\subsection{Extensions to the Domain Name System}\label{sec:dns}

In a distributed context, it is often not feasible to rely on a central,
authoritative DNS server, and there is usually no easy way to discover services.
The first problem is addressed with \term{Multicast DNS}, and since DNS is
basically a key-value store, it can also be used for service discovery, which is
achieved using \term{DNS-Based Service Discovery}. Both techniques were first
developed by Apple as part of the \term{Bonjour} project, and are now maintained
by the IETF Zeroconf working group.

\subsubsection{Multicast DNS}

\term{Multicast DNS}~\cite{rfc6762} (mDNS) describes an extension to the Domain
Name System that allows DNS resource records to be distributed on multiple hosts
in a network, therefore avoiding central authorities and enabling every host to
publish its own entries. For that purpose, a special domain, usually
named \code{.local}, is used.

Software that supports mDNS listens on the reserved
link-local multicast address \code{224.0.0.251} (for IPv4 queries) or
\code{FF02::FB} (for IPv6 queries) on UDP port 5353 for incoming queries.
Queries sent to those multicast address and port are standard DNS queries.
If a host receives a query and knows about the queried resource, it responds to the
querying host with a standard DNS response. The querying host can then simply
finish and use the result, or wait until other hosts respond to its query. The
latter is typically the case when a record can have multiple values, as it is
the case with \code{SRV} and \code{PTR} records.

Another feature of Multicast DNS is the reduction of traffic through
\term{Known-Answer Suppression}. It allows a querying host to specify already
known resources in its query when querying resources that could exist on more
than one host (e.~g., SRV records). The hosts matching those resources then do
not generate a response.

Finally, hosts may also send unsolicited responses. This can be used to notify
the network of new services available on a host.

\pages{1}

\subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}

As another recent extension for the Domain Name System, \term{DNS-Based Service
Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of types
\code{SRV}~\cite{rfc2782} and \code{PTR} in a way that allows hosts to browse
for services in a domain. As an example, Figure~\ref{fig:dnssd} shows the
process of browsing for all XMPP clients in the domain \code{example.org}.
This is a two-step process, consisting of \term{Service Instance Enumeration}
and \term{Service Instance Resolution}.

\begin{figure}[top]
  \centering
  \includegraphics[width=0.9\textwidth]{fig-dnssd-mock.jpg}
  \caption{DNS-SD: Service Instance Enumeration and Resolution
    \todo[XMPP is a probably not the best example here, use IPP instead]}
  \label{fig:dnssd}
\end{figure}

\paragraph{Service Instance Enumeration} At first, to enumerate the available
services in a domain for a given protocol, a DNS-SD-enabled client queries
resources of type \code{PTR} of the form \code{\_service.\_proto.domain}. The
result of this query is then a list of \term{instance names} of the form
\code{name.\_service.\_proto.domain} which provide the specified service. For
example, in Figure~\ref{fig:dnssd}, by querying for
\code{\_ipp.\_tcp.\_example.org}, all printers supporting the IPP protocol in the
domain \code{example.org} are enumerated, and as a result, the instance names of
three hosts are returned.

\paragraph{Service Instance Resolution} As a second step, the returned instance
names are resolved as \code{SRV} records to retrieve the actual host names and
port numbers of a service. In the example, resolution of one instance name shows
that an IPP server is running at host \code{gutenberg.example.org} on port 5222.
Additionally, an optional \code{TXT} record with the same instance name can
contain further information about the service (e.~g., information about the
supported paper sizes).

Through the usage of \code{SRV} records, it is easily possible for a service to
inform clients about non-standard port numbers, and especially in connection
with Multicast DNS makes it easy to deploy decentralized systems for the
Internet of Things~\cite{Klauck:2012:BCC:2352852.2352881}.

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% vim: set ft=tex et ts=2 sw=2 :
