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+[[!meta copyright="Copyright © 2001 Marcus Brinkmann"]]
+
+[[!meta license="Verbatim copying and distribution of this entire article is
+permitted in any medium, provided this notice is preserved."]]
+
+[[!meta title="The Hurd, a presentation by Marcus Brinkmann"]]
+
+[[!tag stable_URL]]
+
+<H4><A NAME="contents">Table of Contents</A></H4>
+<UL>
+  <LI><A HREF="#int" NAME="TOCint">Introduction</A>
+  <LI><A HREF="#ove" NAME="TOCove">Overview</A>
+  <LI><A HREF="#his" NAME="TOChis">Historicals</A>
+  <LI><A HREF="#ker" NAME="TOCker">Kernel Architectures</A>
+  <LI><A HREF="#mic" NAME="TOCmic">Micro vs Monolithic</A>
+  <LI><A HREF="#sin" NAME="TOCsin">Single Server vs Multi Server</A>
+  <LI><A HREF="#mul" NAME="TOCmul">Multi Server is superior, ...</A>
+  <LI><A HREF="#the" NAME="TOCthe">The Hurd even more so.</A>
+  <LI><A HREF="#mac" NAME="TOCmac">Mach Inter Process Communication</A>
+  <LI><A HREF="#how" NAME="TOChow">How to get a port?</A>
+  <LI><A HREF="#exa" NAME="TOCexa">Example of <SAMP>hurd_file_name_lookup</SAMP></A>
+  <LI><A HREF="#pat" NAME="TOCpat">Pathname resolution example</A>
+  <LI><A HREF="#map" NAME="TOCmap">Mapping the POSIX Interface</A>
+  <LI><A HREF="#filser" NAME="TOCfilser">File System Servers</A>
+  <LI><A HREF="#act" NAME="TOCact">Active vs Passive</A>
+  <LI><A HREF="#aut" NAME="TOCaut">Authentication</A>
+  <LI><A HREF="#ope" NAME="TOCope">Operations on authentication ports</A>
+  <LI><A HREF="#est" NAME="TOCest">Establishing trusted connections</A>
+  <LI><A HREF="#pas" NAME="TOCpas">Password Server</A>
+  <LI><A HREF="#pro" NAME="TOCpro">Process Server</A>
+  <LI><A HREF="#filsys" NAME="TOCfilsys">Filesystems</A>
+  <LI><A HREF="#dev" NAME="TOCdev">Developing the Hurd</A>
+  <LI><A HREF="#sto" NAME="TOCsto">Store Abstraction</A>
+  <LI><A HREF="#deb" NAME="TOCdeb">Debian GNU/Hurd</A>
+  <LI><A HREF="#stabin" NAME="TOCstabin">Status of the Debian GNU/Hurd binary archive</A>
+  <LI><A HREF="#stainf" NAME="TOCstainf">Status of the Debian infrastructure</A>
+  <LI><A HREF="#staarc" NAME="TOCstaarc">Status of the Debian Source archive</A>
+  <LI><A HREF="#debide" NAME="TOCdebide">Debian GNU/Hurd: Good idea, bad idea?</A>
+  <LI><A HREF="#end" NAME="TOCend">End</A>
+</UL>
+<HR>
+<H3>Talk about the Hurd</H3>
+<P>
+This talk about the Hurd was written by Marcus Brinkmann for
+<UL>
+<LI>OSDEM, Brussels, 4. Feb 2001,
+<LI>Frühjahrsfachgespräche, Cologne, 2. Mar 2001 and
+<LI>Libre Software Meeting, Bordeaux, 4. Jul 2001.
+</UL>
+
+<H4><A HREF="#TOCint" NAME="int">Introduction</A></H4>
+<P>
+When we talk about free software, we usually refer to the free
+software licenses.  We also need relief from software patents, so our
+freedom is not restricted by them.  But there is a third type of
+freedom we need, and that's user freedom.
+
+<P>
+Expert users don't take a system as it is.  They like to change the
+configuration, and they want to run the software that works best for
+them.  That includes window managers as well as your favourite text
+editor.  But even on a GNU/Linux system consisting only of free
+software, you can not easily use the filesystem format, network
+protocol or binary format you want without special privileges.  In
+traditional unix systems, user freedom is severly restricted by the
+system administrator.
+
+<P>
+The Hurd removes these restrictions from the user.  It provides an
+user extensible system framework without giving up POSIX compatibility
+and the unix security model.  Throughout this talk, we will see that
+this brings further advantages beside freedom.
+
+<H4><A HREF="#TOCove" NAME="ove">Overview</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+
+<P>
+The Hurd is a POSIX compatible multi-server
+system operating on top of the GNU&nbsp;Mach microkernel.
+
+<P>
+Topics:
+<UL>
+  <LI>GNU&nbsp;Mach</LI>
+  <LI>The Hurd</LI>
+  <LI>Development</LI>
+  <LI>Debian GNU/Hurd</LI>
+</UL>
+</TD></TR></TABLE>
+
+<P>
+The Hurd is a POSIX compatible multi-server system operating on top of
+the GNU&nbsp;Mach Microkernel.
+
+<P>
+I will have to explain what GNU&nbsp;Mach is, so we start with that.  Then
+I will talk about the Hurd's architecture.  After that, I will give a
+short overview on the Hurd libraries.  Finally, I will tell you how
+the Debian project is related to the Hurd.
+
+<H4><A HREF="#TOChis" NAME="his">Historicals</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%">
+<TR><TD VALIGN="TOP" ALIGN="LEFT">
+<UL>
+  <LI>1983: Richard Stallman founds the GNU project.</LI>
+  <LI>1988: Decision is made to use Mach 3.0 as the kernel.</LI>
+  <LI>1991: Mach 3.0 is released under compatible license.</LI>
+  <LI>1991: Thomas Bushnell, BSG, founds the Hurd project.</LI>
+  <LI>1994: The Hurd boots the first time.</LI>
+  <LI>1997: Version 0.2 of the Hurd is released.<BR><BR></LI>
+  <LI>1998: Debian hurd-i386 archive is created.</LI>
+  <LI>2001: Debian GNU/Hurd snapshot fills three CD images.</LI>
+</UL>
+</TD></TR></TABLE>
+
+<P>
+When Richard Stallman founded the GNU project in 1983, he wanted to
+write an operating system consisting only of free software.  Very
+soon, a lot of the essential tools were implemented, and released
+under the GPL.  However, one critical piece was missing: The kernel.
+<P>
+After considering several alternatives, it was decided not to write a
+new kernel from scratch, but to start with the Mach microkernel.  This
+was in 1988, and it was not before 1991 that Mach was released under a
+license allowing the GNU project to distribute it as a part of the
+system.
+<P>
+In 1998, I started the Debian GNU/Hurd project, and in 2001 the number
+of available GNU/Hurd packages fills three CD images.
+
+<H4><A HREF="#TOCker" NAME="ker">Kernel Architectures</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Microkernel:
+<UL>
+  <LI>Enforces resource management (paging, scheduling)</LI>
+  <LI>Manages tasks</LI>
+  <LI>Implements message passing for IPC</LI>
+  <LI>Provides basic hardware support</LI>
+</UL>
+<P>
+Monolithic kernel:
+<UL>
+  <LI>No message passing necessary</LI>
+  <LI>Rich set of features (filesystems, authentication, network
+  sockets, POSIX interface, ...)</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+Microkernels were very popular in the scientific world around that
+time.  They don't implement a full operating system, but only the
+infrastructure needed to enable other tasks to implement most
+features.  In contrast, monolithical kernels like Linux contain
+program code of device drivers, network protocols, process management,
+authentication, file systems, POSIX compatible interfaces and much
+more.
+<P>
+So what are the basic facilities a microkernel provides?  In general,
+this is resource management and message passing.  Resource management,
+because the kernel task needs to run in a special privileged mode of
+the processor, to be able to manipulate the memory management unit and
+perform context switches (also to manage interrupts).  Message
+passing, because without a basic communication facility the other
+tasks could not interact to provide the system services. Some
+rudimentary hardware device support is often necessary to bootstrap
+the system.  So the basic jobs of a microkernel are enforcing the
+paging policy (the actual paging can be done by an external pager
+task), scheduling, message passing and probably basic hardware device
+support.
+<P>
+Mach was the obvious choice back then, as it provides a rich set of
+interfaces to get the job done.  Beside a rather brain-dead device
+interface, it provides tasks and threads, a messaging system allowing
+synchronous and asynchronous operation and a complex interface for
+external pagers.  It's certainly not one of the sexiest microkernels
+that exist today, but more like a big old mama.  The GNU project
+maintains its own version of Mach, called GNU&nbsp;Mach, which is based on
+Mach 4.0.  In addition to the features contained in Mach 4.0, the GNU
+version contains many of the Linux 2.0 block device and network card
+drivers.
+<P>
+A complete treatment of the differences between a microkernel and
+monolithical kernel design can not be provided here.  But a couple of
+advantages of a microkernel design are fairly obvious.
+
+<H4><A HREF="#TOCmic" NAME="mic">Micro vs Monolithic</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Microkernel
+<UL>
+  <LI>Clear cut responsibilities
+  <LI>Flexibility in operating system design, easier debugging</LI>
+  <LI>More stability (less code to break)</LI>
+  <LI>New features are not added to the kernel</LI>
+</UL>
+<P>
+Monolithic kernel
+<UL>
+  <LI>Intolerance or creeping featuritis</LI>
+  <LI>Danger of spaghetti code</LI>
+  <LI>Small changes can have far reaching side effects</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+Because the system is split up into several components, clean
+interfaces have to be developed, and the responsibilities of each part
+of the system must be clear.
+<P>
+Once a microkernel is written, it can be used as the base for several
+different operating systems.  Those can even run in parallel which
+makes debugging easier.  When porting, most of the hardware dependant
+code is in the kernel.
+<P>
+Much of the code that doesn't need to run in the special kernel mode
+of the processor is not part of the kernel, so stability increases
+because there is simply less code to break.
+<P>
+New features are not added to the kernel, so there is no need to hold
+the barrier high for new operating system features.
+<P>
+Compare this to a monolithical kernel, where you either suffer from
+creeping featuritis or you are intolerant of new features (we see both
+in the Linux kernel).
+<P>
+Because in a monolithical kernel, all parts of the kernel can access
+all data structures in other parts, it is more likely that short cuts
+are used to avoid the overhead of a clean interface.  This leads to a
+simple speed up of the kernel, but also makes it less comprehensible
+and more error prone.  A small change in one part of the kernel can
+break remote other parts.
+
+<H4><A HREF="#TOCsin" NAME="sin">Single Server vs Multi Server</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Single Server
+<UL>
+  <LI>A single task implements the functionality of the operating system.</LI>
+</UL>
+<P>
+Multi Server
+<UL>
+  <LI>Many tasks cooperate to provide the system's functionality.</LI>
+  <LI>One server provides only a small but well-defined part of the
+  whole system.</LI>
+  <LI>The responsibilities are distributed logically among the servers.</LI>
+</UL>
+<P>
+A single-server system is comparable to a monolithic kernel system. It
+has similar
+advantages and disadvantages.
+</TD></TR></TABLE>
+<P>
+There exist a couple of operating systems based on Mach, but they all
+have the same disadvantages as a monolithical kernel, because those
+operating systems are implemented in one single process running on top
+of the kernel.  This process provides all the services a monolithical
+kernel would provide.  This doesn't make a whole lot of sense (the
+only advantage is that you can probably run several of such isolated
+single servers on the same machine).  Those systems are also called
+single-server systems.  The Hurd is the only usable multi-server
+system on top of Mach.  In the Hurd, there are many server programs,
+each one responsible for a unique service provided by the operating
+system.  These servers run as Mach tasks, and communicate using the
+Mach message passing facilities.  One of them does only provide a
+small part of the functionality of the system, but together they build
+up a complete and functional POSIX compatible operating system.
+
+<H4><A HREF="#TOCmul" NAME="mul">Multi Server is superior, ...</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Any multi-server has advantages over single-server:
+<UL>
+  <LI>Clear cut responsibilities</LI>
+  <LI>More stability: If one server dies, all others remain</LI>
+  <LI>Easier development cycle: Testing without reboot (or replacing
+  running servers), debugging with gdb</LI>
+  <LI>Easier to make changes and add new features
+</UL>
+</TD></TR></TABLE>
+<P>
+Using several servers has many advantages, if done right.  If a file
+system server for a mounted partition crashes, it doesn't take down
+the whole system.  Instead the partition is "unmounted", and
+you can try to start the server again, probably debugging it this time
+with gdb.  The system is less prone to errors in individual
+components, and over-all stability increases.  The functionality of
+the system can be extended by writing and starting new servers
+dynamically.  (Developing these new servers is easier for the reasons
+just mentioned.)
+<P>
+But even in a multi-server system the barrier between the system and
+the users remains, and special privileges are needed to cross it.  We
+have not achieved user freedom yet.
+
+<H4><A HREF="#TOCthe" NAME="the">The Hurd even more so.</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+The Hurd goes beyond all this, and allows users to write and run their
+servers, too!
+<UL>
+  <LI>Users can replace system servers dynamically with their own
+  implementations.</LI>
+  <LI>Users can decide what parts of the remainder of the system they
+  want to use.</LI>
+  <LI>Users can extend the functionality of the system.</LI>
+  <LI>No mutual trust necessary to make use of other users
+  services.</LI>
+  <LI>Security of the system is not harmed by trusting users
+  services.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+To quote Thomas Bushnell, BSG, from his paper
+[[``Towards_a_New_Strategy_of_OS_design''_(1996)|hurd-paper]]:
+<BLOCKQUOTE>
+The GNU&nbsp;Hurd, by contrast, is designed to make the area of system code
+as limited as possible.  Programs are required to communicate only
+with a few essential parts of the kernel; the rest of the system is
+replaceable dynamically.  Users can use whatever parts of the
+remainder of the system they want, and can easily add components
+themselves for other users to take advantage of.  No mutual trust need
+exist in advance for users to use each other's services, nor does the
+system become vulnerable by trusting the services of arbitrary users.
+</BLOCKQUOTE>
+
+<P>
+<EM>So the Hurd is a set of servers running on top of the Mach
+micro-kernel, providing a POSIX compatible and extensible operating
+system.  What servers are there?  What functionality do they provide,
+and how do they cooperate?</EM>
+
+<H4><A HREF="#TOCmac" NAME="mac">Mach Inter Process Communication</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Ports are message queues which can be used as one-way communication
+channels.
+<UL>
+  <LI>Port rights are receive, send or send-once</LI>
+  <LI>Exactly one receiver</LI>
+  <LI>Potentially many senders</LI>
+</UL>
+<P>
+MIG provides remote procedure calls on top of Mach IPC. RPCs look like
+function calls to the user.
+</TD></TR></TABLE>
+<P>
+Inter-process communication in Mach is based on the ports concept.  A
+port is a message queue, used as a one-way communication channel.  In
+addition to a port, you need a port right, which can be a send right,
+receive right, or send-once right.  Depending on the port right, you
+are allowed to send messages to the server, receive messages from it,
+or send just one single message.
+<P>
+For every port, there exists exactly one task holding the receive
+right, but there can be no or many senders.  The send-once right is
+useful for clients expecting a response message.  They can give a
+send-once right to the reply port along with the message.  The kernel
+guarantees that at some point, a message will be received on the reply
+port (this can be a notification that the server destroyed the
+send-once right).
+<P>
+You don't need to know much about the format a message takes to be
+able to use the Mach IPC.  The Mach interface generator mig hides the
+details of composing and sending a message, as well as receiving the
+reply message.  To the user, it just looks like a function call, but
+in truth the message could be sent over a network to a server running
+on a different computer.  The set of remote procedure calls a server
+provides is the public interface of this server.
+
+
+<H4><A HREF="#TOChow" NAME="how">How to get a port?</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Traditional Mach:
+<UL>
+  <LI>Nameserver provides ports to all registered servers.</LI>
+  <LI>The nameserver port itself is provided by Mach.</LI>
+  <LI>Like a phone book: One list.</LI>
+</UL>
+<P>
+The Hurd:
+<UL>
+  <LI>The filesystem is used as the server namespace.</LI>
+  <LI>Root directory port is inserted into each task.</LI>
+  <LI>The C library finds other ports with hurd_file_name_lookup,
+  performing a pathname resolution.</LI>
+  <LI>Like a tree of phone books.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+So how does one get a port to a server?  You need something like a
+phone book for server ports, or otherwise you can only talk to
+yourself.  In the original Mach system, a special nameserver is
+dedicated to that job.  A task could get a port to the nameserver from
+the Mach kernel and ask it for a port (with send right) to a server
+that registered itself with the nameserver at some earlier time.
+<P>
+In the Hurd, there is no nameserver.  Instead, the filesystem is used
+as the server namespace.  This works because there is always a root
+filesystem in the Hurd (remember that the Hurd is a POSIX compatible
+system); this is an assumption the people who developed Mach couldn't
+make, so they had to choose a different strategy.  You can use the
+function hurd_file_name_lookup, which is part of the C library, to get
+a port to the server belonging to a filename.  Then you can start to
+send messages to the server in the usual way.
+
+<H4><A HREF="#TOCexa" NAME="exa">Example of <SAMP>hurd_file_name_lookup</SAMP></A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT"><PRE>
+mach_port_t identity;
+mach_port_t pwserver;
+kern_return_t err;
+
+pwserver = hurd_file_name_lookup
+                ("/servers/password");
+
+err = password_check_user (pwserver,
+                           0 /* root */, "supass",
+                           &identity);
+</PRE></TD></TR></TABLE>
+<P>
+As a concrete example, the special filename
+<SAMP>/servers/password</SAMP> can be used to request a port to the
+Hurd password server, which is responsible to check user provided
+passwords.
+<P>
+(explanation of the example)
+
+<H4><A HREF="#TOCpat" NAME="pat">Pathname resolution example</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Task: Lookup /mnt/readme.txt where /mnt has a mounted filesystem.
+<UL>
+  <LI>The C library asks the root filesystem server about
+  <SAMP>/mnt/readme.txt</SAMP>.</LI>
+  <LI>The root filesystem returns a port to the mnt filesystem server
+  (matching <SAMP>/mnt</SAMP>) and the retry name
+  <SAMP>/readme.txt</SAMP>.</LI>
+  <LI>The C library asks the mnt filesystem server about
+  <SAMP>/readme.txt</SAMP>.</LI>
+  <LI>The mnt filesystem server returns a port to itself and records
+  that this port refers to the regular file
+  <SAMP>/readme.txt</SAMP>.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The C library itself does not have a full list of all available
+servers.  Instead pathname resolution is used to traverse through a
+tree of servers.  In fact, filesystems themselves are implemented by
+servers (let us ignore the chicken and egg problem here).  So all the
+C library can do is to ask the root filesystem server about the
+filename provided by the user (assuming that the user wants to resolve
+an absolute path), using the <SAMP>dir_lookup</SAMP> RPC.  If the
+filename refers to a regular file or directory on the filesystem, the
+root filesystem server just returns a port to itself and records that
+this port corresponds to the file or directory in question.  But if a
+prefix of the full path matches the path of a server the root
+filesystem knows about, it returns to the C library a port to this
+server and the remaining part of the pathname that couldn't be
+resolved.  The C library than has to retry and query the other server
+about the remaining path component.  Eventually, the C library will
+either know that the remaining path can't be resolved by the last
+server in the list, or get a valid port to the server in question.
+
+<H4><A HREF="#TOCmap" NAME="map">Mapping the POSIX Interface</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<TABLE BORDER="0" CELLPADDING="10">
+<TR>
+<TH>Filedescriptor</TH>
+<TH>Port to server providing the file</TH>
+</TR><TR>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>fd = open(name,...)</SAMP></TD>
+<TD VALIGN="TOP"
+ALIGN="LEFT"><SAMP>dir_lookup(..,name,..,&amp;port)</SAMP><BR>
+[pathname resolution]</TD>
+</TR><TR>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>read(fd, ...)</SAMP></TD>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>io_read(port, ...)</SAMP></TD>
+</TR><TR>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>write(fd, ...)</SAMP></TD>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>io_write(port, ...)</SAMP></TD>
+</TR><TR>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>fstat(fd, ...)</SAMP></TD>
+<TD VALIGN="TOP" ALIGN="LEFT"><SAMP>io_stat(port, ...)</SAMP></TD>
+</TR><TR>
+<TD VALIGN="TOP" ALIGN="LEFT">...</TD><TD></TD>
+</TR>
+</TABLE>
+</TD></TR></TABLE>
+<P>
+It should by now be obvious that the port returned by the server can
+be used to query the files status, content and other information from
+the server, if good remote procedure calls to do that are defined and
+implemented by it.  This is exactly what happens.  Whenever a file is
+opened using the C libraries <SAMP>open()</SAMP> call, the C library
+uses the above pathname resolution to get a port to a server providing
+the file.  Then it wraps a file descriptor around it.  So in the Hurd,
+for every open file descriptor there is a port to a server providing
+this file.  Many other C library calls like <SAMP>read()</SAMP> and
+<SAMP>write()</SAMP> just call a corresponding RPC using the port
+associated with the file descriptor.
+
+<H4><A HREF="#TOCfilser" NAME="filser">File System Servers</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<UL>
+  <LI>Provide file and directory services for ports (and more).</LI>
+  <LI>These ports are returned by a directory lookup.</LI>
+  <LI>Translate filesystem accesses through their root path (hence the
+  name translator).</LI>
+  <LI>The C library maps the POSIX file and directory interface (and
+  more) to RPCs to the filesystem servers ports, but also does work on
+  its own.</LI>
+  <LI>Any user can install file system servers on inodes they own.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+So we don't have a single phone book listing all servers, but rather a
+tree of servers keeping track of each other.  That's really like
+calling your friend and asking for the phone number of the blond girl
+at the party yesterday.  He might refer you to a friend who hopefully
+knows more about it. Then you have to retry.
+<P>
+This mechanism has huge advantages over a single nameserver.  First,
+note that standard unix permissions on directories can be used to
+restrict access to a server (this requires that the filesystems
+providing those directories behave).  You just have to set the
+permissions of a parent directory accordingly and provide no other way
+to get a server port.
+<P>
+But there are much deeper implications.  Most of all, a pathname never
+directly refers to a file, it refers to a port of a server.  That
+means that providing a regular file with static data is just one of
+the many options the server has to service requests on the file port.
+A server can also create the data dynamically.  For example, a server
+associated with <SAMP>/dev/random</SAMP> can provide new random data
+on every <SAMP>io_read()</SAMP> on the port to it.  A server
+associated with <SAMP>/dev/fortune</SAMP> can provide a new fortune
+cookie on every <SAMP>open()</SAMP>.
+<P>
+While a regular filesystem server will just serve the data as stored
+in a filesystem on disk, there are servers providing purely virtual
+information, or a mixture of both.  It is up to the server to behave
+and provide consistent and useful data on each remote procedure call.
+If it does not, the results may not match the expectations of the user
+and confuse him.
+<P>
+A footnote from the Hurd info manual:
+<BLOCKQUOTE>
+(1) You are lost in a maze of twisty little filesystems, all
+alike....
+</BLOCKQUOTE>
+<P>
+Because a server installed in the filesystem namespace translates all
+filesystem operations that go through its root path, such a server is
+also called "active translator".  You can install translators using
+the settrans command with the <SAMP>-a</SAMP> option.
+
+<H4><A HREF="#TOCact" NAME="act">Active vs Passive</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Active Translators:
+<UL>
+  <LI>"<SAMP>settrans -a /cdrom /hurd/isofs /dev/hd2</SAMP>"</LI>
+  <LI>Are running filesystem servers.</LI>
+  <LI>Are attached to the root node they translate.</LI>
+  <LI>Run as a normal process.</LI>
+  <LI>Go away with every reboot, or even time out.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+Many translator settings remain constant for a long time.  It would be
+very lame to always repeat the same couple of dozens settrans calls
+manually or at boot time.  So the Hurd provides a filesystem extension
+that allows to store translator settings inside the filesystem and let
+the filesystem servers do the work to start those servers on demand.
+Such translator settings are called "passive translators".  A passive
+translator is really just a command line string stored in an inode of
+the filesystem.  If during a pathname resolution a server encounters
+such a passive translator, and no active translator does exist already
+(for this node), it will use this string to start up a new translator
+for this inode, and then let the C library continue with the path
+resolution as described above.  Passive translators are installed with
+settrans using the <SAMP>-p</SAMP> option (which is already the
+default).
+
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Passive Translators:
+<UL>
+  <LI>"<SAMP>settrans /mnt /hurd/ext2fs /dev/hd1s1</SAMP>"</LI>
+  <LI>Are stored as command strings into an inode.</LI>
+  <LI>Are used to start a new active translator if there isn't
+  one.</LI>
+  <LI>Startup is transparent to the user.</LI>
+  <LI>Startup happens the first time the server is needed.</LI>
+  <LI>Are permanent across reboots (like file data).</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+So passive translators also serve as a sort of automounting feature,
+because no manual interaction is required.  The server start up is
+deferred until the service is need, and it is transparent to the user.
+<P>
+When starting up a passive translator, it will run as a normal process
+with the same user and group id as those of the underlying inode.  Any
+user is allowed to install passive and active translators on inodes
+that he owns.  This way the user can install new servers into the
+global namespace (for example, in his home or tmp directory) and thus
+extend the functionality of the system (recall that servers can
+implement other remote procedure calls beside those used for files and
+directories).  A careful design of the trusted system servers makes
+sure that no permissions leak out.
+<P>
+In addition, users can provide their own implementations of some of
+the system servers instead the system default.  For example, they can
+use their own exec server to start processes.  The user specific exec
+server could for example start java programs transparently (without
+invoking the interpreter manually).  This is done by setting the
+environment variable <SAMP>EXECSERVERS</SAMP>.  The systems default
+exec server will evaluate this environment variable and forward the
+RPC to each of the servers listed in turn, until some server accepts
+it and takes over.  The system default exec server will only do this
+if there are no security implications.  (XXX There are other ways to
+start new programs than by using the system exec server.  Those are
+still available.)
+<P>
+Let's take a closer look at some of the Hurd servers.  It was already
+mentioned that only few system servers are mandatory for users.  To
+establish your identity within the Hurd system, you have to
+communicate with the trusted systems authentication server
+<SAMP>auth</SAMP>.  To put the system administrator into control over
+the system components, the process server does some global
+bookkeeping.
+<P>
+But even these servers can be ignored.  However, registration with the
+authentication server is the only way to establish your identity
+towards other system servers.  Likewise, only tasks registered as
+processes with the process server can make use of its services.
+
+<H4><A HREF="#TOCaut" NAME="aut">Authentication</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+A user identity is just a port to an authserver. The auth server
+stores four set of ids for it:
+<UL>
+  <LI>effective user ids</LI>
+  <LI>effective group ids</LI>
+  <LI>available user ids</LI>
+  <LI>available group ids</LI>
+</UL>
+<P>
+Basic properties:
+<UL>
+  <LI>Any of these can be empty.</LI>
+  <LI>A 0 among the user ids identifies the superuser.</LI>
+  <LI>Effective ids are used to check if the user has the
+  permission.</LI>
+  <LI>Available ids can be turned into effective ids on user
+  request.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The Hurd auth server is used to establish the identity of a user for a
+server.  Such an identity (which is just a port to the auth server)
+consists of a set of effective user ids, a set of effective group ids,
+a set of available user ids and a set of available group ids.  Any of
+these sets can be empty.
+
+<H4><A HREF="#TOCope" NAME="ope">Operations on authentication ports</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+The auth server provides the following operations on ports:
+<UL>
+  <LI>Merge the ids of two ports into a new one.</LI>
+  <LI>Return a new port containing a subset of the ids in a port.</LI>
+  <LI>Create a new port with arbitrary ids (superuser only).</LI>
+  <LI>Establish a trusted connection between users and servers.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+If you have two identities, you can merge them and request an identity
+consisting of the unions of the sets from the auth server.  You can
+also create a new identity consisting only of subsets of an identity
+you already have.  What you can't do is extending your sets, unless
+you are the superuser which is denoted by having the user id 0.
+
+<H4><A HREF="#TOCest" NAME="est">Establishing trusted connections</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<UL>
+  <LI>User provides a rendezvous port to the server (with
+  <SAMP>io_reauthenticate</SAMP>).</LI>
+  <LI>User calls <SAMP>auth_user_authenticate</SAMP> on the
+  authentication port (his identity), passing the rendezvous port.</LI>
+  <LI>Server calls <SAMP>auth_server_authenticate</SAMP> on its
+  authentication port (to a trusted auth server), passing the
+  rendezvous port and the server port.</LI> 
+  <LI>If both authentication servers are the same, it can match the
+  rendezvous ports and return the server port to the user and the user
+  ids to the server.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+Finally, the auth server can establish the identity of a user for a
+server.  This is done by exchanging a server port and a user identity
+if both match the same rendezvous port.  The server port will be
+returned to the user, while the server is informed about the id sets
+of the user.  The server can then serve or reject subsequent RPCs by
+the user on the server port, based on the identity it received from
+the auth server.
+<P>
+Anyone can write a server conforming to the auth protocol, but of
+course all system servers use a trusted system auth server to
+establish the identity of a user.  If the user is not using the system
+auth server, matching the rendezvous port will fail and no server port
+will be returned to the user.  Because this practically requires all
+programs to use the same auth server, the system auth server is
+minimal in every respect, and additional functionality is moved
+elsewhere, so user freedom is not unnecessarily restricted.
+
+<H4><A HREF="#TOCpas" NAME="pas">Password Server</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+The password server <SAMP>/servers/password</SAMP> runs as root and
+returns a new authentication port in exchange for a unix password.
+<P>
+The ids corresponding to the authentication port match the unix user
+and group ids.
+<P>
+Support for shadow passwords is implemented here.
+</TD></TR></TABLE>
+<P>
+The password server sits at <SAMP>/servers/password</SAMP> and runs as
+root.  It can hand out ports to the auth server in exchange for a unix
+password, matching it against the password or shadow file.  Several
+utilities make use of this server, so they don't need to be setuid
+root.
+
+<H4><A HREF="#TOCpro" NAME="pro">Process Server</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+The superuser must remain control over user tasks, so:
+<UL>
+  <LI>All mach tasks are associated with a PID in the system default
+  proc server.</LI>
+</UL>
+<P>
+Optionally, user tasks can store:
+<UL>
+  <LI>Their environment variables.</LI>
+  <LI>Their argument vector.</LI>
+  <LI>A port, which others can request based on the PID (like a
+  nameserver).</LI>
+</UL>
+<P>
+Also implemented in the proc server:
+<UL>
+  <LI>Sessions and process groups.</LI>
+  <LI>Global configuration not in Mach, like hostname, hostid, system
+  version.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The process server is responsible for some global bookkeeping.  As
+such it has to be trusted and is not replaceable by the user.
+However, a user is not required to use any of its service.  In that
+case the user will not be able to take advantage of the POSIXish
+appearance of the Hurd.
+<P>
+The Mach Tasks are not as heavy as POSIX processes.  For example,
+there is no concept of process groups or sessions in Mach.  The proc
+server fills in the gap.  It provides a PID for all Mach tasks, and
+also stores the argument line, environment variables and other
+information about a process (if the mach tasks provide them, which is
+usually the case if you start a process with the default
+<SAMP>fork()</SAMP>/<SAMP>exec()</SAMP>).  A process can also register
+a message port with the proc server, which can then be requested by
+anyone.  So the proc server also functions as a nameserver using the
+process id as the name.
+<P>
+The proc server also stores some other miscellaneous information not
+provided by Mach, like the hostname, hostid and system version.
+Finally, it provides facilities to group processes and their ports
+together, as well as to convert between pids, process server ports and
+mach task ports.
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+User tasks not registering themselve with proc only have a PID assigned.
+<P>
+Users can run their own proc server in addition to the system default,
+at least for those parts of the interface that don't require superuser
+privileges.
+</TD></TR></TABLE>
+<P>
+Although the system default proc server can't be avoided (all Mach
+tasks spawned by users will get a pid assigned, so the system
+administrator can control them), users can run their own additional
+process servers if they want, implementing the features not requiring
+superuser privileges.
+
+<H4><A HREF="#TOCfilsys" NAME="filsys">Filesystems</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Store based filesystems
+<UL>
+  <LI><SAMP>ext2fs</SAMP></LI>
+  <LI><SAMP>ufs</SAMP></LI>
+  <LI><SAMP>isofs</SAMP> (iso9660, RockRidge, GNU extensions)</LI>
+  <LI><SAMP>fatfs</SAMP> (under development)</LI>
+</UL>
+<P>
+Network file systems
+<UL>
+  <LI><SAMP>nfs</SAMP></LI>
+  <LI><SAMP>ftpfs</SAMP></LI>
+</UL>
+<P>
+Miscellaneous
+<UL>
+  <LI><SAMP>hostmux</SAMP></LI>
+  <LI><SAMP>usermux</SAMP></LI>
+  <LI><SAMP>tmpfs</SAMP> (under development)</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+We already talked about translators and the file system service they
+provide.  Currently, we have translators for the ext2, ufs and iso9660
+filesystems.  We also have an nfs client and an ftp filesystem.
+Especially the latter is intriguing, as it provides transparent access
+to ftp servers in the filesystem.  Programs can start to move away
+from implementing a plethora of network protocols, as the files are
+directly available in the filesystem through the standard POSIX file
+interface.
+
+
+<H4><A HREF="#TOCdev" NAME="dev">Developing the Hurd</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Over a dozen libraries support the development of new servers.
+<P>
+For special server types highly specialized
+libraries require only the implementation of a
+number of callback functions.
+<UL>
+  <LI>Use <SAMP>libdiskfs</SAMP> for store based filesystems.</LI>
+  <LI>Use <SAMP>libnetfs</SAMP> for network filesystems, also for
+  virtual filesystems.</LI>
+  <LI>Use <SAMP>libtrivfs</SAMP> for simple filesystems providing only
+  a single file or directory.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The Hurd server protocols are complex enough to allow for the
+implementation of a POSIX compatible system with GNU extensions.
+However, a lot of code can be shared by all or at least similar
+servers. For example, all storage based filesystems need to be able to
+read and write to a store medium splitted in blocks. The Hurd comes
+with several libraries which make it easy to implement new servers.
+Also, there are already a lot of examples of different server types in
+the Hurd.  This makes writing a new server easier.
+<P>
+<SAMP>libdiskfs</SAMP> is a library that supports writing store based
+filesystems like ext2fs or ufs. It is not very useful for filesystems
+which are purely virtual, like <SAMP>/proc</SAMP> or files in
+<SAMP>/dev</SAMP>.
+<P>
+<SAMP>libnetfs</SAMP> is intended for filesystems which provide a rich
+directory hierarchy, but don't use a backing store (for example ftpfs,
+nfs).
+<P>
+<SAMP>libtrivfs</SAMP> is intended for filesystems which just provide
+a single inode or directory. Most servers which are not intended to
+provide a filesystem but other services (like
+<SAMP>/servers/password</SAMP>) use it to provide a dummy file, so
+that file operations on the servers node will not return errors. But
+it can also be used to provide meaningful data in a single file, like
+a device store or a character device.
+
+<H4><A HREF="#TOCsto" NAME="sto">Store Abstraction</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Another very useful library is libstore, which is used by all store
+based filesystems.  It provides a store media abstraction.  A store
+consists of a store class and a name (which itself can sometimes
+contain stores).
+<P>
+Primitive store classes:
+<UL>
+  <LI>device store like device:hd2, device:hd0s1, device:fd0</LI>
+  <LI>file store like file:/tmp/disk_image</LI>
+  <LI>task store like task:PID</LI>
+  <LI>zero store like zero:4m (like /dev/zero, of size 4 MB)</LI>
+</UL>
+</TD></TR></TABLE>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Composed store classes:
+<UL>
+  <LI>copy store like copy:zero:4m</LI>
+  <LI>gunzip/bunzip2 store like gunzip:device:fd0</LI>
+  <LI>concat store like concat:device:hd0s2:device:hd1s5</LI>
+  <LI>ileave store (RAID-0(2))</LI>
+  <LI>remap store like remap:10+20,50+:file:/tmp/blocks</LI>
+  <LI>...</LI>
+</UL>
+<P>
+Wanted: A similar abstraction for streams (based on channels), which
+can be used by network and character device servers.
+</TD></TR></TABLE>
+<P>
+<SAMP>libstore</SAMP> provides a store abstraction, which is used by
+all store based filesystems. The store is determined by a type and a
+name, but some store types modify another store rather than providing
+a new store, and thus stores can be stacked. For example, the device
+store type expects a Mach device, but the remap store expects a list
+of blocks to pick from another store, like remap:1+:device:hd2, which
+would pick all blocks from hd2 but the first one, which skipped.
+Because this functionality is provided in a library, all libstore
+using filesystems support many different store kinds, and adding a new
+store type is enough to make all store based filesystems support it.
+
+<H4><A HREF="#TOCdeb" NAME="deb">Debian GNU/Hurd</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Goal:
+<UL>
+  <LI>Provide a binary distribution of the Hurd that is easy to
+  install.</LI>
+</UL>
+<P>
+Constraints:
+<UL>
+  <LI>Use the same source packages as Debian GNU/Linux.</LI>
+  <LI>Use the same infrastructure:
+  <UL>
+    <LI>Policy</LI>
+    <LI>Archive</LI>
+    <LI>Bug tracking system</LI>
+    <LI>Release process</LI>
+  </UL></LI>
+</UL>
+<P>
+Side Goal:
+<UL>
+  <LI>Prepare Debian for the future:
+  <UL>
+    <LI>More flexibility in the base system</LI>
+    <LI>Identify dependencies on the Linux kernel</LI>
+  </UL></LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The Debian distribution of the GNU&nbsp;Hurd that I started in 1998 is
+supposed to become a complete binary distribution of the Hurd that is
+easy to install.
+
+<H4><A HREF="#TOCstabin" NAME="stabin">Status of the Debian GNU/Hurd binary archive</A></H4>
+<P>
+See
+<A HREF="http://buildd.debian.org/stats/graph.png">http://buildd.debian.org/stats/graph.png</A>
+for the most current version of the statistic.
+
+<H4><A HREF="#TOCstainf" NAME="stainf">Status of the Debian infrastructure</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Plus:
+<UL>
+  <LI>Source packages can identify build and host OS using
+  dpkg-architecture.</LI>
+</UL>
+<P>
+Minus:
+<UL>
+  <LI>The binary architecture field is insufficient.</LI>
+  <LI>The BTS has no architecture tag.</LI>
+  <LI>The policy/FHS need (small) Hurd specific extensions.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+While good compatibiity can be achieved at the source level,
+the binary packages can not always express their relationship
+to the available architectures sufficiently.
+<P>
+For example, the Linux version of makedev is binary-all, where
+a binary-all-linux relationship would be more appropriate.
+<P>
+More work has to be done here to fix the tools.
+
+<H4><A HREF="#TOCstaarc" NAME="staarc">Status of the Debian Source archive</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<UL>
+  <LI>Most packages just work.</LI>
+  <LI>Maintainers are usually responsive and cooperative.</LI>
+  <LI>Turtle, the autobuilder, crunches through the whole list right
+  now.</LI>
+</UL>
+<P>
+Common pitfalls are POSIX incompatibilities:
+<UL>
+  <LI>Upstream:
+  <UL>
+    <LI>Unconditional use of <SAMP>PATH_MAX</SAMP>
+    (<SAMP>MAXPATHLEN</SAMP>), <SAMP>MAXHOSTNAMELEN</SAMP>.</LI>
+    <LI>Unguarded use of Linux kernel features.</LI>
+    <LI>Use of legacy interfaces (<SAMP>sys_errlist</SAMP>,
+    <SAMP>termio</SAMP>).</LI>
+  </UL></LI>
+  <LI>Debian:
+  <UL>
+    <LI>Unguarded activation of extensions available with Linux.</LI>
+    <LI>Low quality patches.</LI>
+    <LI>Assuming GNU/Linux in package scripts.</LI>
+  </UL></LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+Most packages are POSIX compatible and can be compiled without
+changes on the Hurd.  The maintainers of the Debian source packages
+are usually very kind, responsiver and helpful.
+<P>
+The Turtle autobuilder software (<A
+HREF="http://turtle.sourceforge.net" >http://turtle.sourceforge.net</A>)
+builds the Debian packages on the Hurd automatically.
+
+<H4><A HREF="#TOCdebide" NAME="debide">Debian GNU/Hurd: Good idea, bad idea?</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Upstream benefits:
+<UL>
+  <LI>Software packages become more portable.</LI>
+</UL>
+<P>
+Debian benefits:
+<UL>
+  <LI>Debian becomes more portable.</LI>
+  <LI>Maintainers learn about portability and other systems.</LI>
+  <LI>Debian gets a lot of public recognition.</LI>
+</UL>
+<P>
+GNU/Hurd benefits:
+<UL>
+  <LI>Large software base.</LI>
+  <LI>Great infrastructure.</LI>
+  <LI>Nice community to partner with.</LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+The sheet lists the advantages of all groups involved.
+
+<H4><A HREF="#TOCend" NAME="end">End</A></H4>
+<TABLE BORDER="1" CELLPADDING="5" WIDTH="100%"><TR><TD VALIGN="TOP" ALIGN="LEFT">
+<P>
+Join us at
+<UL>
+  <LI><A HREF="http://hurd.gnu.org/" >http://hurd.gnu.org/</A></LI>
+  <LI><A HREF="http://www.debian.org/ports/hurd"
+  >http://www.debian.org/ports/hurd</A></LI>
+  <LI><A HREF="http://www.hurdfr.org"
+  >http://www.hurdfr.org</A></LI>
+</UL>
+</TD></TR></TABLE>
+<P>
+List of contacts.