This document attempts to answer the questions that most often plague users when they are talking about, installing, using, compiling and developing the GNU Hurd as well as its binary distribution Debian GNU/Hurd. Be sure to read this before asking for help.
The GNU Hurd is under active development and a stable version has not yet been released. Be aware that: there is a lot of work yet to be completed; you will find bugs; your system will crash. That said, there is a lot of room for contributions at all levels: development of the Hurd and Mach proper, porting applications, writing documentation and, most importantly, user feedback.
Should you have a question that is not answered by this document and you feel that it should be, submit it and, if possible, include an answer.
The copyright for each section is by held its respective author(s). Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts and with no Back-Cover Texts. A copy of the license is included in the file COPYING.
Neal H. Walfield neal@cs.uml.edu
{NHW} A Microkernel has nothing to do with the size of the kernel. Rather, it refers to the functionality that the kernel provides. It is generally agreed that this is; a set of interfaces to allow processes to communicate and a way to talk to the hardware. ``Software drivers,'' as I like to call them, are then implemented in user space as servers. The most obvious examples of these are the TCP/IP stack, the ext2 filesystem and NFS. In the case of the Hurd, users now have access to functionality that, in a monolithic kernel, they could never use, but now, because the server runs in user space as the user that started it, they may, for instance, mount an ftp filesystem in their home directory.
For more information about the design of the Hurd, read the paper by Thomas Bushnell, BSG: ``Towards a new strategy on OS design'' available at:
http://www.gnu.org/software/hurd/hurd-paper.html
{MB,NHW} ``Hurd'', as an acronym, stands for ``Hird of Unix-Replacing Daemons''. Hird, in turn, stands for ``Hurd of Interfaces Representing Depth''.
We treat ``Hurd'' as a title rather than as a proper name: it requires an article, as in ``the Hurd''. For instance: ``The ext2 filesystem is provided by the Hurd, not by Mach.'' Note that all of the following are incorrect: ``Hurd'', ``HURD'', ``The HURD'', and ``the hurd''.
We write ``the GNU Hurd'' instead of ``the Hurd'' when we want to emphasize that the Hurd is a GNU package. Once this has been made clear, we usually use the shorter form, without ``GNU''.
The whole operating system includes not only the kernel and the system servers, but also many more programs. This system is called ``GNU'', or ``the GNU operating system''. The GNU programs can also run on other operating system kernels. We say ``GNU/Hurd'' when we want to put emphasis on the fact that this is the GNU system running on top of the Hurd, and to contrast it with the GNU/Linux system which is GNU using Linux as the kernel.
Finally, there is ``Debian GNU/Hurd''. This refers to the distribution of the GNU system as created by the Debian developers. For example: ``What do you run on your laptop? Debian GNU/Hurd, of course.''
The French generally write ``le Hurd'' -- that is, they treat the name as masculine singular, capitalized as in English.
When we are referring to the microkernel, we say ``Mach'' and use it as a proper noun. For example: ``Mach uses the device drivers found in version 2.0.x of Linux.'' We sometimes say ``The Mach microkernel'' instead of just ``Mach''.
{NHW} There are four principle mailing lists:
bug-hurd@gnu.org: Development of the Hurd and Mach.
help-hurd@gnu.org: General questions about the Hurd.
web-hurd@gnu.org: Maintenance of the Hurd webpages at http://www.gnu.org/software/hurd/hurd.html
debian-hurd@lists.debian.org: Maintenance of (including porting) and help with the Debian GNU/Hurd distribution.
Subscribe in the usual manner.
{NHW} Use the Debian bug tracking system. You can find out more about the BTS at http://www.debian.org/Bugs.
{NHW} There are two. Look at the file tasks and TODO in the root of the Hurd source code.
{NHW} No one seems to know. However, the Hurd is definitely runnable, try it and help us get there.
{MB} The distinction between `/' and `/usr' has historical reasons. Back when Unix systems were booted from two tapes, a small root tape and a big user tape. Today, we like to use different partitions for these two spaces. The Hurd throws this historical garbage away. We think that we have found a more flexible solution called shadow filesystems. Unfortunately, support for shadowed filesystems is not yet implemented.
{NHW} As GNU/Hurd is an official unstable Debian port, you can find the binaries at your local Debian mirror. There are, however, several core packages that do not compile cleanly from source and have several, unfortunate, hacks. These can be found at:
ftp://alpha.gnu.org/pub/hurd/debian.
Once you have a working installation, you can point apt at the archives by adding the following lines to your /etc/apt/sources.list file. For example:
deb ftp://alpha.gnu.org/pub/gnu/hurd/debian unstable main deb http://http.us.debian.org/debian unstable main
As downloading all of the packages can be quite long (there is over a gigabyte of files), Philip Charles has created Debian GNU Hurd CDs. Vendors can be found at the following site:
http://www.debian.org/ports/hurd/hurd-cd
{NHW} Consult one of the installation guides. The most up to date guide can be found at:
http://web.walfield.org/papers/hurd-installation-guide/
{NHW} Currently, the GNU Hurd only runs on IA32 machines, however, porting to other architectures should not be difficult, in fact, a port to the PowerPC architecture is currently underway.
With respect to drivers, GNU Mach uses a subset of the drivers from the Linux 2.0.x kernel. A relatively complete hardware compatibility guide can be found at:
http://www.freesoftware.fsf.org/thug/gnumach_hardware.html
If you run into trouble, it is most likely that:
You are using IRQ sharing; GNU Mach does not support this in the least.
A device driver is frobbing the hardware and accidentally activating the wrong device.
In the former case, you should start removing devices or play with the BIOS. As for the latter, GNU Mach does not support loadable kernel modules. Therefore, you will have to compile a new kernel and only activate those device drivers that you actually need.
{MB} You should use an ext2 filesystem. Alternatively, you may use BSD style ufs. The partition type number for ext2fs is 0x83 (this is the same as in Linux), not 0x63 (GNU HURD). Thomas explains why 0x63 is the wrong choice:
One day we may have a new filesystem format, which would probably be called the GNU Hurd filesystem format, and might well use that partition code.
Regardless, as Roland points out, it is always an error to use that code for an ext2fs partition,
{MB,NHW} I knew you would ask this. If I had to reduce this FAQ to only one question, I'd choose this one. It is pretty easy, but you have to know that there are actually several incompatibly naming convention.
First, the Hurd: if the disk is question is a SCSI disk, you must know the SCSI device number; if it is an IDE disk, you must know what controller the disk is on and whether it is a master or a slave. The Hurd uses the BSD naming convention which, is to say, disks are ordered according to their physical location, numerically, starting from zero. This naming scheme is quite similar to that found in Linux. There, the master disk on the primary controller is designated as `hda' and the slave as `hdb'. On the secondary controller, the master and the slave are designated by `hdc' and `hdd' respectively. Under the Hurd, `hda' would become `hd0', `hdb' would be referred to as `hd1', etc.
In the Hurd, like in BSD, partitions are called `slices' and are numbered starting from one. Thus, to name a particular partition, we take the disk name, append a `s' and the partition number. Again, this is similar to Linux except, there is no `s'. For instance, `hda1' would become `hd0s1'.
GRUB, the boot loader, uses a completely different nomenclature: it probes the BIOS and appends each disk in turn to an array. Both disks and partitions are enumerated using zero based arrays. The format is: `hd (<disk>, <partition>)'. Thus, `hd (0, 1)' refers to the second partition on the first drive detected by the BIOS. As Grub now has tab completion, there is not a lot of guess work.
{MB} No, not currently. The filesystem servers need to be changed to not map the whole store into memory, which is not too difficult. For large files, some interfaces need to be changed, which is a bit harder but still doable.
{NHW} Generally, a lot; once you run out, Mach panics. I have at least 128MB of ram and 256MB of swap on all of machines running GNU/Hurd.
{NHW} Yes. The default pager recognizes and respects Linux swap partitions. It will also swap to a raw partition, i.e. anything that it does not recognize as Linux swap. Therefore: BE CAREFUL.
{MB} You forgot to set the file system owner of the GNU/Hurd partition to ``hurd''. The Hurd uses additional information in the inodes to set translators. To make this work, the partition must be marked as ``owned by the Hurd''. This is normally done by passing the `-o hurd' option to `mke2fs' when creating ext2 system from other operating systems (filesystems created in GNU/Hurd automatically enable this option). If you failed to do this, you can still use the `e2os' script.
{MB} Funny, isn't it? In addition to the rescue `term' server in `/tmp/console', another `term' server got started and is clobbing the keyboard input. After a reboot this problem vanishes as only one `term' server will remain. If `tar' would support translator, we would not have this problem... Even if you don't experience this problem right after the installation, reboot immediately so you don't hit this bug by accident.
{MB} A swap partition is also called a paging file. Usually, it is sufficient to add the swap partition to `/etc/fstab', just as you would under Linux. You can swap to a Linux swap partition and the Hurd will honour the Linux swap signature (both versions). The Hurd will just as happily swap to any other raw disk space and overwrite anything it finds. So, be careful!
If you want to swap to a file or make sure that it checks the Linux swap signature before, you need to edit `/boot/servers.boot'. The syntax is the partition device file name plus, optionally, the swap file inside an ext2fs partition, followed by a space and then one of: `$(add-raw-paging-file)', `$(add-linux-paging-file)', `$(add-paging-file)'. The first works with any partition or file and does not honour any swap signature or other data. The second has a safety check and only uses the file if a Linux swap signature is found. The third looks for a swap signature first and falls back to raw paging if it failed to find one. This is also the default for entries in `/etc/fstab'.
{MB} In the former case, be sure that GNU Mach detected your network card. Either way, you need to setup `pfinet'. Documentation can be found at:
http://www.debian.org/ports/hurd/hurd-doc-server#pfinet
Don't forget to fill in `/etc/resolv.conf', `/etc/hosts', etc.
Of course, you only need to do this if the installation routine didn't do it for you.
{MB} Give them to the kernel (i.e. on the GRUB command line) and it will pass them down to `serverboot' automatically.
{MB} Yes, at least since `e2fsprogs-1.05'. Check this with `e2fsck -V' first.
{NHW} Do not try to defrag your partition as this utility does not know about translators.
{MB} `settrans -fgc /servers/socket/1 /hurd/pflocal' should help.
{NHW} There are neither man pages nor info nodes for the Hurd translators and commands. Documentation lives inside of the binaries and can be found by passing the `--help' option to a given command. For instance:
# /hurd/ext2fs --help
will tell you what types of options the ext2fs translator accepts.
{MB} There is a text about translators available at:
http://www.debian.org/ports/hurd/hurd-doc-translator
Read this and ask further questions on the mailing lists.
{MB} The Hurd has the concept of a not-logged in user. This user has neither user ids nor groups ids. This stems from the fact that the Hurd supports uid and gid sets and one possibility is, of course, the empty set. Rather than deny access in this case, filesystems in the Hurd offer a forth permission triplet (i.e. rwx) which is used to determine the privileges for users with no credentials. This, however, needs to be enabled on a file by file basis. By default, the `other' permission triplet is used.
The Hurd login shell is a shell running with neither uids nor gids. To restrict access to your files, either enable and change the fourth permission triplet or change the login shell of the `login' user in the password file to `/bin/loginpr' which implements the standard login prompt.
{NHW} Take a look at:
ftp://alpha.gnu.org/gnu/hurd/contrib/marcus/keymap.tar.gz
{NHW} If you are using the GNU Mach microkernel, you can set your terminal to `mach-color'. For instance:
# export TERM=mach-color
{NHW} There is currently no support for virtual consoles. You can, however, use `screen'. It is much more flexible anyway.
Kalle Olavi Niemitalo <tosi@ees2.oulu.fi> began working on colortext. His aim was to provide a featureful program that multiplexes the console in userspace by reading from the /dev/kdb device. Check:
http://stekt.oulu.fi/~tosi/
{MB} Here are some useful options to add to your ~/.screenrc file to get it to emulate Linux virtual terminals a bit more closely:
bindkey ^[O0 select 0 bindkey ^[O1 select 1 bindkey ^[O2 select 2 bindkey ^[O3 select 3 bindkey ^[O4 select 4 bindkey ^[O5 select 5 bindkey ^[O6 select 6 bindkey ^[O7 select 7 bindkey ^[O8 select 8 bindkey ^[O9 select 9
# Unfortunately, Mach ignores modifiers on Cursor keys. This can be # fixed, however, for now, we shall just use other keys to move back, # and forth. # ^[, is ALT and "," # ^[. is ALT and "." bindkey ^[, prev bindkey ^[. next
{MB} It works! The packages are available at any Debian ftp mirror. XFree86 4.0.2 is available, as are some of the v3 servers. Instructions on how to use the packages are in the mailing list archive (link follow later).
{MB} Try `export LD_LIBRARY_PATH=/usr/X11R6/lib'. This is a linker issue. GNU/Hurd expects that `RPATH' is used, however, Debian takes certain measures to avoid this. Note that this does not cut it for suid binaries because of security implications. We expect to rectify this by using `RUNPATH', which is specified in the new ELF standard.
{MB} Two is the kernel, three is the default pager and four is the root filesystem.
{MB} Try `ps Aux'.
{MB} Interrupt it and pass it the `-M' option.
{NHW} By default, `ps' gathers information from both the proc server and the processes themselves (via their message port). If a process it hung, it will not be able respond to its message port and thus, ps will wait forever. The `-M' option instructs ps to not gather information that would require use of the message port.
{MB} `fdisk' and `dmesg' are not yet ported.
Instead of `free', use `vmstat' and `vminfo'.
For kernel messages, read `/dev/klog' directly. Note, once you read this, it is gone forever.
{NHW} `ifconfig' does not exist. Try:
# /hurd/pfinet --help
As for dhcp, take a look at hurd/trans/pump.c; this is the start of a dhcp client for the Hurd.
{MB} No. Maybe there will be an emulation filesystem someday for programs that rely it. If you are wondering about the empty `/proc' directory, this is a relict from a Debian GNU/Linux package (specifically, `base-files').
You can probe for existing hardware devices with the devprobe utility.
{NHW} It does, you just have to tell it what filesystem to query. E.g.
# df /
{NHW} Try passing the `-ap' flag to settrans. By default, settrans only sets a passive translator, therefore, no output will show up on your terminal. Using `-ap', however, sets both the active and the passive translator which, means that the translator starts immediately and its stderr is connected to you terminal.
Additionally, the biggest problem is passing relative paths to passive translators. You cannot predict what the current working directory of a translator will be when it is setup as a passive translator.
{MB} It is important to understand that there is nothing special about a directory under the Hurd, it is just another file. This fits in with the translator concept where a translator can appear as a directory but provide also as a file.
{MB} APM is not supported in the current version of GNU Mach, sorry. Please disable APM in your BIOS setup.
{MB} The default pager handles virtual memory allocation. If it can't allocate a new memory page because you are out of memory, some terrible things may happen. Whenever you get errors referring to any of these, you either need more memory (make sure you have swap) or you have found a memory leak.
{MB} This comes from `strerror(EGRATUITOUS)'. If you check glibc's documentation, it will say that this error code has no purpose. This, however, is not quite true. You only get this when something terrible happens. Thomas explains:
More precisely `EGRATUITOUS' officially means that some server has responded in an impossible or protocol-violating fashion. There are some cases in the Hurd where `EGRATUITOUS' is returned and probably something else should be chosen instead.
If you can reproduce this error message, please report it.
{NHW} In this case, /dev/hd0s1 was not unmounted cleanly. The Hurd will, on boot up, run ``fsck -p'' on any partitions that it finds in /etc/fstab, so, you may want to consider adding this partition to that file. If you are sure that the partition is fine, you can run:
# fsysopts /home --writable
to ask the translator sitting on /home to change from read-only to read/write mode. Note that the command is being sent to the filesystem and not the store (e.g. /dev/hd0s1).
{MB} Pass the `-H' option to init (add it to the boot command line), and `init' will tell Mach to enter the kernel debugger instead to rebooting it. At the debugger prompt (`db>'), you can type `reboot' any time to reboot the system.
{NHW} A lot, take a look at the Debian archive. Many programs, however, do not necessarily need to be ported; they have just never been compiled.
{NHW} Porting applications to GNU/Hurd is relatively easy assuming the application is POSIX compliant as GNU/Hurd does its best to be a conforming operating system.
The most common error made by programmers is assuming the MAXPATHLEN and PATH_MAX are defined. On most operating systems this is set to a few thousand, however, on GNU/Hurd, there is no maximum and thus, this is not set. The correct thing to do is to submit a patch to the upstream author that allocates memory dynamically.
{NHW} A effort to compile all of the Debian packages is underway by Jeff Bailey. Take a look at:
http://people.debian.org/~jbailey/oasis/group/Debian/index.html
to see what has been done and how you can help.
{NHW} The source is stored in CVS at:
:pserver:anoncvs@subversions.gnu.org:/cvsroot/hurd
You do not need a password to login. The modules that you are interested in are: `hurd', `mach' and `mig'.
A web interface is also available at: http://subversions.gnu.org.
To get the source to the latest debian package, look on any debian mirror.
{NHW} Yes. If you are running Debian GNU/Linux on IA32, this is quite easy as there is a cheap cross compiler available; all that is required is installing the gcc-i386-gnu and mig-i386-gnu Debian packages. When running configure, you will have to specify tools directly:
# MIG=/usr/bin/i386-gnu-mig CC=/usr/bin/i386-gnu-gcc \ ../src/hurd/configure ...
If you are running another distribution, you will have to do this the long way. You can find instructions at the Cross Compiling HOW-TO available at:
http://hurddocs.sourceforge.net/howto/cross.html
Farid Hajji <farid.hajji@ob.kamp.net> also talks about his experiences at:
http://lists.debian.org/debian-hurd-0012/msg00062.html
{NHW} Yeah, building in the source tree is untested. Try:
# ../src/hurd/configure ...
{NHW} There are two versions of Mach: GNU Mach and OSKit-Mach. The former uses the drivers from Linux 2.0.x while the latter uses the University of Utah's OSKit. If all goes well, OSKit-Mach will be stabilized and become the official kernel. You can find out more about the OSKit at:
http://www.cs.utah.edu/flux/oskit
{NHW} There were several books written on the Mach kernel. The information that they contain is still mostly pertinent and should be considered required reading for potential hackers. They can be found at:
http://www.cs.cmu.edu/afs/cs/project/mach/public/www/doc/publications.html
The documentation for the Hurd is quite inadequate. The starting of a book, ``The GNU Hurd'' is in the doc directory in the Hurd source. You can read this using:
# info hurd
The authoritative place is, of course, the source code; that does not, however, mean that we would not welcome more documentation. To get started, take a look at <hurd>/doc/navigating.
{NHW} Unfortunately, you have to buy the POSIX standard from IEEE. The Single Unix Specification version 2, a superset of POSIX, is available for free on the Internet. Try:
http://www.unix-systems.org/online.html
{NHW} If they are against the Hurd, Mach or MiG, send a patch to the bug-hurd mailing list.
If they are against other packages, the Debian BTS is a good place. In this case, be sure to advise the debian-hurd mailing list of the bug.
{MB} All patches should be sent in unified context diff format (option `-u' to GNU diff). It is helpful for us if you also use the `-p' option which includes information about the function changed by a patch. Changes that are similar can be grouped together in one file, but unrelated changes should be sent in seperate files. The patches can be included in the message or as a MIME attachement. They should not be compressed and/or archived unless they are very large, and if they are very large it is probably better to store them on-line at some place and only sent an URL.
Write a ChangeLog entry for each change, following the format of the existing files. Here is an example:
2000-12-02 Marcus Brinkmann <marcus@gnu.org>
* ops.c (op_readlink): Before returning, check if the buffer pointed to by transp is ours. If not, munmap it. (op_read): Likewise for bp. (op_readdir): Don't alloca a buffer here. Instead initialize BUF and BUFSIZE to 0 and let the server (eh, MiG) do it. munmap BUF before returning.
The file name and the name of the function changed should always be spelled out completely, and not abbreviated or otherwise mangled (i.e. avoid foo.{c,h}), because this makes searching for all changes to a file or function nearly impossible. Local variable names are all capitalized. There are two spaces between sentences. You can use ``C-x 4 a'' in Emacs to add a new ChangeLog entry. If you do that with the mark being in a function, Emacs will automatically fill in the file and function name for you.
Do not send in a patch for the ChangeLog file. Rather include the ChangeLog entries in the message that contains the patch. Patches for ChangeLog files often conflict.
If you have the original source tree in the directory `hurd-orig', and the modified source tree in the directory `hurd', the following command will produce a good patch (please make sure there are no extra files like backups in the modified tree, or, in this case, do not use the `-N' option). You will need to collect the ChangeLog entries seperately.
# diff -x ChangeLog -Nurp hurd-orig hurd
Answers were given by:
{NHW} Neal H Walfield <neal@cs.uml.edu>
{MB} Marcus Brinkmann <Marcus.Brinkmann@ruhr-uni-bochum.de>