Security

Consider the following aspects to ensure that your LXD installation is secure:

  • Keep your operating system up-to-date and install all available security patches.

  • Use only supported LXD versions (LTS releases or monthly feature releases).

  • Restrict access to the LXD daemon and the remote API.

  • Do not use privileged containers unless required. If you use privileged containers, put appropriate security measures in place. See the LXC security page for more information.

  • Configure your network interfaces to be secure.

See the following sections for detailed information.

If you discover a security issue, see the LXD security policy for information on how to report the issue.

Supported versions

Never use unsupported LXD versions in a production environment.

LXD has two types of releases:

  • Monthly feature releases

  • LTS releases

For feature releases, only the latest one is supported, and we usually don’t do point releases. Instead, users are expected to wait until the next monthly release.

For LTS releases, we do periodic bugfix releases that include an accumulation of bugfixes from the feature releases. Such bugfix releases do not include new features.

Access to the LXD daemon

LXD is a daemon that can be accessed locally over a UNIX socket or, if configured, remotely over a TLS socket. Anyone with access to the socket can fully control LXD, which includes the ability to attach host devices and file systems or to tweak the security features for all instances.

Therefore, make sure to restrict the access to the daemon to trusted users.

Local access to the LXD daemon

The LXD daemon runs as root and provides a UNIX socket for local communication. Access control for LXD is based on group membership. The root user and all members of the lxd group can interact with the local daemon.

Important

Local access to LXD through the UNIX socket always grants full access to LXD. This includes the ability to attach file system paths or devices to any instance as well as tweak the security features on any instance.

Therefore, you should only give such access to users who you’d trust with root access to your system.

Access to the remote API

By default, access to the daemon is only possible locally. By setting the core.https_address configuration option (see Server configuration), you can expose the same API over the network on a TLS socket. Remote clients can then connect to LXD and access any image that is marked for public use.

There are several ways to authenticate remote clients as trusted clients to allow them to access the API. See Remote API authentication for details.

In a production setup, you should set core.https_address to the single address where the server should be available (rather than any address on the host). In addition, you should set firewall rules to allow access to the LXD port only from authorized hosts/subnets.

Container security

LXD containers can use a wide range of features for security.

By default, containers are unprivileged, meaning that they operate inside a user namespace, restricting the abilities of users in the container to that of regular users on the host with limited privileges on the devices that the container owns.

If data sharing between containers isn’t needed, you can enable security.idmap.isolated (see Instance configuration), which will use non-overlapping uid/gid maps for each container, preventing potential DoS attacks on other containers.

LXD can also run privileged containers. Note, however, that those aren’t root safe, and a user with root access in such a container will be able to DoS the host as well as find ways to escape confinement.

More details on container security and the kernel features we use can be found on the LXC security page.

Network security

Make sure to configure your network interfaces to be secure. Which aspects you should consider depends on the networking mode you decide to use.

Bridged NIC security

The default networking mode in LXD is to provide a “managed” private network bridge that each instance connects to. In this mode, there is an interface on the host called lxdbr0 that acts as the bridge for the instances.

The host runs an instance of dnsmasq for each managed bridge, which is responsible for allocating IP addresses and providing both authoritative and recursive DNS services.

Instances using DHCPv4 will be allocated an IPv4 address, and a DNS record will be created for their instance name. This prevents instances from being able to spoof DNS records by providing false host name information in the DHCP request.

The dnsmasq service also provides IPv6 router advertisement capabilities. This means that instances will auto-configure their own IPv6 address using SLAAC, so no allocation is made by dnsmasq. However, instances that are also using DHCPv4 will also get an AAAA DNS record created for the equivalent SLAAC IPv6 address. This assumes that the instances are not using any IPv6 privacy extensions when generating IPv6 addresses.

In this default configuration, whilst DNS names cannot not be spoofed, the instance is connected to an Ethernet bridge and can transmit any layer 2 traffic that it wishes, which means an untrusted instance can effectively do MAC or IP spoofing on the bridge.

In the default configuration, it is also possible for instances connected to the bridge to modify the LXD host’s IPv6 routing table by sending (potentially malicious) IPv6 router advertisements to the bridge. This is because the lxdbr0 interface is created with /proc/sys/net/ipv6/conf/lxdbr0/accept_ra set to 2, meaning that the LXD host will accept router advertisements even though forwarding is enabled (see /proc/sys/net/ipv4/* Variables for more information).

However, LXD offers several bridged NIC security features that can be used to control the type of traffic that an instance is allowed to send onto the network. These NIC settings should be added to the profile that the instance is using, or they can be added to individual instances, as shown below.

The following security features are available for bridged NICs:

Key

Type

Default

Required

Description

security.mac_filtering

boolean

false

no

Prevent the instance from spoofing another’s MAC address

security.ipv4_filtering

boolean

false

no

Prevent the instance from spoofing another’s IPv4 address (enables mac_filtering)

security.ipv6_filtering

boolean

false

no

Prevent the instance from spoofing another’s IPv6 address (enables mac_filtering)

One can override the default bridged NIC settings from the profile on a per-instance basis using:

lxc config device override <instance> <NIC> security.mac_filtering=true

Used together, these features can prevent an instance connected to a bridge from spoofing MAC and IP addresses. These options are implemented using either xtables (iptables, ip6tables and ebtables) or nftables, depending on what is available on the host.

It’s worth noting that those options effectively prevent nested containers from using the parent network with a different MAC address (i.e using bridged or macvlan NICs).

The IP filtering features block ARP and NDP advertisements that contain a spoofed IP, as well as blocking any packets that contain a spoofed source address.

If security.ipv4_filtering or security.ipv6_filtering is enabled and the instance cannot be allocated an IP address (because ipvX.address=none or there is no DHCP service enabled on the bridge), then all IP traffic for that protocol is blocked from the instance.

When security.ipv6_filtering is enabled, IPv6 router advertisements are blocked from the instance.

When security.ipv4_filtering or security.ipv6_filtering is enabled, any Ethernet frames that are not ARP, IPv4 or IPv6 are dropped. This prevents stacked VLAN QinQ (802.1ad) frames from bypassing the IP filtering.

Routed NIC security

An alternative networking mode is available called “routed”. It provides a veth pair between container and host. In this networking mode, the LXD host functions as a router, and static routes are added to the host directing traffic for the container’s IPs towards the container’s veth interface.

By default, the veth interface created on the host has its accept_ra setting disabled to prevent router advertisements from the container modifying the IPv6 routing table on the LXD host. In addition to that, the rp_filter on the host is set to 1 to prevent source address spoofing for IPs that the host does not know the container has.