diff --git a/internal/authelia/config.go b/internal/authelia/config.go
index ff12f9f..4e739bb 100644
--- a/internal/authelia/config.go
+++ b/internal/authelia/config.go
@@ -32,8 +32,28 @@ type ConfigOpts struct {
 	SMTPPassword   string       // SMTP password (from secrets)
 }
 
-// GenerateConfig builds Authelia's configuration.yml and writes it atomically.
+// ValidateConfigOpts checks that required fields are present and valid.
+func ValidateConfigOpts(opts ConfigOpts) error {
+	if opts.Domain == "" {
+		return fmt.Errorf("auth portal domain is required")
+	}
+	if opts.JWTSecret == "" {
+		return fmt.Errorf("JWT secret is required")
+	}
+	if opts.SessionSecret == "" {
+		return fmt.Errorf("session secret is required")
+	}
+	if len(opts.StorageEncKey) < 20 {
+		return fmt.Errorf("storage encryption key must be at least 20 characters, got %d", len(opts.StorageEncKey))
+	}
+	return nil
+}
+
+// GenerateConfig validates options, builds Authelia's configuration.yml, and writes it atomically.
 func (m *Manager) GenerateConfig(opts ConfigOpts) error {
+	if err := ValidateConfigOpts(opts); err != nil {
+		return fmt.Errorf("config validation: %w", err)
+	}
 	if err := m.EnsureDataDir(); err != nil {
 		return err
 	}
diff --git a/internal/nftables/manager.go b/internal/nftables/manager.go
index 0f1cfeb..2fafc12 100644
--- a/internal/nftables/manager.go
+++ b/internal/nftables/manager.go
@@ -3,6 +3,7 @@ package nftables
 import (
 	"fmt"
 	"log/slog"
+	"net"
 	"os"
 	"os/exec"
 	"strings"
@@ -131,6 +132,18 @@ func (m *Manager) ValidateRules(rulesPath string) error {
 	return nil
 }
 
+// ValidateWANInterface checks that the specified WAN interface exists on the system.
+// Returns nil if empty (no WAN filtering) or if the interface exists.
+func ValidateWANInterface(name string) error {
+	if name == "" {
+		return nil
+	}
+	if _, err := net.InterfaceByName(name); err != nil {
+		return fmt.Errorf("WAN interface %q not found: %w", name, err)
+	}
+	return nil
+}
+
 // SafeApply validates, backs up, writes, and applies nftables rules.
 // On apply failure, rolls back to the previous rules.
 func (m *Manager) SafeApply(content string) error {
diff --git a/internal/tunnel/manager.go b/internal/tunnel/manager.go
index 66b5fc5..cf42f72 100644
--- a/internal/tunnel/manager.go
+++ b/internal/tunnel/manager.go
@@ -132,8 +132,32 @@ func (m *Manager) GenerateConfig(cfg Config, services []PublicDomain) string {
 	return string(data)
 }
 
-// WriteConfig generates and writes the cloudflared config to disk.
+// ValidateConfig checks tunnel config for common errors before generating.
+func ValidateConfig(cfg Config) error {
+	if !cfg.Enabled {
+		return nil // disabled is valid
+	}
+	if cfg.TunnelID == "" {
+		return fmt.Errorf("tunnel ID is required")
+	}
+	if cfg.PublicDomain == "" {
+		return fmt.Errorf("public domain is required")
+	}
+	if cfg.GatewayDomain == "" {
+		return fmt.Errorf("gateway domain is required")
+	}
+	credsFile := credentialsPath(cfg.CredentialsDir, cfg.TunnelID)
+	if _, err := os.Stat(credsFile); err != nil {
+		return fmt.Errorf("credentials file not found: %s", credsFile)
+	}
+	return nil
+}
+
+// WriteConfig validates, generates, and writes the cloudflared config to disk.
 func (m *Manager) WriteConfig(cfg Config, services []PublicDomain) error {
+	if err := ValidateConfig(cfg); err != nil {
+		return fmt.Errorf("config validation: %w", err)
+	}
 	content := m.GenerateConfig(cfg, services)
 	if content == "" {
 		// Remove config if tunnel is disabled or no public services
diff --git a/internal/tunnel/manager_test.go b/internal/tunnel/manager_test.go
index 7e52729..883047d 100644
--- a/internal/tunnel/manager_test.go
+++ b/internal/tunnel/manager_test.go
@@ -7,13 +7,17 @@ import (
 	"testing"
 )
 
-func testConfig() Config {
+func testConfig(t *testing.T) Config {
+	t.Helper()
+	credsDir := filepath.Join(t.TempDir(), "creds")
+	os.MkdirAll(credsDir, 0755)
+	os.WriteFile(filepath.Join(credsDir, "abc-123.json"), []byte(`{"AccountTag":"test"}`), 0600)
 	return Config{
 		Enabled:        true,
 		TunnelID:       "abc-123",
 		PublicDomain:   "pub.payne.io",
 		GatewayDomain:  "payne.io",
-		CredentialsDir: "/tmp/creds",
+		CredentialsDir: credsDir,
 	}
 }
 
@@ -25,7 +29,8 @@ func TestGenerateConfig_Basic(t *testing.T) {
 		{Name: "dashboard"},
 	}
 
-	out := m.GenerateConfig(testConfig(), services)
+	cfg := testConfig(t)
+	out := m.GenerateConfig(cfg, services)
 
 	if out == "" {
 		t.Fatal("expected non-empty config")
@@ -37,7 +42,7 @@ func TestGenerateConfig_Basic(t *testing.T) {
 	}
 
 	// Check credentials file
-	if !strings.Contains(out, "credentials-file: /tmp/creds/abc-123.json") {
+	if !strings.Contains(out, "abc-123.json") {
 		t.Errorf("expected credentials file, got:\n%s", out)
 	}
 
@@ -75,7 +80,7 @@ func TestGenerateConfig_SubdomainOverride(t *testing.T) {
 		{Name: "internal-name", Subdomain: "public-name"},
 	}
 
-	out := m.GenerateConfig(testConfig(), services)
+	out := m.GenerateConfig(testConfig(t), services)
 
 	if !strings.Contains(out, "hostname: public-name.pub.payne.io") {
 		t.Errorf("expected subdomain override, got:\n%s", out)
@@ -85,7 +90,7 @@ func TestGenerateConfig_SubdomainOverride(t *testing.T) {
 func TestGenerateConfig_Disabled(t *testing.T) {
 	m := NewManager(t.TempDir())
 
-	cfg := testConfig()
+	cfg := testConfig(t)
 	cfg.Enabled = false
 
 	out := m.GenerateConfig(cfg, []PublicDomain{{Name: "my-api"}})
@@ -96,7 +101,7 @@ func TestGenerateConfig_Disabled(t *testing.T) {
 
 func TestGenerateConfig_NoServices(t *testing.T) {
 	m := NewManager(t.TempDir())
-	out := m.GenerateConfig(testConfig(), nil)
+	out := m.GenerateConfig(testConfig(t), nil)
 	if out != "" {
 		t.Errorf("expected empty config with no services, got:\n%s", out)
 	}
@@ -104,7 +109,7 @@ func TestGenerateConfig_NoServices(t *testing.T) {
 
 func TestGenerateConfig_MissingTunnelID(t *testing.T) {
 	m := NewManager(t.TempDir())
-	cfg := testConfig()
+	cfg := testConfig(t)
 	cfg.TunnelID = ""
 
 	out := m.GenerateConfig(cfg, []PublicDomain{{Name: "my-api"}})
@@ -118,7 +123,7 @@ func TestWriteConfig(t *testing.T) {
 	m := NewManager(tmpDir)
 
 	services := []PublicDomain{{Name: "my-api"}}
-	if err := m.WriteConfig(testConfig(), services); err != nil {
+	if err := m.WriteConfig(testConfig(t), services); err != nil {
 		t.Fatalf("WriteConfig failed: %v", err)
 	}
 
@@ -139,12 +144,12 @@ func TestWriteConfig_RemovesWhenDisabled(t *testing.T) {
 	m := NewManager(tmpDir)
 
 	// Write config first
-	if err := m.WriteConfig(testConfig(), []PublicDomain{{Name: "my-api"}}); err != nil {
+	if err := m.WriteConfig(testConfig(t), []PublicDomain{{Name: "my-api"}}); err != nil {
 		t.Fatalf("WriteConfig failed: %v", err)
 	}
 
 	// Now disable and write again — should remove the file
-	cfg := testConfig()
+	cfg := testConfig(t)
 	cfg.Enabled = false
 	if err := m.WriteConfig(cfg, []PublicDomain{{Name: "my-api"}}); err != nil {
 		t.Fatalf("WriteConfig (disable) failed: %v", err)
diff --git a/internal/wireguard/manager.go b/internal/wireguard/manager.go
index 50f0054..49b2722 100644
--- a/internal/wireguard/manager.go
+++ b/internal/wireguard/manager.go
@@ -103,8 +103,29 @@ func (m *Manager) GetConfig() (*Config, error) {
 	return &cfg, nil
 }
 
-// SaveConfig writes the server interface config.
+// ValidateConfig checks a WireGuard config for common errors before saving.
+func ValidateConfig(cfg *Config) error {
+	if cfg.ListenPort < 1 || cfg.ListenPort > 65535 {
+		return fmt.Errorf("listenPort must be 1-65535, got %d", cfg.ListenPort)
+	}
+	if cfg.Address != "" {
+		if _, _, err := net.ParseCIDR(cfg.Address); err != nil {
+			return fmt.Errorf("invalid server address CIDR %q: %w", cfg.Address, err)
+		}
+	}
+	if cfg.LanCIDR != "" {
+		if _, _, err := net.ParseCIDR(cfg.LanCIDR); err != nil {
+			return fmt.Errorf("invalid LAN CIDR %q: %w", cfg.LanCIDR, err)
+		}
+	}
+	return nil
+}
+
+// SaveConfig validates and writes the server interface config.
 func (m *Manager) SaveConfig(cfg *Config) error {
+	if err := ValidateConfig(cfg); err != nil {
+		return err
+	}
 	if err := os.MkdirAll(m.vpnDir(), 0755); err != nil {
 		return fmt.Errorf("create vpn dir: %w", err)
 	}