Editing GNU Hurd

{{short description|Operating system kernel designed as a replacement for Unix}}
{{Redirect|Hurd}}
{{Infobox OS
| name = GNU Hurd
| logo = [[File:Hurd-logo.svg|96px]]
| screenshot = [[File:Debian GNU HURD text mode screenshot.png|250px]]
| developer = [[GNU Project]]<br />[[Thomas Bushnell]]<br />Roland McGrath<br />Marcus Brinkmann<br />Neal Walfield<br />Samuel Thibault
| programmed in = [[Assembly language|Assembly]], [[C (programming language)|C]]
| family = [[Unix-like]]
| working state = Current
| source model = [[Free software]] <!-- Shouldn't say this per [[Template:Infobox OS/doc]], but is contentious, see talk page -->
| released = {{Start date and age|1990}}
| latest release version = {{wikidata|property|preferred|references|edit|Q48464|P348|P548=Q2804309}}
| latest release date = {{wikidata|qualifier|preferred|single|Q48464|P348|P548=Q2804309|P577}}
| latest preview version = {{wikidata|property|preferred|references|edit|Q48464|P348|P548=Q51930650}}
| latest preview date = {{wikidata|qualifier|preferred|single|Q48464|P348|P548=Q51930650|P577}}
| supported platforms = [[x86-64]], [[IA-32]], [[P6 (microarchitecture)|i686]], [[ARM architecture family|ARM]]<ref>{{cite web|url=https://www.osnews.com/story/139255/gnu-hurd-ported-to-aarch64-and-more-hurd-news/|title=GNU Hurd ported to AArch64, and more Hurd news|website=osnews.com|access-date=22 April 2024}}</ref>
| kernel type = {{URL|https://www.gnu.org/software/hurd/faq/multiserver_microkernel.html|Multiserver microkernel}}
| license = [[GNU General Public License|GPL-2.0-or-later]]<ref>{{cite web|url=http://git.savannah.gnu.org/cgit/hurd/hurd.git/tree/COPYING|title=COPYING - hurd/hurd.git - Hurd|website=Git.savannah.gnu.org|access-date=25 February 2019}}</ref>
}}

'''GNU Hurd''' is a collection of [[microkernel]] servers written as part of [[GNU]], for the [[GNU Mach]] microkernel. It has been under development since 1990 by the [[GNU Project]] of the [[Free Software Foundation]], designed as a replacement for the [[Unix]] kernel,<ref name="What Is the GNU Hurd" /> and released as [[free software]] under the [[GNU General Public License]]. When the [[Linux kernel]] proved to be a viable solution, development of GNU Hurd slowed, at times alternating between stasis and renewed activity and interest.<ref name=":0">{{Cite web|url=http://thevarguy.com/open-source-application-software-companies/042015/30-years-hurd-lives-gnu-updates-open-source-|title=30 Years On, HURD Lives: GNU Updates Open Source Unix Kernel|last=Tozzi|first=Christopher|date=2015-04-20|archive-url=https://web.archive.org/web/20150424014208/http://thevarguy.com/open-source-application-software-companies/042015/30-years-hurd-lives-gnu-updates-open-source-|archive-date=2015-04-24|language=en |url-status=usurped|website=The VAR Guy }}</ref>

The Hurd's design consists of a set of protocols and [[server (computing)|server processes]] (or [[daemon (computing)|daemons]], in Unix terminology) that run on the GNU Mach microkernel.<ref name="What Is the GNU Hurd"/> The Hurd aims to surpass the Unix kernel in functionality, security, and stability, while remaining largely compatible with it. The GNU Project chose the multiserver microkernel<ref>{{cite web|url=https://www.gnu.org/software/hurd/faq/multiserver_microkernel.html |title=What is a Multiserver Microkernel? |publisher=GNU |date=2013-04-13 |access-date=2015-08-11}}</ref> for the operating system, due to perceived advantages over the traditional Unix [[monolithic kernel]] architecture,<ref name="advantages" /> a view that had been advocated by some developers in the 1980s.<ref name=":0" />

== Name and logo ==
In December 1991 the primary architect of the Hurd described the name as a [[mutual recursion|mutually]] [[recursive acronym]]:<ref name="Koen Vervloesem, 2010">{{cite web |last=Vervloesem |first=Koen |date=July 7, 2010 |title=The Hurd: GNU's quest for the perfect kernel |publisher=[[LWN.net]] |url=https://lwn.net/Articles/395150/ |access-date=October 5, 2012}}</ref>
{{Blockquote|text=It's time [to] explain the meaning of "Hurd". "Hurd" stands for "Hird of Unix-Replacing Daemons". And, then, "Hird" stands for "Hurd of Interfaces Representing Depth". We have here, to my knowledge, the first software to be named by a pair of mutually recursive acronyms.|author=[[Thomas Bushnell|Thomas (then Michael) Bushnell]] }}

As both ''hurd'' and ''hird'' are [[homophone]]s of the English word ''herd'', the full name ''GNU Hurd'' is also a play on the words ''[[herd]] of [[wildebeest|gnus]]'', reflecting how the kernel works.<ref name="hurd-name" />

The logo is called the ''Hurd boxes'' and it also reflects on architecture. The logo is a graph where nodes represent the Hurd kernel's servers and directed edges are [[Inter-process communication|IPC]] messages.<ref name="Koen Vervloesem, 2010"/>

== Development history ==
[[Richard Stallman]] founded the [[GNU Project]] in September 1983 with an aim to create a [[Free software|free]] [[GNU]] operating system.<ref>{{Cite web |title=GNU History |url=https://cs.stanford.edu/people/eroberts/courses/cs181/projects/2000-01/open-source/gnu.htm#:~:text=In%201983,%20the%20GNU%20Project,up%20by%20proprietary%20software%20vendors. |access-date=2024-09-03 |website=cs.stanford.edu}}</ref> Initially the components required for kernel development were written: [[Text editor|editors]], [[Unix shell|shell]], [[compiler]], [[debugger]] etc. By 1989, the [[GNU General Public License|GPL]] came into being and the only major component missing was the kernel.<ref name="Richard Hillesley, 2010">{{cite web |last=Hillesley |first=Richard |title=GNU HURD: Altered visions and lost promise | date=June 30, 2010 |url=http://www.h-online.com/open/features/GNU-HURD-Altered-visions-and-lost-promise-1030942.html |access-date=October 1, 2012}}</ref><ref name="linux-and-gnu" />

Development on the Hurd began in 1990 after an abandoned kernel attempt in 1986, based on the research [[Trix (operating system)|TRIX]] operating system developed by Professor [[Steve Ward (computer scientist)|Steve Ward]] and his group at [[Massachusetts Institute of Technology|MIT's]] [[Laboratory for Computer Science]] (LCS).<ref name="hurd-hist" /> According to [[Thomas Bushnell]], the initial Hurd architect, their early plan was to adapt the [[4.4BSD]]-Lite kernel and, in hindsight, "It is now perfectly obvious to me that this would have succeeded splendidly and the world would be a very different place today."<ref name="hurd-and-bsdi" /> In 1987 Richard Stallman proposed using the [[Mach (kernel)|Mach microkernel]] developed by [[Richard Rashid]] at [[Carnegie Mellon University]]. Work on this was delayed for three years due to uncertainty over whether CMU would release the Mach code under a suitable license.<ref name="hurd-hist" />

With the release of the [[Linux kernel]] in 1991, the primary user of GNU's [[userland (computing)|userland]] components soon became operating systems based on the Linux kernel ([[Linux distribution]]s), prompting the coining of the term [[GNU/Linux naming controversy|''GNU/Linux'']].

Development of the Hurd has proceeded slowly. Despite an optimistic announcement by Stallman in 2002 predicting a release of GNU/Hurd later that year,<ref name="gnu-loose-linux" /> the Hurd is still not considered suitable for production environments. Development in general has not met expectations, and there are still a significant number of bugs and missing features.<ref name="hurd-status" /> This has resulted in a poorer product than many, including Stallman, had expected.<ref name="fsf-future-of-freedom" /> In 2010, after twenty years under development, Stallman said that he was "not very optimistic about the GNU Hurd. It makes some progress, but to be really superior it would require solving a lot of deep problems", but added that "finishing it is not crucial" for the GNU system because a free kernel already existed ([[Linux]]), and completing Hurd would not address the main remaining problem for a free operating system: device support.<ref name="reddit-interview-stallman" />

The [[Debian]] project, among others, have worked on the Hurd project to produce binary distributions of [[Debian GNU/Hurd|Hurd-based GNU operating systems]] for [[IBM PC compatible]] systems.

After years of stagnation, development picked up again in 2015 and 2016, with four releases during these two years,<ref>{{Cite news|url=http://sdtimes.com/free-software-foundation-changes-priorities/|title=Free Software Foundation changes priorities - SD Times|date=2017-01-17|work=SD Times|access-date=2017-04-17|language=en-US}}</ref> but no more since then.

On August 20, 2015, amid the [[Google Summer of Code]], it was announced that [[GNU Guix]] had been ported to GNU Hurd.<ref name="gsoc">{{cite mailing list|url=https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00379.html|title=[GSoC update] Porting Guix to GNU/Hurd|date=August 20, 2015|access-date=August 20, 2015|first=Manolis|last=Ragkousis|publisher=guix-devel}}</ref>

== Architecture ==
[[File:OS-structure2.svg|thumb|600px|General structure of monolithic, microkernel and hybrid kernel-based operating systems, respectively.]]

Unlike most [[Unix-like]] kernels, the Hurd uses a [[server–client architecture]], built on a [[microkernel]] that is responsible for providing the most basic [[kernel service]]s – coordinating access to the [[Computer hardware|hardware]]: the [[Central processing unit|CPU]] (through [[process management (computing)|process management]] and [[scheduling (computing)|scheduling]]), [[Random access memory|RAM]] (via [[memory management]]), and other various [[input/output]] devices (via [[I/O scheduling]]) for sound, graphics, mass storage, etc. In theory, the microkernel design would allow for all device drivers to be built as servers working in [[user space]], but today most drivers of this kind are still contained in the [[GNU Mach]] [[kernel space]].<ref name="device-drivers-in-mach" />

According to Hurd developers, the main advantage of microkernel-based design is the ability to extend the system: developing a new module would not require in depth knowledge of the rest of the kernel, and a bug in one module would not crash the entire system. Hurd provides a concept of ''translators'', a framework of modules used to extend a file system functionality.<ref name="Doeppner2010">{{cite book |last=Doeppner |first=Thomas W. |date=20 December 2010 |title=Operating Systems In Depth: Design and Programming |publisher=John Wiley & Sons |isbn=978-0-471-68723-8 |page=160 |url=https://books.google.com/books?id=xX5tfrAQQ8cC |access-date=29 November 2012 }}</ref>

From early on, the Hurd was developed to use [[GNU Mach]] as the microkernel. This was a technical decision made by [[Richard Stallman]], who thought it would speed up the work by saving a large part of it. He has admitted that he was wrong about that.<ref name="lt-defence-rh" /> Other Unix-like systems working on the [[Mach kernel|Mach microkernel]] include [[OSF/1]], [[Lites]], and [[MkLinux]]. [[macOS]] and [[NeXTSTEP]] use [[hybrid kernel]]s based on Mach.

=== Other microkernels ===
From 2004 onward, various efforts were launched to port the Hurd to more modern microkernels. The [[L4 microkernel family|L4 microkernel]] was the original choice in 2004, but progress slowed to a halt. Nevertheless, during 2005, Hurd developer Neal Walfield finished the initial memory management framework for the L4/Hurd port, and Marcus Brinkmann ported essential parts of [[glibc]]; namely, getting the process startup code working, allowing programs to run, thus allowing the first user programs (trivial ones such as the [[hello world program]]) in C to run.

Since 2005, Brinkmann and Walfield started researching [[EROS (microkernel)|Coyotos]] as a new kernel for HURD.<ref name="shapiro-comment" /><ref name="l4-and-coyotos-mess" /> In 2006, Brinkmann met with Jonathan Shapiro (a primary architect of the Coyotos Operating System) to aid in and discuss the use of the Coyotos kernel for GNU/Hurd. In further discussion HURD developers realised that Coyotos (as well as other similar kernels) are not suitable for HURD.<ref>{{cite web | url = https://www.gnu.org/software/hurd/history/port_to_another_microkernel.html | title = Porting the Hurd to another microkernel | access-date = 2017-05-06 | publisher = Free Software Foundation | website = GNU Hurd }}</ref>

In 2007, Hurd developers Neal Walfield and Marcus Brinkmann gave a critique of the Hurd architecture, known as "the critique",<ref name="critique-mach" /> and a proposal for how a future system may be designed, known as "the position paper".<ref name="position-paper" /> In 2008, Neal Walfield began working on the Viengoos microkernel as a modern native kernel for HURD. {{as of|2009}}, development on Viengoos is paused due to Walfield lacking time to work on it.<ref name="viengoos" />

In the meantime, others have continued working on the Mach variant of Hurd.<ref name="what-happend-l4-coyotos" />

=== Unix extensions ===
{{Unreferenced section|date=March 2007}}

A number of traditional Unix concepts are replaced or extended in the Hurd.

Under Unix, every running program has an associated [[user id]], which normally corresponds to the user that started the [[process (computing)|process]]. This id largely dictates the actions permitted to the program. No outside process can change the user id of a running program. A Hurd process, on the other hand, runs under a ''set'' of user ids, which can contain multiple ids, one, or none. A sufficiently privileged process can add and remove ids to another process. For example, there is a password server that will hand out ids in return for a correct login password.

Regarding the [[file system]], a suitable program can be designated as a ''translator'' for a single file or a whole directory hierarchy. Every access to the translated file, or files below a hierarchy in the second case, is in fact handled by the program. For example, a file translator may simply redirect read and write operations to another file, like a Unix [[symbolic link]]. The effect of Unix ''[[mount (computing)|mounting]]'' is achieved by setting up a filesystem translator (using the "settrans" command). Translators can also be used to provide services to the user. For example, the [[ftpfs]] translator allows a user to encapsulate remote FTP sites within a directory. Then, standard tools such as [[ls]], [[cp (Unix)|cp]], and [[rm (Unix)|rm]] can be used to manipulate files on the remote system. Even more powerful translators are ones such as [[UnionFS]], which allows a user to unify multiple directories into one; thus listing the unified directory reveals the contents of all the directories.

The Hurd requires a [[Multiboot Specification|multiboot]]-compliant [[booting|boot loader]], such as [[GNU GRUB|GRUB]].

=== Architecture of the servers ===
According to the Debian documentation, there are 24 servers (18 core servers and 6 file system servers) named as follows:<ref name="debian-hurd-doc" />

==== Core servers ====
* '''auth''' ([[authentication server]]): Receives requests and passwords from programs and gives them an ID, which changes the privileges of the program.
* '''crash''' ([[Crash (computing)|crash]] server): Handles all fatal errors.
* '''[[Enforce In-order Execution of I/O|eieio]]''' (translation server): TODO
* '''exec''' (execution server): Translates an executable image (currently [[Executable and Linkable Format|ELF]] and [[a.out]] are supported) to a runnable image in memory.
* '''fifo''' ([[FIFO (computing and electronics)|FIFO]] translator): Implements named pipes.
* '''new-fifo''' (new FIFO server): An alternate server for named pipes.
* '''firmlink''' (the firmlink translator): Implements firmlinks ‒ "half-way between a [[symbolic link]] and a [[hard link]]".<ref>{{cite web |date=1996-10-10 |title=GNU/Hurd - Documentation |publisher=Debian |url=http://www.debian.org/ports/hurd/hurd-doc-server#firmlink |access-date=2012-07-12}}</ref>
* '''fwd''' (forward server): Forwards requests to other servers, used by fifo and symlink servers.
* '''hostmux''' (host multiplexer server)
* '''ifsock''' (server for sockets interface): Helps with UNIX domain socket addresses.
* '''init''' ([[init]] server): Basic system booting and configuration.
* '''magic''' ([[Magic (programming)|magic]] server): Signals that a name lookup must be resolved internally by a process when the result involves the process's state.
* '''null''' (null server): Implements [[/dev/null]] and /dev/zero.
* '''pfinet''' (pfinet server): Implements the PF_INET protocol family.
* '''pflocal''' (pflocal server): Implements [[UNIX domain sockets]].
* '''proc''' (process server): Assigns [[Process identifier|PIDs]] and manages process-level actions.
* '''symlink''' ([[symbolic link]] translator): Implements symbolic links for filesystems that do not support them.
* '''term''' (terminal server): A [[POSIX]] terminal.
* '''usermux''' (user multiplexer server): Invokes user-specific translators.

==== Filesystem servers ====
;ext2fs
:The [[ext2]] filesystem translator. It receives disk blocks from the microkernel and gives files and directories to the applications.
;isofs
:The translator for the [[ISO 9660]] filesystem. Translates blocks of a CD or DVD to files and directories for the applications.
;nfs
:See [[Network File System (protocol)|Network File System]].
;ftpfs
: [[File transfer protocol]] filesystem translator.
;storeio
:The storage translator.

The servers collectively implement the [[POSIX]] [[Applications programming interface|API]], with each server implementing a part of the interface. For instance, the various filesystem servers each implement the filesystem calls. The storage server will work as a wrapping layer, similar to the block layer of Linux. The equivalent of [[Virtual file system|VFS]] of [[Linux]] is achieved by libdiskfs and libpager libraries.

==GNU distributions running Hurd==
[[File:Debian GNU HURD XFCE desktop screenshot.png|thumbnail|[[Debian GNU/Hurd]] with [[Xfce]]]]

Hurd-based [[GNU variants|GNU distributions]] include:
* [[Arch Hurd]]
* Bee GNU/Hurd (discontinued)<ref>{{cite web |url=http://www.gnu.org/software/hurd/hurd/running/distrib.html |title= GNU Hurd/ hurd/ running/ distrib |website=GNU |date=2015-05-03 |access-date=2017-09-21}}</ref>
* [[Debian#Debian_GNU/Hurd|Debian GNU/Hurd]]
* [[Gentoo/Alt#GNU Hurd|Gentoo GNU Hurd]] (discontinued)
* GNU/Hurd Live CD<ref name="Live CD">{{cite web |title=GNU Hurd/ hurd/ running/ live cd |website=GNU |url=https://www.gnu.org/software/hurd/hurd/running/live_cd.html |date=2013-04-09 |access-date=2017-09-21}}</ref> (discontinued){{not in citation|date=September 2017}}
* [[GNU Guix System|Guix System]] (under development)

== See also ==
{{Portal|Free and open-source software}}
<!-- Please keep entries in alphabetical order & add a short description [[WP:SEEALSO]] -->
* [[Comparison of operating system kernels]]
* [[GNU variants|GNU distributions]]
* [[Kernel-based Virtual Machine]] (KVM)
* [[Linux-libre]] – a [[Free software|free]] version of the [[Linux kernel]]
* [[Multiboot Specification]]
<!-- please keep entries in alphabetical order -->

== References ==
{{Reflist|refs=
<ref name="What Is the GNU Hurd">{{cite web |title=What Is the GNU Hurd? |work=[[GNU]] |url=https://www.gnu.org/software/hurd/hurd/what_is_the_gnu_hurd.html |access-date=2010-03-04}}</ref>
<ref name="advantages">{{cite web |title=advantages |work=[[GNU]] |url=https://www.gnu.org/software/hurd/advantages.html |access-date=2011-12-07}}</ref>
<ref name="hurd-name">{{cite web |title=GNU Hurd: Origin of the Name |work=[[GNU]] |url=https://www.gnu.org/software/hurd/hurd/what_is_the_gnu_hurd/origin_of_the_name.html |access-date=2010-03-04}}</ref>
<ref name="linux-and-gnu">{{cite web |date=2010-01-26 |title=Linux and the GNU Project |work=[[GNU]] |url=https://www.gnu.org/gnu/linux-and-gnu.html |access-date=2010-03-04 }}</ref>
<ref name="hurd-hist">{{cite web |title=The GNU Hurd History, 'How it Started' |work=[[GNU]] |url=https://www.gnu.org/software/hurd/history.html |access-date=2006-08-27}}</ref>
<ref name="hurd-and-bsdi">{{cite web |last=Salus |first=Peter |author-link=Peter H. Salus |title=The Daemon, the GNU and the Penguin |url=http://www.groklaw.net/article.php?story=20050727225542530 |access-date=2006-08-08}}</ref>
<ref name="gnu-loose-linux">{{cite web |last=Ribeiro |first=John |date=2002-03-11 |title=Free Software Sees GNU Loose of Linux |work=[[PC World (magazine)|PC World]] |url=http://www.pcworld.com/article/88464/article.html |access-date=2012-10-05 |archive-url=https://web.archive.org/web/20160922183417/http://www.pcworld.com/article/88464/article.html |archive-date=2016-09-22 |url-status=dead }}</ref>
<ref name="hurd-status">{{cite web |title=Status |work=[[GNU]] |url=https://www.gnu.org/software/hurd/hurd/status.html |access-date=2010-03-04}}</ref>
<ref name="fsf-future-of-freedom">{{cite web |last=Stallman |first=Richard |author-link=Richard Stallman |date=2006-03-09 |title=The Free Software Movement and the Future of Freedom |location=[[Zagreb]] |work=[[Free Software Foundation]] |url=http://mjesec.ffzg.hr/~dpavlin/stallman2006/free_software_movement_and_the_future_of_freedom_zagreb_09_march_2006.ogg |format=ogg |quote=This is the way, also, that people thought was the cleanest possible way to design kernels back in 1990. Well, it took many many many years to get this kernel to run at all, and it still doesn't run well, and it looks like there may be fundamental problems with this design, which nobody knew about back in 1990.}}
*{{cite web |author=Richard Stallman |title=The Free Software Movement and the Future of Freedom; March 9th 2006 |type=Lecture transcript |url=http://fsfeurope.org/documents/rms-fs-2006-03-09.en.html |archive-url=https://web.archive.org/web/20070112043637/http://fsfeurope.org/documents/rms-fs-2006-03-09.en.html |archive-date=2007-01-12 |website=Free Software Foundation Europe}}</ref>
<ref name="reddit-interview-stallman">{{cite web | url = https://redditblog.com/2010/07/rms-ama.html | title = RMS AMA | first = Richard | last = Stallman | author-link = Richard Stallman | date = 2010-07-29 | publisher = [[Reddit]] | access-date = 2011-12-07 }}</ref><!-- comments are at https://www.reddit.com/tb/cv7sw -->
<ref name="device-drivers-in-mach">{{cite mailing list | url = http://lists.gnu.org/archive/html/bug-hurd/2007-03/msg00089.html | title = Re: Device drivers in Mach? | first = Constantine | last = Kousoulos | mailing-list = bug-hurd | date = 2007-03-21 }}</ref>
<ref name="lt-defence-rh">{{cite web | url = http://www.linuxtoday.com/infrastructure/2000101200421OPCYRH | title = In Defense of Red Hat | first = Richard | last = Stallman | author-link = Richard Stallman | work = Linux Today | date = 2000-10-12 | access-date = 2011-12-07 | quote = I take full responsibility for the technical decision to develop the GNU kernel based on Mach, a decision which seems to have been responsible for the slowness of the development. I thought using Mach would speed the work by saving us a large part of the job, but I was wrong. }}</ref>
<ref name="critique-mach">{{cite web | url = http://walfield.org/papers/200707-walfield-critique-of-the-GNU-Hurd.pdf | title = A Critique of the GNU Hurd Multi-server Operating System | first1 = Neal H. | last1 = Walfield | first2 = Marcus | last2 = Brinkmann | work = [[GNU]] | date = July 2007 | access-date = 2011-12-07 }}</ref>
<ref name="position-paper">{{cite web |last1=Walfield |first1=Neal H. |last2=Brinkmann |first2=Marcus |date=2007-01-04 |title=Improving Usability via Access Decomposition and Policy Refinement |work=[[GNU]] |url=http://walfield.org/papers/20070104-walfield-access-decomposition-policy-refinement.pdf |access-date=2011-12-07}}</ref>
<ref name="shapiro-comment">{{cite mailing list | url = http://lists.gnu.org/archive/html/l4-hurd/2005-10/msg00755.html | title = Re: A comment about changing kernels | first = Jonathan S. | last = Shapiro | mailing-list = l4-hurd | date = 2005-10-27 }}</ref>
<ref name="l4-and-coyotos-mess">{{cite mailing list | url = http://lists.gnu.org/archive/html/l4-hurd/2006-07/msg00004.html | title = Re: seL4, L4.sec and coyotos mess | first = Tom | last = Bachmann | mailing-list = l4-hurd | date = 2006-07-07 }}</ref>
<ref name="viengoos">{{cite web |title=viengoos |publisher=[[GNU]] |url=https://www.gnu.org/software/hurd/microkernel/viengoos.html |access-date=2010-03-04 }}</ref>
<ref name="what-happend-l4-coyotos">{{cite web |title=What happened to the L4/Coyotos/viengoos micro-kernels? |publisher=[[GNU]] |url=https://www.gnu.org/software/hurd/faq/which_microkernel.html |access-date=2011-01-07 }}</ref>
<ref name="debian-hurd-doc">{{cite web |title=Preliminary GNU/Hurd User Interface Description |work=[[Debian]] |date=1996-10-10 |url=http://www.debian.org/ports/hurd/hurd-doc-server |access-date=2010-03-04}}</ref>
}}

== External links ==
{{Commons category|GNU Hurd}}
* {{Official website|https://www.gnu.org/software/hurd/}}

{{GNU}}
{{Unix-like}}
{{Free Software Foundation}}
{{Microkernel}}

[[Category:Free software operating systems]]
[[Category:GNU Project software]]
[[Category:Microkernel-based operating systems]]
[[Category:Microkernels]]
[[Category:Operating system distributions bootable from read-only media]]
[[Category:Unix variants]]
[[Category:X86-64 operating systems]]