Frequently Asked Questions about the GNU Hurd

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.
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, with an answer.

Each section is copyright 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 COPYRIGHT.

                                                     Neal H Walfield
? Installation

??	What partition type should I use for my GNU/Hurd partitions?

{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, 

??	How do I name partitions?

{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.  Disks are enumerated
using zero based arrays. GRUB 2 enumerates partitions from one, while GRUB 1
used to enumerate them from zero.
The format is:
`hd (<disk>, <partition>)'.  Thus, in GRUB 2 `hd (0, 1)' refers to the first
partition on the first drive detected by the BIOS (or the second partition with
GRUB 1).  As Grub now has tab
completion, there is not a lot of guess work.

??	How much swap do I need?

{ST} The usual rule of thumb applies: the same amount as RAM, for instance.

??	Can I share swap space between GNU/Linux and GNU/Hurd?

{NHW} Yes.  The default pager recognises 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.

??	Why do I get ``Operation not permitted error''?

{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.

??	After `native-install' is finished, I had to write tthhiiss
	wwaayy.  In particular, I had to type `rreebboooott' to reboot.

{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

? Setup

??	How do I add a swap partition?

{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

??	How do I set up a network?
	How do I set up a loopback device?

{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:

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.

??	Can I use the GNU/Linux version of `e2fsck' on a GNU/Hurd partition?

{MB} Yes, at least since `e2fsprogs-1.05'.  Check this with `e2fsck -V'

{NHW} Do not try to defrag your partition as this utility does not know
about translators.

??	Why are pipes not working?

{MB} `settrans -fgc /servers/socket/1 /hurd/pflocal' should help.

? Usage

??	Where is the documentation?

{NHW,MM} 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.

The GNU/Hurd User's Guide and the GNU Hurd Reference Manual both
provide some help about the usage of and concepts behind the GNU Hurd.
You can find them, among others, at:

??	What is a translator?

{MB} There is a text about translators available at:

Read this and ask further questions on the mailing lists.

??	What is the login shell?

{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
fourth 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

??	How do I use non-US keyboard bindings?

{ST} On Debian Hurd, you can configure the console layout from

??	How do I enable color on the console?

{NHW} If you are using the GNU Mach microkernel, you can set your
terminal to `mach-gnu-color'.  For instance:

	# export TERM=mach-gnu-color

??	How can I enable virtual consoles?

{ST} On Debian Hurd, it is already enabled by default, and configured from

{AMS} This can be done by running the following command:

	console -d vga -d pc_kbd -d generic_speaker /dev/vcs

If something went wrong, or if you just wish to exit the Hurd console
then hitting C-A-<backspace> will exit it.

??	What is the status of X?

{MB} It works!  The packages are available at any Debian ftp mirror.
Xorg is available, as are some of the v3 servers.  Instructions
on how to use the packages are in the mailing list archive (link follow

??	What are these strange pids `0`, `2', `3', `4`, and `5`?

{MB,ST} Zero is the kernel, one is the root filesystem server, two is the
`exec` server (which handles starting programs), three is the init process, four
is the `proc` processus server, five is the `auth` server (which
handles user authentication).

??	Why does `ps aux' give me strange output?

{MB,MM} Try `ps Aux'.  Indeed, under GNU/Hurd, `ps aux' doesn't list
all processes: it omits the session and group leaders, and the
processes without parent.

??	I have a hung process that I want to kill, however, `ps' is now
	hanging too.

{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.

??	Why are my translators dying?

{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.

??	Why can I `read' a directory?

{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.

? Trouble shooting

??	When the APM support in the BIOS spins down my disk drives, the
	Hurd is unable to wake up.  What's wrong?

{MB} APM is not supported in the current version of GNU Mach, sorry.
Please disable APM in your BIOS setup.

??	What are these messages referring to `default pager', `paging',
	and `pager request'?

{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.

??	What is a gratuitous error?

{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.

??	What does ``computer bought the farm'' mean ?

{FH} This message is the text that corresponds to the errno code
`EIEIO'.  Roland McGrath explains:

	That message is not output by any particular servers at
	particular times; rather it is the perror text for the errno
	code EIEIO, which is returned by various RPCs and functions
	for a variety of "hopeless" error conditions.

??	What does ``/dev/hd0s1: MOUNTED READ-ONLY; MUST USE `fsysopts
	--writable''' mean?

{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).

??	When GNU/Hurd crashes, GNU Mach automatically reboots.  Is
	there anyway I can make it pause so I can write down the error?

{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.

? Porting

??	Is porting easy?

{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.

? Compiling

??	Where can I get the source?

{AMS} Instructions on how to download the CVS tree from Savanah are
available at

{NHW} To get the source to the latest debian package, look on any
debian mirror.

??	Can I cross compile?


{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:

Farid Hajji <> also talks about his experiences

??	Any general tips?

{NHW} Yeah, building in the source tree is untested.  Try:

	# ../src/hurd/configure ...

? Development

??	What is OSKit-Mach?

{NHW,FH} There have once been two versions of GNU Mach developed: GNU Mach
1.x and GNU Mach 2.x, formerly known as OSKit-Mach.  The former uses
the drivers from Linux 2.0.x while the latter uses the University of
Utah's OSKit library for drivers.  You can find out more about the
OSKit library at:

The project didn't work out too well, so development has again been halted.

??	Where is the documentation?

{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

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.

??	How do I make sure that my code is POSIX compliant?

{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:

??	Who do I submit patches to?

{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.

??	In what format should patches for the Hurd and GNU Mach be?

{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  <>

		* 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 (like
foo.{c,h}), because that would make searching for all changes to a
file or function 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 leave away the option
`-N').  You will need to collect the ChangeLog entries seperately.

	# diff -x ChangeLog -Nurp hurd-orig hurd

Answers were given by (in chronological order):
* {NHW} Neal H Walfield  <>
* {MB} Marcus Brinkmann  <>
* {AMS} Alfred M. Szmidt <>
* {OK} Ognyan Kulev	 <>
* {FH} Frédéric Henry	 <>
* {MM} Manuel Menal	 <>
* {ST} Samuel Thibault	 <>