Refactor iptables chain detection to centralize and default to DOCKER-USER

Move chain detection logic from firewall to iptables manager for better encapsulation. The manager now auto-detects both the iptables binary and chain (DOCKER-USER or FORWARD) based on the presence of the Docker-managed chain, but always defaults to DOCKER-USER for consistency. This simplifies firewall code and ensures proper Docker integration regardless of iptables version.

network-go/firewall/firewall.go

@@ -204,11 +204,8 @@ func (o *Orchestrator) applyForwardRule(ctx context.Context, cfg *config.Network
 	logger.Info("FIREWALL: forward rule: from=%q (IP=%s) to=%q (IP=%s) proto=%s port=%s",
 		policy.From, sourceIP, policy.To, targetIP, proto, port)
 
-	// Determine the chain: use DOCKER-USER (iptables-legacy) or FORWARD
-	chain := "FORWARD"
-	if o.iptablesMgr.Binary() == "/usr/sbin/iptables-legacy" {
-		chain = "DOCKER-USER"
-	}
+	// Use the auto-detected chain (DOCKER-USER or FORWARD)
+	chain := o.iptablesMgr.Chain()
 	logger.Debug("FIREWALL: using iptables chain=%s (binary=%s)", chain, o.iptablesMgr.Binary())
 
 	// Ensure established/related rule exists at the top

network-go/firewall/firewall_test.go

@@ -108,9 +108,9 @@ func TestReconcilePoliciesForwardRule(t *testing.T) {
 		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)
+	// Should use DOCKER-USER chain (default, even with non-legacy iptables)
+	if iptables.InsertForwardAcceptChain != "DOCKER-USER" {
+		t.Errorf("expected DOCKER-USER chain, got %s", iptables.InsertForwardAcceptChain)
 	}
 }
 

network-go/iptables/iptables.go

@@ -11,6 +11,7 @@ import (
 // IPTablesAPI defines the interface for iptables operations, enabling mock implementations for testing
 type IPTablesAPI interface {
 	Binary() string
+	Chain() string
 	EnsureIPForward() error
 	EnsureEstablishedRelated(chain string) error
 	DeleteLine(chain string, lineNum string) error
@@ -27,6 +28,7 @@ type IPTablesAPI interface {
 // Manager manages iptables rules via the iptables/iptables-legacy CLI
 type Manager struct {
 	binary    string
+	chain     string // "DOCKER-USER" or "FORWARD"
 	debug     bool
 }
 
@@ -40,18 +42,34 @@ func NewManager(debug bool) *Manager {
 	return m
 }
 
-// detectBinary checks if iptables-legacy is available (matching shell script logic)
+// detectBinary checks which iptables binary has the DOCKER-USER chain (Docker-managed)
 func (m *Manager) detectBinary() {
-	logger.Info("IPTABLES: detecting iptables binary")
+	logger.Info("IPTABLES: detecting iptables binary and Docker chain")
+
+	// Check iptables-legacy first
 	cmd := exec.Command("/usr/sbin/iptables-legacy", "-L")
 	output, err := cmd.CombinedOutput()
 	if err == nil && strings.Contains(string(output), "DOCKER-USER") {
 		m.binary = "/usr/sbin/iptables-legacy"
-		logger.Info("IPTABLES: detected iptables-legacy (DOCKER-USER chain present)")
+		m.chain = "DOCKER-USER"
+		logger.Info("IPTABLES: detected iptables-legacy with DOCKER-USER chain")
 		return
 	}
+
+	// Check default iptables (may be nft-based)
+	cmd2 := exec.Command("/usr/sbin/iptables", "-L")
+	output2, err2 := cmd2.CombinedOutput()
+	if err2 == nil && strings.Contains(string(output2), "DOCKER-USER") {
 		m.binary = "/usr/sbin/iptables"
-	logger.Info("IPTABLES: using default iptables binary")
+		m.chain = "DOCKER-USER"
+		logger.Info("IPTABLES: detected iptables with DOCKER-USER chain")
+		return
+	}
+
+	// No DOCKER-USER chain found — default to DOCKER-USER for ACCEPT rules
+	m.binary = "/usr/sbin/iptables"
+	m.chain = "DOCKER-USER"
+	logger.Info("IPTABLES: no DOCKER-USER chain detected, defaulting to DOCKER-USER with default iptables")
 }
 
 // Binary returns the detected iptables binary path
@@ -59,6 +77,11 @@ func (m *Manager) Binary() string {
 	return m.binary
 }
 
+// Chain returns the firewall chain to use for FORWARD rules (DOCKER-USER or FORWARD)
+func (m *Manager) Chain() string {
+	return m.chain
+}
+
 // run executes an iptables command on the host
 func (m *Manager) run(args ...string) error {
 	cmdStr := m.binary + " " + strings.Join(args, " ")

network-go/mock/mock.go

@@ -163,6 +163,11 @@ func (m *MockIPTablesManager) Binary() string {
 	return m.BinaryResult
 }
 
+func (m *MockIPTablesManager) Chain() string {
+	// Default to DOCKER-USER (matches production behavior)
+	return "DOCKER-USER"
+}
+
 func (m *MockIPTablesManager) EnsureIPForward() error {
 	m.EnsureIPForwardCalled = true
 	return m.EnsureIPForwardErr