keycloak-scim/docs/guides/server/hostname.adoc

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<@tmpl.guide
title="Configuring the hostname (v2)"
summary="Learn how to configure the frontend and backchannel endpoints exposed by {project_name}."
includedOptions="hostname hostname-* proxy"
deniedCategories="hostname_v1">

== The importance of setting the hostname option

By default, {project_name} mandates the configuration of the `hostname` option and does not dynamically resolve URLs. This is a security measure.

{project_name} freely discloses its own URLs, for instance through the OIDC Discovery endpoint, or as part of the password reset link in an email. If the hostname was dynamically interpreted from a hostname header, it could provide a potential attacker with an opportunity to manipulate a URL in the email, redirect a user to the attacker's fake domain, and steal sensitive data such as action tokens, passwords, etc.

By explicitly setting the `hostname` option, we avoid a situation where tokens could be issued by a fraudulent issuer. The server can be started with an explicit hostname using the following command:

<@kc.start parameters="--hostname my.keycloak.org"/>

NOTE: The examples start the {project_name} instance in production mode, which requires a public certificate and private key in order to secure communications. For more information, refer to the <@links.server id="configuration-production"/>.

== Defining specific parts of the hostname option

As demonstrated in the previous example, the scheme and port are not explicitly required. In such cases, {project_name} automatically handles these aspects. For instance, the server would be accessible at `https://my.keycloak.org:8443` in the given example. However, a reverse proxy will typically expose {project_name} at the default ports, e.g. `443`. In that case it’s desirable to specify the full URL in the `hostname` option rather than keeping the parts of the URL dynamic. The server can then be started with:

<@kc.start parameters="--hostname https://my.keycloak.org"/>

Similarly, your reverse proxy might expose {project_name} at a different context path. It is possible to configure {project_name} to reflect that via the `hostname` and `hostname-admin` options. See the following example:

<@kc.start parameters="--hostname https://my.keycloak.org:123/auth"/>

== Utilizing an internal URL for communication among clients

{project_name} has the capability to offer a separate URL for backchannel requests, enabling internal communication while maintaining the use of a public URL for frontchannel requests. Moreover, the backchannel is dynamically resolved based on incoming headers. Consider the following example:

<@kc.start parameters="--hostname https://my.keycloak.org --hostname-backchannel-dynamic true"/>

In this manner, your applications, referred to as clients, can connect with {project_name} through your local network, while the server remains publicly accessible at `https://my.keycloak.org`.

== Using edge TLS termination

As you can observe, the HTTPS protocol is the default choice, adhering to {project_name}'s commitment to security best practices. However, {project_name} also provides the flexibility for users to opt for HTTP if necessary. This can be achieved simply by specifying the HTTP listener, consult the <@links.server id="enabletls"/> for details. With an edge TLS-termination proxy you can start the server as follows:

<@kc.start parameters="--hostname https://my.keycloak.org --http-enabled true"/>

The result of this configuration is that you can continue to access {project_name} at `https://my.keycloak.org` via HTTPS, while the proxy interacts with the instance using HTTP and port `8080`.

== Using a reverse proxy

When a proxy is forwarding http or reencrypted TLS requests, the `proxy-headers` option should be set. Depending on the hostname settings, some or all of the URL, may be dynamically determined.

WARNING: If either `forwarded` or `xforwarded` is selected, make sure your reverse proxy properly sets and overwrites the `Forwarded` or `X-Forwarded-*` headers respectively. To set these headers, consult the documentation for your reverse proxy. Misconfiguration will leave {project_name} exposed to security vulnerabilities.

=== Fully dynamic URLs.

For example if your reverse proxy correctly sets the Forwarded header, and you don't want to hardcode the hostname, {project_name} can accommodate this. You simply need to initiate the server as follows:

<@kc.start parameters="--hostname-strict false --proxy-headers forwarded"/>

With this configuration, the server respects the value set by the Forwarded header. This also implies that all endpoints are dynamically resolved.

=== Partially dynamic URLs

The `proxy-headers` option can be also used to resolve the URL partially dynamically when the `hostname` option is not specified as a full URL. For example:

<@kc.start parameters="--hostname my.keycloak.org --proxy-headers xforwarded"/>

In this case, scheme, port and context path are resolved dynamically from X-Forwarded-* headers, while hostname is statically defined as `my.keycloak.org`.

=== Fixed URLs

The `proxy-headers` is still relevant even when the `hostname` is set to a full URL as the headers are used to determine the origin of the request. For example:

<@kc.start parameters="--hostname https://my.keycloak.org --proxy-headers xforwarded"/>

In this case, while nothing is dynamically resolved from the X-Forwarded-* headers, the X-Forwarded-* headers are used to determine the correct origin of the request.

== Exposing the Administration Console on a separate hostname

If you wish to expose the Admin Console on a different host, you can do so with the following command:

<@kc.start parameters="--hostname https://my.keycloak.org --hostname-admin https://admin.my.keycloak.org:8443"/>

This allows you to access {project_name} at `https://my.keycloak.org` and the Admin Console at `https://admin.my.keycloak.org:8443`, while the backend continues to use `https://my.keycloak.org`.


NOTE: Keep in mind that hostname and proxy options do not change the ports on which the server listens. Instead it changes only the ports of static resources like JavaScript and CSS links, OIDC well-known endpoints, redirect URIs, etc. that will be used in front of the proxy. You need to use HTTP configuration options to change the actual ports the server is listening on. Refer to the <@links.server id="all-config"/> for details.

WARNING: Using the `hostname-admin` option does not prevent accessing the Administration REST API endpoints via the frontend URL specified by the `hostname` option. If you want to restrict access to the Administration REST API, you need to do it on the reverse proxy level. Administration Console implicitly accesses the API using the URL as specified by the `hostname-admin` option.

== Background - server endpoints

{project_name} exposes several endpoints, each with a different purpose. They are typically used for communication among applications or for managing the server. We recognize 3 main endpoint groups:

* Frontend
* Backend
* Administration

If you want to work with either of these endpoints, you need to set the base URL. The base URL consists of a several parts:

* a scheme (e.g. https protocol)
* a hostname (e.g. example.keycloak.org)
* a port (e.g. 8443)
* a path (e.g. /auth)

The base URL for each group has an important impact on how tokens are issued and validated, on how links are created for actions that require the user to be redirected to {project_name} (for example, when resetting password through email links), and, most importantly, how applications will discover these endpoints when fetching the OpenID Connect Discovery Document from `realms/{realm-name}/.well-known/openid-configuration`.

=== Frontend

Users and applications use the frontend URL to access {project_name} through a front channel. The front channel is a publicly accessible communication channel. For example browser-based flows (accessing the login page, clicking on the link to reset a password or binding the tokens) can be considered as frontchannel requests.

In order to make {project_name} accessible via the frontend URL, you need to set the `hostname` option:

<@kc.start parameters="--hostname my.keycloak.org"/>

=== Backend

The backend endpoints are those accessible through a public domain or through a private network. They're related to direct backend communication between {project_name} and a client (an application secured by {project_name}). Such communication might be over a local network, avoiding a reverse proxy. Examples of the endpoints that belong to this group are the authorization endpoint, token and token introspection endpoint, userinfo endpoint, JWKS URI endpoint, etc.

The default value of `hostname-backchannel-dynamic` option is `false`, which means that the backchannel URLs are same as the frontchannel URLs. Dynamic resolution of backchannel URLs from incoming request headers can be enabled by setting the following options:

<@kc.start parameters="--hostname https://my.keycloak.org --hostname-backchannel-dynamic true"/>

Note that `hostname` option must be set to a URL. For more information, refer to the <<Validations>> section below.

=== Administration

Similarly to the base frontend URL, you can also set the base URL for resources and endpoints of the administration console. The server exposes the administration console and static resources using a specific URL. This URL is used for redirect URLs, loading resources (CSS, JS), Administration REST API etc. It can be done by setting the `hostname-admin` option:

<@kc.start parameters="--hostname https://my.keycloak.org --hostname-admin https://admin.my.keycloak.org:8443"/>

Again, the `hostname` option must be set to a URL. For more information, refer to the <<Validations>> section below.

== Sources for resolving the URL

As indicated in the previous sections, URLs can be resolved in several ways: they can be dynamically generated, hardcoded, or a combination of both:

* Dynamic from an incoming request:
** Host header, scheme, server port, context path
** Proxy-set headers: `Forwarded` and `X-Forwarded-*`
* Hardcoded:
** Server-wide config (e.g `hostname`, `hostname-admin`, etc.)
** Realm configuration for frontend URL

== Validations

* `hostname` URL and `hostname-admin` URL are verified that full URL is used, incl. scheme and hostname. Port is validated only if present, otherwise default port for given protocol is assumed (80 or 443).
* In production profile (`kc.sh|bat start`), either `--hostname` or `--hostname-strict false` must be explicitly configured.
** This does not apply for dev profile (`kc.sh|bat start-dev`) where `--hostname-strict false` is the default value.
* If `--hostname` is not configured:
** `hostname-backchannel-dynamic` must be set to false.
** `hostname-strict` must be set to false.
* If `hostname-admin` is configured, `hostname` must be set to a URL (not just hostname). Otherwise {project_name} would not know what is the correct frontend URL (incl. port etc.) when accessing the Admin Console.
* If `hostname-backchannel-dynamic` is set to true, `hostname` must be set to a URL (not just hostname). Otherwise {project_name} would not know what is the correct frontend URL (incl. port etc.) when being access via the dynamically resolved bachchannel.

Additionally if hostname is configured, then hostname-strict is ignored.

== Troubleshooting

To troubleshoot the hostname configuration, you can use a dedicated debug tool which can be enabled as:

.{project_name} configuration:
<@kc.start parameters="--hostname=mykeycloak --hostname-debug=true"/>

After {project_name} starts properly, open your browser and go to: `http://mykeycloak:8080/realms/<your-realm>/hostname-debug`

.By default, this endpoint is disabled (`--hostname-debug=false`)

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