QEMU-KVM KVM Command Line Practical Guide

I am going to build this based on a series of small howto QEMU / KVM posts I've made as I feel much of the information is actually hard to find and piece together from the rest of the web.

What I'm going to focus on is how to use virtio as the NIC because if you don't you get very slow NIC speeds but with the virtio NIC model you basically get host speeds.

/usr/libexec/qemu-kvm -enable-kvm -smp 8 -m 16000 -net user -net nic,model=virtio -drive file=ubuntu-gpt2large.img,if=virtio

How To Create and Start a QEMU VM Manually:

Using QEMU / KVM /dev/kvm without root access

On most distributions you'll find /dev/kvm is owned by the root user and group kvm.

Just add yourself to the kvm group and logoff and logon.

sudo usermod -a -G kvm username

If you don't want to logout you can try this:

exec su -l $USER

How can you boot off a physical SSD/HDD/USB drive?

Just use

-drive file=/dev/sdb

If /dev/sdb was the drive you wanted to boot from.

Need to install a physical machine, you can use a VM to install to a physical drive and then take the drive to the destination computer with this guide.

How can you boot a Linux kernel and initrd/initramfs directly?

This is an excellent way to save development time and boot Linux directly.

-kernel /arch/x86_64/boot/bzImage

Just pass the kernel option to the location of the kernel file.

If you need an initrd then pass it -initrd:

-initrd /path/to/initrd.img

How do I specify local NAT network only?

By default if you don't specify "-net" as network type it defaults to user mode networking. Basically you get a standard NAT IP that allows the VM to surf the net, download etc.. but it's not possible to remotely access the VM.

How to not have any NIC on the VM

-nic none

For security or testing reasons, you may not want the VM to have any NIC to prevent any access to the LAN/WAN.

Use this option to have no NIC, as by default prevents a NIC with usermode networking, which is essentially a NAT IP that takes you out to the internet.

How do specify my NIC as being virtio?

-net nic,model=virtio

How do you start in bridged mode and use the virtio accelerated network card/driver?

-netdev bridge,id=hn1 -device virtio-net-pci,netdev=hn1

You can also specify the mac, which is necessary as if you start multiple VMs they will default to the same MAC which breaks things of course

-netdev bridge,id=hn1 -device virtio-net-pci,netdev=hn1,mac="DE:AD:BE:EF:06:02"

*Newer QEMU requires the macaddr instead of mac:

-netdev bridge,id=hn1 -device virtio-net-pci,netdev=hn1,macaddr="DE:AD:BE:EF:06:02"

An example of using user mode networking instead of bridge:

qemu-system-x86_64 -enable-kvm -m 8096 -smp 8 Debian11-Wazuh.qcow2 -net nic,macaddr=DE:AD:BE:EF:AF:E8,netdev=hn1 -netdev user,id=hn1

For networking you must specify a netdev device netname and then on the -net nic line you must specify the same netname (eg. hn1), or you get errors like this and networking that is broken:

qemu-system-x86_64: warning: hub 0 is not connected to host network qemu-system-x86_64: warning: netdev hn1 has no peer

How can you enable audio?

Below enables the AC97 driver

-audiodev driver=pa,id=pa1,server=unix:/run/user/1000/pulse/native -device AC97,audiodev=pa1

To enable Intel HD audio with microphone, do this:

-audiodev driver=pa,id=pa1,server=unix:/run/user/1000/pulse/native -device ich9-intel-hda -device hda-duplex,audiodev=pa1

How can you boot in EFI mode?

Use the -bios switch and pass it the BIOS firmware location.

-bios /usr/share/ovmf/OVMF.fd

In Ubuntu/Debian you can install OVMF EFI firmware with:

sudo apt install ovmf

The bios file will be located in /usr/share/ovmf/OVMF.fd

How to use text/console port without GUI from the terminal?

Add these two options and you can boot a kernel or Linux iso straight from the terminal without needing any GUI (great for embedded/industrial solutions).

-append console=ttyS0 -nographic

QEMU Enable VNC Port

This will enable a VNC port on localhost 5900. The number indicates 59xx so -vnc :0 means port 5900 or :1 means port 5901:

-vnc :1

You can securely connect to it by using SSH port forwarding eg.

ssh -L 6000:localhost:5901 user@host

The left side 6000 means the port on your local machine will take you to port 5901 on the remote machine that runs a QEMU VM on VNC port 5901.

QEMU How To Port Forward

This example forwards host port 2222 to port 22 on the VM for SSH. Of course it can used for any number of services and ports, with the exception that the host port must be unused of course (since ports cannot be shared).

Note that you do need the - after the 2222 or it will not work.

-net nic,netdev=hn1 -netdev user,hostfwd=tcp::2222-:22,id=hn1

QEMU How To Share Folder/File Using SMB:

-net nic,netdev=hn1 -netdev user,smb=/path/to/folder,id=hn1

QEMU How To Boot from the Network / PXE with UEFI

You will need the OVMF firmware:

sudo apt install ovmf

The firmware will be installed to: /usr/share/ovmf/OVMF.fd

To boot from the network you need the "-boot n" flag which means boot from the network.

If you don't specify hte OVMF firmware the default BIOS image will be MBR so EFI won't work.

Specify the OVMF EFI BIOS with: -bios /usr/share/ovmf/OVMF.fd

qemu-system-x86_64 -smp 8 -m 4096 -enable-kvm -net nic,netdev=hn1 -netdev bridge,id=hn1 -boot n -bios /usr/share/ovmf/OVMF.fd -vnc :1

QEMU How To Boot over Network using PXE MBR Mode:

It's the same as EFI except we just don't specify the -bios as OVMF which was for EFI only. Without this flag and specifying an EFI BIOS we will boot in MBR mode.

qemu-system-x86_64 -smp 8 -m 4096 -enable-kvm -net nic,netdev=hn1 -netdev bridge,id=hn1 -boot n -vnc :1

How Can You Access and Partition a .qcow2 file without booting it

By using the nbd driver we can do something similar to kpartx on a raw image, only with qcow2.

First load the nbd module

sudo modprobe nbd

Let's create the nbd device from the .qcow2 file

Change the DebianEFItest.qcow2 to the name and path of your qcow2 file.

sudo qemu-nbd -c /dev/nbd0 DebianEFItest.qcow2

Once you are done with the nbd0 device, make sure you disconnect it to prevent issues with corruption on the .qcow2

sudo qemu-nbd -d /dev/nbd0 /dev/nbd0 disconnected

Now we can access and partition the qcow2 through /dev/nbd0 like a normal disk/block device

sudo fdisk -l /dev/nbd0 Disk /dev/nbd0: 10 GiB, 10737418240 bytes, 20971520 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes

Full example of manipulating the qcow2 via the nbd block device:

realtechtalk.com VMs$:sudo fdisk /dev/nbd0

Welcome to fdisk (util-linux 2.34).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xc335a2a1.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p):

Using default response p.
Partition number (1-4, default 1):
First sector (2048-20971519, default 2048):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971519, default 20971519): +500M

Created a new partition 1 of type 'Linux' and of size 500 MiB.

Command (m for help): n
Partition type
   p   primary (1 primary, 0 extended, 3 free)
   e   extended (container for logical partitions)
Select (default p):

Using default response p.
Partition number (2-4, default 2):
First sector (1026048-20971519, default 1026048):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (1026048-20971519, default 20971519):

Created a new partition 2 of type 'Linux' and of size 9.5 GiB.

Command (m for help): p
Disk /dev/nbd0: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc335a2a1

Device      Boot   Start      End Sectors Size Id Type
/dev/nbd0p1         2048 1026047 1024000 500M 83 Linux
/dev/nbd0p2      1026048 20971519 19945472 9.5G 83 Linux

Command (m for help): t
Partition number (1,2, default 2): 1
Hex code (type L to list all codes): L

0 Empty           24 NEC DOS         81 Minix / old Lin bf Solaris
1 FAT12           27 Hidden NTFS Win 82 Linux swap / So c1 DRDOS/sec (FAT-
2 XENIX root      39 Plan 9          83 Linux           c4 DRDOS/sec (FAT-
3 XENIX usr       3c PartitionMagic 84 OS/2 hidden or c6 DRDOS/sec (FAT-
4 FAT16 <32M      40 Venix 80286     85 Linux extended c7 Syrinx
5 Extended        41 PPC PReP Boot   86 NTFS volume set da Non-FS data
6 FAT16           42 SFS             87 NTFS volume set db CP/M / CTOS / .
7 HPFS/NTFS/exFAT 4d QNX4.x          88 Linux plaintext de Dell Utility
8 AIX             4e QNX4.x 2nd part 8e Linux LVM       df BootIt
9 AIX bootable    4f QNX4.x 3rd part 93 Amoeba          e1 DOS access
a OS/2 Boot Manag 50 OnTrack DM      94 Amoeba BBT      e3 DOS R/O
b W95 FAT32       51 OnTrack DM6 Aux 9f BSD/OS          e4 SpeedStor
c W95 FAT32 (LBA) 52 CP/M            a0 IBM Thinkpad hi ea Rufus alignment
e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a5 FreeBSD         eb BeOS fs
f W95 Ext'd (LBA) 54 OnTrackDM6      a6 OpenBSD         ee GPT
10 OPUS            55 EZ-Drive        a7 NeXTSTEP        ef EFI (FAT-12/16/
11 Hidden FAT12    56 Golden Bow      a8 Darwin UFS      f0 Linux/PA-RISC b
12 Compaq diagnost 5c Priam Edisk     a9 NetBSD          f1 SpeedStor
14 Hidden FAT16 <3 61 SpeedStor       ab Darwin boot     f4 SpeedStor
16 Hidden FAT16    63 GNU HURD or Sys af HFS / HFS+      f2 DOS secondary
17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs         fb VMware VMFS
18 AST SmartSleep 65 Novell Netware b8 BSDI swap       fc VMware VMKCORE
1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fd Linux raid auto
1c Hidden W95 FAT3 75 PC/IX           bc Acronis FAT32 L fe LANstep
1e Hidden W95 FAT1 80 Old Minix       be Solaris boot    ff BBT
Hex code (type L to list all codes): ef

Changed type of partition 'Linux' to 'EFI (FAT-12/16/32)'.

Command (m for help): p
Disk /dev/nbd0: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc335a2a1

Device      Boot   Start      End Sectors Size Id Type
/dev/nbd0p1         2048 1026047 1024000 500M ef EFI (FAT-12/16/32)
/dev/nbd0p2      1026048 20971519 19945472 9.5G 83 Linux

Command (m for help): wq
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

realtechtalk.com VMs$:sudo mkfs.vfat /dev/nbd0p1
mkfs.fat 4.1 (2017-01-24)
realtechtalk.com VMs$:sudo mkfs.ext4 /dev/nbd0p2
mke2fs 1.45.5 (07-Jan-2020)
Discarding device blocks: failed - Input/output error
Creating filesystem with 2493184 4k blocks and 623392 inodes
Filesystem UUID: 77ab4895-80ea-45c1-b08f-975024f8b231
Superblock backups stored on blocks:
   32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done