SSH. Expect. Named pipes. Input/output to the host terminal. Early boot messages.
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While struggling to automate QEMU guest (communicate and control with the shell scripts), I faced a lot of incomplete, partially working solutions around the Internet. Now, I've got a pretty decent collection of working recipes to tune up a QEMU guest, so I decided to organize all that stuff here, and it could be definitely useful for anyone else. Each scenario has been tested on the binaries, links on which I put below in the annex: Binaries used in the examples, so you could check it out on your own.
- Input/output to the host terminal
- Early boot messages in the host terminal
- Input/output through a named pipe (file)
- Automate QEMU guest using expect tool
- Automate QEMU guest using ssh
- Binaries used in examples
-serial stdio
qemu-system-x86_64 -serial stdio wheezy.qcow2
-serial stdio redirects the virtual serial port to the host's terminal input/output. You will see a welcome string after a successful boot.
-nographic
qemu-system-x86_64 -nographic wheezy.qcow2
-nographic does the same as "-serial stdio" and also hides a QEMU's graphical window.
Cautions:
- You will not see any early boot logs in the host's console. To get them, see Early boot messages in the host terminal below.
- To exit the guest system without GUI, using stdio redirected to the terminal, login as a root (user: root, password: root) and shutdown the system (wait after that for a while):# Guest shutdown -h now
console=ttyS0
If you want to see early boot logs, you should pass console=ttyS0 parameter to a Linux kernel command line:
qemu-system-x86_64 -nographic -kernel vmlinuz -hda wheezy.img -append "root=/dev/sda console=ttyS0"or
qemu-system-x86_64 -serial stdio -kernel vmlinuz -hda wheezy.img -append "root=/dev/sda console=ttyS0"or
qemu-system-x86_64 -serial stdio wheezy.qcow2 # 1. Wait for a GRUB menu to show. # 2. Press `e`. # 3. Find the line starting with "linux". # 4. Add "console=ttyS0".qemu-system-x86_64 -serial stdio -kernel vmlinuz -hda wheezy.img -append "root=/dev/sda console=ttyS0":
- -serial stdio or -nographic redirects input/output to the current terminal.
- -append "root=/dev/sda console=ttyS0": console=ttyS0 forces the guest kernel to send output to the first UART serial port ttyS0, which is redirected to the host by the -serial stdio option, and root=/dev/sda points the kernel to use a /dev/sda device to load the wheezy.img.
Other options:
- -kernel vmlinuz loads the kernel from the local "./vmlinuz" file.
- -hda wheezy.img is a raw image which is suitable for booting with vmlinuz binary (wheezy.qcow2 won't be recognized in the block device).
Create a named pipe
mkfifo /tmp/guest.in /tmp/guest.outStart QEMU
qemu-system-x86_64 -serial pipe:/tmp/guest -kernel vmlinuz -hda wheezy.img -append "root=/dev/sda console=ttyS0"-serial pipe:/tmp/guest redirects a guest's output to a /tmp/guest.out and allows to send input from host to guest via /tmp/guest.in.
Take an output from the guest
cat /tmp/guest.outSend a command to the guest
When login screen appears, send a login string:
printf "root\n" > /tmp/guest.inWait until some string
Wait until SSH Daemon starts.
while read line; do echo "${line}" if [[ ${line} == *"Secure Shell server: sshd"* ]]; then break; fi done < /tmp/quest.outInstall "expect" tool
sudo apt install expectCreate an expect script
example.exp:
#!/usr/bin/expect -f # Wait enough (forever) until a long-time boot set timeout -1 # Start the guest VM spawn qemu-system-x86_64 -serial stdio wheezy.qcow2 expect "login: " send "root\n" expect "Password: " send "root\n" expect "# " send "shutdown -h now"Original script is found there: https://stacoverflow.com/questions/314613/qemu-guest-automation, but be careful, symbol of quotes “ (which is not a ") in the original stackoverflow answer cannot be recognized by the expect utility (send "root\n").
Execute "expect" script
chmod +x example.exp ./example.expSet up port forwarding
qemu-system-x86_64 -netdev user,id=net0,hostfwd=tcp::10022-:22 -device e1000,netdev=net0 wheezy.qcow2Connect via ssh
ssh root@localhost -p 10022 'uptime; ls; echo Test;'- To apply server's public key automatically use -o "StrictHostKeyChecking no":ssh root@localhost -p 10022 -o "StrictHostKeyChecking no" 'uptime; ls; echo Test;'
Troubleshooting
- QEMU guest has to be able to recognize a network card device (NIC, Network Interface Card): -netdev user,id=net0 -device e1000,netdev=net0.# Without port forwarding qemu-system-x86_64 -netdev user,id=net0 -device e1000,netdev=net0 wheezy.qcow2
- Boot and check that the new interface has appeared on the guest system:# Guest ifconfig -a Linux kernel on the guest must support a network card emulated by QEMU. In the opposite case the guest won't get a new Ethernet interface. After booting you should find "eth0" (running broadcast device, not loopback) on the guest. It depends solely on the guest Linux kernel and on the kernel modules.
- Check the 10022 port on the host:# Host netstat -tanp | grep 10022 tcp 0 0 0.0.0.0:10022 0.0.0.0:* LISTEN 16589/qemu-system-x
- Check the 22 port on the guest:# Guest netstat -tanp | grep 22 tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2430/sshd
- You can forward telnet port 23 and verify the connection:qemu-system-x86_64 -netdev user,id=net0,hostfwd=tcp::10023-:23 -device e1000,netdev=net0 wheezy.qcow2
- Guest (server):# Guest nc -v -l -p 23 Listening on [0.0.0.0] (family 0, port 23)
- Host (client):# Host echo asdf | nc localhost 10023
Establish passwordless login via ssh
- Generate host SSH keys:# Host ssh-keygen -b 2048 -t rsa -q -N "" -f ./qemukey
- Set up a public key to the guest as a trusted (authorized) key.
- Via ssh-copy-id
- You need a root with password. You the guest root is passwordless, go to the guest system and set up the password:# Guest sudo passwd
- Send the generated public key:# Host ssh-copy-id -p 10022 -i ~/.ssh/qemukey root@localhost
- Reset the password in the guest system:# Guest sudo passwd -l root
- Manually
- Send a public key via scp:# Host scp -P 10022 ./qemukey.pub root@localhost:/root/.ssh/
- Login to the guest and set up new authorized key:# Guest cat /root/.ssh/qemukey.pub >> /root/.ssh/authorized_keys /etc/init.d/ssh restart
- Or mount device locally, put the public key to the .ssh directory, and concatenate to authorized_keys.
- Via ssh-copy-id
- Fix the /etc/ssh/sshd_config on the guest:PasswordAuthentication no PermitRootLogin without-password
- Restart SSH daemon on the guest:# Guest /etc/init.d/ssh restart
- Connect via ssh:# Host ssh root@localhost -p 10022 -i ./qemukey Viola! You don't need the password and you can automate the remote QEMU guest.
- wheezy.qcow2 (i386): bootable Debian "Wheezy" image a QEMU copy-on-write format. Login/password: "root"/"root", and "user"/"user".wget https://people.debian.org/~aurel32/qemu/i386/debian_wheezy_i386_standard.qcow2 -O wheezy.qcow2
- wheezy.img (i386): non-bootable Debian "Wheezy" image (without kernel) to use with own kernel (-kernel vmlinuz).wget https://storage.googleapis.com/syzkaller/wheezy.img
- vmlinuz (i386): compressed bootable Linux kernel. Options:
- Build from the scratch: Build Android Kernel and Run on QEMU with Minimal Environment: Step by Step.
- Download from Ubuntu repository (WARNING! Port forwarding will NOT work):wget http://security.ubuntu.com/ubuntu/pool/main/l/linux-signed-azure/linux-image-4.15.0-1036-azure_4.15.0-1036.38~14.04.2_amd64.deb ar x linux-image-4.15.0-1036-azure_4.15.0-1036.38~14.04.2_amd64.deb tar xf data.tar.xz ./boot/vmlinuz-4.15.0-1036-azure cp ./boot/vmlinuz-4.15.0-1036-azure ./vmlinuz
- You can try your host's linux kernel passing one to the QEMU guest (WARNING! You could have problems either with port forwarding, or with a block device):sudo cp /boot/vmlinuz-$(uname -r) ./
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