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, thus reducing the messages in the network and saving
  34 bandwidth, which is usually a scarce resource in wireless networks.
  35
  36 Finally, hosts may also send unsolicited responses. This can be used to notify
  37 the network of new services available on a host.
  38
  39 \pages{1}
  40
  41 \subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
  42
  43 As another recent extension for the Domain Name System, \term{DNS-Based Service
  44 Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
  45 \code{SRV}~\cite{rfc2782} and \code{PTR} in a way that allows hosts to browse
  46 for services in a domain. As an example, Figure~\ref{fig:dnssd} shows the
  47 process of browsing for all XMPP clients in the domain \code{example.org}.
  48 This is a two-step process, consisting of \term{Service Instance Enumeration}
  49 and \term{Service Instance Resolution}.
  50
  51 \todo{XMPP is a probably not the best example here, use IPP instead}
  52 \begin{figure}[top]
  53   \centering
  54   \includegraphics[width=0.9\textwidth]{fig-dnssd-mock.jpg}
  55   \caption{DNS-SD: Service Instance Enumeration and Resolution}
  56   \label{fig:dnssd}
  57 \end{figure}
  58
  59 \paragraph{Service Instance Enumeration} At first, to enumerate the available
  60 services in a domain for a given protocol, a DNS-SD-enabled client queries
  61 resources of type \code{PTR} of the form \code{\_service.\_proto.domain}. The
  62 result of this query is then a list of \term{instance names} of the form
  63 \code{name.\_service.\_proto.domain} which provide the specified service. For
  64 example, in Figure~\ref{fig:dnssd}, by querying for
  65 \code{\_ipp.\_tcp.\_example.org}, all printers supporting the IPP protocol in the
  66 domain \code{example.org} are enumerated, and as a result, the instance names of
  67 three hosts are returned.
  68
  69 \paragraph{Service Instance Resolution} As a second step, the returned instance
  70 names are resolved as \code{SRV} records to retrieve the actual host names and
  71 port numbers of a service. In the example, resolution of one instance name shows
  72 that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
  73 Additionally, an optional \code{TXT} record with the same instance name can
  74 contain further information about the service (e.~g., information about the
  75 supported paper sizes).
  76
  77 Through the usage of \code{SRV} records, it is easily possible for a service to
  78 inform clients about non-standard port numbers, and especially in connection
  79 with Multicast DNS makes it easy to deploy decentralized systems for the
  80 Internet of Things~\cite{Klauck:2012:BCC:2352852.2352881}.
  81
  82 \pages{1}
  83 % vim: set ft=tex et ts=2 sw=2 :