Container setup for SADM

This page explains how to run and test the Prologin SADM infrastruction using containers.

Note

TL;DR run container_setup_host.sh then container_setup_gw.sh from install_scripts/containers/

Why containers?

Containers are lightweight isolation mechanisms. You can think of them as “starting a new userland on the same kernel”, contrarily to virtual machines, where you “start a whole new system”. You may know them from tools such as docker, kubernetes or rkt. In this guide we will use system-nspawn(1), which you may already have installed if you are using systemd. Its main advantages compared to other container managers are:

• Its simplicity. It does one thing well: configuring namespaces, the core of containers. No configuration file, daemon (other than systemd), managed filesystem or hidden network configuration. Everything is on the command line and all the options are in the man page.

• Its integrated with the systemd ecosystem. A container started with systemd-nspawn is registered and managable with machinectl(1) <https://www.freedesktop.org/software/systemd/man/machinectl.html>. You can use the -M of many systemd utilities (e.g. systemctl, journalctl) to control it.

• Its feature set. You can configure the filesystem mapping, network interfaces, resources limits and security properties you want. Just look at the man page to see the options.

Containers compare favorably to virtual machines on the following points:

• Startup speed. The containers share the devices of the host, these are already initialised and running therefore the boot time is reduced.

• Memory and CPU resources usage. No hypervisor and extra kernel overhead.

• Storage. The content of the container is stored in your existing file system and is actually completely editable from outside of the container. It’s very useful for inspecting what’s going on.

• Configuration. For system-nspawn, the only configuration you’ll have is the command line.

Overview

This guides starts by discussing the virtual network setup, then we build and start the systems.

Networkd in your base system

The container setup requires systemd-networkd to be running on your system. That said, you might not want it to be managing your main network interfaces.

If you want to tell systemd-networkd to not manage your other interfaces, you can run this command:

cat >/etc/systemd/network/00-ignore-all.network <<EOF
[Match]
Name=!vz*

[Link]
Unmanaged=yes
EOF

Automated container setup

If you want to setup SADM in containers to test something else than the install procedure, you can use the automated container install scripts. They will create and manage the containers for you and perform a full SADM install as you would do manually. They are intended for automated and end-to-end tests of SADM.

Requirements:

• The host system should be Arch Linux. Experimental support has been added for non Arch Linux hosts (CoreOS) and will be used if the script detects you are not running Arch.

• For convenience, /var/lib/machines should be a btrfs volume. The scripts will run without that but you will not have the ability to restore intermediate snapshots of the install. Note that if you don’t want to use a btrfs volume you can use:

  echo 'USE_BTRFS=false' > install_scripts/containers/container_setup.conf
  

To start, run the host setup script, you are strongly advised to check its content beforehand, as it does some substantial changes to your system setup:

cd install_scripts/containers/
./container_setup_host.sh

Then, run the container install scripts:

./container_setup_gw.sh
./container_setup_rhfs.sh
./container_setup_web.sh
./container_setup_pas-r11p11.sh

That’s it!

You should be able to see the containers listed by machinectl, and you can get a shell on the system using machinectl shell CONTAINER_NAME.

What do the scripts do?

They automate setups of Arch Linux and SADM components in containers. The commands in the scripts are taken from the main setup documentation. We expect the container setup to follow the manual setup as strictly as possible.

BTRFS snapshots

Each stage of the system setups we are building can take a substantial amount of time to complete. To iterate faster we user file system snapshots at each stage so that the system can be rollback the stage just before what you want to test or debug.

Each stage_* shell function ends by a call to container_snapshot $FUNCNAME.

Cleaning up

If you want to clean up what these scripts did, you must stop the currently running containers. List the containers with machinectl list and machinectl kill all of them. You can then remove the containers’ data by deleting the content of /var/lib/machines. List bind-mounted directories: findmnt | grep /var/lib/machines/ and umount them. Then delete the BTRFS snapshots. List them using btrfs subvolume list . and delete them using btrfs subvolume delete.

Containers deep dive

As mentioned above, these scripts setup containers using machinectl. It’s not necessary to understand how the containers work to test features in prologin-sadm, but you may encounter weird bugs caused by them. The following sections discuss some internals of the containers setup.

A key design decision is that the container setup should not require special cases added to the normal setup. This is to avoid bloating the code and keep it as simple as possible. The container setup can highlight potential fixes, for example how to make the setup more generic or how to decouple the services from the underlying system or network setup.

We note that containers do require special configuration. It should be applied in the container scripts themselves.

Virtual network setup

The first step consists in creating a virtual network. Doing it with containers is not that different compared to using virtual machines. We can still use bridge type interfaces to wire all the systems together, but we also have new possibilities, as the container is running on the same kernel as the host system.

One interesting thing is that we will be able to start one system as a container, let say gw.prolo, and others as virtual machines, for example the contestant systems, to test network boot for example.

We will use a bridge interface, the next problem to solve is to give this interface an uplink: a way to forward packets to the internet, and back again. To do that, we have multiple choices, here are two:

• Masquerade (“NAT”) the vz-prolo bridge type interface behind your uplink. With this technique the packets arriving on vz-prolo will be rewritten, tracked and moved to your uplink to be routed as if they originated from it. The machines behind the NAT will not be accessible directly and you will have to setup port forwarding to access them from outside your system. From within your system they will be accessible directly using their local IP address. In this guide we will use the “zone” network type of systemd-nspawn and systemd-networkd as the main system network manager. systemd-networkd will manage the NAT in iptables for us. Be careful, if you shadowed the 80-container-vz.network rule with a catch-all (Name=*) .network configuration files, the NAT will not be created.

• Bridge your uplink interface with vz-prolo. This will have the bad effect to link your LAN, which is most likely already using your router DHCP server, to SADM network, which has its own DHCP server. Depending on various parameters your machine and those on your LAN might get IPs and DNS configuration from Prologin SADM. Be careful if you choose this option, as bridging your uplink will down the interface, vz-prolo will get an IP from your DHCP server if you use one and you may have to clean your routes to remove the old ones. It is still the fastest to setup, especially if you just want to give internet to a container. Note: as of 2016, some wireless drivers such as broadcom’s wl do not support bridging 802.11 interfaces.

The NAT setup is simpler and more flexible, that’s what we will use.

All the containers will be connected to their own L2 network using a bridge interface. This interface is managed by systemd, created when the first container using it is spawned. Here is a small diagram to explain how we want the network to look like:

   /---------\
   |   WAN   |
   \---------/
        |
        | < NAT
        |
 +----------------------------------------------------+
 |                 bridge: vz-prolo                   |
 +-+===========+----+============+------+===========+-+
   | if: vb-gw |    | if: vb-web |      | if: vnet0 |
   +-----------+    +------------+      +-----------+
       |                 |                   |
       | < veth          | < veth            | < VM interface
       |                 |                   |
   +-------+         +-------+           +------+
   | host0 |         | host0 |           | ens3 |
o--+=======+----o o--+=======+-----o  o--+======+--o
| container: gw | | container: web |  | VM: r00p01 |
o---------------o o----------------o  o------------o

Veth type interfaces what we will use) linked to a bridge will have the name host0. systemd-networkd provides a default configuration (80-container-host0.network) file that enable DHCP on them. With the NAT rule managed by systemd-networkd and that, the internet will be accessible out-of-the-box in the containers. The only remaining configuration to do being the DNS resolver (/etc/resolv.conf).

Setting up gw manually

Let’s boot the first container: gw

Everything starts with an empty directory. This is where we will instantiate the file system used by gw:

$ mkdir gw

Use the Arch Linux install script from the sadm repository to populate it. Here is how to use it:

# ./install_scripts/bootstrap_arch_linux.sh /path/to/container machine_name ./file_containing_plaintest_root_pass

We suggest storing the password in a text file. It’s a good way to be able to to reproduce the setup quickly. If you don’t want that, just create the file on the fly or delete it afterwards.

The first system we build is gw, so let’s create the container accordingly. Run it as root:

# ./install_scripts/bootstrap_arch_linux.sh /path/to/gw gw ./plaintest_root_pass

Packages will get installed a few scripts run to configure the Arch Linux system. This is the same script we use for the bare metal or VM setup.

Then, start the container with a virtual ethernet interface connected to the vz-prolo network zone, a bridge interface managed by systemd, as well an ipvlan interface linked to your uplink:

# systemd-nspawn --boot --directory /path/to/gw --network-zone=prologin

Note

To exit the container, press ‘ctrl+]’ three time. systemd-nspawn told you that when it started, but there is good chance you missed it, so we are putting it here just for you :)

You should see systemd booting, all the units should be OK except Create Volatile Files and Directories. which fails because /sys/ is mounted read-only by systemd-nspawn. After the startup you should get a login prompt. Login as root and check that you see the virtual interface named host0 in the container using ip link:

# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: host0@if3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether e6:28:86:d2:de:6e brd ff:ff:ff:ff:ff:ff link-netnsid 0

The host system should have two new interfaces:

• vz-prolo, a bridge type interface.

• vb-gw, a veth device whose master is vz-prolo, meaning it’s wired in this bridge.

Both these interface have an extra @... suffix. It is not part of the interface name and simply indicates their peer interface.

If you are running systemd-networkd on your host system, with the default configuration files, the vz-prolo interface will get an IP from a private subnet and a MASQUERADE rule will be inserted into iptables. You can start systemd-networkd inside the container to get an IP in the vz-prologin network, which will be NAT’ed to your uplink.

For some reason host0 cannot be renamed to prologin by a systemd-networkd .link file. What needs to be changed to account for that is:

• The firewall configuration

You can do the usual install, with the following changes:

• In prologin.network, in [Match], set Name=host0 to match the virtualized interface.

What will not work:

• Some services are disabled when run in a container, for example systemd-timesyncd.service.

• nic-configuration@host0.service will fail (Cannot get device pause settings: Operation not supported) as this is a virtual interface.

Note

When you exit the container everything you started inside it is killed. If you want a persistent container, run:

# systemd-run systemd-nspawn --keep-unit --boot --directory /full/path/to/gw --network-zone=prologin
Running as unit run-r10cb0f7202be483b88ea75f6d3686ff6.service.

And then monitor it using the transient unit name:

# systemctl status run-r10cb0f7202be483b88ea75f6d3686ff6.service

Manual network configuration

This section is a do-it-yourself version of the --network-veth --network-bridge=prologin nspawn’s arguments. The main advantage of doing so is that the interfaces are not deleted when the container is shut down. Its useful if you have iptables rules you want to keep.

First let’s make sure we have ip forwarding enabled, without that the bridge will move packets around:

# echo 1 > /proc/sys/net/ipv4/ip_forward

We will create a bridge interface named prologin that will represent the isolated L2 network for SADM:

# ip link add prologin type bridge

You can now see the prologin interface using:

# ip link show
...
4: prologin: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000

For each system we want to start, we create a veth and plug one end to the bridge. For example for the gw:

# ip link add gw.local type veth peer name gw.bridge
# ip link show label 'gw*'

Here we create the two virtual ethernet interfaces, gw.local@gw.local and gw.bridge@@gw.bridge. On veth pairs, a packet arriving to one these interface is dispatched to the other. When manipulating them only the part of the name before the @ is required, the other is just a reminder of what interface is at the other end.

Let’s wire gw.bridge to the bridge:

# ip link set gw.bridge master prologin

You can see that the interface is connected to the bridge with the master prologin keyword on the following command:

$ ip link show gw.bridge

The interface is not running (state DOWN), we have to enable it:

# ip link set dev prologin up

Going further/discussion

What could make your container usage better?

• Use the --overlay option from systemd-nspawn. Have only one base Arch Linux distro and build other systems form it. It reduces the time to install and disk usage (if that’s your concern).