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} project, and are now maintained
   9 by the IETF Zeroconf working group.
  10
  11 \subsubsection{Multicast DNS}
  12
  13 \term{Multicast DNS}~\cite{rfc6762} (mDNS) describes an extension to the Domain
  14 Name System that allows DNS resource records to be distributed on multiple hosts
  15 in a network, therefore avoiding central authorities and enabling every host to
  16 publish its own entries.
  17
  18 Software that supports resolution of mDNS records listens on the reserved
  19 link-local multicast address \code{224.0.0.251} (for IPv4 queries) or
  20 \code{FF02::FB} (for IPv6 queries) on UDP port 5353 for incoming queries.
  21 Queries sent to those multicast address and port are standard DNS queries.
  22 If a host receives a query and knows about the queried resource, it responds to the
  23 querying host with a standard DNS response. The querying host can then simply
  24 finish and use the result, or wait until other hosts respond to its query. The
  25 latter is typically the case when a record can have multiple values, as it is
  26 the case with \code{SRV} and \code{PTR} records.
  27
  28 Another feature of Multicast DNS is the reduction of traffic through
  29 \term{Known-Answer Suppression}. It allows a querying host to specify already
  30 known resources in its query when querying resources that could exist on more
  31 than one host (e.~g., SRV records). The hosts matching those resources then do
  32 not generate a response.
  33
  34 Finally, hosts may also send unsolicited responses. This can be used to notify
  35 the network of new services available on a host.
  36
  37 \pages{1}
  38
  39 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
  40
  41 As another recent extension for the Domain Name System, \term{DNS-Based Service
  42 Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
  43 \code{SRV}~\cite{rfc2782} and \code{PTR} in a way that allows hosts to browse
  44 for services in a domain. As an example, Figure~\ref{fig:dnssd} shows the
  45 process of browsing for all XMPP clients in the domain \code{example.org}.
  46 This is a two-step process, consisting of \term{Service Instance Enumeration}
  47 and \term{Service Instance Resolution}.
  48
  49 \begin{figure}[htop]
  50   \centering
  51   \includegraphics[width=0.9\textwidth]{dnssd-mock.jpg}
  52   \caption{DNS-SD: Service Instance Enumeration and Resolution
  53     \todo[XMPP is a bad example here, use IPP]}
  54   \label{fig:dnssd}
  55 \end{figure}
  56
  57 \paragraph{Service Instance Enumeration} At first, to enumerate the available
  58 services in a domain for a given protocol, a DNS-SD-enabled client queries
  59 resources of type \code{PTR} of the form \code{\_service.\_proto.domain}. The
  60 result of this query is then a list of \term{instance names} of the form
  61 \code{name.\_service.\_proto.domain} which provide the specified service. For
  62 example, in Figure~\ref{fig:dnssd}, by querying for
  63 \code{\_ipp.\_tcp.\_example.org}, all printers supporting the IPP protocol in the
  64 domain \code{example.org} are enumerated, and as a result, the instance names of
  65 three hosts are returned.
  66
  67 \paragraph{Service Instance Resolution} As a second step, the returned instance
  68 names are resolved as \code{SRV} records to retrieve the actual host names and
  69 port numbers of a service. In the example, resolution of one instance name shows
  70 that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
  71 Additionally, an optional \code{TXT} record with the same instance name can
  72 contain further information about the service (e.~g., information about the
  73 supported paper sizes).
  74
  75 Through the usage of \code{SRV} records, it is easily possible for a service to
  76 inform clients about non-standard port numbers, and especially in connection
  77 with Multicast DNS makes it easy to deploy decentralized systems for the
  78 Internet of Things~\cite{Klauck:2012:BCC:2352852.2352881}.
  79
  80 \pages{1}
  81 % vim: set ft=tex et ts=2 sw=2 :