Understanding how VMs differ

Earlier, we started talking about VMs and what they are. We're now going to look at a couple of ways of working out if we're in a VM, from inside the machine itself.

I would generally do this if I'd got a new virtual private server (VPS) from a hosting provider, and wanted to know what software was being used to virtualize my new machine.

dmidecode

One of my favourite tools, dmidecode, can be used to dump a computer's desktop management interface (DMI) table. In practice, this means that it can be used to find out what kind of hardware you're running in a machine.

This command requires root access, so we'll be using sudo throughout these examples.

First, we're going to list the valid types we can pass to dmidecode:

$ dmidecode --type
dmidecode: option '--type' requires an argument
Type number or keyword expected
Valid type keywords are:
  bios
  system
  baseboard
  chassis
  processor
  memory
  cache
  connector
  slot

Starting at the top, we're going to use bios and see if it gives us anything useful:

$ sudo dmidecode --type bios
# dmidecode 3.0
Getting SMBIOS data from sysfs.
SMBIOS 2.5 present.

Handle 0x0000, DMI type 0, 20 bytes
BIOS Information
 Vendor: innotek GmbH
 Version: VirtualBox
 Release Date: 12/01/2006
 Address: 0xE0000
 Runtime Size: 128 kB
 ROM Size: 128 kB
 Characteristics:
 ISA is supported
 PCI is supported
 Boot from CD is supported
 Selectable boot is supported
 8042 keyboard services are supported (int 9h)
 CGA/mono video services are supported (int 10h)
 ACPI is supported

Instantly, we can see VirtualBox next to Version, which is a pretty strong hint that we're dealing with a VM.

Next, we will choose something else, system:

$ sudo dmidecode --type system
# dmidecode 3.0
Getting SMBIOS data from sysfs.
SMBIOS 2.5 present.

Handle 0x0001, DMI type 1, 27 bytes
System Information
    Manufacturer: innotek GmbH
    Product Name: VirtualBox
    Version: 1.2
    Serial Number: 0
    UUID: BDC643B8-8D4D-4288-BDA4-A72F606CD0EA
    Wake-up Type: Power Switch
    SKU Number: Not Specified
    Family: Virtual Machine

Again, the Product Name seen here is VirtualBox, and the Family is Virtual Machine, both of which are pretty damning pieces of evidence.

Lastly, we're going to look at the Chassis Information:

$ sudo dmidecode --type chassis
# dmidecode 3.0
Getting SMBIOS data from sysfs.
SMBIOS 2.5 present.

Handle 0x0003, DMI type 3, 13 bytes
Chassis Information
    Manufacturer: Oracle Corporation
    Type: Other
    Lock: Not Present
    Version: Not Specified
    Serial Number: Not Specified
    Asset Tag: Not Specified
    Boot-up State: Safe
    Power Supply State: Safe
    Thermal State: Safe
    Security Status: None

Oracle corporation is, again, a significant piece of information that leads us to believe we're in a virtualized environment.

If we don't want a lot of other information, we can fine-tune our search using dmidecode's -s option.

Running this option without an argument outputs a list of potential arguments we can use:

$ sudo dmidecode -s
dmidecode: option requires an argument -- 's'
String keyword expected
Valid string keywords are:
 bios-vendor
 bios-version
 bios-release-date
 system-manufacturer
 system-product-name
 system-version
 system-serial-number
 system-uuid
 baseboard-manufacturer
 baseboard-product-name
 baseboard-version
 baseboard-serial-number
 baseboard-asset-tag
 chassis-manufacturer
 chassis-type
 chassis-version
 chassis-serial-number
 chassis-asset-tag
 processor-family
 processor-manufacturer
 processor-version
 processor-frequency

Here, we can instantly see bios-version, and as we know from earlier, it should be VirtualBox:

$ sudo dmidecode -s bios-version
VirtualBox

These types of short-output commands are very useful for scripting, where succinctness is sometimes desirable.

dmidecode is usually installed by default, at least on Ubuntu and CentOS installations.

lshw

Should dmidecode not be available, you can also make use of lshw, a command for listing hardware. Again, it makes use of the DMI table on a device.

Very quickly, we can use a format option of lshw to show the bus information of a system:

$ sudo lshw -businfo
Bus info Device Class Description
=====================================================
 system VirtualBox
 bus VirtualBox
 memory 128KiB BIOS
 memory 1GiB System memory
cpu@0 processor Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
pci@0000:00:00.0 bridge 440FX - 82441FX PMC [Natoma]
pci@0000:00:01.0 bridge 82371SB PIIX3 ISA [Natoma/Triton II]
pci@0000:00:01.1 scsi1 storage 82371AB/EB/MB PIIX4 IDE
scsi@1:0.0.0 /dev/cdrom disk CD-ROM
pci@0000:00:02.0 display VirtualBox Graphics Adapter
pci@0000:00:03.0 enp0s3 network 82540EM Gigabit Ethernet Controller
pci@0000:00:04.0 generic VirtualBox Guest Service
pci@0000:00:05.0 multimedia 82801AA AC'97 Audio Controller
pci@0000:00:06.0 bus KeyLargo/Intrepid USB
usb@1 usb1 bus OHCI PCI host controller
pci@0000:00:07.0 bridge 82371AB/EB/MB PIIX4 ACPI
pci@0000:00:0d.0 scsi2 storage 82801HM/HEM (ICH8M/ICH8M-E) SATA Controller [AHCI mode]
scsi@2:0.0.0 /dev/sda disk 8589MB VBOX HARDDISK
scsi@2:0.0.0,1 /dev/sda1 volume 1GiB Linux filesystem partition
scsi@2:0.0.0,2 /dev/sda2 volume 7167MiB Linux LVM Physical Volume partition
 input PnP device PNP0303
 input PnP device PNP0f03

This gives us information that instantly suggests a VM, such as the system, bus, and display entries.

We also have an easy-to-read breakdown of the classes available, meaning that we can query those directly, starting with disk in this example:

$ sudo lshw -c disk
  *-cdrom                   
       description: DVD reader
       product: CD-ROM
       vendor: VBOX
       physical id: 0.0.0
       bus info: scsi@1:0.0.0
       logical name: /dev/cdrom
       logical name: /dev/sr0
       version: 1.0
       capabilities: removable audio dvd
       configuration: ansiversion=5 status=nodisc
  *-disk
       description: ATA Disk
       product: VBOX HARDDISK
       vendor: VirtualBox
       physical id: 0.0.0
       bus info: scsi@2:0.0.0
       logical name: /dev/sda
       version: 1.0
       serial: VB5cbf266c-3015878d
       size: 8GiB (8589MB)
       capabilities: partitioned partitioned:dos
       configuration: ansiversion=5 logicalsectorsize=512 sectorsize=512 signature=000b6a88

Alternatively, if we think that's too much information, we could query the system class:

$ sudo lshw -c system
localhost.localdomain 
 description: Computer
 product: VirtualBox
 vendor: innotek GmbH
 version: 1.2
 serial: 0
 width: 64 bits
 capabilities: smbios-2.5 dmi-2.5 vsyscall32
 configuration: family=Virtual Machine uuid=BDC643B8-8D4D-4288-BDA4-A72F606CD0EA