# Image registries An image registry is a server-side entity that points to a global, read-only source of images, such as a SimpleStreams server or another LXD server. A registry does not store any images itself; it records the location of the image source and the protocol used to reach it, so that LXD can download images from that source. See [How to manage image registries](https://canonical.com/lxd/docs/latest/howto/image_registries/index.html.md#howto-image-registries) for instructions on how to create and manage image registries, and [Built-in image registries](https://canonical.com/lxd/docs/latest/reference/remote_image_servers/index.html.md#remote-image-servers) for the registries that are built in to LXD. ## Registry protocols An image registry uses one of the following protocols, depending on the type of image source it points to: `simplestreams` : The image source is a pure image server that uses the [simple streams format](https://git.launchpad.net/simplestreams/tree/). The built-in registries are all `simplestreams` registries.
You configure a `simplestreams` registry with the `url` of the image server (which must use HTTPS). `lxd` : The image source is another LXD server, reached through a [cluster link](https://canonical.com/lxd/docs/latest/howto/cluster_links_create/index.html.md#howto-cluster-links-create). This can be a LXD server that is used solely to serve images, or a regular LXD server that also serves images in addition to running instances.
You configure a `lxd` registry with the name of the `cluster` link that connects to the source server. Use a public cluster link for a public LXD server, or a unidirectional or bidirectional cluster link to authenticate to a private server. For a public LXD server that is used solely to serve images, set the [`core.https_address`](https://canonical.com/lxd/docs/latest/server/index.html.md#server-core:core.https_address) configuration option and mark the images that you want to share as `public` (see [How to expose LXD to the network](https://canonical.com/lxd/docs/latest/howto/server_expose/index.html.md#server-expose) for more information). For a private LXD server, restrict access to the remote API and configure an authentication method (see [Remote API authentication](https://canonical.com/lxd/docs/latest/authentication/index.html.md#authentication)). ## Configuration options Image registries can be configured through a set of key/value configuration options. See [Configure an image registry](https://canonical.com/lxd/docs/latest/howto/image_registries/index.html.md#howto-image-registries-configure) for instructions on how to set these options. The following options are available: `cluster` Cluster link name for image registry using “LXD” protocol [](#image-registry-image-registry-conf:cluster) | **Key:** | `cluster` | |---------------|-------------| | **Type:** | string | | **Required:** | no | `source_project` Source project for image registry using “LXD” protocol [](#image-registry-image-registry-conf:source_project) | **Key:** | `source_project` | |---------------|--------------------| | **Type:** | string | | **Required:** | no | `url` Source URL for image registry using “SimpleStreams” protocol [](#image-registry-image-registry-conf:url) | **Key:** | `url` | |---------------|---------| | **Type:** | string | | **Required:** | no | `user.*` User-provided free-form key/value pairs [](#image-registry-image-registry-conf:user.*) | **Key:** | `user.*` | |-------------|------------| | **Type:** | string | ## Related topics How-to guides: - [Images](https://canonical.com/lxd/docs/latest/images/index.html.md#images) Explanation: - [Local and remote images](https://canonical.com/lxd/docs/latest/image-handling/index.html.md#about-images)