Merge pull request #14494 from poettering/container-interface
Let's import the container interface description text from the fdo wiki
This commit is contained in:
Zbigniew Jędrzejewski-Szmek
GitHub
commit
af6a2a1e57
294
docs/CONTAINER_INTERFACE.md
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294
docs/CONTAINER_INTERFACE.md
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---
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title: The 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 should just set it before invoking systemd, and systemd
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will leave it unmodified (that is unless there's an explicit hostname
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configured in `/etc/hostname` which overrides whatever is pre-initialized by
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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 avoid that the container can alter the host kernel's
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configuration settings. (As special exception, if your container has network
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namespaces enabled, feel free to make `/proc/sys/net/` writable). systemd
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and various other subsystems (such as the SELinux userspace) have been
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modified to detect whether these file systems are read-only, and will behave
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accordingly. (It's OK to mount `/sys/` as `tmpfs` btw, and only mount a
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subset of its sub-trees from the real `sysfs` to hide `/sys/firmware/`,
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`/sys/kernel/` and so on. If you do that, still make sure to mark `/sys/`
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read-only, as that condition is what systemd looks for, and is what is
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considered to be the API 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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these days use `PrivateDevices=`, which means that systemd will set up a
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private `/dev/` for them for which it needs to be able to create these
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device nodes. Dropping `CAP_MKNOD` for containers is hence generally not
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advisable, but see below.
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4. `systemd-udevd` is not available in containers (and refuses to start), and hence device
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dependencies are unavailable. The `systemd-udevd` unit files will check for `/sys/`
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being read-only, as an indication whether device management can work. Hence
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make sure to mount `/sys/` read-only in the container (see above). Various
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clients to `systemd-udevd` also check the read-only state of `/sys/` in some cases,
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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) must 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
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container. Do not add new cgroup directories to the top of the tree. This
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will not only confuse systemd and the admin, but also ensure your
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implementation is not "stackable".
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## Environment Variables
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1. To allow systemd (and other code) to identify that it is executed within a
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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
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environment variable for the container's PID 1 to the container UUID it
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wants to set. (This is similar to the effect of `qemu`'s `-uuid`
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switch). Note that you should pass only a UUID here that is actually unique
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(i.e. only one running container should have a specific UUID), and gets
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changed when a container gets duplicated. Also note that systemd will try to
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persistently store the UUID in `/etc/machine-id` (if writable) when this
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option is used, hence you should always pass the same UUID here. Keeping the
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externally used UUID for a container and the internal one in sync is
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hopefully useful to minimize 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). Note
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that this variable should not include the pty that `/dev/console` maps to if
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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 additionally 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 by making it relative it looks nice even if people list
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the containers' `/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
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directory. More precisely, link `/var/log/journal/<container-machine-id>` of
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the 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 you can determine 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 do a
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clean shutdown. Note however, that only systemd understands `SIGRTMIN+3` like
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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 set `$LISTEN_PID` to for 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 a container manager to a 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 `name=systemd` cgroup hierarchy
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outside of the unit they created for their container. That's private
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property of systemd, and no other code should interfere with 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 do DHCPv4 client, DHCPv6
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client and IPv4LL 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 automatically
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configured networked, with no manual interference from outside.
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2. Outside of a container, if a `veth` link is prefixed "ve-" will by default do
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DHCPv4 server and DHCPv6 serer on it, as well as IPv4LL. It is thus recommended
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that container managers that add a `veth` link to a container name the external
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side `ve-` followed by the container name.
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3. It is recommended to configure stable MAC addresses to 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, and this are increasingly many, as we encourage
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people to make use of this functionality. Use `BPF_PROG_TYPE_CGROUP_DEVICE`
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BPF programs — on cgroupv2 — or the `devices` controller — on cgroupv1 — to
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restrict what device nodes the container can create instead of taking away
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the capability wholesale. (Also see section about fully unprivileged
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containers below.)
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2. Do not drop `CAP_SYS_ADMIN` from the container. A number of file system
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namespacing related settings, such as `PrivateDevices=`, `ProtectHome=`,
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`ProtectSystem=`, `MountFlags=`, `PrivateTmp=`, `ReadWriteDirectories=`,
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`ReadOnlyDirectories=`, `InaccessibleDirectories=`, `MountFlags=` need to be
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able to open new mount namespaces and the mount certain file system into
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it. You break all services that make use of these flags if you drop the
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flag. Note that already quite a number of services make use of this as we
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actively encourage users to make use of this security functionality. Also
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note that logind mounts `XDG_RUNTIME_DIR` as `tmpfs` for all logged in users
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and won't work either if you take away the capability. (Also see section
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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 would be available in the containers. The
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VT subsystem consists of all 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
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understand. Hence, it is explicitly not OK to mount a pty to `/dev/tty1`,
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`/dev/tty2`, `/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 a 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 affect
|
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the host and other containers, and vice versa. This is a major security hole,
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and actually is a real-life problem: since Avahi sets `RLIMIT_NPROC` of its user
|
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to 2 (to effectively disallow `fork()`ing) you cannot run more than one Avahi
|
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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
|
||||
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
|
||||
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
|
||||
everything documented above, please contact us.
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|
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@ -28,29 +28,20 @@
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<refsect1>
|
||||
<title>Description</title>
|
||||
|
||||
<para><filename>systemd-getty-generator</filename> is a generator
|
||||
that automatically instantiates
|
||||
<filename>serial-getty@.service</filename> on the kernel
|
||||
console(s), if they can function as ttys and are not provided by
|
||||
the virtual console subsystem. It will also instantiate
|
||||
<filename>serial-getty@.service</filename> instances for
|
||||
virtualizer consoles, if execution in a virtualized environment is
|
||||
detected. If execution in a container environment is detected, it
|
||||
will instead enable <filename>console-getty.service</filename> for
|
||||
<filename>/dev/console</filename>, and
|
||||
<filename>container-getty@.service</filename> instances for
|
||||
additional container pseudo TTYs as requested by the container
|
||||
manager (see <ulink
|
||||
url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/"><filename>Container
|
||||
Interface</filename></ulink>). This should ensure that the user is
|
||||
shown a login prompt at the right place, regardless of which
|
||||
environment the system is started in. For example, it is
|
||||
sufficient to redirect the kernel console with a kernel command
|
||||
line argument such as <varname>console=</varname> to get both
|
||||
kernel messages and a getty prompt on a serial TTY. See <ulink
|
||||
<para><filename>systemd-getty-generator</filename> is a generator that automatically instantiates
|
||||
<filename>serial-getty@.service</filename> on the kernel console(s), if they can function as ttys and are
|
||||
not provided by the virtual console subsystem. It will also instantiate
|
||||
<filename>serial-getty@.service</filename> instances for virtualizer consoles, if execution in a
|
||||
virtualized environment is detected. If execution in a container environment is detected, it will instead
|
||||
enable <filename>console-getty.service</filename> for <filename>/dev/console</filename>, and
|
||||
<filename>container-getty@.service</filename> instances for additional container pseudo TTYs as requested
|
||||
by the container manager (see <ulink url="https://systemd.io/CONTAINER_INTERFACE"><filename>Container
|
||||
Interface</filename></ulink>). This should ensure that the user is shown a login prompt at the right
|
||||
place, regardless of which environment the system is started in. For example, it is sufficient to
|
||||
redirect the kernel console with a kernel command line argument such as <varname>console=</varname> to
|
||||
get both kernel messages and a getty prompt on a serial TTY. See <ulink
|
||||
url="https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt"><filename>kernel-parameters.txt</filename></ulink>
|
||||
for more information on the <varname>console=</varname> kernel
|
||||
parameter.</para>
|
||||
for more information on the <varname>console=</varname> kernel parameter.</para>
|
||||
|
||||
<para><filename>systemd-getty-generator</filename> implements
|
||||
<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
|
||||
|
|
|
|||
|
|
@ -55,12 +55,9 @@
|
|||
and is different for every booted instance of the
|
||||
VM.</para></listitem>
|
||||
|
||||
<listitem><para>Similarly, if run inside a Linux container
|
||||
environment and a UUID is configured for the container, this is
|
||||
used to initialize the machine ID. For details, see the
|
||||
documentation of the <ulink
|
||||
url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container
|
||||
Interface</ulink>.</para></listitem>
|
||||
<listitem><para>Similarly, if run inside a Linux container environment and a UUID is configured for the
|
||||
container, this is used to initialize the machine ID. For details, see the documentation of the <ulink
|
||||
url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink>.</para></listitem>
|
||||
|
||||
<listitem><para>Otherwise, a new ID is randomly
|
||||
generated.</para></listitem>
|
||||
|
|
|
|||
|
|
@ -111,8 +111,7 @@
|
|||
container.</para>
|
||||
|
||||
<para><command>systemd-nspawn</command> implements the <ulink
|
||||
url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink>
|
||||
specification.</para>
|
||||
url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> specification.</para>
|
||||
|
||||
<para>While running, containers invoked with <command>systemd-nspawn</command> are registered with the
|
||||
<citerefentry><refentrytitle>systemd-machined</refentrytitle><manvolnum>8</manvolnum></citerefentry> service that
|
||||
|
|
|
|||
|
|
@ -280,10 +280,9 @@
|
|||
files or parameters passed on the kernel command line. For details, see
|
||||
<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
|
||||
|
||||
<para>Systems which invoke systemd in a container or initrd
|
||||
environment should implement the
|
||||
<ulink url="https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface">Container Interface</ulink> or
|
||||
<ulink url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
|
||||
<para>Systems which invoke systemd in a container or initrd environment should implement the <ulink
|
||||
url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> or <ulink
|
||||
url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
|
||||
specifications, respectively.</para>
|
||||
</refsect1>
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue