December 2025 Improvements - Azure AD, Internal Networks, Startup Race Condition (#100)

* Allow internal IPs for OIDC configuration via extra flag.

Addresses issue #97

* Allow for internal IPs in OIDC configuration.

Addresses issue #97.

* feat: Add allowPrivateIPAddresses config option for internal networks

Adds a new configuration option `allowPrivateIPAddresses` that allows
OIDC provider URLs to use private IP addresses (10.x.x.x, 172.16-31.x.x,
192.168.x.x). This is useful for internal deployments where Keycloak or
other OIDC providers run on private networks without DNS resolution.

Security considerations:
- Loopback addresses (127.0.0.1, localhost, ::1) remain blocked
- Link-local addresses (169.254.x.x) remain blocked
- Default is false (secure by default)

Fixes #97

* feat: Support non-email user identifiers for Azure AD

Add userIdentifierClaim configuration option to support Azure AD users
without email addresses. This allows using alternative JWT claims like
"sub", "oid", "upn", or "preferred_username" for user identification.

- Default behavior uses "email" claim (backward compatible)
- Falls back to "sub" claim if configured claim is missing
- allowedUsers matches against the configured claim value
- allowedUserDomains only applies when using email-based identification

Fixes #95

* Race condition on traefik pod startup

When the plugin initializes and calls GetMetadataWithRecovery():

1. Checks cache first (if metadata is cached, returns immediately)
2. Creates a retry executor with startup-optimized settings (10 attempts, 1s delays)
3. Attempts to fetch metadata from the OIDC provider
4. If the fetch fails with a retryable error (connection refused, EOF, TLS/certificate errors, Traefik default cert), it waits and retries
5. After 10 attempts or on a non-retryable error, returns the error

This allows the plugin to handle the race condition where:
- Traefik initializes the plugin before routes are established
- Traefik serves its default certificate before loading real ones
- The OIDC provider pod isn't fully ready yet

Fixes issue #90

* Race condition on traefik pod startup

When the plugin initializes and calls GetMetadataWithRecovery():

1. Checks cache first (if metadata is cached, returns immediately)
2. Creates a retry executor with startup-optimized settings (10 attempts, 1s delays)
3. Attempts to fetch metadata from the OIDC provider
4. If the fetch fails with a retryable error (connection refused, EOF, TLS/certificate errors, Traefik default cert), it waits and retries
5. After 10 attempts or on a non-retryable error, returns the error

This allows the plugin to handle the race condition where:
- Traefik initializes the plugin before routes are established
- Traefik serves its default certificate before loading real ones
- The OIDC provider pod isn't fully ready yet

Fixes issue #90

* Headers too big and 431 responses

Added new option `minimalHeaders` to reduce the size of forwarded headers from the auth middleware to backend services.

  - When minimalHeaders: false (default): All headers are forwarded as before
    - X-Forwarded-User (always set)
    - X-Auth-Request-Redirect
    - X-Auth-Request-User
    - X-Auth-Request-Token (the large ID token)
    - X-User-Groups, X-User-Roles (if configured)
  - When minimalHeaders: true: Reduces header overhead
    - X-Forwarded-User (always set)
    - X-User-Groups, X-User-Roles (still forwarded if configured)
    - Custom templated headers (still processed)
    - Skipped: X-Auth-Request-Token, X-Auth-Request-User, X-Auth-Request-Redirect

Fixes issues #64 and #86

docs/PROVIDER_CONFIGURATIONS.md

@@ -437,6 +437,21 @@ http:
 4. Configure client scopes and mappers
 5. Generate client secret in Credentials tab
 
+### Internal Network Deployment
+
+If your Keycloak instance runs on an internal network with private IP addresses (e.g., Docker networks, Kubernetes internal services), set `allowPrivateIPAddresses: true`:
+
+```yaml
+traefikoidc:
+  providerUrl: "https://192.168.1.100:8443/auth/realms/your-realm"  # Private IP
+  allowPrivateIPAddresses: true  # Required for private IP addresses
+  clientId: "your-client-id"
+  clientSecret: "your-client-secret"
+  # ... other config
+```
+
+> **Security Warning**: Only enable `allowPrivateIPAddresses` in trusted network environments where you control the OIDC provider. This setting reduces SSRF protection.
+
 ---
 
 ## Okta