-\subsection{Extensions to the DNS protocol}
+\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 of the Domain
+\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 the need of a central server.
+in a network, therefore avoiding central authorities and enabling every host to
+publish its own entries.
Software that supports resolution of mDNS records 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, and
-if a host receiving a query knows about the queried resource, it responds to the
+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 querying for \code{SRV} records to obtain a
-list of services which are present in the network.
+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
\subsubsection{DNS-Based Service Discovery}\label{sec:dnssd}
-As a recent extension for the Domain Name System, \term{DNS-Based Service
-Discovery (DNS-SD)}~\cite{rfc6763} uses DNS records of type
+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}.
\begin{figure}[htop]
\centering
\includegraphics[width=0.9\textwidth]{dnssd-mock.jpg}
- \caption{DNS-SD: Service Instance Enumeration and Resolution}
+ \caption{DNS-SD: Service Instance Enumeration and Resolution
+ \todo[XMPP is a bad example here, use IPP]}
\label{fig:dnssd}
\end{figure}
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}, all XMPP clients in the domain
-\code{example.org} are enumerated, and as a result, the instance names of three
-hosts are returned.
+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 XMPP client is running at host \code{juliet.example.org} on port 5222.
+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
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