New in virt tools: virt-diff

virt-diff is a simple tool I wrote yesterday to display the differences between two virtual machine disk images. Some ways you could use this include:

• Run an installer on a virtual machine and see what files the installer creates or modifies.
• See what changes a tool like virt-sysprep is making to a disk image.
• Reverse engineer changes made by config programs like wicd.

It should work on any Linux or Windows disk image, but it’s a bit rough and ready at the moment.

Anyway I’m going to use it to show what changes virt-builder makes to the template when it builds an operating system image.

Run virt-builder to create an OS:

$ virt-builder fedora-19 \
    --root-password password:123456 \
    --edit '/etc/fedora-release: s/Schrödinger/Henry/'
[   1.0] Downloading: http://libguestfs.org/download/builder/fedora-19.xz
[   2.0] Planning how to build this image
[   2.0] Uncompressing
[  16.0] Opening the new disk
[  36.0] Setting a random seed
[  36.0] Setting root password
[  36.0] Editing: /etc/fedora-release
[  36.0] Finishing off
Output: fedora-19.img
Output size: 4.0G
Output format: raw
Total usable space: 1.9G
Free space: 1.2G (65%)

When virt-builder ran, it downloaded or used a Fedora 19 template, which it saved in its cache. Let’s fish out that template:

$ xzcat ~/.cache/virt-builder/fedora-19.1 > fedora-19-original.img

Finally use virt-diff to show the differences (what virt-builder changed compared to the downloaded template):

$ virt-diff -a fedora-19-original.img -A fedora-19.img
= - 0644         41 /etc/fedora-release
@@ -1 +1 @@
-Fedora release 19 (Schrödinger’s Cat)
+Fedora release 19 (Henry’s Cat)
@@ End of diff @@
= - 0000        672 /etc/shadow
@@ -1,4 +1,4 @@
-root:$6$/hQ0mnIY.tM0f/FB$6bjUN2Dophh9VTN3MBeeSHCgAClesYsdLcb/u/8b5sSbmWAgQyUfBPCcHBV6KfbzsfHiIhWtW155b096t31P70:15979:0:99999:7:::
+root:$6$vU.BupZx3RCtreCl$6EJnzCB4MhjnYmNX7sdi//IqysqKzsp2PmtYgykFfzIlIF5gnqPANVXRTFV2CdflyOpBSCIR7rAmv/s4etWHZ/:15979:0:99999:7:::
 bin:*:15863:0:99999:7:::
 daemon:*:15863:0:99999:7:::
 adm:*:15863:0:99999:7:::
@@ End of diff @@
= - 0600          8 /var/lib/random-seed
@@ End of diff @@

As expected, virt-builder set root’s password, edited the file, and set a new random seed (not shown because it’s binary).