrastructure.





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   There are some other small nits too; for example, Passive Observation
   Of Failure (POOF) will not work given stateful proxying.  Therefore,
   it leads to requiring somewhat shorter TTLs, perhaps.

Appendix D.  Acknowledgements

   Thanks to Stuart Cheshire for the original hybrid proxy draft and
   interesting discussion in Orlando, where I was finally convinced that
   stateful Multicast DNS proxying is a bad idea.

   Also thanks to Mark Baugher, Ole Troan and Shwetha Bhandari for
   review comments.

Author's Address

   Markus Stenberg
   Helsinki  00930
   Finland

   Email: markus.stenberg@iki.fi































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