395 lines
22 KiB
XML
395 lines
22 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE chapter [
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<!ENTITY % openstack SYSTEM "../common/entities/openstack.ent">
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%openstack;
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]>
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<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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version="5.0"
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xml:id="ch_modifying_images">
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<title>Modify images</title>
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<?dbhtml stop-chunking?>
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<para>Once you have obtained a virtual machine image, you may want
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to make some changes to it before uploading it to the
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OpenStack Image service. Here we describe several tools
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available that allow you to modify images.<warning>
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<para>Do not attempt to use these tools to modify an image
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that is attached to a running virtual machine. These
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tools are designed to only modify images that are not
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currently running.</para>
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</warning></para>
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<section xml:id="guestfish">
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<title>guestfish</title>
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<para>The <command>guestfish</command> program is a tool from
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the <link xlink:href="http://libguestfs.org/"
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>libguestfs</link> project that allows you to modify
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the files inside of a virtual machine image.</para>
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<note>
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<para><command>guestfish</command> does not mount the
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image directly into the local file system. Instead, it
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provides you with a shell interface that enables you
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to view, edit, and delete files. Many of
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<command>guestfish</command> commands, such as
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<command>touch</command>,
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<command>chmod</command>, and <command>rm</command>,
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resemble traditional bash commands.</para>
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</note>
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<simplesect>
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<title>Example guestfish session</title>
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<para>Sometimes, you must modify a virtual machine image
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to remove any traces of the MAC address that was
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assigned to the virtual network interface card when
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the image was first created, because the MAC address
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will be different when it boots the next time. This
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example shows how to use guestfish to remove
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references to the old MAC address by deleting the
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<filename>/etc/udev/rules.d/70-persistent-net.rules</filename>
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file and removing the <literal>HWADDR</literal> line
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from the
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<filename>/etc/sysconfig/network-scripts/ifcfg-eth0</filename>
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file.</para>
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<para>Assume that you have a CentOS qcow2 image called
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<filename>centos63_desktop.img</filename>. Mount
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the image in read-write mode as root, as
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follows:</para>
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<screen><prompt>#</prompt> <userinput>guestfish --rw -a centos63_desktop.img</userinput>
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<computeroutput>
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Welcome to guestfish, the libguestfs filesystem interactive shell for
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editing virtual machine filesystems.
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Type: 'help' for help on commands
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'man' to read the manual
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'quit' to quit the shell
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><fs></computeroutput></screen>
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<para>This starts a guestfish session. Note that the
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guestfish prompt looks like a fish: <literal>>
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<fs></literal>.</para>
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<para>We must first use the <command>run</command> command
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at the guestfish prompt before we can do anything
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else. This will launch a virtual machine, which will
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be used to perform all of the file
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manipulations.<screen><prompt>><fs></prompt> <userinput>run</userinput></screen>
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We can now view the file systems in the image using the
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<command>list-filesystems</command>
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command:<screen><prompt>><fs></prompt> <userinput>list-filesystems</userinput>
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<computeroutput>/dev/vda1: ext4
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/dev/vg_centosbase/lv_root: ext4
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/dev/vg_centosbase/lv_swap: swap</computeroutput></screen>We
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need to mount the logical volume that contains the
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root partition:
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<screen><prompt>><fs></prompt> <userinput>mount /dev/vg_centosbase/lv_root /</userinput></screen></para>
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<para>Next, we want to delete a file. We can use the
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<command>rm</command> guestfish command, which
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works the same way it does in a traditional
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shell.</para>
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<para><screen><prompt>><fs></prompt> <userinput>rm /etc/udev/rules.d/70-persistent-net.rules</userinput></screen>We
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want to edit the <filename>ifcfg-eth0</filename> file
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to remove the <literal>HWADDR</literal> line. The
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<command>edit</command> command will copy the file
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to the host, invoke your editor, and then copy the
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file back.
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<screen><prompt>><fs></prompt> <userinput>edit /etc/sysconfig/network-scripts/ifcfg-eth0</userinput></screen></para>
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<para>If you want to modify this image to load the 8021q
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kernel at boot time, you must create an executable
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script in the
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<filename>/etc/sysconfig/modules/</filename>
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directory. You can use the <command>touch</command>
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guestfish command to create an empty file, the
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<command>edit</command> command to edit it, and
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the <command>chmod</command> command to make it
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executable.<screen><prompt>><fs></prompt> <userinput>touch /etc/sysconfig/modules/8021q.modules</userinput>
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<prompt>><fs></prompt> <userinput>edit /etc/sysconfig/modules/8021q.modules</userinput></screen>
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We add the following line to the file and save
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it:<programlisting>modprobe 8021q</programlisting>Then
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we set to executable:
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<screen>><fs> <userinput>chmod 0755 /etc/sysconfig/modules/8021q.modules</userinput></screen></para>
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<para>We're done, so we can exit using the
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<command>exit</command>
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command:<screen><prompt>><fs></prompt> <userinput>exit</userinput></screen></para>
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</simplesect>
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<simplesect>
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<title>Go further with guestfish</title>
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<para>There is an enormous amount of functionality in
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guestfish and a full treatment is beyond the scope of
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this document. Instead, we recommend that you read the
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<link
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xlink:href="http://libguestfs.org/guestfs-recipes.1.html"
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>guestfs-recipes</link> documentation page for a
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sense of what is possible with these tools.</para>
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</simplesect>
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</section>
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<section xml:id="guestmount">
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<title>guestmount</title>
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<para>For some types of changes, you may find it easier to
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mount the image's file system directly in the guest. The
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<command>guestmount</command> program, also from the
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libguestfs project, allows you to do so.</para>
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<para>For example, to mount the root partition from our
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<filename>centos63_desktop.qcow2</filename> image to
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<filename>/mnt</filename>, we can do:</para>
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<para>
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<screen><prompt>#</prompt> <userinput>guestmount -a centos63_desktop.qcow2 -m /dev/vg_centosbase/lv_root --rw /mnt</userinput></screen>
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</para>
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<para>If we didn't know in advance what the mount point is in
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the guest, we could use the <literal>-i</literal>(inspect)
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flag to tell guestmount to automatically determine what
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mount point to
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use:<screen><prompt>#</prompt> <userinput>guestmount -a centos63_desktop.qcow2 -i --rw /mnt</userinput></screen>Once
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mounted, we could do things like list the installed
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packages using
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rpm:<screen><prompt>#</prompt> <userinput>rpm -qa --dbpath /mnt/var/lib/rpm</userinput></screen>
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Once done, we
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unmount:<screen><prompt>#</prompt> <userinput>umount /mnt</userinput></screen></para>
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</section>
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<section xml:id="virt-tools">
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<title>virt-* tools</title>
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<para>The <link xlink:href="http://libguestfs.org/"
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>libguestfs</link> project has a number of other
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useful tools, including:<itemizedlist>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-edit.1.html"
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>virt-edit</link> for editing a file
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inside of an image.</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-df.1.html"
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>virt-df</link> for displaying free space
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inside of an image.</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-resize.1.html"
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>virt-resize</link> for resizing an
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image.</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-sysprep.1.html"
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>virt-sysprep</link> for preparing an
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image for distribution (for example, delete
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SSH host keys, remove MAC address info, or
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remove user accounts).</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-sparsify.1.html"
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>virt-sparsify</link> for making an image
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sparse.</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-v2v/"
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>virt-p2v</link> for converting a physical
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machine to an image that runs on KVM.</para>
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</listitem>
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<listitem>
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<para><link
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xlink:href="http://libguestfs.org/virt-v2v/"
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>virt-v2v</link> for converting Xen and
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VMware images to KVM images.</para>
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</listitem>
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</itemizedlist></para>
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<simplesect>
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<title>Modify a single file inside of an image</title>
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<para>This example shows how to use
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<command>virt-edit</command> to modify a file. The
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command can take either a filename as an argument with
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the <literal>-a</literal> flag, or a domain name as an
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argument with the <literal>-d</literal> flag. The
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following examples shows how to use this to modify the
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<filename>/etc/shadow</filename> file in instance
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with libvirt domain name
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<literal>instance-000000e1</literal> that is
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currently running:</para>
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<para>
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<screen><prompt>#</prompt> <userinput>virsh shutdown instance-000000e1</userinput>
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<prompt>#</prompt> <userinput>virt-edit -d instance-000000e1 /etc/shadow</userinput>
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<prompt>#</prompt> <userinput>virsh start instance-000000e1</userinput></screen>
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</para>
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</simplesect>
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<simplesect>
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<title>Resize an image</title>
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<para>Here is an example of how to use
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<command>virt-resize</command> to resize an image.
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Assume we have a 16 GB Windows image in qcow2 format
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that we want to resize to 50 GB. First, we use
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<command>virt-filesystems</command> to identify
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the
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partitions:<screen><prompt>#</prompt> <userinput>virt-filesystems --long --parts --blkdevs -h -a /data/images/win2012.qcow2</userinput>
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<computeroutput>Name Type MBR Size Parent
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/dev/sda1 partition 07 350M /dev/sda
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/dev/sda2 partition 07 16G /dev/sda
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/dev/sda device - 16G -
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</computeroutput></screen></para>
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<para>In this case, it's the
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<filename>/dev/sda2</filename> partition that we
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want to resize. We create a new qcow2 image and use
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the <command>virt-resize</command> command to write a
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resized copy of the original into the new
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image:
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<screen><prompt>#</prompt> <userinput>qemu-img create -f qcow2 /data/images/win2012-50gb.qcow2 50G</userinput>
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<prompt>#</prompt> <userinput>virt-resize --expand /dev/sda2 /data/images/win2012.qcow2 \
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/data/images/win2012-50gb.qcow2</userinput>
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<computeroutput>Examining /data/images/win2012.qcow2 ...
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**********
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Summary of changes:
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/dev/sda1: This partition will be left alone.
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/dev/sda2: This partition will be resized from 15.7G to 49.7G. The
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filesystem ntfs on /dev/sda2 will be expanded using the
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'ntfsresize' method.
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**********
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Setting up initial partition table on /data/images/win2012-50gb.qcow2 ...
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Copying /dev/sda1 ...
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100% ⟦▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓⟧ 00:00
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Copying /dev/sda2 ...
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100% ⟦▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓⟧ 00:00
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Expanding /dev/sda2 using the 'ntfsresize' method ...
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Resize operation completed with no errors. Before deleting the old
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disk, carefully check that the resized disk boots and works correctly.
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</computeroutput></screen></para>
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</simplesect>
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</section>
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<section xml:id="losetup-kpartx-nbd">
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<title>Loop devices, kpartx, network block devices</title>
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<para>If you don't have access to libguestfs, you can mount
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image file systems directly in the host using loop
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devices, kpartx, and network block devices.<warning>
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<para>Mounting untrusted guest images using the tools
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described in this section is a security risk,
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always use libguestfs tools such as guestfish and
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guestmount if you have access to them. See <link
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xlink:href="https://www.berrange.com/posts/2013/02/20/a-reminder-why-you-should-never-mount-guest-disk-images-on-the-host-os/"
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>A reminder why you should never mount guest
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disk images on the host OS</link> by Daniel
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Berrangé for more details.</para>
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</warning></para>
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<simplesect>
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<title>Mount a raw image (without LVM)</title>
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<para>If you have a raw virtual machine image that is not
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using LVM to manage its partitions, use the
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<command>losetup</command> command to find an
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unused loop device.
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<screen><prompt>#</prompt> <userinput>losetup -f</userinput>
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<computeroutput>/dev/loop0</computeroutput></screen></para>
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<para>In this example, <filename>/dev/loop0</filename> is
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free. Associate a loop device with the raw
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image:<screen><prompt>#</prompt> <userinput>losetup /dev/loop0 fedora17.img</userinput></screen></para>
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<para>If the image only has a single partition, you can
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mount the loop device
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directly:<screen><prompt>#</prompt> <userinput>mount /dev/loop0 /mnt</userinput></screen></para>
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<para>If the image has multiple partitions, use
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<command>kpartx</command> to expose the partitions
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as separate devices (for example,
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<filename>/dev/mapper/loop0p1</filename>), then
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mount the partition that corresponds to the root file
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system:<screen><prompt>#</prompt> <userinput>kpartx -av /dev/loop0</userinput></screen></para>
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<para>If the image has, say three partitions (/boot, /,
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swap), there should be one new device created per
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partition:<screen><prompt>$</prompt> <userinput>ls -l /dev/mapper/loop0p*</userinput>
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<computeroutput>brw-rw---- 1 root disk 43, 49 2012-03-05 15:32 /dev/mapper/loop0p1
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brw-rw---- 1 root disk 43, 50 2012-03-05 15:32 /dev/mapper/loop0p2
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brw-rw---- 1 root disk 43, 51 2012-03-05 15:32 /dev/mapper/loop0p3</computeroutput></screen>To
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mount the second partition, as
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root:<screen><prompt>#</prompt> <userinput>mkdir /mnt/image</userinput>
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<prompt>#</prompt> <userinput>mount /dev/mapper/loop0p2 /mnt</userinput></screen>Once
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you're done, to clean
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up:<screen><prompt>#</prompt> <userinput>umount /mnt</userinput>
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<prompt>#</prompt> <userinput>kpartx -d /dev/loop0</userinput>
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<prompt>#</prompt> <userinput>losetup -d /dev/loop0</userinput></screen></para>
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</simplesect>
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<simplesect>
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<title>Mount a raw image (with LVM)</title>
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<para>If your partitions are managed with LVM, use losetup
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and kpartx as in the previous example to expose the
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partitions to the host.</para>
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<screen><prompt>#</prompt> <userinput>losetup -f</userinput>
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<computeroutput>/dev/loop0</computeroutput>
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<prompt>#</prompt> <userinput>losetup /dev/loop0 rhel62.img</userinput>
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<prompt>#</prompt> <userinput>kpartx -av /dev/loop0</userinput></screen>
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<para>Next, you need to use the <command>vgscan</command>
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command to identify the LVM volume groups and then
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<command>vgchange</command> to expose the volumes
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as devices:</para>
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<screen><prompt>#</prompt> <userinput>vgscan</userinput>
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<computeroutput>Reading all physical volumes. This may take a while...
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Found volume group "vg_rhel62x8664" using metadata type lvm2</computeroutput>
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<prompt>#</prompt> <userinput>vgchange -ay</userinput>
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<computeroutput> 2 logical volume(s) in volume group "vg_rhel62x8664" now active</computeroutput>
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<prompt>#</prompt> <userinput>mount /dev/vg_rhel62x8664/lv_root /mnt</userinput></screen>
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<para>Clean up when you're done:</para>
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<screen><prompt>#</prompt> <userinput>umount /mnt</userinput>
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<prompt>#</prompt> <userinput>vgchange -an vg_rhel62x8664</userinput>
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<prompt>#</prompt> <userinput>kpartx -d /dev/loop0</userinput>
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<prompt>#</prompt> <userinput>losetup -d /dev/loop0</userinput></screen>
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</simplesect>
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<simplesect>
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<title>Mount a qcow2 image (without LVM)</title>
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<para>You need the <literal>nbd</literal> (network block
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device) kernel module loaded to mount qcow2 images.
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This will load it with support for 16 block devices,
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which is fine for our purposes. As
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root:<screen><prompt>#</prompt> <userinput>modprobe nbd max_part=16</userinput></screen></para>
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<para>Assuming the first block device
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(<filename>/dev/nbd0</filename>) is not currently
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in use, we can expose the disk partitions using the
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<command>qemu-nbd</command> and
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<command>partprobe</command> commands. As
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root:<screen><prompt>#</prompt> <userinput>qemu-nbd -c /dev/nbd0 image.qcow2</userinput>
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<prompt>#</prompt> <userinput>partprobe /dev/nbd0</userinput></screen></para>
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<para>If the image has, say three partitions (/boot, /,
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swap), there should be one new device created for
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each partition:</para>
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<screen><prompt>$</prompt> <userinput>ls -l /dev/nbd3*</userinput>
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<computeroutput>brw-rw---- 1 root disk 43, 48 2012-03-05 15:32 /dev/nbd0
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brw-rw---- 1 root disk 43, 49 2012-03-05 15:32 /dev/nbd0p1
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brw-rw---- 1 root disk 43, 50 2012-03-05 15:32 /dev/nbd0p2
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brw-rw---- 1 root disk 43, 51 2012-03-05 15:32 /dev/nbd0p3</computeroutput></screen>
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<note>
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<para>If the network block device you selected was
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already in use, the initial
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<command>qemu-nbd</command> command will fail
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silently, and the
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<filename>/dev/nbd3p{1,2,3}</filename> device
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files will not be created.</para>
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</note>
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<para>If the image partitions are not managed with LVM,
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they can be mounted directly:</para>
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<screen><prompt>#</prompt> <userinput>mkdir /mnt/image</userinput>
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<prompt>#</prompt> <userinput>mount /dev/nbd3p2 /mnt</userinput></screen>
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<para>When you're done, clean up:</para>
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<screen><prompt>#</prompt> <userinput>umount /mnt</userinput>
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<prompt>#</prompt> <userinput>qemu-nbd -d /dev/nbd0</userinput></screen>
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</simplesect>
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<simplesect>
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<title>Mount a qcow2 image (with LVM)</title>
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<para>If the image partitions are managed with LVM, after
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you use <command>qemu-nbd</command> and
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<command>partprobe</command>, you must use
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<command>vgscan</command> and <command>vgchange
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-ay</command> in order to expose the LVM
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partitions as devices that can be
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mounted:<screen><prompt>#</prompt> <userinput>modprobe nbd max_part=16</userinput>
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<prompt>#</prompt> <userinput>qemu-nbd -c /dev/nbd0 image.qcow2</userinput>
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<prompt>#</prompt> <userinput>partprobe /dev/nbd0</userinput><prompt>#</prompt> <userinput>vgscan</userinput>
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<computeroutput> Reading all physical volumes. This may take a while...
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Found volume group "vg_rhel62x8664" using metadata type lvm2</computeroutput>
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<prompt>#</prompt> <userinput>vgchange -ay</userinput>
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<computeroutput> 2 logical volume(s) in volume group "vg_rhel62x8664" now active</computeroutput>
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<prompt>#</prompt> <userinput>mount /dev/vg_rhel62x8664/lv_root /mnt</userinput></screen></para>
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<para>When you're done, clean
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up:<screen><prompt>#</prompt> <userinput>umount /mnt</userinput>
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<prompt>#</prompt> <userinput>vgchange -an vg_rhel62x8664</userinput>
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<prompt>#</prompt> <userinput>qemu-nbd -d /dev/nbd0</userinput></screen></para>
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</simplesect>
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</section>
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</chapter>
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