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} 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. For that purpose, a special domain, usually
  17 named \code{.local}, is used.
  18
  19 Software that supports mDNS listens on the reserved
  20 link-local multicast address \code{224.0.0.251} (for IPv4 queries) or
  21 \code{FF02::FB} (for IPv6 queries) on UDP port 5353 for incoming queries.
  22 Queries sent to those multicast address and port are standard DNS queries.
  23 If a host receives a query and knows about the queried resource, it responds to the
  24 querying host with a standard DNS response. The querying host can then simply
  25 finish and use the result, or wait until other hosts respond to its query. The
  26 latter is typically the case when a record can have multiple values, as it is
  27 the case with \code{SRV} and \code{PTR} records.
  28
  29 Another feature of Multicast DNS is the reduction of traffic through
  30 \term{Known-Answer Suppression}. It allows a querying host to specify already
  31 known resources in its query when querying resources that could exist on more
  32 than one host (e.~g., SRV records). The hosts matching those resources then do
  33 not generate a response.
  34
  35 Finally, hosts may also send unsolicited responses. This can be used to notify
  36 the network of new services available on a host.
  37
  38 \pages{1}
  39
  40 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
  41
  42 As another recent extension for the Domain Name System, \term{DNS-Based Service
  43 Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
  44 \code{SRV}~\cite{rfc2782} and \code{PTR} in a way that allows hosts to browse
  45 for services in a domain. As an example, Figure~\ref{fig:dnssd} shows the
  46 process of browsing for all XMPP clients in the domain \code{example.org}.
  47 This is a two-step process, consisting of \term{Service Instance Enumeration}
  48 and \term{Service Instance Resolution}.
  49
  50 \begin{figure}[top]
  51   \centering
  52   \includegraphics[width=0.9\textwidth]{fig-dnssd-mock.jpg}
  53   \caption{DNS-SD: Service Instance Enumeration and Resolution
  54     \todo[XMPP is a bad example here, use IPP]}
  55   \label{fig:dnssd}
  56 \end{figure}
  57
  58 \paragraph{Service Instance Enumeration} At first, to enumerate the available
  59 services in a domain for a given protocol, a DNS-SD-enabled client queries
  60 resources of type \code{PTR} of the form \code{\_service.\_proto.domain}. The
  61 result of this query is then a list of \term{instance names} of the form
  62 \code{name.\_service.\_proto.domain} which provide the specified service. For
  63 example, in Figure~\ref{fig:dnssd}, by querying for
  64 \code{\_ipp.\_tcp.\_example.org}, all printers supporting the IPP protocol in the
  65 domain \code{example.org} are enumerated, and as a result, the instance names of
  66 three hosts are returned.
  67
  68 \paragraph{Service Instance Resolution} As a second step, the returned instance
  69 names are resolved as \code{SRV} records to retrieve the actual host names and
  70 port numbers of a service. In the example, resolution of one instance name shows
  71 that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
  72 Additionally, an optional \code{TXT} record with the same instance name can
  73 contain further information about the service (e.~g., information about the
  74 supported paper sizes).
  75
  76 Through the usage of \code{SRV} records, it is easily possible for a service to
  77 inform clients about non-standard port numbers, and especially in connection
  78 with Multicast DNS makes it easy to deploy decentralized systems for the
  79 Internet of Things~\cite{Klauck:2012:BCC:2352852.2352881}.
  80
  81 \pages{1}
  82 % vim: set ft=tex et ts=2 sw=2 :