Add configuration option to select iptables implementation

When running gocast in a container, the default iptables implementation
may not match that used on the underlying host kernel. The current
container uses the legacy iptables implementation and calls the
`iptables` binary. This fails with exit code 3 when running on a host
using the newer nftables implementation. The container already has
`iptables-nft` binary included, so just needs a way to call this instead
of the default `iptables` binary.

This change implements a new `iptables_binary` config option, defaulting
to `iptables`, and calls this when adding or removing NAT rules.

Fixes #32

This change was written using AI LLM.

Authored-By: Claude Code (Sonnet 4.5)

controller/monitor.go

@@ -98,6 +98,11 @@ func NewMonitor(config *c.Config) *MonitorMgr {
 	if config.Agent.CleanupTimer == 0 {
 		config.Agent.CleanupTimer = defaultCleanupTimer
 	}
+	// Set iptables binary (defaults to "iptables" if not specified)
+	if config.Agent.IptablesBinary == "" {
+		config.Agent.IptablesBinary = "iptables"
+	}
+	SetIptablesBinary(config.Agent.IptablesBinary)
 	mon.config = config
 	// add apps defined in config
 	for _, a := range config.Apps {
@@ -356,6 +361,15 @@ func (m *MonitorMgr) Reload(configPath string) error {
 	if newConfig.Agent.CleanupTimer == 0 {
 		newConfig.Agent.CleanupTimer = defaultCleanupTimer
 	}
+	if newConfig.Agent.IptablesBinary == "" {
+		newConfig.Agent.IptablesBinary = "iptables"
+	}
+
+	// Update iptables binary if changed
+	if m.config.Agent.IptablesBinary != newConfig.Agent.IptablesBinary {
+		glog.Infof("Iptables binary changed from %s to %s", m.config.Agent.IptablesBinary, newConfig.Agent.IptablesBinary)
+		SetIptablesBinary(newConfig.Agent.IptablesBinary)
+	}
 
 	// Check if BGP configuration has changed
 	bgpChanged := m.bgpConfigChanged(m.config.Bgp, newConfig.Bgp)

controller/system.go

@@ -8,6 +8,7 @@ import (
 )
 
 var execCmd = "bash"
+var iptablesBinary = "iptables"
 
 func getCmdList(mainCmd string) []string {
 	cmdList := []string{}
@@ -88,8 +89,8 @@ func deleteLoopback(addr *net.IPNet) error {
 
 func natRule(op string, vip, localAddr net.IP, protocol, lport, dport string) error {
 	cmd := fmt.Sprintf(
-		"iptables -t nat -%s PREROUTING -p %s -d %s --dport %s -j DNAT --to-destination %s:%s",
-		op, protocol, vip.String(), lport, localAddr.String(), dport,
+		"%s -t nat -%s PREROUTING -p %s -d %s --dport %s -j DNAT --to-destination %s:%s",
+		iptablesBinary, op, protocol, vip.String(), lport, localAddr.String(), dport,
 	)
 	cmdList := getCmdList(cmd)
 	_, err := exec.Command(execCmd, cmdList...).Output()
@@ -98,3 +99,10 @@ func natRule(op string, vip, localAddr net.IP, protocol, lport, dport string) er
 	}
 	return nil
 }
+
+// SetIptablesBinary sets the iptables binary to use for NAT rules
+func SetIptablesBinary(binary string) {
+	if binary != "" {
+		iptablesBinary = binary
+	}
+}

controller/system_test.go

@@ -43,6 +43,71 @@ func TestAddLoopback(t *testing.T) {
 	assert.NotNil(t, err)
 }
 
+func TestSetIptablesBinary(t *testing.T) {
+	a := assert.New(t)
+
+	// Save original value
+	originalBinary := iptablesBinary
+	defer func() {
+		iptablesBinary = originalBinary
+	}()
+
+	// Test setting custom binary
+	SetIptablesBinary("iptables-nft")
+	a.Equal("iptables-nft", iptablesBinary)
+
+	// Test setting back to default
+	SetIptablesBinary("iptables")
+	a.Equal("iptables", iptablesBinary)
+
+	// Test that empty string doesn't change the value
+	SetIptablesBinary("iptables-custom")
+	a.Equal("iptables-custom", iptablesBinary)
+	SetIptablesBinary("")
+	a.Equal("iptables-custom", iptablesBinary, "Empty string should not change the binary")
+}
+
+func TestNatRuleCommandFormat(t *testing.T) {
+	a := assert.New(t)
+
+	// Save original values
+	originalBinary := iptablesBinary
+	originalExecCmd := execCmd
+	defer func() {
+		iptablesBinary = originalBinary
+		execCmd = originalExecCmd
+	}()
+
+	vip := net.ParseIP("192.0.2.1")
+	localAddr := net.ParseIP("10.0.0.1")
+
+	// Test with default iptables
+	SetIptablesBinary("iptables")
+	err := natRule("A", vip, localAddr, "tcp", "80", "8080")
+	// We expect this to fail in test environment, but we can check the error message
+	// contains our command
+	if err != nil {
+		a.Contains(err.Error(), "iptables -t nat -A PREROUTING")
+	}
+
+	// Test with iptables-nft
+	SetIptablesBinary("iptables-nft")
+	err = natRule("A", vip, localAddr, "tcp", "80", "8080")
+	if err != nil {
+		a.Contains(err.Error(), "iptables-nft -t nat -A PREROUTING")
+	}
+
+	// Test with custom binary path
+	SetIptablesBinary("/usr/sbin/iptables")
+	err = natRule("D", vip, localAddr, "udp", "53", "5353")
+	if err != nil {
+		a.Contains(err.Error(), "/usr/sbin/iptables -t nat -D PREROUTING")
+		a.Contains(err.Error(), "udp")
+		a.Contains(err.Error(), "53")
+		a.Contains(err.Error(), "5353")
+	}
+}
+
 func TestMain(m *testing.M) {
 	switch os.Getenv("test_name") {
 	case "test_gateway":