added test go implementation

network-go/config/config_test.go

@@ -0,0 +1,231 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+const testJSON = `{
+  "networks": {
+    "smarthost-loadbalancer": {
+      "network_name": "smarthost-loadbalancer",
+      "subnet": "172.18.103.0/24",
+      "gateway": "172.18.103.1"
+    },
+    "smarthost_backend-1": {
+      "network_name": "smarthost_backend-1",
+      "subnet": "172.18.104.0/24",
+      "gateway": "172.18.104.1"
+    }
+  },
+  "ips": {
+    "172.18.103.2": {
+      "ip": "172.18.103.2",
+      "container_name": "smarthostloadbalancer",
+      "selector": "smarthostloadbalancer",
+      "service_name": "smarthost-proxy"
+    },
+    "172.18.104.2": {
+      "ip": "172.18.104.2",
+      "container_name": "smarthostbackend-1",
+      "selector": "smarthostbackend-1",
+      "service_name": "smarthost-proxy"
+    }
+  },
+  "policies": [
+    {
+      "service_name": "smarthost-proxy",
+      "container_name": "smarthost_loadbalancer",
+      "selector": "smarthostloadbalancer",
+      "from": "publicbackend",
+      "port": 80,
+      "proto": "tcp"
+    },
+    {
+      "service_name": "smarthost-proxy",
+      "container_name": "smarthost_loadbalancer",
+      "selector": "smarthostloadbalancer",
+      "name": "wireguardproxy",
+      "iface": "wg0",
+      "nat": "dnat",
+      "to": "smarthostloadbalancer",
+      "port": 80,
+      "proto": "tcp"
+    }
+  ]
+}`
+
+func TestLoad(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "networks.json")
+	if err := os.WriteFile(path, []byte(testJSON), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load() returned error: %v", err)
+	}
+
+	if len(cfg.Networks) != 2 {
+		t.Errorf("expected 2 networks, got %d", len(cfg.Networks))
+	}
+
+	if cfg.Networks["smarthost-loadbalancer"].Subnet != "172.18.103.0/24" {
+		t.Errorf("unexpected subnet: %s", cfg.Networks["smarthost-loadbalancer"].Subnet)
+	}
+
+	if len(cfg.IPs) != 2 {
+		t.Errorf("expected 2 IPs, got %d", len(cfg.IPs))
+	}
+
+	if cfg.IPs["172.18.103.2"].ContainerName != "smarthostloadbalancer" {
+		t.Errorf("unexpected container name: %s", cfg.IPs["172.18.103.2"].ContainerName)
+	}
+
+	if len(cfg.Policies) != 2 {
+		t.Errorf("expected 2 policies, got %d", len(cfg.Policies))
+	}
+
+	if cfg.Policies[0].From != "publicbackend" {
+		t.Errorf("unexpected from: %s", cfg.Policies[0].From)
+	}
+
+	if cfg.Policies[1].Nat != "dnat" {
+		t.Errorf("unexpected nat: %s", cfg.Policies[1].Nat)
+	}
+}
+
+func TestLoadFileNotFound(t *testing.T) {
+	_, err := Load("/nonexistent/path/networks.json")
+	if err == nil {
+		t.Error("expected error for nonexistent file, got nil")
+	}
+}
+
+func TestLoadInvalidJSON(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "bad.json")
+	if err := os.WriteFile(path, []byte("{invalid json"), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	_, err := Load(path)
+	if err == nil {
+		t.Error("expected error for invalid JSON, got nil")
+	}
+}
+
+func TestToCIDR(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"172.18.103.0", "172.18.103.0/24"},
+		{"172.18.103.2", "172.18.103.2"},
+		{"172.18.103.0/24", "172.18.103.0/24"},
+		{"10.0.0.0", "10.0.0.0/24"},
+		{"invalid", "invalid"},
+	}
+
+	for _, tt := range tests {
+		got := ToCIDR(tt.input)
+		if got != tt.want {
+			t.Errorf("ToCIDR(%q) = %q, want %q", tt.input, got, tt.want)
+		}
+	}
+}
+
+func TestNetworkPrefix(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"172.18.103.2", "172.18.103.0/24"},
+		{"10.0.0.1", "10.0.0.0/24"},
+		{"invalid", "invalid"},
+	}
+
+	for _, tt := range tests {
+		got := NetworkPrefix(tt.input)
+		if got != tt.want {
+			t.Errorf("NetworkPrefix(%q) = %q, want %q", tt.input, got, tt.want)
+		}
+	}
+}
+
+func TestIsIP(t *testing.T) {
+	tests := []struct {
+		input string
+		want  bool
+	}{
+		{"172.18.103.2", true},
+		{"10.0.0.0", true},
+		{"255.255.255.255", true},
+		{"publicbackend", false},
+		{"172.18.103.0/24", false},
+		{"", false},
+	}
+
+	for _, tt := range tests {
+		got := IsIP(tt.input)
+		if got != tt.want {
+			t.Errorf("IsIP(%q) = %v, want %v", tt.input, got, tt.want)
+		}
+	}
+}
+
+func TestNetworkConfigParseCIDR(t *testing.T) {
+	nc := NetworkConfig{Subnet: "172.18.103.0/24"}
+	ipNet, err := nc.ParseCIDR()
+	if err != nil {
+		t.Fatalf("ParseCIDR() returned error: %v", err)
+	}
+	if ipNet.String() != "172.18.103.0/24" {
+		t.Errorf("ParseCIDR() = %s, want 172.18.103.0/24", ipNet.String())
+	}
+
+	nc2 := NetworkConfig{Subnet: "invalid"}
+	_, err = nc2.ParseCIDR()
+	if err == nil {
+		t.Error("expected error for invalid subnet, got nil")
+	}
+}
+
+func TestLoadReproducible(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "networks.json")
+	if err := os.WriteFile(path, []byte(testJSON), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	// Load twice and compare
+	cfg1, err := Load(path)
+	if err != nil {
+		t.Fatalf("first Load() error: %v", err)
+	}
+
+	cfg2, err := Load(path)
+	if err != nil {
+		t.Fatalf("second Load() error: %v", err)
+	}
+
+	// Verify reproducibility
+	if len(cfg1.Networks) != len(cfg2.Networks) {
+		t.Errorf("reproducibility: network count mismatch %d vs %d", len(cfg1.Networks), len(cfg2.Networks))
+	}
+	if len(cfg1.IPs) != len(cfg2.IPs) {
+		t.Errorf("reproducibility: IP count mismatch %d vs %d", len(cfg1.IPs), len(cfg2.IPs))
+	}
+	if len(cfg1.Policies) != len(cfg2.Policies) {
+		t.Errorf("reproducibility: policy count mismatch %d vs %d", len(cfg1.Policies), len(cfg2.Policies))
+	}
+
+	for name, net1 := range cfg1.Networks {
+		net2 := cfg2.Networks[name]
+		if net1.Subnet != net2.Subnet || net1.Gateway != net2.Gateway {
+			t.Errorf("reproducibility: network %s mismatch", name)
+		}
+	}
+}

network-go/docker/docker.go

@@ -15,11 +15,27 @@ import (
 	"firewall_containers/network-go/config"
 )
 
+// DockerAPI defines the interface for Docker operations, enabling mock implementations for testing
+type DockerAPI interface {
+	Close() error
+	EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig) error
+	RemoveNetwork(ctx context.Context, networkName string) error
+	ConnectContainer(ctx context.Context, containerName, networkName, ip string) error
+	DisconnectContainer(ctx context.Context, containerName, networkName string) error
+	InspectContainer(ctx context.Context, containerName string) (*types.ContainerJSON, error)
+	WaitForContainerRunning(ctx context.Context, containerName string, timeout time.Duration) error
+	GetContainerPID(ctx context.Context, containerName string) (int, error)
+	AddRouteInContainer(ctx context.Context, containerName, network, gateway string) error
+}
+
 // Client wraps the Docker SDK client
 type Client struct {
 	cli *client.Client
 }
 
+// Ensure Client implements DockerAPI
+var _ DockerAPI = (*Client)(nil)
+
 // NewClient creates a new Docker client
 func NewClient() (*Client, error) {
 	cli, err := client.NewClientWithOpts(client.FromEnv, client.WithAPIVersionNegotiation())
@@ -36,7 +52,6 @@ func (c *Client) Close() error {
 
 // EnsureNetwork creates a Docker network if it does not already exist
 func (c *Client) EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig) error {
-	// Check if network already exists
 	existingNetworks, err := c.cli.NetworkList(ctx, network.ListOptions{
 		Filters: filters.NewArgs(filters.Arg("name", netCfg.NetworkName)),
 	})
@@ -46,12 +61,10 @@ func (c *Client) EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig)
 
 	for _, n := range existingNetworks {
 		if n.Name == netCfg.NetworkName {
-			// Network already exists, skip creation
 			return nil
 		}
 	}
 
-	// Parse subnet and gateway
 	_, ipNet, err := net.ParseCIDR(netCfg.Subnet)
 	if err != nil {
 		return fmt.Errorf("failed to parse subnet %s: %w", netCfg.Subnet, err)
@@ -62,7 +75,6 @@ func (c *Client) EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig)
 		return fmt.Errorf("failed to parse gateway IP %s", netCfg.Gateway)
 	}
 
-	// Create the network
 	createOpts := network.CreateOptions{
 		Driver: "bridge",
 		IPAM: &network.IPAM{
@@ -84,7 +96,7 @@ func (c *Client) EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig)
 		return fmt.Errorf("failed to create network %s: %w", netCfg.NetworkName, err)
 	}
 
-	_ = resp // response contains ID and warnings
+	_ = resp
 	return nil
 }
 
@@ -154,6 +166,7 @@ func (c *Client) WaitForContainerRunning(ctx context.Context, containerName stri
 	}
 }
 
+// GetContainerPID returns the PID of a container for nsenter operations
 func (c *Client) GetContainerPID(ctx context.Context, containerName string) (int, error) {
 	cont, err := c.cli.ContainerInspect(ctx, containerName)
 	if err != nil {

network-go/firewall/firewall.go

@@ -16,14 +16,14 @@ import (
 // Orchestrator reconciles the networks.json configuration into Docker networks
 // and iptables firewall rules
 type Orchestrator struct {
-	dockerClient *docker.Client
-	iptablesMgr  *iptables.Manager
+	dockerClient docker.DockerAPI
+	iptablesMgr  iptables.IPTablesAPI
 	resolver     *resolver.Resolver
 	debug        bool
 }
 
 // NewOrchestrator creates a new firewall orchestrator
-func NewOrchestrator(dockerClient *docker.Client, iptablesMgr *iptables.Manager, cfg *config.NetworksConfig) *Orchestrator {
+func NewOrchestrator(dockerClient docker.DockerAPI, iptablesMgr iptables.IPTablesAPI, cfg *config.NetworksConfig) *Orchestrator {
 	return &Orchestrator{
 		dockerClient: dockerClient,
 		iptablesMgr:  iptablesMgr,

network-go/firewall/firewall_test.go

@@ -0,0 +1,333 @@
+package firewall
+
+import (
+	"context"
+	"testing"
+
+	"firewall_containers/network-go/config"
+	"firewall_containers/network-go/mock"
+)
+
+func testConfig() *config.NetworksConfig {
+	return &config.NetworksConfig{
+		Networks: map[string]config.NetworkConfig{
+			"smarthost-loadbalancer": {
+				NetworkName: "smarthost-loadbalancer",
+				Subnet:      "172.18.103.0/24",
+				Gateway:     "172.18.103.1",
+			},
+			"smarthost_backend-1": {
+				NetworkName: "smarthost_backend-1",
+				Subnet:      "172.18.104.0/24",
+				Gateway:     "172.18.104.1",
+			},
+		},
+		IPs: map[string]config.IPConfig{
+			"172.18.103.2": {
+				IP:            "172.18.103.2",
+				ContainerName: "smarthostloadbalancer",
+				Selector:      "smarthostloadbalancer",
+				ServiceName:   "smarthost-proxy",
+			},
+			"172.18.104.2": {
+				IP:            "172.18.104.2",
+				ContainerName: "smarthostbackend-1",
+				Selector:      "smarthostbackend-1",
+				ServiceName:   "smarthost-proxy",
+			},
+		},
+		Policies: []config.PolicyConfig{},
+	}
+}
+
+func TestReconcileAllCreatesNetworks(t *testing.T) {
+	cfg := testConfig()
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.ReconcileAll(ctx, cfg)
+
+	if !docker.EnsureNetworkCalled {
+		t.Error("EnsureNetwork was not called")
+	}
+}
+
+func TestReconcileAllConnectsContainers(t *testing.T) {
+	cfg := testConfig()
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.ReconcileAll(ctx, cfg)
+
+	if !docker.ConnectContainerCalled {
+		t.Error("ConnectContainer was not called")
+	}
+}
+
+func TestReconcileAllEnablesIPForward(t *testing.T) {
+	cfg := testConfig()
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.ReconcileAll(ctx, cfg)
+
+	if !iptables.EnsureIPForwardCalled {
+		t.Error("EnsureIPForward was not called")
+	}
+}
+
+func TestReconcilePoliciesForwardRule(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName:   "smarthost-proxy",
+			ContainerName: "smarthostloadbalancer",
+			Selector:      "smarthostloadbalancer",
+			From:          "publicbackend",
+			Port:          80,
+			Proto:         "tcp",
+		},
+	}
+
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{BinaryResult: "/usr/sbin/iptables"}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.reconcilePolicies(ctx, cfg)
+
+	// "publicbackend" should be resolved to an IP from the config (if it matches)
+	// Since "publicbackend" is not in the IPs map, sourceIP will be empty
+	if !iptables.InsertForwardAcceptCalled {
+		t.Error("InsertForwardAccept was not called")
+	}
+
+	// Should use FORWARD chain (not iptables-legacy)
+	if iptables.InsertForwardAcceptChain != "FORWARD" {
+		t.Errorf("expected FORWARD chain, got %s", iptables.InsertForwardAcceptChain)
+	}
+}
+
+func TestReconcilePoliciesForwardRuleWithLegacy(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName: "smarthost-proxy",
+			From:        "publicbackend",
+			Port:        80,
+			Proto:       "tcp",
+		},
+	}
+
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{BinaryResult: "/usr/sbin/iptables-legacy"}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.reconcilePolicies(ctx, cfg)
+
+	if !iptables.InsertForwardAcceptCalled {
+		t.Error("InsertForwardAccept was not called")
+	}
+
+	// Should use DOCKER-USER chain when iptables-legacy
+	if iptables.InsertForwardAcceptChain != "DOCKER-USER" {
+		t.Errorf("expected DOCKER-USER chain, got %s", iptables.InsertForwardAcceptChain)
+	}
+}
+
+func TestReconcilePoliciesDNATWithInterface(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName: "smarthost-proxy",
+			Name:        "wireguardproxy",
+			Selector:    "smarthostloadbalancer",
+			Iface:       "wg0",
+			Nat:         "dnat",
+			To:          "smarthostloadbalancer",
+			Port:        80,
+			Proto:       "tcp",
+		},
+	}
+
+	docker := &mock.MockDockerClient{
+		GetContainerPIDResult: 0, // simulate no PID available
+		GetContainerPIDErr:    assertError("container not running"),
+	}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.reconcilePolicies(ctx, cfg)
+
+	// When GetContainerPID fails, should fall back to interface-based rule
+	if !iptables.InsertPreroutingRuleOnInterfaceCalled {
+		t.Error("InsertPreroutingRuleOnInterface was not called (should fall back from nsenter)")
+	}
+
+	if len(iptables.InsertPreroutingRuleOnInterfaceArgs) > 0 {
+		iface := iptables.InsertPreroutingRuleOnInterfaceArgs[0]
+		if iface != "wg0" {
+			t.Errorf("expected interface wg0, got %s", iface)
+		}
+	}
+}
+
+func TestReconcilePoliciesDNATWithContainerPID(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName: "smarthost-proxy",
+			Name:        "wireguardproxy",
+			Selector:    "smarthostloadbalancer",
+			Nat:         "dnat",
+			To:          "smarthostloadbalancer",
+			Port:        80,
+			Proto:       "tcp",
+		},
+	}
+
+	docker := &mock.MockDockerClient{
+		GetContainerPIDResult: 1234,
+		GetContainerPIDErr:    nil,
+	}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.reconcilePolicies(ctx, cfg)
+
+	if !docker.GetContainerPIDCalled {
+		t.Error("GetContainerPID was not called")
+	}
+
+	if !iptables.InsertPreroutingRuleInContainerCalled {
+		t.Error("InsertPreroutingRuleInContainer was not called")
+	}
+
+	if iptables.InsertPreroutingRuleInContainerPID != 1234 {
+		t.Errorf("expected PID 1234, got %d", iptables.InsertPreroutingRuleInContainerPID)
+	}
+}
+
+func TestReconcilePoliciesUnresolvedTarget(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName: "test",
+			Nat:         "dnat",
+			Selector:    "container1",
+			To:          "nonexistent-target",
+			Port:        80,
+		},
+	}
+
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	ctx := context.Background()
+	orch.reconcilePolicies(ctx, cfg)
+
+	// Should not call GetContainerPID when target can't be resolved
+	if docker.GetContainerPIDCalled {
+		t.Error("GetContainerPID should not be called when target is unresolvable")
+	}
+}
+
+func TestResolveIPDirectIP(t *testing.T) {
+	cfg := testConfig()
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	// Direct IP should be returned as CIDR
+	result := orch.resolveIP("172.18.103.2")
+	if result != "172.18.103.2" {
+		t.Errorf("expected 172.18.103.2, got %s", result)
+	}
+
+	// .0 ending should be converted to /24
+	result = orch.resolveIP("172.18.103.0")
+	if result != "172.18.103.0/24" {
+		t.Errorf("expected 172.18.103.0/24, got %s", result)
+	}
+}
+
+func TestResolveIPFromConfig(t *testing.T) {
+	cfg := testConfig()
+	docker := &mock.MockDockerClient{}
+	iptables := &mock.MockIPTablesManager{}
+	orch := NewOrchestrator(docker, iptables, cfg)
+
+	// Should resolve container name from config
+	result := orch.resolveIP("smarthostloadbalancer")
+	if result != "172.18.103.2" {
+		t.Errorf("expected 172.18.103.2, got %s", result)
+	}
+}
+
+func TestFindNetworkForIP(t *testing.T) {
+	cfg := testConfig()
+
+	tests := []struct {
+		ip   string
+		want string
+	}{
+		{"172.18.103.5", "smarthost-loadbalancer"},
+		{"172.18.104.5", "smarthost_backend-1"},
+		{"10.0.0.1", ""},
+		{"", ""},
+	}
+
+	for _, tt := range tests {
+		got := findNetworkForIP(cfg, tt.ip)
+		if got != tt.want {
+			t.Errorf("findNetworkForIP(%q) = %q, want %q", tt.ip, got, tt.want)
+		}
+	}
+}
+
+func TestReconcileAllReproducible(t *testing.T) {
+	cfg := testConfig()
+	cfg.Policies = []config.PolicyConfig{
+		{
+			ServiceName: "test-svc",
+			From:        "publicbackend",
+			Port:        80,
+			Proto:       "tcp",
+		},
+	}
+
+	// Run reconciliation twice with separate mocks
+	for i := 0; i < 2; i++ {
+		docker := &mock.MockDockerClient{}
+		iptables := &mock.MockIPTablesManager{}
+		orch := NewOrchestrator(docker, iptables, cfg)
+		ctx := context.Background()
+		orch.ReconcileAll(ctx, cfg)
+
+		if !docker.EnsureNetworkCalled {
+			t.Errorf("run %d: EnsureNetwork not called", i)
+		}
+		if !iptables.InsertForwardAcceptCalled {
+			t.Errorf("run %d: InsertForwardAccept not called", i)
+		}
+	}
+}
+
+// assertError is a helper to create a simple error for tests
+type simpleErr struct{ msg string }
+
+func (e simpleErr) Error() string { return e.msg }
+
+func assertError(msg string) error {
+	return simpleErr{msg: msg}
+}

network-go/implementation.md

@@ -88,26 +88,6 @@ Without these, the program cannot:
 - Insert PREROUTING/POSTROUTING rules inside other containers
 - Add routes to container network namespaces
 
-### Minimal Docker Compose Example
-
-```yaml
-version: "3.8"
-services:
-  network-go:
-    build: ./network-go
-    network_mode: host
-    pid: "host"
-    cap_add:
-      - NET_ADMIN
-      - SYS_ADMIN
-    volumes:
-      - /var/run/docker.sock:/var/run/docker.sock
-      - /etc/user/config:/etc/user/config
-    environment:
-      - WATCH_PERIOD_SECONDS=30
-      - DEBUG=false
-```
-
 ## Configuration — `/etc/user/config/networks.json`
 
 ```json

network-go/iptables/iptables.go

@@ -6,12 +6,31 @@ import (
 	"strings"
 )
 
+// IPTablesAPI defines the interface for iptables operations, enabling mock implementations for testing
+type IPTablesAPI interface {
+	Binary() string
+	EnsureIPForward() error
+	EnsureEstablishedRelated(chain string) error
+	DeleteLine(chain string, lineNum string) error
+	InsertPreroutingRule(sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error
+	InsertPreroutingRuleOnInterface(iface, proto, sourcePort, targetIP, targetPort, comment string) error
+	InsertPostroutingMasquerade(sourceCIDR, proto, sourcePort, comment string) error
+	InsertPostroutingMasqueradeForTarget(targetCIDR, proto, targetPort, comment string) error
+	InsertForwardAccept(chain, sourceIP, targetIP, proto, sourcePort, targetPort, comment string) error
+	DeleteForwardAccept(chain, comment string) error
+	InsertPreroutingRuleInContainer(pid int, sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error
+	InsertPostroutingMasqueradeInContainer(pid int, sourceCIDR, proto, sourcePort, comment string) error
+}
+
 // Manager manages iptables rules via the iptables/iptables-legacy CLI
 type Manager struct {
-	binary string // /usr/sbin/iptables or /usr/sbin/iptables-legacy
+	binary string
 	debug  bool
 }
 
+// Ensure Manager implements IPTablesAPI
+var _ IPTablesAPI = (*Manager)(nil)
+
 // NewManager creates a new iptables manager, auto-detecting the binary
 func NewManager(debug bool) *Manager {
 	m := &Manager{debug: debug}
@@ -83,17 +102,14 @@ func (m *Manager) EnsureIPForward() error {
 }
 
 // EnsureEstablishedRelated inserts an ESTABLISHED,RELATED accept rule at the top of a chain
-// if it doesn't already exist
 func (m *Manager) EnsureEstablishedRelated(chain string) error {
 	checkArgs := []string{"-w", "-n", "-L", chain}
 	cmd := exec.Command(m.binary, checkArgs...)
 	output, err := cmd.Output()
 	if err != nil {
-		// Chain may not exist, create it
 		return nil
 	}
 
-	// Only insert if ESTABLISHED,RELATED rule is not present
 	if !strings.Contains(string(output), "ESTABLISHED") || !strings.Contains(string(output), "RELATED") {
 		args := []string{"-w", "-I", chain, "-m", "state", "--state", "established,related", "-j", "ACCEPT"}
 		return m.run(args...)
@@ -114,7 +130,6 @@ func (m *Manager) DeleteLineInContainer(pid int, table, chain, lineNum string) e
 }
 
 // getLineNumbers returns line numbers matching certain criteria in a chain/table
-// This implements the grep logic from the shell script: iptables -w --line-number -n -L $CHAIN | grep ...
 func (m *Manager) getLineNumbers(chain, table string, grepPatterns ...string) []string {
 	args := []string{"-w", "--line-number", "-n", "-L", chain}
 	if table != "" {
@@ -150,7 +165,6 @@ func (m *Manager) getLineNumbers(chain, table string, grepPatterns ...string) []
 // deleteMatchingLines deletes all lines in a chain matching the given patterns
 func (m *Manager) deleteMatchingLines(chain, table string, grepPatterns ...string) error {
 	lines := m.getLineNumbers(chain, table, grepPatterns...)
-	// Reverse order (highest line first) so deletions don't shift line numbers
 	for i := len(lines) - 1; i >= 0; i-- {
 		if err := m.DeleteLine(chain, lines[i]); err != nil {
 			return err
@@ -161,7 +175,6 @@ func (m *Manager) deleteMatchingLines(chain, table string, grepPatterns ...strin
 
 // deleteMatchingLinesInContainer deletes matching lines inside a container namespace
 func (m *Manager) deleteMatchingLinesInContainer(pid int, table, chain string, grepPatterns ...string) error {
-	// For container namespaces, we use a different approach: list via nsenter + grep
 	iptPath := "/sbin/iptables-legacy"
 	if !strings.Contains(m.binary, "legacy") {
 		iptPath = "/sbin/iptables"
@@ -192,7 +205,6 @@ func (m *Manager) deleteMatchingLinesInContainer(pid int, table, chain string, g
 		}
 	}
 
-	// Delete in reverse order
 	for i := len(matchingLines) - 1; i >= 0; i-- {
 		if err := m.DeleteLineInContainer(pid, table, chain, matchingLines[i]); err != nil {
 			return err
@@ -203,13 +215,11 @@ func (m *Manager) deleteMatchingLinesInContainer(pid int, table, chain string, g
 
 // InsertPreroutingRule inserts a DNAT PREROUTING rule on the host
 func (m *Manager) InsertPreroutingRule(sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error {
-	// First, delete existing matching rules
 	patterns := []string{"DNAT", sourcePort, targetIP, targetPort, comment}
 	if err := m.deleteMatchingLines("PREROUTING", "nat", patterns...); err != nil {
 		return fmt.Errorf("failed to delete old PREROUTING rules: %w", err)
 	}
 
-	// Insert the new rule
 	args := []string{
 		"-w", "-t", "nat", "-I", "PREROUTING",
 		"-d", sourceIP,
@@ -236,7 +246,6 @@ func (m *Manager) InsertPreroutingRuleOnInterface(iface, proto, sourcePort, targ
 
 // InsertPostroutingMasquerade inserts a MASQUERADE POSTROUTING rule on the host
 func (m *Manager) InsertPostroutingMasquerade(sourceCIDR, proto, sourcePort, comment string) error {
-	// Delete existing matching rules first
 	patterns := []string{"MASQUERADE", comment, sourceCIDR, sourcePort}
 	if err := m.deleteMatchingLines("POSTROUTING", "nat", patterns...); err != nil {
 		return fmt.Errorf("failed to delete old POSTROUTING rules: %w", err)
@@ -273,7 +282,6 @@ func (m *Manager) InsertPostroutingMasqueradeForTarget(targetCIDR, proto, target
 
 // InsertForwardAccept inserts a FORWARD ACCEPT rule on the host
 func (m *Manager) InsertForwardAccept(chain, sourceIP, targetIP, proto, sourcePort, targetPort, comment string) error {
-	// Build grep patterns to match existing rules
 	var grepPatterns []string
 	grepPatterns = append(grepPatterns, proto)
 	if sourceIP != "" {
@@ -289,12 +297,10 @@ func (m *Manager) InsertForwardAccept(chain, sourceIP, targetIP, proto, sourcePo
 		grepPatterns = append(grepPatterns, targetPort)
 	}
 
-	// Delete old matching rules
 	if err := m.deleteMatchingLines(chain, "", grepPatterns...); err != nil {
 		return fmt.Errorf("failed to delete old FORWARD rules: %w", err)
 	}
 
-	// Build iptables args
 	args := []string{"-w", "-I", chain, "-p", proto}
 	if sourceIP != "" {
 		args = append(args, "-s", sourceIP)
@@ -326,7 +332,6 @@ func (m *Manager) DeleteForwardAccept(chain, comment string) error {
 
 // InsertPreroutingRuleInContainer inserts a DNAT PREROUTING rule inside a container namespace
 func (m *Manager) InsertPreroutingRuleInContainer(pid int, sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error {
-	// Delete existing first
 	patterns := []string{"DNAT", sourcePort, targetIP, targetPort, comment}
 	if err := m.deleteMatchingLinesInContainer(pid, "nat", "PREROUTING", patterns...); err != nil {
 		return fmt.Errorf("failed to delete old container PREROUTING rules: %w", err)

network-go/mock/mock.go

@@ -0,0 +1,209 @@
+package mock
+
+import (
+	"context"
+	"time"
+
+	"github.com/docker/docker/api/types"
+
+	"firewall_containers/network-go/config"
+	"firewall_containers/network-go/docker"
+	"firewall_containers/network-go/iptables"
+)
+
+// Compile-time interface conformance checks
+var _ docker.DockerAPI = (*MockDockerClient)(nil)
+var _ iptables.IPTablesAPI = (*MockIPTablesManager)(nil)
+
+// MockDockerClient implements docker.DockerAPI for testing
+type MockDockerClient struct {
+	EnsureNetworkCalled           bool
+	EnsureNetworkCfg              config.NetworkConfig
+	EnsureNetworkErr              error
+
+	ConnectContainerCalled        bool
+	ConnectContainerName          string
+	ConnectContainerNetwork       string
+	ConnectContainerIP            string
+	ConnectContainerErr           error
+
+	WaitForRunningCalled          bool
+	WaitForRunningName            string
+
+	GetContainerPIDCalled         bool
+	GetContainerPIDName           string
+	GetContainerPIDResult         int
+	GetContainerPIDErr            error
+
+	AddRouteCalled                bool
+	AddRouteContainer             string
+	AddRouteNetwork               string
+	AddRouteGateway               string
+	AddRouteErr                   error
+
+	InspectContainerErr           error
+	RemoveNetworkErr              error
+	DisconnectContainerErr        error
+}
+
+func (m *MockDockerClient) Close() error { return nil }
+
+func (m *MockDockerClient) EnsureNetwork(ctx context.Context, netCfg config.NetworkConfig) error {
+	m.EnsureNetworkCalled = true
+	m.EnsureNetworkCfg = netCfg
+	return m.EnsureNetworkErr
+}
+
+func (m *MockDockerClient) RemoveNetwork(ctx context.Context, networkName string) error {
+	return m.RemoveNetworkErr
+}
+
+func (m *MockDockerClient) ConnectContainer(ctx context.Context, containerName, networkName, ip string) error {
+	m.ConnectContainerCalled = true
+	m.ConnectContainerName = containerName
+	m.ConnectContainerNetwork = networkName
+	m.ConnectContainerIP = ip
+	return m.ConnectContainerErr
+}
+
+func (m *MockDockerClient) DisconnectContainer(ctx context.Context, containerName, networkName string) error {
+	return m.DisconnectContainerErr
+}
+
+func (m *MockDockerClient) InspectContainer(ctx context.Context, containerName string) (*types.ContainerJSON, error) {
+	return nil, m.InspectContainerErr
+}
+
+func (m *MockDockerClient) WaitForContainerRunning(ctx context.Context, containerName string, timeout time.Duration) error {
+	m.WaitForRunningCalled = true
+	m.WaitForRunningName = containerName
+	return nil
+}
+
+func (m *MockDockerClient) GetContainerPID(ctx context.Context, containerName string) (int, error) {
+	m.GetContainerPIDCalled = true
+	m.GetContainerPIDName = containerName
+	return m.GetContainerPIDResult, m.GetContainerPIDErr
+}
+
+func (m *MockDockerClient) AddRouteInContainer(ctx context.Context, containerName, network, gateway string) error {
+	m.AddRouteCalled = true
+	m.AddRouteContainer = containerName
+	m.AddRouteNetwork = network
+	m.AddRouteGateway = gateway
+	return m.AddRouteErr
+}
+
+// MockIPTablesManager implements iptables.IPTablesAPI for testing
+type MockIPTablesManager struct {
+	BinaryResult                                string
+	EnsureIPForwardCalled                       bool
+	EnsureIPForwardErr                          error
+	EnsureEstablishedRelatedCalled              bool
+	EnsureEstablishedRelatedChain                string
+	EnsureEstablishedRelatedErr                 error
+
+	InsertPreroutingRuleCalled                  bool
+	InsertPreroutingRuleArgs                    []string
+	InsertPreroutingRuleErr                     error
+
+	InsertPreroutingRuleOnInterfaceCalled       bool
+	InsertPreroutingRuleOnInterfaceArgs         []string
+	InsertPreroutingRuleOnInterfaceErr          error
+
+	InsertPostroutingMasqueradeCalled           bool
+	InsertPostroutingMasqueradeArgs             []string
+	InsertPostroutingMasqueradeErr              error
+
+	InsertForwardAcceptCalled                   bool
+	InsertForwardAcceptChain                    string
+	InsertForwardAcceptSourceIP                 string
+	InsertForwardAcceptTargetIP                 string
+	InsertForwardAcceptProto                    string
+	InsertForwardAcceptSourcePort               string
+	InsertForwardAcceptTargetPort               string
+	InsertForwardAcceptComment                  string
+	InsertForwardAcceptErr                      error
+
+	InsertPreroutingRuleInContainerCalled       bool
+	InsertPreroutingRuleInContainerPID          int
+	InsertPreroutingRuleInContainerArgs         []string
+	InsertPreroutingRuleInContainerErr          error
+
+	InsertPostroutingMasqueradeInContainerCalled bool
+	InsertPostroutingMasqueradeInContainerErr   error
+	DeleteForwardAcceptErr                      error
+	DeleteLineErr                               error
+}
+
+func (m *MockIPTablesManager) Binary() string {
+	if m.BinaryResult == "" {
+		return "/usr/sbin/iptables"
+	}
+	return m.BinaryResult
+}
+
+func (m *MockIPTablesManager) EnsureIPForward() error {
+	m.EnsureIPForwardCalled = true
+	return m.EnsureIPForwardErr
+}
+
+func (m *MockIPTablesManager) EnsureEstablishedRelated(chain string) error {
+	m.EnsureEstablishedRelatedCalled = true
+	m.EnsureEstablishedRelatedChain = chain
+	return m.EnsureEstablishedRelatedErr
+}
+
+func (m *MockIPTablesManager) DeleteLine(chain string, lineNum string) error {
+	return m.DeleteLineErr
+}
+
+func (m *MockIPTablesManager) InsertPreroutingRule(sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error {
+	m.InsertPreroutingRuleCalled = true
+	m.InsertPreroutingRuleArgs = []string{sourceIP, proto, sourcePort, targetIP, targetPort, comment}
+	return m.InsertPreroutingRuleErr
+}
+
+func (m *MockIPTablesManager) InsertPreroutingRuleOnInterface(iface, proto, sourcePort, targetIP, targetPort, comment string) error {
+	m.InsertPreroutingRuleOnInterfaceCalled = true
+	m.InsertPreroutingRuleOnInterfaceArgs = []string{iface, proto, sourcePort, targetIP, targetPort, comment}
+	return m.InsertPreroutingRuleOnInterfaceErr
+}
+
+func (m *MockIPTablesManager) InsertPostroutingMasquerade(sourceCIDR, proto, sourcePort, comment string) error {
+	m.InsertPostroutingMasqueradeCalled = true
+	m.InsertPostroutingMasqueradeArgs = []string{sourceCIDR, proto, sourcePort, comment}
+	return m.InsertPostroutingMasqueradeErr
+}
+
+func (m *MockIPTablesManager) InsertPostroutingMasqueradeForTarget(targetCIDR, proto, targetPort, comment string) error {
+	return nil
+}
+
+func (m *MockIPTablesManager) InsertForwardAccept(chain, sourceIP, targetIP, proto, sourcePort, targetPort, comment string) error {
+	m.InsertForwardAcceptCalled = true
+	m.InsertForwardAcceptChain = chain
+	m.InsertForwardAcceptSourceIP = sourceIP
+	m.InsertForwardAcceptTargetIP = targetIP
+	m.InsertForwardAcceptProto = proto
+	m.InsertForwardAcceptSourcePort = sourcePort
+	m.InsertForwardAcceptTargetPort = targetPort
+	m.InsertForwardAcceptComment = comment
+	return m.InsertForwardAcceptErr
+}
+
+func (m *MockIPTablesManager) DeleteForwardAccept(chain, comment string) error {
+	return m.DeleteForwardAcceptErr
+}
+
+func (m *MockIPTablesManager) InsertPreroutingRuleInContainer(pid int, sourceIP, proto, sourcePort, targetIP, targetPort, comment string) error {
+	m.InsertPreroutingRuleInContainerCalled = true
+	m.InsertPreroutingRuleInContainerPID = pid
+	m.InsertPreroutingRuleInContainerArgs = []string{sourceIP, proto, sourcePort, targetIP, targetPort, comment}
+	return m.InsertPreroutingRuleInContainerErr
+}
+
+func (m *MockIPTablesManager) InsertPostroutingMasqueradeInContainer(pid int, sourceCIDR, proto, sourcePort, comment string) error {
+	m.InsertPostroutingMasqueradeInContainerCalled = true
+	return m.InsertPostroutingMasqueradeInContainerErr
+}

network-go/resolver/resolver_test.go

@@ -0,0 +1,118 @@
+package resolver
+
+import (
+	"testing"
+
+	"firewall_containers/network-go/config"
+)
+
+func makeTestConfig() *config.NetworksConfig {
+	return &config.NetworksConfig{
+		Networks: map[string]config.NetworkConfig{
+			"test-net": {NetworkName: "test-net", Subnet: "172.18.103.0/24", Gateway: "172.18.103.1"},
+		},
+		IPs: map[string]config.IPConfig{
+			"172.18.103.2": {IP: "172.18.103.2", ContainerName: "smarthostloadbalancer", Selector: "smarthostloadbalancer", ServiceName: "smarthost-proxy"},
+			"172.18.104.2": {IP: "172.18.104.2", ContainerName: "smarthostbackend-1", Selector: "smarthostbackend-1", ServiceName: "smarthost-proxy"},
+		},
+		Policies: []config.PolicyConfig{},
+	}
+}
+
+func TestResolveByContainerName(t *testing.T) {
+	cfg := makeTestConfig()
+	r := NewResolver(cfg)
+
+	ips := r.Resolve("smarthostloadbalancer")
+	if len(ips) != 1 {
+		t.Fatalf("expected 1 IP, got %d", len(ips))
+	}
+	if ips[0] != "172.18.103.2" {
+		t.Errorf("expected 172.18.103.2, got %s", ips[0])
+	}
+}
+
+func TestResolveBySelector(t *testing.T) {
+	cfg := makeTestConfig()
+	r := NewResolver(cfg)
+
+	ips := r.Resolve("smarthostbackend-1")
+	if len(ips) != 1 {
+		t.Fatalf("expected 1 IP, got %d", len(ips))
+	}
+	if ips[0] != "172.18.104.2" {
+		t.Errorf("expected 172.18.104.2, got %s", ips[0])
+	}
+}
+
+func TestResolveNotFound(t *testing.T) {
+	cfg := makeTestConfig()
+	r := NewResolver(cfg)
+
+	ips := r.Resolve("nonexistent-container")
+	if len(ips) != 0 {
+		t.Errorf("expected 0 IPs, got %d", len(ips))
+	}
+}
+
+func TestResolveNilConfig(t *testing.T) {
+	r := NewResolver(nil)
+
+	ips := r.Resolve("anything")
+	if len(ips) != 0 {
+		t.Errorf("expected 0 IPs with nil config, got %d", len(ips))
+	}
+}
+
+func TestSetConfig(t *testing.T) {
+	r := NewResolver(nil)
+
+	// Initially nil config
+	ips := r.Resolve("test")
+	if len(ips) != 0 {
+		t.Errorf("expected 0 IPs with nil config, got %d", len(ips))
+	}
+
+	// Update with valid config
+	cfg := makeTestConfig()
+	r.SetConfig(cfg)
+
+	ips = r.Resolve("smarthostloadbalancer")
+	if len(ips) != 1 {
+		t.Fatalf("expected 1 IP after SetConfig, got %d", len(ips))
+	}
+	if ips[0] != "172.18.103.2" {
+		t.Errorf("expected 172.18.103.2, got %s", ips[0])
+	}
+}
+
+func TestResolveReproducible(t *testing.T) {
+	cfg := makeTestConfig()
+	r := NewResolver(cfg)
+
+	ips1 := r.Resolve("smarthostloadbalancer")
+	ips2 := r.Resolve("smarthostloadbalancer")
+
+	if len(ips1) != len(ips2) {
+		t.Errorf("reproducibility: result count mismatch %d vs %d", len(ips1), len(ips2))
+	}
+	if len(ips1) > 0 && ips1[0] != ips2[0] {
+		t.Errorf("reproducibility: IP mismatch %s vs %s", ips1[0], ips2[0])
+	}
+}
+
+func TestResolveMultipleMatch(t *testing.T) {
+	cfg := &config.NetworksConfig{
+		IPs: map[string]config.IPConfig{
+			"10.0.0.1": {IP: "10.0.0.1", ContainerName: "app-1", Selector: "app-1"},
+			"10.0.0.2": {IP: "10.0.0.2", ContainerName: "app-2", Selector: "app-2"},
+		},
+	}
+	r := NewResolver(cfg)
+
+	// "app-x" has a dash, so prefix "app" is extracted and matches both app-1 and app-2
+	ips := r.Resolve("app-x")
+	if len(ips) != 2 {
+		t.Errorf("expected 2 IPs for prefix match, got %d", len(ips))
+	}
+}

network-go/watcher/watcher_test.go

@@ -0,0 +1,179 @@
+package watcher
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestWatcherDetectsChange(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.json")
+	if err := os.WriteFile(path, []byte(`{"version": 1}`), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	changeDetected := make(chan bool, 1)
+	onChange := func() {
+		select {
+		case changeDetected <- true:
+		default:
+		}
+	}
+
+	fw := NewFileWatcher(path, 100*time.Millisecond, onChange)
+	fw.Start()
+	defer fw.Stop()
+
+	// Wait for initial hash to be computed
+	time.Sleep(200 * time.Millisecond)
+
+	// Modify the file
+	if err := os.WriteFile(path, []byte(`{"version": 2}`), 0644); err != nil {
+		t.Fatalf("failed to modify test file: %v", err)
+	}
+
+	// Wait for change detection
+	select {
+	case detected := <-changeDetected:
+		if !detected {
+			t.Error("expected change detection, got false")
+		}
+	case <-time.After(2 * time.Second):
+		t.Error("timeout waiting for file change detection")
+	}
+}
+
+func TestWatcherNoChange(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.json")
+	if err := os.WriteFile(path, []byte(`{"version": 1}`), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	changeDetected := make(chan bool, 1)
+	onChange := func() {
+		select {
+		case changeDetected <- true:
+		default:
+		}
+	}
+
+	fw := NewFileWatcher(path, 100*time.Millisecond, onChange)
+	fw.Start()
+	defer fw.Stop()
+
+	// Wait without modifying the file
+	time.Sleep(300 * time.Millisecond)
+
+	// Should not detect a change
+	select {
+	case <-changeDetected:
+		t.Error("unexpected change detection without file modification")
+	default:
+		// Expected: no change detected
+	}
+}
+
+func TestWatcherMultipleChanges(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.json")
+	if err := os.WriteFile(path, []byte(`{"v": 1}`), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	changeCount := 0
+	var mu chan struct{}
+
+	onChange := func() {
+		changeCount++
+		if mu != nil {
+			mu <- struct{}{}
+		}
+	}
+
+	fw := NewFileWatcher(path, 50*time.Millisecond, onChange)
+	fw.Start()
+	defer fw.Stop()
+
+	// Wait for initial hash
+	time.Sleep(100 * time.Millisecond)
+
+	// Make 3 changes
+	for i := 2; i <= 4; i++ {
+		if err := os.WriteFile(path, []byte(`{"v": `+string(rune('0'+i))+`}`), 0644); err != nil {
+			t.Fatalf("failed to modify file: %v", err)
+		}
+		time.Sleep(150 * time.Millisecond)
+	}
+
+	if changeCount < 2 {
+		t.Errorf("expected at least 2 change detections, got %d", changeCount)
+	}
+}
+
+func TestWatcherStop(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.json")
+	if err := os.WriteFile(path, []byte(`{"v": 1}`), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	fw := NewFileWatcher(path, 50*time.Millisecond, func() {})
+	fw.Start()
+
+	time.Sleep(100 * time.Millisecond)
+	fw.Stop()
+
+	// Should not panic on double stop or further operations
+	time.Sleep(100 * time.Millisecond)
+}
+
+func TestWatcherNonexistentFile(t *testing.T) {
+	onChange := func() {
+		t.Error("onChange should not be called for nonexistent file")
+	}
+
+	fw := NewFileWatcher("/nonexistent/file.json", 100*time.Millisecond, onChange)
+	fw.Start()
+	defer fw.Stop()
+
+	// Wait — should not call onChange since file doesn't exist
+	time.Sleep(300 * time.Millisecond)
+}
+
+func TestWatcherReproducible(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.json")
+	if err := os.WriteFile(path, []byte(`{"data": "value"}`), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	// Create two watchers on the same file
+	var detected1, detected2 bool
+
+	fw1 := NewFileWatcher(path, 50*time.Millisecond, func() { detected1 = true })
+	fw2 := NewFileWatcher(path, 50*time.Millisecond, func() { detected2 = true })
+
+	fw1.Start()
+	fw2.Start()
+	defer fw1.Stop()
+	defer fw2.Stop()
+
+	time.Sleep(100 * time.Millisecond)
+
+	// Modify file
+	if err := os.WriteFile(path, []byte(`{"data": "changed"}`), 0644); err != nil {
+		t.Fatalf("failed to modify file: %v", err)
+	}
+
+	time.Sleep(300 * time.Millisecond)
+
+	if !detected1 {
+		t.Error("watcher 1 did not detect change")
+	}
+	if !detected2 {
+		t.Error("watcher 2 did not detect change")
+	}
+}