[keycloak-dev] Keycloak URL and backchannel requests

[Stian Thorgersen]

Keycloak needs to know its public URL for a number of reasons. For example
in the OIDC well-known endpoint, issuer in tokens and emails sent to users.

At the moment we retrieve this from the request headers and have documented
how to block requests with invalid URLs (
https://www.keycloak.org/docs/latest/server_admin/index.html#host). The
latter is important in production installations.

There's two problems with the current approach:

1) Clients in the same subnet can't use internal IP to communicate with
Keycloak
2) Configuring the valid URLs is an important step to secure a production
installation, but the current approach is probably not well known and is
rather messy to configure

I propose we introduce an BaseURL SPI to allow a flexible way to determine
the base URL. We would add two implementations OOTB:

1) Request - this would be set as the default-provider and would determine
the base url from request headers as we do today
2) Fixed - this would allow setting a hardcoded public URL

In standalone.xml it would look like this:

<spi name="baseURL">
    <default-provider>request</default-provider>
    <provider name="fixed">
       <properties>
         <property name="baseURL" value="localhost:8080/auth"/>
       </properties>
   </provider>
</spi>

For production the default-provider should be changed to fixed and the
baseURL value should be changed. This can be handled either through manual
editing standalone.xml or with jboss-cli.

Once this is incorporated it will make it easy to add support for
backchannel URL back to the adapters to allow adapters to use internal IP
address when communicating with Keycloak.

Implementation shouldn't be hard. It's mainly about making sure that
UriInfo is retrieved from KeycloakSession#KeycloakContext and not injected
directly.

If anyone has more complex requirements for determining the public URL they
can implement their own provider to handle it.

One thing I'm not sure about is if it's common to use different URLs for
different realms. If that's the case we may consider adding some support
for mapping different realms to different baseURLs. I don't think this is
common though.