kilabit.info
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Introduction

Use case: I want to give SSH access to someone to setup or install anything without breaking up my current setup. What should I do?

Option 1

Setup a chroot directory and then set sshd_config ChrootDirectory to jail SSH user to that directory.

Advantages,

  • Easy and simple.

Disadvantages,

  • New SSH user need to be created on host.

  • Service running inside chroot need to be managed from the host machine.

Option 2

Setup container using systemd-nspawn and use sshd_config ForceCommand to redirect user SSH to that container.

Advantages,

  • Any services can be managed and run like usual systemd services inside the container.

  • Host machine can limit the resources (RAM, CPU) on container.

Disadvantages,

  • Required a extra steps to setup the container (enable auto login)

  • Both host and container require adding the same user.

  • scp or rsync does not works from the client side.

Option 3

Setup container using systemd-nspawn and run SSH server on different port.

Advantages: same as Option 2 but scp/rsync works seamlessly and the user only need to be setup on the container side.

Disadvantages:

  • Required a little extra steps to setup the container.

In this article, I want to explore option 3, using systemd-nspawn to run container and open an independent SSH server on container with different port.

The SSH flow after setup,

            +----------------+
            | host           |
            |   | (1)        |
            |   v            |
            | /-----------\  |
        (2) | | container |  |
 user ----->| \-----------/  |
            +----------------+

 (1) host: run the container (include SSH server)
 (2) user: ssh into container using "user@host:ssh-port-container".

My host OS is Arch Linux and the container/guest/machine also Arch Linux.

Step 1: install required package

In Arch Linux we need the arch-install-scripts package to bootstrap a new system, Since the host machine is on VPS, we need to update the system and reboot to make sure every things are up to date.

$ sudo pacman -Sy --noconfirm archlinux-keyring
$ sudo pacman -Su --noconfirm
$ sudo pacman -S  --noconfirm arch-install-scripts
$ sudo reboot

The systemd version after update is 251.2-1.

Step 2: setup container

Goals,

  • The container is stored inside the /var/lib/machines to allow the host automatically start it at boot.

  • The container name is test.

  • The container use Host networking, which means it use the same IP addresses as host.

  • The container run its own SSH server on different port than host.

  • The container has one user that can ssh, named test.

All the commands and configurations are executed/modified from the host.

Step 2.1: bootstrapping

Executes the following commands to bootstrap the container,

$ sudo mkdir -p /var/lib/machines/test
$ sudo pacstrap -c /var/lib/machines/test base openssh sudo vim tmux

We install sudo package to allow users inside the container to run command as root. You can add additional packages to be used by user in this step.

Step 2.2: Add configuration for container

Create directory /etc/systemd/nspawn to configure the container,

$ sudo mkdir -p /etc/systemd/nspawn

Create file test.nspawn inside that directory with the following content [3],

[Network]
VirtualEthernet=no

The VirtualEthernet=no means the container will use the host networking. The file name must match with the container name, for example if the container name is "xyz", then the file name should be "xyz.nspawn".

Step 2.3: update sudoers file inside container

Edit the /var/lib/machines/test/etc/sudoers to allow user with group wheel to run sudo without password,

## Same thing without a password
%wheel ALL=(ALL:ALL) NOPASSWD: ALL

Step 2.4: Enable SSH on container on port 2022

On the container, edit /var/lib/machines/test/etc/ssh/sshd_config and changes the following options,

Port 2022
PasswordAuthentication no

Then enable the sshd service to start on boot,

$ sudo systemd-nspawn --machine=test systemctl enable sshd.service
Spawning container test on /var/lib/machines/test.
Press ^] three times within 1s to kill container.
Created symlink /etc/systemd/system/multi-user.target.wants/sshd.service → /usr/lib/systemd/system/sshd.service.
Container test exited successfully.

Step 2.5: create new user with SSH key inside container

Create new user test on container,

$ sudo useradd --create-home --groups wheel \
	--root /var/lib/machines/test \
	test

Create SSH key for user test and authorized it,

$ sudo mkdir -p /var/lib/machines/test/home/test/.ssh

$ sudo ssh-keygen -t ed25519 -q -N "" \
    -f /var/lib/machines/test/home/test/.ssh/id_ed25519

$ sudo cp /var/lib/machines/test/home/test/.ssh/id_ed25519.pub \
	/var/lib/machines/test/home/test/.ssh/authorized_keys

$ sudo chmod 0700 /var/lib/machines/test/home/test/.ssh
$ sudo chmod 0600 /var/lib/machines/test/home/test/.ssh/id_ed25519
$ sudo chmod 0600 /var/lib/machines/test/home/test/.ssh/id_ed25519.pub
$ sudo chmod 0600 /var/lib/machines/test/home/test/.ssh/authorized_keys

$ sudo systemd-nspawn --machine=test chown -R test:test /home/test

Step 3: enable and start container

The last step is enabling the container to auto start at boot,

$ sudo machinectl enable test
Created symlink /etc/systemd/system/machines.target.wants/systemd-nspawn@test.service → /usr/lib/systemd/system/systemd-nspawn@.service.

$ sudo machinectl list
MACHINE   CLASS     SERVICE        OS   VERSION ADDRESSES
test      container systemd-nspawn arch -       -

1 machines listed.

Start the container,

$ sudo machinectl start test

Testing

Now that every things setup and running, ssh into the container from the host machine,

$ sudo ssh -i /var/lib/machines/test/home/test/.ssh/id_ed25519 \
    -p 2022 test@127.0.0.1
ED25519 key fingerprint is SHA256:QSH7wbNf6Lak/zKHvVhN8c1LmFkcLecNkGANwCHIykg.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[127.0.0.1]:2022' (ED25519) to the list of known hosts.
[test@test ~]$

Lets check the process inside this container,

[test@test ~]$ ps -e
    PID TTY          TIME CMD
      1 ?        00:00:00 systemd
     17 ?        00:00:00 systemd-journal
     29 ?        00:00:00 dbus-daemon
     31 ?        00:00:00 systemd-logind
     33 pts/0    00:00:00 agetty
     34 ?        00:00:00 sshd
     35 ?        00:00:00 sshd
     38 ?        00:00:00 systemd
     39 ?        00:00:00 (sd-pam)
     45 ?        00:00:00 sshd
     46 pts/1    00:00:00 bash
     49 pts/1    00:00:00 ps

You can see the top PID is 1, run by systemd itself.

Check if the networking is match with the host (the output should be different with your host),

[test@test ~]$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 00:50:56:45:ac:c2 brd ff:ff:ff:ff:ff:ff
    inet <redacted>/22 brd 194.233.71.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 2407:3640:2083:2556::1/64 scope global
       valid_lft forever preferred_lft forever
    inet6 <redacted>/64 scope link
       valid_lft forever preferred_lft forever
3: wg0: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1420 qdisc noqueue state UNKNOWN group default qlen 1000
    link/none
    inet 10.8.0.1/24 scope global wg0
       valid_lft forever preferred_lft forever

Check if the container can connect to Internet,

[test@test ~]$ ping kilabit.info
PING kilabit.info (<redacted>) 56(84) bytes of data.
64 bytes from test (<redacted>): icmp_seq=1 ttl=64 time=0.039 ms
64 bytes from test (<redacted>): icmp_seq=2 ttl=64 time=0.119 ms
Note
for security reasons, some output has been <redacted>.

Check the hostname,

[test@test ~]$ hostnamectl
   Static hostname: n/a
Transient hostname: test
         Icon name: computer
        Machine ID: cff56de396714debaed8fe8b9435449a
           Boot ID: ecd3f864180b497897169735581805af
    Virtualization: systemd-nspawn
  Operating System: Arch Linux
            Kernel: Linux 5.15.48-1-lts
      Architecture: x86-64
  Firmware Version: rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org

That’s it, happy hacking!