Ubuntu Manpage: guestfs-faq - libguestfs Frequently Asked Questions (FAQ)

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       guestfs-faq - libguestfs Frequently Asked Questions (FAQ)

ABOUT LIBGUESTFS

What is libguestfs?
libguestfs is a way to create, access and modify disk images. You can look inside disk images, modify
the files they contain, create them from scratch, resize them, and much more. It’s especially useful
from scripts and programs and from the command line.

libguestfs is a C library (hence "lib-"), and a set of tools built on this library, and bindings for many
common programming languages.

For more information about what libguestfs can do read the introduction on the home page
(http://libguestfs.org).

What are the virt tools?
Virt tools (website: http://virt-tools.org) are a whole set of virtualization management tools aimed at
system administrators. Some of them come from libguestfs, some from libvirt and many others from other
open source projects. So virt tools is a superset of libguestfs. However libguestfs comes with many
important tools. See http://libguestfs.org for a full list.

Does libguestfs need { libvirt / KVM / Red Hat / Fedora }?
No!

libvirt is not a requirement for libguestfs.

libguestfs works with any disk image, including ones created in VMware, KVM, qemu, VirtualBox, Xen, and
many other hypervisors, and ones which you have created from scratch.

Red Hat sponsors (ie. pays for) development of libguestfs and a huge number of other open source
projects. But you can run libguestfs and the virt tools on many different Linux distros and Mac OS X.
We try our best to support all Linux distros as first-class citizens. Some virt tools have been ported
to Windows.

How does libguestfs compare to other tools?
vs. kpartx
Libguestfs takes a different approach from kpartx. kpartx needs root, and mounts filesystems on the
host kernel (which can be insecure - see guestfs-security(1)). Libguestfs isolates your host kernel
from guests, is more flexible, scriptable, supports LVM, doesn't require root, is isolated from other
processes, and cleans up after itself. Libguestfs is more than just file access because you can use
it to create images from scratch.

vs. vdfuse
vdfuse is like kpartx but for VirtualBox images. See the kpartx comparison above. You can use
libguestfs on the partition files exposed by vdfuse, although it’s not necessary since libguestfs can
access VirtualBox images directly.

vs. qemu-nbd
NBD (Network Block Device) is a protocol for exporting block devices over the network. qemu-nbd is
an NBD server which can handle any disk format supported by qemu (eg. raw, qcow2). You can use
libguestfs and qemu-nbd or nbdkit together to access block devices over the network, for example:
"guestfish -a nbd://remote"

vs. mounting filesystems in the host
Mounting guest filesystems in the host is insecure and should be avoided completely for untrusted
guests. Use libguestfs to provide a layer of protection against filesystem exploits. See also
guestmount(1).

vs. parted
Libguestfs supports LVM. Libguestfs uses parted and provides most parted features through the
libguestfs API.

GETTING HELP AND REPORTING BUGS

   How do I know what version I'm using?
       The simplest method is:

        guestfish --version

       Libguestfs  development happens along an unstable branch and we periodically create a stable branch which
       we backport stable patches to.  To find out more, read "LIBGUESTFS VERSION NUMBERS" in guestfs(3).

   How can I get help?
   What mailing lists or chat rooms are available?
       If you  are  a  Red Hat  customer  using  Red  Hat  Enterprise  Linux,  please  contact  Red Hat Support:
       http://redhat.com/support

       There  is  a  mailing  list,  mainly  for  development, but users are also welcome to ask questions about
       libguestfs and the virt tools: https://lists.libguestfs.org

       You can also talk to us on IRC channel "#guestfs" on Libera Chat.  We're not  always  around,  so  please
       stay in the channel after asking your question and someone will get back to you.

       For  other  virt  tools  (not  ones supplied with libguestfs) there is a general virt tools mailing list:
       https://www.redhat.com/mailman/listinfo/virt-tools-list

   How do I report bugs?
       Please use the following link to enter a bug in Bugzilla:

       https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools

       Include as much detail as you can and a way to reproduce the problem.

       Include the full output of libguestfs-test-tool(1).

COMMON PROBLEMS

See also "LIBGUESTFS GOTCHAS" in guestfs(3) for some "gotchas" with using the libguestfs API.

"Could not allocate dynamic translator buffer"
This obscure error is in fact an SELinux failure. You have to enable the following SELinux boolean:

setsebool -P virt_use_execmem=on

For more information see https://bugzilla.redhat.com/show_bug.cgi?id=806106.

"child process died unexpectedly"
[This error message was changed in libguestfs 1.21.18 to something more explanatory.]

This error indicates that qemu failed or the host kernel could not boot. To get further information
about the failure, you have to run:

libguestfs-test-tool

If, after using this, you still don’t understand the failure, contact us (see previous section).

libguestfs: error: cannot find any suitable libguestfs supermin, fixed or old-style appliance on
LIBGUESTFS_PATH
febootstrap-supermin-helper: ext2: parent directory not found
supermin-helper: ext2: parent directory not found
[This issue is fixed permanently in libguestfs ≥ 1.26.]

If you see any of these errors on Debian/Ubuntu, you need to run the following command:

sudo update-guestfs-appliance

"Permission denied" when running libguestfs as root
You get a permission denied error when opening a disk image, even though you are running libguestfs as
root.

This is caused by libvirt, and so only happens when using the libvirt backend. When run as root, libvirt
decides to run the qemu appliance as user "qemu.qemu". Unfortunately this usually means that qemu cannot
open disk images, especially if those disk images are owned by root, or are present in directories which
require root access.

There is a bug open against libvirt to fix this: https://bugzilla.redhat.com/show_bug.cgi?id=1045069

You can work around this by one of the following methods:

• Switch to the direct backend:

export LIBGUESTFS_BACKEND=direct

• Don’t run libguestfs as root.

• Chmod the disk image and any parent directories so that the qemu user can access them.

• (Nasty) Edit /etc/libvirt/qemu.conf and change the "user" setting.

execl: /init: Permission denied
Note: If this error happens when you are using a distro package of libguestfs (eg. from Fedora, Debian,
etc) then file a bug against the distro. This is not an error which normal users should ever see if the
distro package has been prepared correctly.

This error happens during the supermin boot phase of starting the appliance:

supermin: mounting new root on /root
supermin: chroot
execl: /init: Permission denied
supermin: debug: listing directory /
[...followed by a lot of debug output...]

This is a complicated bug related to supermin(1) appliances. The appliance is constructed by copying
files like /bin/bash and many libraries from the host. The file "hostfiles" lists the files that should
be copied from the host into the appliance. If some files don't exist on the host then they are missed
out, but if these files are needed in order to (eg) run /bin/bash then you'll see the above error.

Diagnosing the problem involves studying the libraries needed by /bin/bash, ie:

ldd /bin/bash

comparing that with "hostfiles", with the files actually available in the host filesystem, and with the
debug output printed in the error message. Once you've worked out which file is missing, install that
file using your package manager and try again.

You should also check that files like /init and /bin/bash (in the appliance) are executable. The debug
output shows file modes.

DOWNLOADING, INSTALLING, COMPILING LIBGUESTFS

Where can I get the latest binaries for ...?
Fedora ≥ 11
Use:

yum install '*guestf*'

For the latest builds, see: http://koji.fedoraproject.org/koji/packageinfo?packageID=8391

Red Hat Enterprise Linux
RHEL 6
RHEL 7
It is part of the default install. On RHEL 6 and 7 (only) you have to install
"libguestfs-winsupport" to get Windows guest support.

Debian and Ubuntu
For libguestfs < 1.26, after installing libguestfs you need to do:

sudo update-guestfs-appliance

(This script has been removed on Debian/Ubuntu with libguestfs ≥ 1.26 and instead the appliance is
built on demand.)

On Ubuntu only:

sudo chmod 0644 /boot/vmlinuz*

You may need to add yourself to the "kvm" group:

sudo usermod -a -G kvm yourlogin

Debian Squeeze (6)
Hilko Bengen has built libguestfs in squeeze backports:
http://packages.debian.org/search?keywords=guestfs&searchon=names&section=all&suite=squeeze-backports

Debian Wheezy and later (7+)
Hilko Bengen supports libguestfs on Debian. Official Debian packages are available:
http://packages.debian.org/search?keywords=libguestfs

Ubuntu
We don’t have a full time Ubuntu maintainer, and the packages supplied by Canonical (which are
outside our control) are sometimes broken.

Canonical decided to change the permissions on the kernel so that it's not readable except by
root. This is completely stupid, but they won't change it
(https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725). So every user should do this:

sudo chmod 0644 /boot/vmlinuz*

Ubuntu 12.04
libguestfs in this version of Ubuntu works, but you need to update febootstrap and seabios to
the latest versions.

You need febootstrap ≥ 3.14-2 from: http://packages.ubuntu.com/precise/febootstrap

After installing or updating febootstrap, rebuild the appliance:

sudo update-guestfs-appliance

You need seabios ≥ 0.6.2-0ubuntu2.1 or ≥ 0.6.2-0ubuntu3 from:
http://packages.ubuntu.com/precise-updates/seabios or
http://packages.ubuntu.com/quantal/seabios

Also you need to do (see above):

sudo chmod 0644 /boot/vmlinuz*

Gentoo
Libguestfs was added to Gentoo in 2012-07 by Andreis Vinogradovs (libguestfs) and Maxim Koltsov
(mainly hivex). Do:

emerge libguestfs

Mageia
Libguestfs was added to Mageia in 2013-08. Do:

urpmi libguestfs

SuSE
Libguestfs was added to SuSE in 2012 by Olaf Hering.

ArchLinux
Libguestfs was added to the AUR in 2010.

Other Linux distro
Compile from source (next section).

Other non-Linux distro
You'll have to compile from source, and port it.

How can I compile and install libguestfs from source?
You can compile libguestfs from git or a source tarball. Read the README file before starting.

Git: https://github.com/libguestfs/libguestfs Source tarballs: http://libguestfs.org/download

Don’t run "make install"! Use the "./run" script instead (see README).

How can I compile and install libguestfs if my distro doesn't have new enough qemu/supermin/kernel?
Libguestfs needs supermin 5. If supermin 5 hasn't been ported to your distro, then see the question
below.

First compile qemu, supermin and/or the kernel from source. You do not need to "make install" them.

In the libguestfs source directory, create two files. "localconfigure" should contain:

source localenv
#export PATH=/tmp/qemu/x86_64-softmmu:$PATH
./configure --prefix /usr "$@"

Make "localconfigure" executable.

"localenv" should contain:

#export SUPERMIN=/tmp/supermin/src/supermin
#export LIBGUESTFS_HV=/tmp/qemu/x86_64-softmmu/qemu-system-x86_64
#export SUPERMIN_KERNEL=/tmp/linux/arch/x86/boot/bzImage
#export SUPERMIN_KERNEL_VERSION=4.XX.0
#export SUPERMIN_MODULES=/tmp/lib/modules/4.XX.0

Uncomment and adjust these lines as required to use the alternate programs you have compiled.

Use "./localconfigure" instead of "./configure", but otherwise you compile libguestfs as usual.

Don’t run "make install"! Use the "./run" script instead (see README).

How can I compile and install libguestfs without supermin?
If supermin 5 supports your distro, but you don’t happen to have a new enough supermin installed, then
see the previous question.

If supermin 5 doesn't support your distro at all, you will need to use the "fixed appliance method" where
you use a pre-compiled binary appliance. To build libguestfs without supermin, you need to pass
"--disable-appliance --disable-daemon" to either ./configure or ./configure (depending whether you are
building respectively from git or from tarballs). Then, when using libguestfs, you must set the
"LIBGUESTFS_PATH" environment variable to the directory of a pre-compiled appliance, as also described in
"FIXED APPLIANCE" in guestfs-internals(1).

For pre-compiled appliances, see also: http://libguestfs.org/download/binaries/appliance/.

Patches to port supermin to more Linux distros are welcome.

How can I add support for sVirt?
Note for Fedora/RHEL users: This configuration is the default starting with Fedora 18 and RHEL 7. If you
find any problems, please let us know or file a bug.

SVirt provides a hardened appliance using SELinux, making it very hard for a rogue disk image to "escape"
from the confinement of libguestfs and damage the host (it's fair to say that even in standard libguestfs
this would be hard, but sVirt provides an extra layer of protection for the host and more importantly
protects virtual machines on the same host from each other).

Currently to enable sVirt you will need libvirt ≥ 0.10.2 (1.0 or later preferred), libguestfs ≥ 1.20, and
the SELinux policies from recent Fedora. If you are not running Fedora 18+, you will need to make
changes to your SELinux policy - contact us on the mailing list.

Once you have the requirements, do:

./configure --with-default-backend=libvirt # libguestfs >= 1.22
./configure --with-default-attach-method=libvirt # libguestfs <= 1.20
make

Set SELinux to Enforcing mode, and sVirt should be used automatically.

All, or almost all, features of libguestfs should work under sVirt. There is one known shortcoming:
virt-rescue(1) will not use libvirt (hence sVirt), but falls back to direct launch of qemu. So you won't
currently get the benefit of sVirt protection when using virt-rescue.

You can check if sVirt is being used by enabling libvirtd logging (see /etc/libvirt/libvirtd.log),
killing and restarting libvirtd, and checking the log files for "Setting SELinux context on ..."
messages.

In theory sVirt should support AppArmor, but we have not tried it. It will almost certainly require
patching libvirt and writing an AppArmor policy.

Libguestfs has a really long list of dependencies!
The base library doesn't depend on very much, but there are three causes of the long list of other
dependencies:

1. Libguestfs has to be able to read and edit many different disk formats. For example, XFS support
requires XFS tools.

2. There are language bindings for many different languages, all requiring their own development tools.
All language bindings (except C) are optional.

3. There are some optional library features which can be disabled.

Since libguestfs ≥ 1.26 it is possible to split up the appliance dependencies (item 1 in the list above)
and thus have (eg) "libguestfs-xfs" as a separate subpackage for processing XFS disk images. We
encourage downstream packagers to start splitting the base libguestfs package into smaller subpackages.

Errors during launch on Fedora ≥ 18, RHEL ≥ 7
In Fedora ≥ 18 and RHEL ≥ 7, libguestfs uses libvirt to manage the appliance. Previously (and upstream)
libguestfs runs qemu directly:

┌──────────────────────────────────┐
│ libguestfs │
├────────────────┬─────────────────┤
│ direct backend │ libvirt backend │
└────────────────┴─────────────────┘
↓ ↓
┌───────┐ ┌──────────┐
│ qemu │ │ libvirtd │
└───────┘ └──────────┘
↓
┌───────┐
│ qemu │
└───────┘

upstream Fedora 18+
non-Fedora RHEL 7+
non-RHEL

The libvirt backend is more sophisticated, supporting SELinux/sVirt (see above) and more. It is,
however, more complex and so less robust.

If you have permissions problems using the libvirt backend, you can switch to the direct backend by
setting this environment variable:

export LIBGUESTFS_BACKEND=direct

before running any libguestfs program or virt tool.

How can I switch to a fixed / prebuilt appliance?
This may improve the stability and performance of libguestfs on Fedora and RHEL.

Any time after installing libguestfs, run the following commands as root:

mkdir -p /usr/local/lib/guestfs/appliance
libguestfs-make-fixed-appliance /usr/local/lib/guestfs/appliance
ls -l /usr/local/lib/guestfs/appliance

Now set the following environment variable before using libguestfs or any virt tool:

export LIBGUESTFS_PATH=/usr/local/lib/guestfs/appliance

Of course you can change the path to any directory you want. You can share the appliance across machines
that have the same architecture (eg. all x86-64), but note that libvirt will prevent you from sharing the
appliance across NFS because of permissions problems (so either switch to the direct backend or don't use
NFS).

How can I speed up libguestfs builds?
By far the most important thing you can do is to install and properly configure Squid. Note that the
default configuration that ships with Squid is rubbish, so configuring it is not optional.

A very good place to start with Squid configuration is here:
https://fedoraproject.org/wiki/Extras/MockTricks#Using_Squid_to_Speed_Up_Mock_package_downloads

Make sure Squid is running, and that the environment variables $http_proxy and $ftp_proxy are pointing to
it.

With Squid running and correctly configured, appliance builds should be reduced to a few minutes.

How can I speed up libguestfs builds (Debian)?

Hilko Bengen suggests using "approx" which is a Debian archive proxy (http://packages.debian.org/approx).
This tool is documented on Debian in the approx(8) manual page.

SPEED, DISK SPACE USED BY LIBGUESTFS

Note: Most of the information in this section has moved: guestfs-performance(1).

Upload or write seem very slow.
If the underlying disk is not fully allocated (eg. sparse raw or qcow2) then writes can be slow because
the host operating system has to do costly disk allocations while you are writing. The solution is to use
a fully allocated format instead, ie. non-sparse raw, or qcow2 with the "preallocation=metadata" option.

Libguestfs uses too much disk space!
libguestfs caches a large-ish appliance in:

/var/tmp/.guestfs-<UID>

If the environment variable "TMPDIR" is defined, then $TMPDIR/.guestfs-<UID> is used instead.

It is safe to delete this directory when you are not using libguestfs.

virt-sparsify seems to make the image grow to the full size of the virtual disk
If the input to virt-sparsify(1) is raw, then the output will be raw sparse. Make sure you are measuring
the output with a tool which understands sparseness such as "du -sh". It can make a huge difference:

$ ls -lh test1.img
-rw-rw-r--. 1 rjones rjones 100M Aug 8 08:08 test1.img
$ du -sh test1.img
3.6M test1.img

(Compare the apparent size 100M vs the actual size 3.6M)

If all this confuses you, use a non-sparse output format by specifying the --convert option, eg:

virt-sparsify --convert qcow2 disk.raw disk.qcow2

Why doesn't virt-resize work on the disk image in-place?
Resizing a disk image is very tricky -- especially making sure that you don't lose data or break the
bootloader. The current method effectively creates a new disk image and copies the data plus bootloader
from the old one. If something goes wrong, you can always go back to the original.

If we were to make virt-resize work in-place then there would have to be limitations: for example, you
wouldn't be allowed to move existing partitions (because moving data across the same disk is most likely
to corrupt data in the event of a power failure or crash), and LVM would be very difficult to support
(because of the almost arbitrary mapping between LV content and underlying disk blocks).

Another method we have considered is to place a snapshot over the original disk image, so that the
original data is untouched and only differences are recorded in the snapshot. You can do this today
using "qemu-img create" + "virt-resize", but qemu currently isn't smart enough to recognize when the same
block is written back to the snapshot as already exists in the backing disk, so you will find that this
doesn't save you any space or time.

In summary, this is a hard problem, and what we have now mostly works so we are reluctant to change it.

Why doesn't virt-sparsify work on the disk image in-place?
In libguestfs ≥ 1.26, virt-sparsify can now work on disk images in place. Use:

virt-sparsify --in-place disk.img

But first you should read "IN-PLACE SPARSIFICATION" in virt-sparsify(1).

PROBLEMS OPENING DISK IMAGES

Remote libvirt guests cannot be opened.
Opening remote libvirt guests is not supported at this time. For example this won't work:

guestfish -c qemu://remote/system -d Guest

To open remote disks you have to export them somehow, then connect to the export. For example if you
decided to use NBD:

remote$ qemu-nbd -t -p 10809 guest.img
local$ guestfish -a nbd://remote:10809 -i

Other possibilities include ssh (if qemu is recent enough), NFS or iSCSI. See "REMOTE STORAGE" in
guestfs(3).

How can I open this strange disk source?
You have a disk image located inside another system that requires access via a library / HTTP / REST /
proprietary API, or is compressed or archived in some way. (One example would be remote access to
OpenStack glance images without actually downloading them.)

We have a sister project called nbdkit (https://github.com/libguestfs/nbdkit). This project lets you
turn any disk source into an NBD server. Libguestfs can access NBD servers directly, eg:

guestfish -a nbd://remote

nbdkit is liberally licensed, so you can link it to or include it in proprietary libraries and code. It
also has a simple, stable plugin API so you can easily write plugins against the API which will continue
to work in future.

Error opening VMDK disks: "uses a vmdk feature which is not supported by this qemu version: VMDK version 3"
Qemu (and hence libguestfs) only supports certain VMDK disk images. Others won't work, giving this or
similar errors.

Ideally someone would fix qemu to support the latest VMDK features, but in the meantime you have three
options:

1. If the guest is hosted on a live, reachable ESX server, then locate and download the disk image
called somename-flat.vmdk. Despite the name, this is a raw disk image, and can be opened by
anything.

If you have a recent enough version of qemu and libguestfs, then you may be able to access this disk
image remotely using either HTTPS or ssh. See "REMOTE STORAGE" in guestfs(3).

2. Use VMware’s proprietary vdiskmanager tool to convert the image to raw format.

3. Use nbdkit with the proprietary VDDK plugin to live export the disk image as an NBD source. This
should allow you to read and write the VMDK file.

UFS disks (as used by BSD) cannot be opened.
The UFS filesystem format has many variants, and these are not self-identifying. The Linux kernel has to
be told which variant of UFS it has to use, which libguestfs cannot know.

You have to pass the right "ufstype" mount option when mounting these filesystems.

See https://www.kernel.org/doc/Documentation/filesystems/ufs.txt

Windows ReFS
Windows ReFS is Microsoft’s ZFS/Btrfs copy. This filesystem has not yet been reverse engineered and
implemented in the Linux kernel, and therefore libguestfs doesn't support it. At the moment it seems to
be very rare "in the wild".

Non-ASCII characters don’t appear on VFAT filesystems.
Typical symptoms of this problem:

• You get an error when you create a file where the filename contains non-ASCII characters,
particularly non 8-bit characters from Asian languages (Chinese, Japanese, etc). The filesystem is
VFAT.

• When you list a directory from a VFAT filesystem, filenames appear as question marks.

This is a design flaw of the GNU/Linux system.

VFAT stores long filenames as UTF-16 characters. When opening or returning filenames, the Linux kernel
has to translate these to some form of 8 bit string. UTF-8 would be the obvious choice, except for Linux
users who persist in using non-UTF-8 locales (the user’s locale is not known to the kernel because it’s a
function of libc).

Therefore you have to tell the kernel what translation you want done when you mount the filesystem. The
two methods are the "iocharset" parameter (which is not relevant to libguestfs) and the "utf8" flag.

So to use a VFAT filesystem you must add the "utf8" flag when mounting. From guestfish, use:

><fs> mount-options utf8 /dev/sda1 /

or on the guestfish command line:

guestfish [...] -m /dev/sda1:/:utf8

or from the API:

guestfs_mount_options (g, "utf8", "/dev/sda1", "/");

The kernel will then translate filenames to and from UTF-8 strings.

We considered adding this mount option transparently, but unfortunately there are several problems with
doing that:

• On some Linux systems, the "utf8" mount option doesn't work. We don't precisely understand what
systems or why, but this was reliably reported by one user.

• It would prevent you from using the "iocharset" parameter because it is incompatible with "utf8". It
is probably not a good idea to use this parameter, but we don't want to prevent it.

Non-ASCII characters appear as underscore (_) on ISO9660 filesystems.
The filesystem was not prepared correctly with mkisofs or genisoimage. Make sure the filesystem was
created using Joliet and/or Rock Ridge extensions. libguestfs does not require any special mount options
to handle the filesystem.

Cannot open Windows guests which use NTFS.
You see errors like:

mount: unknown filesystem type 'ntfs'

On Red Hat Enterprise Linux or CentOS < 7.2, you have to install the libguestfs-winsupport package. In
RHEL ≥ 7.2, "libguestfs-winsupport" is part of the base RHEL distribution, but see the next question.

"mount: unsupported filesystem type" with NTFS in RHEL ≥ 7.2
In RHEL 7.2 we were able to add "libguestfs-winsupport" to the base RHEL distribution, but we had to
disable the ability to use it for opening and editing filesystems. It is only supported when used with
virt-v2v(1). If you try to use guestfish(1) or guestmount(1) or some other programs on an NTFS
filesystem, you will see the error:

mount: unsupported filesystem type

This is not a supported configuration, and it will not be made to work in RHEL. Don't bother to open a
bug about it, as it will be immediately "CLOSED -> WONTFIX".

You may compile your own libguestfs removing this restriction, but that won't be endorsed or supported by
Red Hat.

Cannot open or inspect RHEL 7 guests.
Cannot open Linux guests which use XFS.
RHEL 7 guests, and any other guests that use XFS, can be opened by libguestfs, but you have to install
the "libguestfs-xfs" package.

USING LIBGUESTFS IN YOUR OWN PROGRAMS

   The API has hundreds of methods, where do I start?
       We recommend you start by reading the API overview: "API OVERVIEW" in guestfs(3).

       Although the API overview covers the C API, it is still worth reading  even  if  you  are  going  to  use
       another  programming language, because the API is the same, just with simple logical changes to the names
       of the calls:

                         C  guestfs_ln_sf (g, target, linkname);
                    Python  g.ln_sf (target, linkname);
                     OCaml  g#ln_sf target linkname;
                      Perl  $g->ln_sf (target, linkname);
         Shell (guestfish)  ln-sf target linkname
                       PHP  guestfs_ln_sf ($g, $target, $linkname);

       Once you're familiar with the API overview, you should look at this list of  starting  points  for  other
       language bindings: "USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES" in guestfs(3).

   Can I use libguestfs in my proprietary / closed source / commercial program?
       In  general, yes.  However this is not legal advice - read the license that comes with libguestfs, and if
       you have specific questions contact a lawyer.

       In the source tree the license is in the file "COPYING.LIB" (LGPLv2+ for the library  and  bindings)  and
       "COPYING" (GPLv2+ for the standalone programs).

DEBUGGING LIBGUESTFS

Help, it’s not working!
If no libguestfs program seems to work at all, run the program below and paste the complete, unedited
output into an email to "guestfs@lists.libguestfs.org":

libguestfs-test-tool

If a particular operation fails, supply all the information in this checklist, in an email to
"libguestfs" @ "redhat.com":

1. What are you trying to do?

2. What exact command(s) did you run?

3. What was the precise error or output of these commands?

4. Enable debugging, run the commands again, and capture the complete output. Do not edit the output.

export LIBGUESTFS_DEBUG=1
export LIBGUESTFS_TRACE=1

5. Include the version of libguestfs, the operating system version, and how you installed libguestfs
(eg. from source, "yum install", etc.)

How do I debug when using any libguestfs program or tool (eg. virt-customize or virt-df)?
There are two "LIBGUESTFS_*" environment variables you can set in order to get more information from
libguestfs.

"LIBGUESTFS_TRACE"
Set this to 1 and libguestfs will print out each command / API call in a format which is similar to
guestfish commands.

"LIBGUESTFS_DEBUG"
Set this to 1 in order to enable massive amounts of debug messages. If you think there is some
problem inside the libguestfs appliance, then you should use this option.

To set these from the shell, do this before running the program:

export LIBGUESTFS_TRACE=1
export LIBGUESTFS_DEBUG=1

For csh/tcsh the equivalent commands would be:

setenv LIBGUESTFS_TRACE 1
setenv LIBGUESTFS_DEBUG 1

For further information, see: "ENVIRONMENT VARIABLES" in guestfs(3).

How do I debug when using guestfish?
You can use the same environment variables above. Alternatively use the guestfish options -x (to trace
commands) or -v (to get the full debug output), or both.

For further information, see: guestfish(1).

How do I debug when using the API?
Call "guestfs_set_trace" in guestfs(3) to enable command traces, and/or "guestfs_set_verbose" in
guestfs(3) to enable debug messages.

For best results, call these functions as early as possible, just after creating the guestfs handle if
you can, and definitely before calling launch.

How do I capture debug output and put it into my logging system?
Use the event API. For examples, see: "SETTING CALLBACKS TO HANDLE EVENTS" in guestfs(3) and the
examples/debug-logging.c program in the libguestfs sources.

Digging deeper into the appliance boot process.
Enable debugging and then read this documentation on the appliance boot process: guestfs-internals(1).

libguestfs hangs or fails during run/launch.
Enable debugging and look at the full output. If you cannot work out what is going on, file a bug
report, including the complete output of libguestfs-test-tool(1).

Debugging libvirt
If you are using the libvirt backend, and libvirt is failing, then you can enable debugging by editing
/etc/libvirt/libvirtd.conf.

If you are running as non-root, then you have to edit a different file. Create
~/.config/libvirt/libvirtd.conf containing:

log_level=1
log_outputs="1:file:/tmp/libvirtd.log"

Kill any session (non-root) libvirtd that is running, and next time you run the libguestfs command, you
should see a large amount of useful debugging information from libvirtd in /tmp/libvirtd.log

Broken kernel, or trying a different kernel.
You can choose a different kernel for the appliance by setting some supermin environment variables:

export SUPERMIN_KERNEL_VERSION=4.8.0-1.fc25.x86_64
export SUPERMIN_KERNEL=/boot/vmlinuz-$SUPERMIN_KERNEL_VERSION
export SUPERMIN_MODULES=/lib/modules/$SUPERMIN_KERNEL_VERSION
rm -rf /var/tmp/.guestfs-*
libguestfs-test-tool

Broken qemu, or trying a different qemu.
You can choose a different qemu by setting the hypervisor environment variable:

export LIBGUESTFS_HV=/path/to/qemu-system-x86_64
libguestfs-test-tool

DESIGN/INTERNALS OF LIBGUESTFS

DEVELOPERS

Where do I send patches?
Please send patches to the libguestfs mailing list https://lists.libguestfs.org. You don't have to be
subscribed, but there will be a delay until your posting is manually approved.

Please don’t use github pull requests - they will be ignored. The reasons are (a) we want to discuss and
dissect patches on the mailing list, and (b) github pull requests turn into merge commits but we prefer
to have a linear history.

How do I propose a feature?
Large new features that you intend to contribute should be discussed on the mailing list first
(https://lists.libguestfs.org). This avoids disappointment and wasted work if we don't think the feature
would fit into the libguestfs project.

If you want to suggest a useful feature but don’t want to write the code, you can file a bug (see
"GETTING HELP AND REPORTING BUGS") with "RFE: " at the beginning of the Summary line.

Who can commit to libguestfs git?
About 5 people have commit access to github. Patches should be posted on the list first and ACKed. The
policy for ACKing and pushing patches is outlined here:

https://www.redhat.com/archives/libguestfs/2012-January/msg00023.html

Can I fork libguestfs?
Of course you can. Git makes it easy to fork libguestfs. Github makes it even easier. It’s nice if you
tell us on the mailing list about forks and the reasons for them.

MISCELLANEOUS QUESTIONS

   Can I monitor the live disk activity of a virtual machine using libguestfs?
       A common request is to be able to use libguestfs to monitor the  live  disk  activity  of  a  guest,  for
       example, to get notified every time a guest creates a new file.  Libguestfs does not work in the way some
       people imagine, as you can see from this diagram:

                   ┌─────────────────────────────────────┐
                   │ monitoring program using libguestfs │
                   └─────────────────────────────────────┘
                                    ↓
        ┌───────────┐    ┌──────────────────────┐
        │ live VM   │    │ libguestfs appliance │
        ├───────────┤    ├──────────────────────┤
        │ kernel (1)│    │ appliance kernel (2) │
        └───────────┘    └──────────────────────┘
             ↓                      ↓ (r/o connection)
             ┌──────────────────────┐
             |      disk image      |
             └──────────────────────┘

       This  scenario  is  safe  (as  long  as  you set the "readonly" flag when adding the drive).  However the
       libguestfs appliance kernel (2) does not see all the changes made to the disk image, for two reasons:

       i.  The VM kernel (1) can cache data in memory, so it doesn't appear in the disk image.

       ii. The libguestfs appliance kernel (2) doesn't expect that the disk image is changing underneath it,  so
           its own cache is not magically updated even when the VM kernel (1) does update the disk image.

       The  only  supported  solution is to restart the entire libguestfs appliance whenever you want to look at
       changes in the disk image.  At the API level that corresponds to calling "guestfs_shutdown"  followed  by
       "guestfs_launch", which is a heavyweight operation (see also guestfs-performance(3)).

       There are some unsupported hacks you can try if relaunching the appliance is really too costly:

       •   Call  "guestfs_drop_caches  (g,  3)".   This  causes all cached data help by the libguestfs appliance
           kernel (2) to be discarded, so it goes back to the disk image.

           However this on its own is not sufficient, because qemu also caches some data.  You will also need to
           patch      libguestfs       to       (re-)enable       the       "cache=none"       mode.        See:
           https://rwmj.wordpress.com/2013/09/02/new-in-libguestfs-allow-cache-mode-to-be-selected/

       •   Use a tool like virt-bmap instead.

       •   Run an agent inside the guest.

       Nothing  helps  if  the  guest is making more fundamental changes (eg.  deleting filesystems).  For those
       kinds of things you must relaunch the appliance.

       (Note there is a third problem that you need to use consistent snapshots  to  really  examine  live  disk
       images, but that’s a general problem with using libguestfs against any live disk image.)

AUTHORS

       Richard W.M. Jones ("rjones at redhat dot com")

COPYRIGHT

       Copyright (C) 2012-2023 Red Hat Inc.

LICENSE

       This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser
       General  Public License as published by the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.

       This library is distributed in the hope that it will be useful, but WITHOUT ANY  WARRANTY;  without  even
       the  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General
       Public License for more details.

       You should have received a copy of the GNU Lesser General Public License along with this library; if not,
       write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

BUGS

       To     get      a      list      of      bugs      against      libguestfs,      use      this      link:
       https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools

       To       report       a       new       bug       against       libguestfs,      use      this      link:
       https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools

       When reporting a bug, please supply:

       •   The version of libguestfs.

       •   Where you got libguestfs (eg. which Linux distro, compiled from source, etc)

       •   Describe the bug accurately and give a way to reproduce it.

       •   Run libguestfs-test-tool(1) and paste the complete, unedited output into the bug report.

libguestfs-1.54.1                                  2025-02-18                                     guestfs-faq(1)

NAME

ABOUT LIBGUESTFS

GETTING HELP AND REPORTING BUGS

COMMON PROBLEMS

DOWNLOADING, INSTALLING, COMPILING LIBGUESTFS

SPEED, DISK SPACE USED BY LIBGUESTFS

PROBLEMS OPENING DISK IMAGES

USING LIBGUESTFS IN YOUR OWN PROGRAMS

DEBUGGING LIBGUESTFS

DESIGN/INTERNALS OF LIBGUESTFS

DEVELOPERS

MISCELLANEOUS QUESTIONS

SEE ALSO

AUTHORS

COPYRIGHT

LICENSE

BUGS