# API extensions

The changes below were introduced to the LXC API after the 3.0 API was finalized.

They are all backward compatible and can be detected by client tools by
called the `lxc_has_api_extension` function.

## lxc\_log

This introduces a way to initialize a logging instance from the API for a given
container.

## lxc\_config\_item\_is\_supported

This introduces the `lxc_config_item_is_supported` function. It allows users to
check whether their LXC instance supports a given configuration key.

## console\_log

This adds support to container's console log. The console log is implemented as
an efficient ringbuffer.

## reboot2

This adds `reboot2()` as a new API extension. This function properly waits
until a reboot succeeded. It takes a timeout argument. When set to `> 0`
`reboot2()` will block until the timeout is reached, if timeout is set to zero
`reboot2()` will not block, if set to -1 `reboot2()` will block indefinitely.

## mount\_injection

This adds support for injecting and removing mounts into/from a running
containers. Two new API functions `mount()` and `umount()` are added. They
mirror the current mount and umount API of the kernel.

## seccomp\_allow\_nesting

This adds support for seccomp filters to be stacked regardless of whether a seccomp profile is already loaded. This allows nested containers to load their own seccomp profile.

## seccomp\_notify

This adds "notify" as seccomp action that will cause LXC to register a seccomp listener and retrieve a listener file descriptor from the kernel. When a syscall is made that is registered as "notify" the kernel will generate a poll event and send a message over the file descriptor.

The caller can read this message, inspect the syscalls including its arguments. Based on this information the caller is expected to send back a message informing the kernel which action to take. Until that message is sent the kernel will block the calling process. The format of the messages to read and sent is documented in seccomp itself.

A new API function `seccomp_notify_fd()` has been added which allows callers to retrieve the notifier fd for the container's seccomp filter.

## network\_veth\_routes

This introduces the `lxc.net.[i].veth.ipv4.route` and `lxc.net.[i].veth.ipv6.route` properties
on `veth` type network interfaces. This allows adding static routes on host to the container's
network interface.

## network\_ipvlan

This introduces the `ipvlan` network type.

Example usage:

```
lxc.net[i].type=ipvlan
lxc.net[i].ipvlan.mode=[l3|l3s|l2] (defaults to l3)
lxc.net[i].ipvlan.isolation=[bridge|private|vepa] (defaults to bridge)
lxc.net[i].link=eth0
lxc.net[i].flags=up
```

## network\_l2proxy

This introduces the `lxc.net.[i].l2proxy` that can be either `0` or `1`. Defaults to `0`.
This, when used with `lxc.net.[i].link`, will add IP neighbour proxy entries on the linked device
for any IPv4 and IPv6 addresses on the container's network device.

For IPv4 addresses it will check the following sysctl values and fail with an error if not set:

```
net.ipv4.conf.[link].forwarding=1
```

For IPv6 addresses it will check the following sysctl values and fail with an error if not set:

```
net.ipv6.conf.[link].proxy_ndp=1
net.ipv6.conf.[link].forwarding=1
```

## network\_gateway\_device\_route

This introduces the ability to specify `lxc.net.[i].ipv4.gateway` and/or
`lxc.net.[i].ipv6.gateway` with a value of `dev` which will cause the default gateway
inside the container to be created as a device route without destination gateway IP needed.
This is primarily intended for use with layer 3 networking devices, such as IPVLAN.

## network\_phys\_macvlan\_mtu

This introduces the ability to specify a custom MTU for `phys` and `macvlan` devices using the
`lxc.net.[i].mtu` property.

## network\_veth\_router

This introduces the ability to specify a `lxc.net.[i].veth.mode` setting, which takes a value of "bridge" or "router". This defaults to "bridge".

In "router" mode static routes are created on the host for the container's IP addresses pointing to the host side veth interface. In addition to the routes, a static IP neighbour proxy is added to the host side veth interface for the IPv4 and IPv6 gateway IPs.


## cgroup2\_devices

This enables `LXC` to make use of the new devices controller in the unified cgroup hierarchy. `LXC` will now create, load, and attach bpf program to the cgroup of the container when the controller is available.

## cgroup2

This enables `LXC` to make complete use of the unified cgroup hierarchy. With this extension it is possible to run `LXC` containers on systems that use a pure unified cgroup layout.

## init\_pidfd

This adds a new API function `init_pidfd()` which allows one to retrieve a pidfd for the container's init process allowing process management interactions such as sending signals to be completely reliable and race free.

## pidfd

When running on kernels that support pidfds LXC will rely on them for most operations. This makes interacting with containers not just more reliable it also makes it significantly safer and eliminates various races inherent to PID-based kernel APIs. LXC will require that the running kernel at least support `pidfd_send_signal()`, `CLONE_PIDFD`, `P_PIDFD`, and pidfd polling support. Any kernel starting with `Linux 5.4` should have full support for pidfds.

## cgroup\_advanced\_isolation

Privileged containers will usually be able to override the cgroup limits given to them. This introduces three new configuration keys `lxc.cgroup.dir.monitor`, `lxc.cgroup.dir.container`, and `lxc.cgroup.dir.container.inner`. The `lxc.cgroup.dir.monitor` and `lxc.cgroup.dir.container` keys can be used to set to place the `monitor` and the `container` into different cgroups. The `lxc.cgroup.dir.container.inner` key can be set to a cgroup that is concatenated with `lxc.cgroup.dir.container`. When `lxc.cgroup.dir.container.inner` is set the container will be placed into the `lxc.cgroup.dir.container.inner` cgroup but the limits will be set in the `lxc.cgroup.dir.container` cgroup. This way privileged containers cannot escape their cgroup limits.


## time\_namespace

This adds time namespace support to LXC.

## seccomp\_allow\_deny\_syntax

This adds the ability to use "denylist" and "allowlist" in seccomp v2 policies.

## devpts\_fd

This adds the ability to allocate a file descriptor for the devpts instance of
the container.

## seccomp\_notify\_fd\_active

Retrieve the seccomp notifier fd from a running container.

## seccomp\_proxy\_send\_notify\_fd

Whether the seccomp notify proxy sends a long a notify fd file descriptor.

## idmapped\_mounts

Whether this LXC instance can handle idmapped mounts for the rootfs.

## idmapped\_mounts\_v2

Whether this LXC instance can handle idmapped mounts for lxc.mount.entry
entries.

## cgroup2\_auto_mounting

This adds the new options `cgroup2`, `cgroup2:ro`, `cgroup2:force`,
`cgroup2:ro:force` for the `lxc.mount.auto` configuration key. For example, if
a user specifies `cgroup2:force` LXC will pre-mount a pure `cgroup2` layout for
the container even if the host is running with a hybrid layout.