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I think it makes sense to keep the "The" in place for the actual page's title, but let's drop it from the categorization header, to make it easier to find stuff, as the "The" isn't helpful to that. In particular as we sometimes do it this and sometimes the other way so far, hence let's stick to one common rule.
291 lines
17 KiB
Markdown
291 lines
17 KiB
Markdown
---
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title: Container Interface
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category: Interfaces
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layout: default
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---
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# The Container Interface
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Also consult [Writing Virtual Machine or Container
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Managers](http://www.freedesktop.org/wiki/Software/systemd/writing-vm-managers).
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systemd has a number of interfaces for interacting with container managers,
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when systemd is used inside of an OS container. If you work on a container
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manager, please consider supporting the following interfaces.
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## Execution Environment
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1. If the container manager wants to control the hostname for a container
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running systemd it may just set it before invoking systemd, and systemd will
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leave it unmodified when there is no hostname configured in `/etc/hostname`
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(that file overrides whatever is pre-initialized by the container manager).
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2. Make sure to pre-mount `/proc/`, `/sys/`, and `/sys/fs/selinux/` before
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invoking systemd, and mount `/proc/sys/`, `/sys/`, and `/sys/fs/selinux/`
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read-only in order to prevent the container from altering the host kernel's
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configuration settings. (As a special exception, if your container has
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network namespaces enabled, feel free to make `/proc/sys/net/` writable).
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systemd and various other subsystems (such as the SELinux userspace) have
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been modified to behave accordingly when these file systems are read-only.
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(It's OK to mount `/sys/` as `tmpfs` btw, and only mount a subset of its
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sub-trees from the real `sysfs` to hide `/sys/firmware/`, `/sys/kernel/` and
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so on. If you do that, make sure to mark `/sys/` read-only, as that
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condition is what systemd looks for, and is what is considered to be the API
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in this context.)
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3. Pre-mount `/dev/` as (container private) `tmpfs` for the container and bind
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mount some suitable TTY to `/dev/console`. Also, make sure to create device
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nodes for `/dev/null`, `/dev/zero`, `/dev/full`, `/dev/random`,
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`/dev/urandom`, `/dev/tty`, `/dev/ptmx` in `/dev/`. It is not necessary to
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create `/dev/fd` or `/dev/stdout`, as systemd will do that on its own. Make
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sure to set up a `BPF_PROG_TYPE_CGROUP_DEVICE` BPF program — on cgroupv2 —
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or the `devices` cgroup controller — on cgroupv1 — so that no other devices
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but these may be created in the container. Note that many systemd services
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use `PrivateDevices=`, which means that systemd will set up a private
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`/dev/` for them for which it needs to be able to create these device nodes.
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Dropping `CAP_MKNOD` for containers is hence generally not advisable, but
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see below.
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4. `systemd-udevd` is not available in containers (and refuses to start), and
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hence device dependencies are unavailable. The `systemd-udevd` unit files
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will check for `/sys/` being read-only, as an indication whether device
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management can work. Therefore make sure to mount `/sys/` read-only in the
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container (see above). Various clients of `systemd-udevd` also check the
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read-only state of `/sys/`, including PID 1 itself and `systemd-networkd`.
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5. If systemd detects it is run in a container it will spawn a single shell on
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`/dev/console`, and not care about VTs or multiple gettys on VTs. (But see
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`$container_ttys` below.)
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6. Either pre-mount all cgroup hierarchies in full into the container, or leave
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that to systemd which will do so if they are missing. Note that it is
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explicitly *not* OK to just mount a sub-hierarchy into the container as that
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is incompatible with `/proc/$PID/cgroup` (which lists full paths). Also the
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root-level cgroup directories tend to be quite different from inner
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directories, and that distinction matters. It is OK however, to mount the
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"upper" parts read-only of the hierarchies, and only allow write-access to
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the cgroup sub-tree the container runs in. It's also a good idea to mount
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all controller hierarchies with exception of `name=systemd` fully read-only
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(this only applies to cgroupv1, of course), to protect the controllers from
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alteration from inside the containers. Or to turn this around: only the
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cgroup sub-tree of the container itself (on cgroupv2 in the unified
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hierarchy, and on cgroupv1 in the `name=systemd` hierarchy) may be writable
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to the container.
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7. Create the control group root of your container by either running your
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container as a service (in case you have one container manager instance per
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container instance) or creating one scope unit for each container instance
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via systemd's transient unit API (in case you have one container manager
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that manages all instances. Either way, make sure to set `Delegate=yes` in
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it. This ensures that that the unit you created will be part of all cgroup
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controllers (or at least the ones systemd understands). The latter may also
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be done via `systemd-machined`'s `CreateMachine()` API. Make sure to use the
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cgroup path systemd put your process in for all operations of the container.
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Do not add new cgroup directories to the top of the tree. This will not only
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confuse systemd and the admin, but also prevent your implementation from
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being "stackable".
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## Environment Variables
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1. To allow systemd (and other programs) to identify that it is executed within
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a container, please set the `$container` environment variable for PID 1 in
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the container to a short lowercase string identifying your
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implementation. With this in place the `ConditionVirtualization=` setting in
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unit files will work properly. Example: `container=lxc-libvirt`
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2. systemd has special support for allowing container managers to initialize
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the UUID for `/etc/machine-id` to some manager supplied value. This is only
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enabled if `/etc/machine-id` is empty (i.e. not yet set) at boot time of the
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container. The container manager should set `$container_uuid` as environment
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variable for the container's PID 1 to the container UUID. (This is similar
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to the effect of `qemu`'s `-uuid` switch). Note that you should pass only a
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UUID here that is actually unique (i.e. only one running container should
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have a specific UUID), and gets changed when a container gets duplicated.
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Also note that systemd will try to persistently store the UUID in
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`/etc/machine-id` (if writable) when this option is used, hence you should
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always pass the same UUID here. Keeping the externally used UUID for a
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container and the internal one in sync is hopefully useful to minimize
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surprise for the administrator.
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3. systemd can automatically spawn login gettys on additional ptys. A container
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manager can set the `$container_ttys` environment variable for the
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container's PID 1 to tell it on which ptys to spawn gettys. The variable
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should take a space separated list of pty names, without the leading `/dev/`
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prefix, but with the `pts/` prefix included. Note that despite the
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variable's name you may only specify ptys, and not other types of ttys. Also
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you need to specify the pty itself, a symlink will not suffice. This is
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implemented in
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[systemd-getty-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-getty-generator.html).
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Note that this variable should not include the pty that `/dev/console` maps
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to if it maps to one (see below). Example: if the container receives
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`container_ttys=pts/7 pts/8 pts/14` it will spawn three additional login
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gettys on ptys 7, 8, and 14.
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## Advanced Integration
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1. Consider syncing `/etc/localtime` from the host file system into the
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container. Make it a relative symlink to the containers's zoneinfo dir, as
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usual. Tools rely on being able to determine the timezone setting from the
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symlink value, and making it relative looks nice even if people list the
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container's `/etc/` from the host.
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2. Make the container journal available in the host, by automatically
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symlinking the container journal directory into the host journal directory.
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More precisely, link `/var/log/journal/<container-machine-id>` of the
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container into the same dir of the host. Administrators can then
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automatically browse all container journals (correctly interleaved) by
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issuing `journalctl -m`. The container machine ID can be determined from
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`/etc/machine-id` in the container.
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3. If the container manager wants to cleanly shutdown the container, it might
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be a good idea to send `SIGRTMIN+3` to its init process. systemd will then
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do a clean shutdown. Note however, that since only systemd understands
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`SIGRTMIN+3` like this, this might confuse other init systems.
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4. To support [Socket Activated
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Containers](http://0pointer.de/blog/projects/socket-activated-containers.html)
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the container manager should be capable of being run as a systemd
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service. It will then receive the sockets starting with FD 3, the number of
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passed FDs in `$LISTEN_FDS` and its PID as `$LISTEN_PID`. It should take
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these and pass them on to the container's init process, also setting
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$LISTEN_FDS and `$LISTEN_PID` (basically, it can just leave the FDs and
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`$LISTEN_FDS` untouched, but it needs to adjust `$LISTEN_PID` to the
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container init process). That's all that's necessary to make socket
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activation work. The protocol to hand sockets from systemd to services is
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hence the same as from the container manager to the container systemd. For
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further details see the explanations of
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[sd_listen_fds(1)](http://0pointer.de/public/systemd-man/sd_listen_fds.html)
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and the [blog story for service
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developers](http://0pointer.de/blog/projects/socket-activation.html).
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5. Container managers should stay away from the cgroup hierarchy outside of the
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unit they created for their container. That's private property of systemd,
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and no other code should modify it.
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## Networking
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1. Inside of a container, if a `veth` link is named `host0`, `systemd-networkd`
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running inside of the container will by default run DHCPv4, DHCPv6, and
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IPv4LL clients on it. It is thus recommended that container managers that
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add a `veth` link to a container name it `host0`, to get an automatically
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configured network, with no manual setup.
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2. Outside of a container, if a `veth` link is prefixed "ve-", `systemd-networkd`
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will by default run DHCPv4 and DHCPv6 servers on it, as well as IPv4LL. It
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is thus recommended that container managers that add a `veth` link to a
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container name the external side `ve-` + the container name.
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3. It is recommended to configure stable MAC addresses for container `veth`
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devices, for example hashed out of the container names. That way it is more
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likely that DHCP and IPv4LL will acquire stable addresses.
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## What You Shouldn't Do
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1. Do not drop `CAP_MKNOD` from the container. `PrivateDevices=` is a commonly
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used service setting that provides a service with its own, private, minimal
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version of `/dev/`. To set this up systemd in the container needs this
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capability. If you take away the capability than all services that set this
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flag will cease to work. Use `BPF_PROG_TYPE_CGROUP_DEVICE` BPF programs — on
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cgroupv2 — or the `devices` controller — on cgroupv1 — to restrict what
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device nodes the container can create instead of taking away the capability
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wholesale. (Also see the section about fully unprivileged containers below.)
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2. Do not drop `CAP_SYS_ADMIN` from the container. A number of the most
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commonly used file system namespacing related settings, such as
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`PrivateDevices=`, `ProtectHome=`, `ProtectSystem=`, `MountFlags=`,
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`PrivateTmp=`, `ReadWriteDirectories=`, `ReadOnlyDirectories=`,
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`InaccessibleDirectories=`, and `MountFlags=` need to be able to open new
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mount namespaces and the mount certain file systems into them. You break all
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services that make use of these options if you drop the capability. Also
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note that logind mounts `XDG_RUNTIME_DIR` as `tmpfs` for all logged in users
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and that won't work either if you take away the capability. (Also see
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section about fully unprivileged containers below.)
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3. Do not cross-link `/dev/kmsg` with `/dev/console`. They are different things,
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you cannot link them to each other.
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4. Do not pretend that the real VTs are available in the container. The VT
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subsystem consists of all the devices `/dev/tty*`, `/dev/vcs*`, `/dev/vcsa*`
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plus their `sysfs` counterparts. They speak specific `ioctl()`s and
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understand specific escape sequences, that other ptys don't understand.
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Hence, it is explicitly not OK to mount a pty to `/dev/tty1`, `/dev/tty2`,
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`/dev/tty3`. This is explicitly not supported.
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5. Don't pretend that passing arbitrary devices to containers could really work
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well. For example, do not pass device nodes for block devices to the
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container. Device access (with the exception of network devices) is not
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virtualized on Linux. Enumeration and probing of meta information from
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`/sys/` and elsewhere is not possible to do correctly in a container. Simply
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adding a specific device node to a container's `/dev/` is *not* *enough* to
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do the job, as `systemd-udevd` and suchlike are not available at all, and no
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devices will appear available or enumerable, inside the container.
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6. Don't mount only a sub-tree of the `cgroupfs` into the container. This will not
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work as `/proc/$PID/cgroup` lists full paths and cannot be matched up with
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the actual `cgroupfs` tree visible, then. (You may "prune" some branches
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though, see above.)
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7. Do not make `/sys/` writable in the container. If you do,
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`systemd-udevd.service` is started to manage your devices — inside the
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container, but that will cause conflicts and errors given that the Linux
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device model is not virtualized for containers on Linux and thus the
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containers and the host would try to manage the same devices, fighting for
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ownership. Multiple other subsystems of systemd similarly test for `/sys/`
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being writable to decide whether to use `systemd-udevd` or assume that
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device management is properly available on the instance. Among them
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`systemd-networkd` and `systemd-logind`. The conditionalization on the
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read-only state of `/sys/` enables a nice automatism: as soon as `/sys/` and
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the Linux device model are changed to be virtualized properly the container
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payload can make use of that, simply by marking `/sys/` writable. (Note that
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as special exception, the devices in `/sys/class/net/` are virtualized
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already, if network namespacing is used. Thus it is OK to mount the relevant
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sub-directories of `/sys/` writable, but make sure to leave the root of
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`/sys/` read-only.)
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## Fully Unprivileged Container Payload
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First things first, to make this clear: Linux containers are not a security
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technology right now. There are more holes in the model than in swiss cheese.
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For example: if you do not use user namespacing, and share root and other users
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between container and host, the `struct user` structures will be shared between
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host and container, and hence `RLIMIT_NPROC` and so of the container users
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affect the host and other containers, and vice versa. This is a major security
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hole, and actually is a real-life problem: since Avahi sets `RLIMIT_NPROC` of
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its user to 2 (to effectively disallow `fork()`ing) you cannot run more than
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one Avahi instance on the entire system...
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People have been asking to be able to run systemd without `CAP_SYS_ADMIN` and
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`CAP_SYS_MKNOD` in the container. This is now supported to some level in
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systemd, but we recommend against it (see above). If `CAP_SYS_ADMIN` and
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`CAP_SYS_MKNOD` are missing from the container systemd will now gracefully turn
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off `PrivateTmp=`, `PrivateNetwork=`, `ProtectHome=`, `ProtectSystem=` and
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others, because those capabilities are required to implement these options. The
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services using these settings (which include many of systemd's own) will hence
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run in a different, less secure environment when the capabilities are missing
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than with them around.
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With user namespacing in place things get much better. With user namespaces the
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`struct user` issue described above goes away, and containers can keep
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`CAP_SYS_ADMIN` safely for the user namespace, as capabilities are virtualized
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and having capabilities inside a container doesn't mean one also has them
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outside.
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## Final Words
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If you write software that wants to detect whether it is run in a container,
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please check `/proc/1/environ` and look for the `container=` environment
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variable. Do not assume the environment variable is inherited down the process
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tree. It generally is not. Hence check the environment block of PID 1, not your
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own. Note though that that file is only accessible to root. systemd hence early
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on also copies the value into `/run/systemd/container`, which is readable for
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everybody. However, that's a systemd-specific interface and other init systems
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are unlikely to do the same.
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Note that it is our intention to make systemd systems work flawlessly and
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out-of-the-box in containers. In fact we are interested to ensure that the same
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OS image can be booted on a bare system, in a VM and in a container, and behave
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correctly each time. If you notice that some component in systemd does not work
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in a container as it should, even though the container manager implements
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everything documented above, please contact us.
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