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\typeout{****************************************************************}
\typeout{* Linux Network Administrators' Guide, Version 1.0 }
\typeout{* Olaf Kirch, <okir@monad.swb.de> }
\typeout{*************************************************************** }
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\title{The \linux{} Network Administrators' Guide}
\author{Olaf Kirch}
\years{1992-1994}
\iftex
\newsavebox{\disclaimer}
\savebox{\disclaimer}[\textwidth]{
 \parbox{\textwidth}{
 \sf
 This is the last pre-release before the new Networking Guide
 goes public. I consider the technical chapters mostly complete.
 I have tried to catch up with the current development, but
 in some section, there have been minor changes or additions
 to the software since I wrote them. I don't think I will update
 them unless you tell me I've missed something big.

 \hspace{1em}
 If you have any comments, suggestions, or complaints, please
 mail me at \email{okir@monad.swb.de}.
 \hfill -- Olaf
 \hspace{5mm}
 }
}
\fi

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{\newpage
\clearpage
\samepage % latex2html id marker 10344
\fbox{
 
 \small
 \begin{minipage}{0.9\textwidth}
  {\Large\bf Legal Notice}

  \medskip\noindent
  UNIX is a trademark of Univel.\\ 
  \linux{} is not a trademark, and has no connection to
  UNIX\tm\ or Univel.\\ 
  \vspace*{\fill}
  
  \noindent
  Copyright \copyright \ 1994 \ Olaf Kirch\\ 
  Kattreinstr. 38, 64295 Darmstadt, Germany\\ 
  {\tt okir@monad.swb.de}
  
  \vspace{.2in}\noindent
  \title{} may be reproduced and distributed in whole or in part, subject
  to the following conditions:

  \begin{enumerate}
  \setcounter{enumi}{-1}
  \item
        The copyright notice above and this permission notice must be
        preserved complete on all complete or partial copies.
  
  \item Any translation or derivative work of \title{} must be approved
        by the author in writing before distribution.
  
  \item If you distribute \title{} in part, instructions for obtaining
        the complete version of \title{} must be included, and a means for
        obtaining a complete version provided.
  
  \item Small portions may be reproduced as illustrations for reviews
        or {\bf quotes} in other works without this permission notice if
        proper citation is given.
  
  \item If you print and distribute \title{}, you may not refer to it
  	as the ``Official Printed Version''.

  \item The GNU General Public License referenced below may be
        reproduced under the conditions given within it.
  
  \item Several sections of this document are held under separate
        copyright.  When these sections are covered by a different copyright,
        the seperate copyright is noted.  {\bf If you distribute \title{} in
        part, and that part is, in whole, covered under a seperate, noted
        copyright, the conditions of that copyright apply.}
  \end{enumerate}
  
  Exceptions to these rules may be granted for academic purposes: Write to
  Olaf Kirch at the above address, or email {\tt okir@monad.swb.de}, and
  ask.  These restrictions are here to protect us as authors, not to
  restrict you as educators and learners.

  \vspace{.2in}\noindent All source code in \title{} is placed under the
  GNU General Public License.  See appendix~\ref{appendix.gpl} for a copy
  of the GNU ``GPL.''

  The author is not liable for any damages, direct or indirect,
  resulting from the use of information provided in this
  document.
 \end{minipage}
}
}

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{\newpage
\clearpage
\samepage \begin{dispitems}% latex2html id marker 3012
\ditem[G]
        The route uses a gateway.

\ditem[U]
        The interface to be used is up.

\ditem[H]
        Only a single host can be reached through the route. For
        example, this is the case for the loopback entry
        \host{127.0.0.1}.

\ditem[D]
        This is set if the table entry has been generated
        by an ICMP redirect message (see section~\ref{tcpip.icmp}).

\ditem[M]
        This is set if the table entry was modified by an ICMP
        redirect message.
\end{dispitems}
}

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\samepage \begin{dispitems}% latex2html id marker 3521
\ditem[SOA]
\index{authoritative name server}
\index{SOA (DNS record)}
\index{DNS!zone}

This describes a zone of authority (SOA means ``Start of
Authority''). It signals that the records following the SOA
RR contain authoritative information for the domain. Every master file
included by a \keyword{primary} statement must contain an SOA
record for this zone. The resource data contains the following fields:


\begin{dispitems}
\ditem[\var{origin}]
        This is the canonical hostname of the primary name server
        for this domain. It is usually given as an absolute name.

\ditem[\var{contact}]
        This is the email address of the person responsible for
        maintaining the domain, with the `\keyword{@}' character
        replaced by a dot. For instance, if the responsible person at
        the Virtual Brewery is \user{janet}, then this field would
        contain \keyword{janet.vbrew.com}.

\ditem[\var{serial}]
        This is the version number of the zone file, expressed as a
        single decimal number. Whenever data is changed in the zone
        file, this number should be incremented.

        The serial number is used by secondary name servers to recognize
        when zone information has changed. To stay up to date, secondary
        servers request the primary server's SOA record at certain
        intervals, and compare the serial number to that of the cached
        SOA record.  If the number has changed, the secondary servers
        transfers the whole zone database from the primary server.

\ditem[\var{refresh}]
        This specifies the interval in seconds that the secondary
        servers should wait between checking the SOA record of the
        primary server. Again, this is a decimal number with at most
        eight digits.

        Generally, the network topology doesn't change too often, so
        that this number should specify an interval of roughly a day for
        larger networks, and even more for smaller ones.

\ditem[\var{retry}]
        This number determines the intervals at which a secondary
        server should retry contacting the primary server if
        a request or a zone refresh fails. It must not be too low,
        or else a temporary failure of the server or a network problem
        may cause the secondary server to waste network resources.
        One hour, or perhaps one half hour, might be a good choice.

\ditem[\var{expire}]
        This specifies the time in seconds after which the server should
        finally discard all zone data if it hasn't been able to contact
        the primary server. It should normally be very large.  Craig
        Hunt (\cite{hunt-tcpip}) recommends 42~days.

\ditem[\var{minimum}]
        This is the default ttl value for resource records that do not
        explicitly specify one. This requires other name servers to
        discard the RR after a certain amount of time.  It has however
        nothing to do with the time after which a secondary server tries
        to update the zone information.

        \var{minimum} should be a large value, especially for LANs where
        the network topology almost never changes.  A value of around a
        week or a month is probably a good choice. In the case that
        single RRs may change more frequently, you can still assign them
        different ttl's.

\end{dispitems}


\ditem[A]
\index{A (DNS record)}
\index{address!DNS resource record}

This associates an IP~address with a hostname. The resource data field
contains the address in dotted quad notation.

\index{hostname!canonical}
\index{canonical hostname}
\index{CNAME (DNS record)}
\index{hostname!aliases}
\index{alias!hostname}
For each host, there must be only one A record. The hostname used in
this A record is considered the official or \emph{canonical} hostname.
All other hostnames are aliases and must be mapped onto the canonical
hostname using a CNAME record.



\ditem[NS]

This points to a master name server of a subordinate zone. For an
explanation why one has to have NS records, see section~\ref{tcpip.dns}.
The resource data field contains the hostname of the name server. To
resolve the hostname, an additional A record is needed, the so-called
\emph{glue record} which gives the name server's IP~address.


\ditem[CNAME]
\index{CNAME (DNS record)}

This associates an alias for a host with its \emph{canonical hostname}.
The canonical hostname is the one the master file provides an A record
for; aliases are simply linked to that name by a CNAME record, but don't
have any other records of their own.


\ditem[PTR]
\index{PTR (DNS record)}

This type of record is used to associate names in the \host{in-addr.arpa}
domain with hostnames. This is used for reverse mapping of IP~addresses
to hostnames. The hostname given must be the canonical hostname.


\ditem[MX]
\index{MX (DNS record)}

This RR announces a \emph{mail exchanger} for a domain. The reasons to
have mail exchangers are discussed in section~\ref{mail.routing.internet}
in chapter~\ref{mail}.  The syntax of an MX record is


\begin{screen}\st
[\var{domain}] [\var{ttl}] [\var{class}] MX \var{preference} \var{host}
\end{screen}


\var{host} names the mail exchanger for \var{domain}. Every
mail exchanger has an integer \var{preference} associated with it.
A mail transport agent who desires to deliver mail to \var{domain}
will try all hosts who have an MX record for this domain
until it succeeds. The one with the lowest preference value is
tried first, then the others in order of increasing preference
value.




\ditem[HINFO]
This record provides information on the system's hardware
and software. Its syntax is


\begin{screen}\st
[\var{domain}] [\var{ttl}] [\var{class}] HINFO \var{hardware software}
\end{screen}


The \var{hardware} field identifies the hardware used by this host.
There are special conventions to specify this. A list of valid names is
given in the ``Assigned Numbers'' (RFC~1340). If the field contains any
blanks, it must be enclosed in double quotes.  The \var{software} field
names the operating system software used by the system. Again, a valid
name from the ``Assigned Numbers'' RFC should be chosen.
\end{dispitems}
}

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\ditem[\var{service}]
        \index{services@\file{services}}
        gives the service name. The service name has to be translated to
        a port number by looking it up in the \file{/etc/services} file.
        This file will be described in section \ref{appl.services}
        below.

\ditem[\var{type}]
        specifies a socket type, either \keyword{stream} (for
        connection-oriented protocols) or \keyword{dgram} (for datagram
        protocols).  TCP-based services should therefore always use
        \keyword{stream}, while UDP-based services should always use
        \keyword{dgram}.

\ditem[\var{protocol}]
        \index{protocols@\file{protocols}}
        names the transport protocol used by the service. This must be a
        valid protocol name found in the \file{protocols} file, also
        explained below.

\ditem[\var{wait}]
        This option applies only to \keyword{dgram} sockets. It may
        be either \keyword{wait} or \keyword{nowait}. If \keyword{wait} is
        specified, \prog{inetd} will only execute one server for
        the specified port at any time. Otherwise, it will immediately
        continue to listen on the port after executing the server.

        This is useful for ``single-threaded'' servers that read all
        incoming datagrams until no more arrive, and then exit. Most RPC
        servers are of this type and should therefore specify
        \keyword{wait}.  The opposite type, ``multi-threaded'' servers,
        allow an unlimited number of instances to run concurrently; this
        is only rarely used. These servers should specify
        \keyword{nowait}.

        \keyword{stream} sockets should always use \keyword{nowait}.

\ditem[\var{user}]
        This is the login id of the user the process is executed under.
        This will frequently be the \user{root} user, but some services
        may use different accounts. It is a very good idea to apply the
        principle of least privilege here, which states that you shouldn't
        run a command under a privileged account if the program doesn't
        require this for proper functioning.
        For example, the NNTP news server will run as \user{news}, while
        services that may pose a security risk (such as \prog{tftp} or
        \prog{finger}) are often run as \user{nobody}.

\ditem[\var{server}]
        gives the full path name of the server program to be executed.
        Internal services are marked by the keyword \keyword{internal}.

\ditem[\var{cmdline}]
        This is the command line to be passed to the server. This includes
        argument~0, that is the command name. Usually, this will be
        the program name of the server, unless the program behaves
        differently when invoked by a different name.

        This field is empty for internal services.

\end{dispitems}
}

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\ditem[\var{site}]
	This is the name of the site the entry applies to. One usually
	chooses the site's UUCP name for this.  There has to be an entry
	for your site in the \file{sys} file, too, else you will not
	receive any articles yourself.


	\index{C News!receiving news}
	\index{receiving news}
	\index{news!receiving}
	The special site name \keyword{ME} denotes your site. The
	\keyword{ME} entry defines all groups you are willing to store
	locally. Articles that aren't matched by the \keyword{ME} line
	will go to the \ngrp{junk} group.


	\index{C News!excluding sites}
	\index{C News!hostname aliases}
	\index{alias!and C News}
	Since C~News checks \var{site} against the site names in
	the \mhdr{Path:} header field, you have to make sure they really
	match. Some sites use their fully qualified domain name in this
	field, or an alias like \ngrp{news.}\var{site.domain}. To prevent
	any articles from being returned to these sites, you have to add
	these to the exclusion list, separated by commas.


	For the entry applying to site \host{moria}, for instance, the
	site field would contain \host{moria/}\host{moria.orcnet.org}.

\ditem[\var{grouplist}]
	\index{C News!exchanging news}
	\index{C News!limit a feed}
	This is a comma-separated subscription list of groups and
	hierarchies for that particular site. A hierarchy may be specified
	by giving the hierarchy's prefix (such as \ngrp{comp.os}
	for all groups whose name starts with this prefix), optionally
	followed by the keyword \ngrp{all} (e.g. \ngrp{comp.os.all}).


	A hierarchy or group is excluded from forwarding by preceding it
	with an exclamation mark. If a newsgroup is checked against the
	list, the longest match applies. For example, if \var{grouplist}
	contains

	\begin{tscreen}
	!comp,comp.os.linux,comp.folklore.computers
	\end{tscreen}

	\noindent
        no groups from the \ngrp{comp} hierarchy except
	\ngrp{comp.folklore.computers} and all groups below
	\ngrp{comp.os.linux} will be fed to that site.


	If the site requests to be forwarded all news
	you receive yourself, enter \keyword{all} as \var{grouplist}.


\ditem[\var{distlist}]
	\index{C News!limit a feed}
	\index{C News!limit a feed}
	\index{news!distributions}
	is offset from the \var{grouplist} by a slash, and contains a
	list of distributions to be forwarded.  Again, you may exclude
	certain distributions by preceding them with an exclamation
	mark. All distributions are denoted by \keyword{all}. Omitting
	\var{distlist} implies a list of \keyword{all}.


	For example, you may use a distribution list of
	\keyword{all,!local} to prevent news for local use only from
	being sent to remote sites.


	There are usually at least two distributions: \keyword{world},
	which is often the default distribution used when none is
	specified by the user, and \keyword{local}. There may be other
	distributions that apply to a certain region, state, country,
	etc. Finally, there are two distributions used by C~News only;
	these are \keyword{sendme} and \keyword{ihave}, and are used for
	the sendme/ihave protocol.


	The use of distributions is a subject of debate. For one, some
	newsreaders create bogus distributions by simply using the top
	level hierarchy, for example \ngrp{comp} when posting to
	\ngrp{comp.os.linux}.  Distributions that apply to regions are
	often questionable, too, because news may travel outside of your
	region when sent across the Internet.\footnote{
	  It is not uncommon for an article posted in, say Hamburg,
	  to go to Frankfurt via \host{reston.ans.net} in the 
	  Netherlands, or even via some site in the U.S.
	} Distributions applying to an organization, however, are very
	meaningful, for example to prevent confidential information from
	leaving the company network. This purpose, however, is generally
	served better by creating a separate newsgroup or hierarchy.


\ditem[\var{flags}]

	This describes certain parameters for the feed. It may be
	empty, or a combination of the following:

	\begin{dispitems}
	\ditem[\keyword{F}]
		\index{C News!batching}
		This flag enables batching.

	\ditem[\keyword{f}]
		\index{C News!batching}
		This is almost identical to the \keyword{F} flag, but allows 
		C~News to calculate the size of outgoing batches more
		precisely.

	\ditem[\keyword{I}]
		\index{C News!ihave/sendme}
		This flag makes C~News produce an article list suitable
		for use by ihave/sendme. Additional modifications to the
		\file{sys} and the \file{batchparms} file are required
		to enable ihave/sendme.

	\ditem[\keyword{n}]
		\index{C News!NNTP support}
		This creates batch files for active NNTP transfer
		clients like \prog{nntpxmit} (see chapter~\ref{nntp}).
		The batch files contain the article's filename along
		with its message id.

	\ditem[\keyword{L}]
		This tells C~News to transmit only articles posted at your
		site. This flag may be followed by a decimal number
		\var{n}, which makes C~News only transfer articles
		posted within \var{n} hops from your site. C~News
		determines the number of hops from the \mhdr{Path:} field.

	\ditem[\keyword{u}]
		This tells C~News to batch only articles from unmoderated
		groups. 

	\ditem[\keyword{m}]
		This tells C~News to batch only articles from moderated
		groups. 
	\end{dispitems}

	You may use at most one of \keyword{F}, \keyword{f},
	\keyword{I}, or \keyword{n}.

\ditem[\var{cmds}]
	\index{C News!rnews@\prog{rnews}}
	\index{C News!exchanging news}
	\index{C News!sending news}
	This field contains a command to be executed for each article,
	unless batching is enabled. The article will be fed to the
	command on standard input. This should only be used for very
	small feeds; otherwise the load on both systems will be too high.


	The default command is

	\begin{tscreen}
	 uux - -r -z \var{system}\/!rnews
	\end{tscreen}

	\noindent
	which invokes \prog{rnews} on the remote system, feeding it
	the article on standard input.


	The default search path for commands given in this field is
	\prog{/bin:/usr/bin:\file{/usr/lib/news/bin}/batch}. The latter directory
	contains a number of shell scripts whose name starts with
	\prog{via}; they are briefly described later in this chapter.


	\index{C News!batching}
	\index{C News!togo file@\file{togo} file}
	If batching is enabled using either of the \keyword{F} or
	\keyword{f}, \keyword{I} or \keyword{n} flags, C~News expects to
	find a file name in this field rather than a command. If the
	file name does not begin with a slash (\file{/}), it is assumed
	to be relative to \file{/var/spool/news}\file{/out.going}. If the field is
	empty, it defaults to \file{\var{system}\//togo}.


\end{dispitems}
}

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\ditem[\file{newsgroups}]
        \index{C News!list of current groups}
        This is a companion file of \file{active} which contains a list
        of newsgroup names, along with a one-line description of its
        main topic.  This file is automatically updated when C~News
        receives a \keyword{checknews} control message (see
        section~\ref{cnews.control}).

\ditem[\file{localgroups}]
        If you have a number of local groups that you don't want C~News
        to complain about every time you receive a \keyword{checknews}
        message, put their names and descriptions in this file, just like
        they would appear in \file{newsgroups}.

\ditem[\file{mailpaths}]
        \index{C News!moderated groups}
        This file contains the moderator's address for each moderated
        group. Each line contains the group name, followed by the
        moderator's email address (offset by a tab).

        Two special entries are provided as default. These are
        \keyword{backbone} and \keyword{internet}. Both provide~--- in
        bang-path notation~--- the path to the nearest backbone site,
        and the site that understands RFC~822-style addresses
        (\email{user@host}).  The default entries are

        \begin{tscreen}
	 internet\hspace{6ex}backbone\hspace{6ex}\end{tscreen}

        You will not have to change the \keyword{internet} entry if you
        have \prog{smail} or \prog{sendmail} installed, because they
        understand RFC~822-addressing.

        The \keyword{backbone} entry is used whenever a user posts to a
        moderated group whose moderator is not listed explicitly.  If
        the newsgroup's name is \ngrp{alt.sewer}, and the
        \keyword{backbone} entry contains \email{\var{path}!\%s}, C~News
        will mail the article to \email{\var{path}!alt-sewer}, hoping
        that the backbone machine is able to forward the article. To
        find out which path to use, ask the news admins at the site that
        feeds you.  As a last resort, you can also use
        \email{uunet.uu.net!\%s}.

\ditem[\file{distributions}]
        \index{C News!limit a feed}
        This file is not really a C~News file, but it is used by some
        newsreaders, and \prog{nntpd}. It contains the list of
        distributions recognized by your site, and a description of its
        (intended) effect. For example, Virtual Brewery has the
        following file:

        \begin{tscreen}
         
	 world\hspace{11ex}    everywhere in the world\\ 
	 local\hspace{11ex}    Only local to this site\\ 
	 nl\hspace{15ex}       Netherlands only\\ 
	 mugnet\hspace{10ex}   MUGNET only\\ 
	 fr\hspace{15ex}       France only\\ 
	 de\hspace{15ex}       Germany only\\ 
	 brewery\hspace{9ex}   Virtual Brewery only
        \end{tscreen}


\ditem[\file{log}]
        \index{C News!log files}
        This file contains a log of all C~News activities. It is culled
        regularly by running \prog{newsdaily}; copies of the old logfiles
        are kept in \file{log.o}, \file{log.oo}, etc.

\ditem[\file{errlog}]
        This is a log of all error messages created by C~News. These do
        not include articles junked due to wrong group, etc. This file
        is mailed to the newsmaster (\user{usenet} by default)
        automatically by \prog{newsdaily} if it is found to be
        non-empty.

        \file{errlog} is cleared by \prog{newsdaily}. Old copies are kept
        in \file{errlog.o} and companions.

\ditem[\file{batchlog}]
        This logs all runs of \prog{sendbatches}. It is usually of scant
        interest only. It is also attended by \prog{newsdaily}.

\ditem[\file{watchtime}]
        This is an empty file created each time \prog{newswatch} is run.

\end{dispitems}
}

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\samepage \begin{dispitems}% latex2html id marker 10064
\ditem[ACU]
        Automatic Call Unit. A modem.\footnote{
         Alternatively: A teenager with a telephone.
        }

\ditem[ARP]
        Address Resolution Protocol. Used to map IP~addresses
        to Ethernet addresses.

\ditem[ARPA]
	Advanced Research Project Agency, later DARPA. Founder
	of the Internet.

\ditem[ARPANET]
	The ancestor of today's Internet; an experimental network
	funded by the U.S. Defense Advanced Research Project Agency
	(DARPA).

\ditem[Assigned Numbers]
	The title of an \emph{RFC} published regularly that lists
	the publicly allocated numbers used for various things in
	TCP/IP networking. For example, it contains the list of all
	port numbers of well-known services like \prog{rlogin}, 
	\prog{telnet}, etc. The most recent release of this
	document is RFC~1340.

\ditem[bang path]
	In UUCP networks, a special notation for the path from
	one UUCP site to another. The name derives from the
	use of exclamation marks (`bangs') to separate the
	host names. Example: \email{foo!bar!ernie!bert} denotes
	a path to host \user{bert}, travelling (in this order)
	\host{foo}, \host{bar}, and \host{ernie}.

\ditem[BBS]
	Bulletin Board System. A dial-up mailbox system.

\ditem[BGP]
	Border Gateway Protocol. A protocol for exchanging
	routing information between autonomous systems.

\ditem[BIND]
        The Berkeley Internet Name Domain server. An implementation
        of a DNS server.

\ditem[BNU]
	Basic Networking Utilities. This is the most common UUCP
	variety at the moment.  It is also known as HoneyDanBer UUCP.
	This name is derived from the authors' names: P.~Honeyman,
	D.A.~Novitz, and B.E.~Redman.

\ditem[broadcast network]
	A network that allows one station to address a datagram
	to all other stations on the network simultaneously.

\ditem[BSD]
	Berkeley Software Distribution. A \unix{} flavor.

\ditem[canonical hostname]
	A host's primary name within the Domain Name System. This
	is the host's only name that has an A record 
	associated with it, and which is returned when performing
	a reverse lookup.

\ditem[CCITT]
        Comite\'e Consultatif International de  T\'el\'egraphique
        et T\'el\'ephonique. An International organization of
        telephone services, etc.

\ditem[CSLIP]
	Compressed Serial Line IP. A protocol for exchanging IP
	packets over a serial line, using header compression of
	most TCP/IP datagrams.

\ditem[DNS]
        Domain name system. This is a distributed database used on the
        Internet for mapping of host names to IP~addresses.

\ditem[EGP]
	External Gateway Protocol. A protocol for exchanging
	routing information between autonomous systems.

\ditem[Ethernet]
	In colloquial terms, the name of a sort of network equipment.
	Technically, Ethernet is part of a set of standards set forth by
	the IEEE. The Ethernet hardware uses a single piece of cable,
	frequently coax cable, to connect a number of hosts, and
	allows transfer rates of up to 10Mbps. The Ethernet protocol
	defines the manner in which hosts may communicate over
	this cable.\footnote{
	 As an aside, the Ethernet \emph{protocol} commonly used by TCP/IP
	 is \emph{not} exactly the same as IEEE~802.3. Ethernet frames
	 have a type field where IEEE~802.3 frames have a length field.
	}

\ditem[FQDN]
	Fully Qualified Domain Name. A hostname with a domain name
	tacked onto it, so that it is a valid index into the Domain Name
	database.

\ditem[FTP]
        File Transfer Protocol. The protocol one of the best-known
        file transfer service is based on and named after.

\ditem[FYI]
        ``For Your Information.'' Series of documents with informal
        information on Internet topics.

\ditem[GMU]
	Groucho Marx University. Fictitious University used as an example
	throughout this book.

\ditem[GNU]
	GNU's not Unix~-- this recursive acronym is the name of a project
	by the Free Software Association to provide a coherent set of
	\unix{}-tools that may be used and copied free of charge. All
	GNU software is covered by a special Copyright notice, also
	called the GNU General Public License (GPL), or Copyleft.
	The GPL is reproduced in section~\ref{appendix.gpl}.

\ditem[HoneyDanBer]
	The name of a UUCP variety. See also BNU.

\ditem[host]
	Generally, a network node: something that is able to receive and
	transmit network messages. This will usually be a computer, but
	you can also think of X-Terminals, or smart printers.

\ditem[ICMP]
        Internet Control Message Protocol. A networking protocol used
        by IP to return error information to the sending host, etc.

\ditem[IEEE]
	Institute of Electrical and Eletronics Engineers. Another standards
	organization. From a UNIX user's point of view, their most
	important achievement are probably the POSIX standards which
	define aspects of a UNIX systems, ranging from system call
	interfaces and semantics to administration tools.

	Apart from this, the IEEE developed the specifications for
	Ethernet, Token Ring, and Token Bus networks. A widely-used
	standard for binary representation of real numbers is also due to
	the IEEE.

\ditem[IETF]
        Internet Engineering Task Force.

\ditem[internet]
	A computer network formed of a collection of individual smaller
	networks.

\ditem[Internet]
	A particular world-wide internet.

\ditem[IP]
        Internet Protocol. A networking protocol.

\ditem[ISO]
	International Standards Organization.

\ditem[ISDN]
	Integrated Services Digital Network. New telecommunications
	technology using digital instead of analogue circuitry.

\ditem[LAN]
	Local Area Network. A small computer network.

\ditem[MX]
        Mail Exchanger. A DNS resource record type used for marking
        a host as mail gateway for a domain.

\ditem[network, packet-switched]
	A variety of networks that provide instantaneous forwarding
	of data by all data up in small packets, which are tramsported
	to their destination individually. Packet-switched networks
	rely on permanent or semi-permanent connections.

\ditem[network, store-and-forward]
	They are pretty much the opposite of packet-switched networks.
	These networks transfer data as entire files, and don't
	use permanent connections. Instead, hosts conect to each other
	at certain intervals only, and transfer all data at once.
	This requires that data be stored intermediately until a
	connection is established.

\ditem[NFS]
        Network File System. A standard networking protocol and
        software suite for accessing data on remote disks
        transparently.

\ditem[NIS]
        Network Information System. An RPC-based application that
        allows to share configuration files such as the password file
        between several hosts. See also the entry under YP.

\ditem[NNTP]
        Network News Transfer Protocol. Used to transfer news over
        TCP network connections.

\ditem[octet]
	On the Internet, the technical term referring to a quantity of
	eight bits. It is used rather than \emph{byte}, because there
	are machines on the Internet that have byte sizes other than eight
	bits.

\ditem[OSI]
	Open Systems Interconnection. An ISO standard on network software.

\ditem[path]
	Often used in UUCP networks as a synonym for \emph{route}. Also
	see \emph{bang path}.

\ditem[PLIP]
	Parallel Line IP. A protocol for exchanging IP packets over
	a parallel line such as a printer port.

\ditem[port, TCP or UDP]
	Ports are TCP's and UDP's abstraction of a service endpoint.
	Before a process can provide or access some networking service,
	it must claim (bind) a port. Together with the hosts' IP
	addresses, ports uniquely identify the two peers of a TCP connection.

\ditem[portmapper]
	The portmapper is the mediator between the program numbers used
	by RPC as an identification of individual RPC servers, and the
	TCP and UDP port numbers those services are listening to.

\ditem[PPP]
	The point-to-point protocol. PPP is a flexible and
	fast link-layer protocol to send various network protocols
	such as IP or IPX across a point-to-point connection.  Apart
	from being used on serial (modem) links, PPP can also be
	employed as the link-level protocol on top of ISDN.

\ditem[RARP]
        Reverse Address Resolution Protocol. It permits hosts to
        find out their IP~address at boot time.

\ditem[resolver]
	This is a library responsible for mapping hostnames to
	IP~addresses and vice versa.

\ditem[resource record]
	This is the basic unit of information in the DNS database, 
	commonly abbreviated as RR.
	Each record has a certain type and class associated with it,
	for instance a record mapping a host name to an IP~address
	has a type of A (for address), and a class of IN
	(for the Internet Protocol).

\ditem[reverse lookup]
	The act of looking up a host's name based on a given IP~address.
	Within DNS, this is done by looking up the host's IP~address
	in the \host{in-addr.arpa} domain.

\ditem[RFC]
        Request For Comments. Series of documents describing
        Internet standards.

\ditem[RIP]
        Routing Information Protocol. This is a routing protocol used
        dynamically adjust routes inside a (small) network.

\ditem[route]
	The sequence of hosts a piece of information has to travel
	from the originating host to the destination host. Finding
	an appropriate route is also called \emph{routing}.

\ditem[routing daemon]
	In larger networks, network topology changes are hard to adapt
	to manually, so facilities are used to distribute current
	routing information to the network's member hosts. This is
	called dynamic routing; the routing information is exchanged
	by \emph{routing daemons} running on central hosts in the network.
	The protocols they employ are called \emph{routing protocols}.

\ditem[RPC]
        Remote Procedure Call. Protocol for executing procdures inside
        a process on a remote host.

\ditem[RR]
	Short for \emph{resource record}.

\ditem[RS-232]
	This is a very common standard for serial interfaces.

\ditem[RTS/CTS]
	A colloquial name for the hardware handshake performed by
	two devices communicating over RS-232. The name derives from
	the two cicuits involved, RTS (``Ready To Send''), and
	CTS (``Clear To Send'').

\ditem[RTM Internet Worm]
	A Virus-like program that used several flaws in VMS and
	BSD~4.3 Unix to spread through the Internet. Several ``mistakes''
	in the program caused it to multiply without bound, and so
	effectively bringing down large parts of the Internet.
	RTM are the author's initials (Robert T.~Morris), which he left
	in the program.

\ditem[site]
	An agglomeration of hosts which, to the outside, behave almost
	like a single network node. For example, when speaking from an
	Internet point of view, one would call a Groucho Marx University
	a site, regardless of the complexity of its interior network.

\ditem[SLIP]
	Serial Line IP. This is a protocol for exchanging IP packets over
	a serial line, see also CSLIP.

\ditem[SMTP]
        Simple Mail Transfer Protocol. Used for mail transport
        over TCP connections, but also for mail batches transported
        over UUCP links (batched SMTP).

\ditem[SOA]
        Start of Authority. A DNS resource record type.

\ditem[System V]
	A \unix{} flavor.

\ditem[TCP]
        Transmission Control Protocol. A networking protocol.

\ditem[TCP/IP]
        Sloppy description of the Internet protocol suite
        as a whole.

\ditem[UDP]
        User Datagram Protocol. A networking protocol.

\ditem[UUCP]
        Unix to Unix Copy.
        A suite of network transport commands for dial-up
        networks.

\ditem[Version 2 UUCP]
	An aging UUCP variety.

\ditem[virtual beer]
	Every Linuxer's favorite drink. The first mention of virtual
	beer I remember was in the release note of the \linux{} 0.98.X kernel,
	where Linus listed the ``Oxford Beer Trolls'' in his credits section
	for sending along some virtual beer.

\ditem[well-known services]
	This term is frequently used to refer to common networking
	services such as \prog{telnet} and \prog{rlogin}.  In a more
	technical sense, it describes all services that have been
	assigned an official port number in the ``Assigned Numbers''
	RFC.

\ditem[YP]
	Yellow Pages. An older name for NIS which is no longer used,
	because Yellow Pages is a trademark of British Telecom.
	Nevertheless, most NIS utilities have retained names with
	a prefix of \prog{yp}.

\end{dispitems}
}

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