vault: add OpenBao OIDC integration with Kanidm

Enable Kanidm users to authenticate to OpenBao via OIDC for Web UI access.
Members of the admins group get full read/write access to secrets.

Changes:
- Add OIDC auth backend in Terraform (oidc.tf)
- Add oidc-admin and oidc-default policies
- Add openbao OAuth2 client to Kanidm
- Enable legacy crypto (RS256) for OpenBao compatibility
- Allow imperative group membership management in Kanidm

Limitations:
- CLI login not supported (Kanidm requires HTTPS for confidential client redirects)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/kanidm/default.nix

@@ -24,9 +24,10 @@
       idmAdminPasswordFile = config.vault.secrets.kanidm-idm-admin.outputDir;
 
       groups = {
-        admins = { };
-        users = { };
-        ssh-users = { };
+        # overwriteMembers = false allows imperative member management via CLI
+        admins = { overwriteMembers = false; };
+        users = { overwriteMembers = false; };
+        ssh-users = { overwriteMembers = false; };
       };
 
       # Regular users (persons) are managed imperatively via kanidm CLI
@@ -40,6 +41,20 @@
         preferShortUsername = true;
         scopeMaps.users = [ "openid" "profile" "email" "groups" ];
       };
+
+      systems.oauth2.openbao = {
+        displayName = "OpenBao Secrets";
+        # Web UI callback only (CLI localhost not supported with confidential clients)
+        originUrl = "https://vault.home.2rjus.net:8200/ui/vault/auth/oidc/oidc/callback";
+        originLanding = "https://vault.home.2rjus.net:8200/";
+        basicSecretFile = config.vault.secrets.openbao-oauth2.outputDir;
+        preferShortUsername = true;
+        # Enable RS256 signing algorithm (required by OpenBao)
+        enableLegacyCrypto = true;
+        # Allow groups scope for role binding
+        scopeMaps.admins = [ "openid" "profile" "email" "groups" ];
+        scopeMaps.users = [ "openid" "profile" "email" "groups" ];
+      };
     };
   };
 
@@ -72,6 +87,15 @@
     group = "kanidm";
   };
 
+  # Vault secret for OpenBao OAuth2 client secret
+  vault.secrets.openbao-oauth2 = {
+    secretPath = "services/openbao/oauth2-client-secret";
+    extractKey = "password";
+    services = [ "kanidm" ];
+    owner = "kanidm";
+    group = "kanidm";
+  };
+
   # Note: Kanidm does not expose Prometheus metrics
   # If metrics support is added in the future, uncomment:
   # homelab.monitoring.scrapeTargets = [

terraform/vault/approle.tf

@@ -106,6 +106,7 @@ locals {
         "secret/data/hosts/kanidm01/*",
         "secret/data/kanidm/*",
         "secret/data/services/grafana/*",
+        "secret/data/services/openbao/*",
       ]
     }
 

terraform/vault/oidc.tf

@@ -0,0 +1,50 @@
+# OIDC authentication backend for Kanidm integration
+# Web UI only - CLI localhost redirects not supported with confidential clients
+resource "vault_jwt_auth_backend" "oidc" {
+  path               = "oidc"
+  type               = "oidc"
+  oidc_discovery_url = "https://auth.home.2rjus.net/oauth2/openid/openbao"
+  oidc_client_id     = "openbao"
+  oidc_client_secret = random_password.auto_secrets["services/openbao/oauth2-client-secret"].result
+  default_role       = "default"
+
+  tune {
+    listing_visibility = "unauth"
+    default_lease_ttl  = "1h"
+    max_lease_ttl      = "24h"
+    token_type         = "default-service"
+  }
+}
+
+# Admin role - maps Kanidm admins group to admin policy
+resource "vault_jwt_auth_backend_role" "admin" {
+  backend        = vault_jwt_auth_backend.oidc.path
+  role_name      = "admin"
+  token_policies = ["oidc-admin"]
+
+  user_claim   = "preferred_username"
+  groups_claim = "groups"
+  bound_claims = { groups = "admins@home.2rjus.net" }
+  role_type    = "oidc"
+  oidc_scopes  = ["openid", "profile", "email", "groups"]
+
+  allowed_redirect_uris = [
+    "https://vault.home.2rjus.net:8200/ui/vault/auth/oidc/oidc/callback",
+  ]
+}
+
+# Default role - any authenticated user (limited access)
+resource "vault_jwt_auth_backend_role" "default" {
+  backend        = vault_jwt_auth_backend.oidc.path
+  role_name      = "default"
+  token_policies = ["oidc-default"]
+
+  user_claim   = "preferred_username"
+  groups_claim = "groups"
+  role_type    = "oidc"
+  oidc_scopes  = ["openid", "profile", "email", "groups"]
+
+  allowed_redirect_uris = [
+    "https://vault.home.2rjus.net:8200/ui/vault/auth/oidc/oidc/callback",
+  ]
+}

terraform/vault/policies.tf

@@ -8,3 +8,50 @@ path "sys/metrics" {
 }
 EOT
 }
+
+# OIDC admin policy - full read/write to all secrets
+resource "vault_policy" "oidc_admin" {
+  name = "oidc-admin"
+
+  policy = <<EOT
+# Full access to KV secrets
+path "secret/*" {
+  capabilities = ["create", "read", "update", "delete", "list"]
+}
+
+# Read system health and metrics
+path "sys/health" {
+  capabilities = ["read"]
+}
+
+path "sys/metrics" {
+  capabilities = ["read"]
+}
+
+# List auth methods and mounts
+path "sys/auth" {
+  capabilities = ["read"]
+}
+
+path "sys/mounts" {
+  capabilities = ["read"]
+}
+EOT
+}
+
+# OIDC default policy - minimal access for authenticated users
+resource "vault_policy" "oidc_default" {
+  name = "oidc-default"
+
+  policy = <<EOT
+# Read own token info
+path "auth/token/lookup-self" {
+  capabilities = ["read"]
+}
+
+# Read system health
+path "sys/health" {
+  capabilities = ["read"]
+}
+EOT
+}

terraform/vault/secrets.tf

@@ -115,6 +115,12 @@ locals {
       password_length = 64
     }
 
+    # OpenBao OAuth2 client secret (for Kanidm OIDC)
+    "services/openbao/oauth2-client-secret" = {
+      auto_generate   = true
+      password_length = 64
+    }
+
     # NKey for nixos-exporter NATS cache sharing
     "shared/nixos-exporter/nkey" = {
       auto_generate = false