sec-dns-extensions.tex

   1 \subsection{Extensions to the Domain Name System}\label{sec:dns}
   2
   3 In a distributed context, it is often not feasible to rely on a central,
   4 authoritative DNS server, and there is usually no easy way to discover services.
   5 The first problem is addressed with \term{Multicast DNS}, and since DNS is
   6 basically a key-value store, it can also be used for service discovery, which is
   7 achieved using \term{DNS-Based Service Discovery}. Both techniques were first
   8 developed by Apple as part of the \term{Bonjour}
   9 project\footnote{\url{https://developer.apple.com/bonjour/}}, and are now
  10 maintained by the IETF Zeroconf working
  11 group\footnote{\url{http://zeroconf.org}}.
  12
  13 \subsubsection{Multicast DNS}
  14
  15 \term{Multicast DNS} (mDNS)~\cite{rfc6762} describes an extension to the Domain
  16 Name System that allows DNS resource records to be distributed on multiple hosts
  17 in a network, therefore avoiding central authorities and enabling every host to
  18 publish its own entries. For that purpose, a special domain, usually
  19 named \code{.local}, is used.
  20
  21 Software that supports mDNS listens on the reserved
  22 link-local multicast address \code{224.0.0.251} (for IPv4 queries) or
  23 \code{FF02::FB} (for IPv6 queries) on UDP port 5353 for incoming queries.
  24 Queries sent to those multicast address and port are standard DNS queries.
  25 If a host receives a query and knows about the queried resource, it responds to the
  26 querying host with a standard DNS response. The querying host can then simply
  27 finish and use the result, or wait until other hosts respond to its query. The
  28 latter is typically the case when a record can have multiple values, as it is
  29 the case with \code{SRV} and \code{PTR} records (which will be discussed in the
  30 next section).
  31
  32 Another feature of Multicast DNS is the reduction of traffic through
  33 \term{Known-Answer Suppression}. It allows a querying host to specify already
  34 known resources in its query when querying resources that could exist on more
  35 than one host (e.~g., SRV records). The hosts matching those resources then do
  36 not generate a response, thus reducing the messages in the network and saving
  37 bandwidth, which is usually a scarce resource in wireless networks.
  38
  39 \enlargethispage{2\baselineskip}
  40 Finally, hosts may also send unsolicited responses. This can be used to notify
  41 the network of new services available on a host.
  42
  43 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
  44
  45 As another recent extension for the Domain Name System, \term{DNS-Based Service
  46 Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
  47 SRV~\cite{rfc2782} and PTR~\cite{rfc1035} in a way that allows hosts to browse
  48 for services in a domain. While SRV records specify the location of services on
  49 a host, PTR records hold a reverse mapping from IP address to host name.
  50 DNS-SD now relies on a two-step process, consisting of
  51 \term{Service Instance Enumeration} and \term{Service Instance Resolution}.
  52
  53 \paragraph{1. Service Instance Enumeration} At first, to enumerate the available
  54 services in a domain for a given protocol, a DNS-SD-enabled client queries
  55 PTR resources of the form \code{\_service.\_proto.domain}. The result of
  56 this query is then a list of \term{instance names} of the form
  57 \code{name.\_service.\_proto.domain} which point to the hosts providing the
  58 service. For example, by querying for \code{\_ipp.\_tcp.\_example.org}, the
  59 instance names for all printers supporting the IPP protocol in the domain
  60 \code{example.org} are returned.
  61
  62 \paragraph{2. Service Instance Resolution} As a second step, the returned instance
  63 names are resolved as SRV records to retrieve the actual host names and
  64 port numbers of a service. For example, resolution of one instance name shows
  65 that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
  66 Additionally, an optional TXT record with the same instance name can
  67 contain further information about the service (e.~g. information about the
  68 supported paper sizes).
  69
  70 Through the usage of SRV records, it is easily possible for a service to
  71 inform clients about non-standard port numbers, and especially in connection
  72 with Multicast DNS, this makes it easy to deploy decentralized systems for the
  73 Internet of Things. \cite{Klauck:2012:BCC:2352852.2352881}
  74
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