Keycloak

Keycloak (Wikipedia) is identity and access management software, and can serve as an authentication server for applications (providing support for OpenID Connect, OAuth 2.0, and SAML.)

Keycloak is...

covered in the NixOS manual
packaged for Nix
available as a NixOS service
written in Java
maintained by Red Hat

Troubleshooting

Installing on system without X11

If, when you perform:

nixos-rebuild switch

... you encounter errors like:

building Nix...

...

checking for CAIRO_BACKEND... no
configure: error: Package requirements (cairo-xlib >= 1.6) were not met:

No package 'cairo-xlib' found

...

error: build of '/nix/store/vfz...2a0-nixos-system-nixos-21.11pre322478.e4ef597edfd.drv' failed

... it would be because the package expects X11 to be installed. The environment.noXlibs NixOS option will specify to not require the X11 libraries:

/etc/nixos/configuration.nix

{ config, pkgs, ... }:

{
  environment.noXlibs = false;
}

Installation in subdirectory

Keycloak may be installed in a subdirectory of a domain. Thus you don't need to configure and expose a subdomain. For example with the following configuration, remember to edit domain.tld, reflecting your used domain.

/etc/nixos/configuration.nix

{

  services.nginx = {
    enable = true;

    # enable recommended settings
    recommendedGzipSettings = true;
    recommendedOptimisation = true;
    recommendedTlsSettings = true;
    recommendedProxySettings = true;

    virtualHosts = {
      "domain.tld" = {
        forceSSL = true;
        enableACME = true;
        locations = {
          "/cloak/" = {
            proxyPass = "http://localhost:${toString config.services.keycloak.settings.http-port}/cloak/";
          };
        };
      };
    };
  };

  services.postgresql.enable = true;

  services.keycloak = {
    enable = true;

    database = {
      type = "postgresql";
      createLocally = true;

      username = "keycloak";
      passwordFile = "/etc/nixos/secrets/keycloak_psql_pass";
    };

    settings = {
      hostname = "domain.tld";
      http-relative-path = "/cloak";
      http-port = 38080;
      proxy = "passthrough";
      http-enabled = true;
    };
  };

}