bridge: Add unit tests for outgoing NAT rules
**- What I did**

Added unit tests for outgoing NAT rules.

This commit is preparation for [my work-in-progress fix](https://github.com/rhansen/moby/commit/host_ipv6) for #46469.

**- How to verify it**

```shell
make TESTDIRS='github.com/docker/docker/libnetwork/drivers/bridge/...' test-unit
```

**- Description for the changelog**

(not needed)

libnetwork/drivers/bridge/setup_ip_tables_linux_test.go
package bridge

import (
	"net"
	"testing"

	"github.com/docker/docker/internal/testutils/netnsutils"
	"github.com/docker/docker/libnetwork/iptables"
	"github.com/docker/docker/libnetwork/netlabel"
	"github.com/docker/docker/libnetwork/portmapper"
	"github.com/vishvananda/netlink"
)

const (
	iptablesTestBridgeIP = "192.168.42.1"
)

func TestProgramIPTable(t *testing.T) {
	// Create a test bridge with a basic bridge configuration (name + IPv4).
	defer netnsutils.SetupTestOSContext(t)()

	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}

	createTestBridge(getBasicTestConfig(), &bridgeInterface{nlh: nh}, t)

	// Store various iptables chain rules we care for.
	rules := []struct {
		rule  iptRule
		descr string
	}{
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-d", "127.1.2.3", "-i", "lo", "-o", "lo", "-j", "DROP"}}, "Test Loopback"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Nat, chain: "POSTROUTING", args: []string{"-s", iptablesTestBridgeIP, "!", "-o", DefaultBridgeName, "-j", "MASQUERADE"}}, "NAT Test"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-o", DefaultBridgeName, "-m", "conntrack", "--ctstate", "RELATED,ESTABLISHED", "-j", "ACCEPT"}}, "Test ACCEPT INCOMING"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "!", "-o", DefaultBridgeName, "-j", "ACCEPT"}}, "Test ACCEPT NON_ICC OUTGOING"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "-o", DefaultBridgeName, "-j", "ACCEPT"}}, "Test enable ICC"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "-o", DefaultBridgeName, "-j", "DROP"}}, "Test disable ICC"},
	}

	// Assert the chain rules' insertion and removal.
	for _, c := range rules {
		assertIPTableChainProgramming(c.rule, c.descr, t)
	}
}

func TestSetupIPChains(t *testing.T) {
	// Create a test bridge with a basic bridge configuration (name + IPv4).
	defer netnsutils.SetupTestOSContext(t)()

	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}

	driverconfig := configuration{
		EnableIPTables: true,
	}
	d := &driver{
		config: driverconfig,
	}
	assertChainConfig(d, t)

	config := getBasicTestConfig()
	br := &bridgeInterface{nlh: nh}
	createTestBridge(config, br, t)

	assertBridgeConfig(config, br, d, t)

	config.EnableIPMasquerade = true
	assertBridgeConfig(config, br, d, t)

	config.EnableICC = true
	assertBridgeConfig(config, br, d, t)

	config.EnableIPMasquerade = false
	assertBridgeConfig(config, br, d, t)
}

func getBasicTestConfig() *networkConfiguration {
	config := &networkConfiguration{
		BridgeName:  DefaultBridgeName,
		AddressIPv4: &net.IPNet{IP: net.ParseIP(iptablesTestBridgeIP), Mask: net.CIDRMask(16, 32)},
	}
	return config
}

func createTestBridge(config *networkConfiguration, br *bridgeInterface, t *testing.T) {
	if err := setupDevice(config, br); err != nil {
		t.Fatalf("Failed to create the testing Bridge: %s", err.Error())
	}
	if err := setupBridgeIPv4(config, br); err != nil {
		t.Fatalf("Failed to bring up the testing Bridge: %s", err.Error())
	}
	if config.EnableIPv6 {
		if err := setupBridgeIPv6(config, br); err != nil {
			t.Fatalf("Failed to bring up the testing Bridge: %s", err.Error())
		}
	}
}

// Assert base function which pushes iptables chain rules on insertion and removal.
func assertIPTableChainProgramming(rule iptRule, descr string, t *testing.T) {
	// Add
	if err := programChainRule(rule, descr, true); err != nil {
		t.Fatalf("Failed to program iptable rule %s: %s", descr, err.Error())
	}

	if !rule.Exists() {
		t.Fatalf("Failed to effectively program iptable rule: %s", descr)
	}

	// Remove
	if err := programChainRule(rule, descr, false); err != nil {
		t.Fatalf("Failed to remove iptable rule %s: %s", descr, err.Error())
	}
	if rule.Exists() {
		t.Fatalf("Failed to effectively remove iptable rule: %s", descr)
	}
}

// Assert function which create chains.
func assertChainConfig(d *driver, t *testing.T) {
	var err error

	d.natChain, d.filterChain, d.isolationChain1, d.isolationChain2, err = setupIPChains(d.config, iptables.IPv4)
	if err != nil {
		t.Fatal(err)
	}
	if d.config.EnableIP6Tables {
		d.natChainV6, d.filterChainV6, d.isolationChain1V6, d.isolationChain2V6, err = setupIPChains(d.config, iptables.IPv6)
		if err != nil {
			t.Fatal(err)
		}
	}
}

// Assert function which pushes chains based on bridge config parameters.
func assertBridgeConfig(config *networkConfiguration, br *bridgeInterface, d *driver, t *testing.T) {
	nw := bridgeNetwork{
		portMapper:   portmapper.New(),
		portMapperV6: portmapper.New(),
		config:       config,
	}
	nw.driver = d

	// Attempt programming of ip tables.
	err := nw.setupIP4Tables(config, br)
	if err != nil {
		t.Fatalf("%v", err)
	}
	if d.config.EnableIP6Tables {
		if err := nw.setupIP6Tables(config, br); err != nil {
			t.Fatalf("%v", err)
		}
	}
}

// Regression test for https://github.com/moby/moby/issues/46445
func TestSetupIP6TablesWithHostIPv4(t *testing.T) {
	defer netnsutils.SetupTestOSContext(t)()
	d := newDriver()
	dc := &configuration{
		EnableIPTables:  true,
		EnableIP6Tables: true,
	}
	if err := d.configure(map[string]interface{}{netlabel.GenericData: dc}); err != nil {
		t.Fatal(err)
	}
	nc := &networkConfiguration{
		BridgeName:         DefaultBridgeName,
		AddressIPv4:        &net.IPNet{IP: net.ParseIP(iptablesTestBridgeIP), Mask: net.CIDRMask(16, 32)},
		EnableIPMasquerade: true,
		EnableIPv6:         true,
		AddressIPv6:        &net.IPNet{IP: net.ParseIP("2001:db8::1"), Mask: net.CIDRMask(64, 128)},
		HostIPv4:           net.ParseIP("192.0.2.2"),
	}
	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}
	br := &bridgeInterface{nlh: nh}
	createTestBridge(nc, br, t)
	assertBridgeConfig(nc, br, d, t)
}

package bridge

import (
	"net"
	"testing"

	"github.com/docker/docker/internal/testutils/netnsutils"
	"github.com/docker/docker/libnetwork/driverapi"
	"github.com/docker/docker/libnetwork/iptables"
	"github.com/docker/docker/libnetwork/netlabel"
	"github.com/docker/docker/libnetwork/portmapper"
	"github.com/vishvananda/netlink"
	"gotest.tools/v3/assert"
)

const (
	iptablesTestBridgeIP = "192.168.42.1"
)

// A testRegisterer implements the driverapi.Registerer interface.
type testRegisterer struct {
	t *testing.T
	d *driver
}

func (r *testRegisterer) RegisterDriver(name string, di driverapi.Driver, _ driverapi.Capability) error {
	if got, want := name, "bridge"; got != want {
		r.t.Fatalf("got driver name %s, want %s", got, want)
	}
	d, ok := di.(*driver)
	if !ok {
		r.t.Fatalf("got driver type %T, want %T", di, &driver{})
	}
	r.d = d
	return nil
}

func TestProgramIPTable(t *testing.T) {
	// Create a test bridge with a basic bridge configuration (name + IPv4).
	defer netnsutils.SetupTestOSContext(t)()

	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}

	createTestBridge(getBasicTestConfig(), &bridgeInterface{nlh: nh}, t)

	// Store various iptables chain rules we care for.
	rules := []struct {
		rule  iptRule
		descr string
	}{
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-d", "127.1.2.3", "-i", "lo", "-o", "lo", "-j", "DROP"}}, "Test Loopback"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Nat, chain: "POSTROUTING", args: []string{"-s", iptablesTestBridgeIP, "!", "-o", DefaultBridgeName, "-j", "MASQUERADE"}}, "NAT Test"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-o", DefaultBridgeName, "-m", "conntrack", "--ctstate", "RELATED,ESTABLISHED", "-j", "ACCEPT"}}, "Test ACCEPT INCOMING"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "!", "-o", DefaultBridgeName, "-j", "ACCEPT"}}, "Test ACCEPT NON_ICC OUTGOING"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "-o", DefaultBridgeName, "-j", "ACCEPT"}}, "Test enable ICC"},
		{iptRule{ipv: iptables.IPv4, table: iptables.Filter, chain: "FORWARD", args: []string{"-i", DefaultBridgeName, "-o", DefaultBridgeName, "-j", "DROP"}}, "Test disable ICC"},
	}

	// Assert the chain rules' insertion and removal.
	for _, c := range rules {
		assertIPTableChainProgramming(c.rule, c.descr, t)
	}
}

func TestSetupIPChains(t *testing.T) {
	// Create a test bridge with a basic bridge configuration (name + IPv4).
	defer netnsutils.SetupTestOSContext(t)()

	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}

	driverconfig := configuration{
		EnableIPTables: true,
	}
	d := &driver{
		config: driverconfig,
	}
	assertChainConfig(d, t)

	config := getBasicTestConfig()
	br := &bridgeInterface{nlh: nh}
	createTestBridge(config, br, t)

	assertBridgeConfig(config, br, d, t)

	config.EnableIPMasquerade = true
	assertBridgeConfig(config, br, d, t)

	config.EnableICC = true
	assertBridgeConfig(config, br, d, t)

	config.EnableIPMasquerade = false
	assertBridgeConfig(config, br, d, t)
}

func getBasicTestConfig() *networkConfiguration {
	config := &networkConfiguration{
		BridgeName:  DefaultBridgeName,
		AddressIPv4: &net.IPNet{IP: net.ParseIP(iptablesTestBridgeIP), Mask: net.CIDRMask(16, 32)},
	}
	return config
}

func createTestBridge(config *networkConfiguration, br *bridgeInterface, t *testing.T) {
	if err := setupDevice(config, br); err != nil {
		t.Fatalf("Failed to create the testing Bridge: %s", err.Error())
	}
	if err := setupBridgeIPv4(config, br); err != nil {
		t.Fatalf("Failed to bring up the testing Bridge: %s", err.Error())
	}
	if config.EnableIPv6 {
		if err := setupBridgeIPv6(config, br); err != nil {
			t.Fatalf("Failed to bring up the testing Bridge: %s", err.Error())
		}
	}
}

// Assert base function which pushes iptables chain rules on insertion and removal.
func assertIPTableChainProgramming(rule iptRule, descr string, t *testing.T) {
	// Add
	if err := programChainRule(rule, descr, true); err != nil {
		t.Fatalf("Failed to program iptable rule %s: %s", descr, err.Error())
	}

	if !rule.Exists() {
		t.Fatalf("Failed to effectively program iptable rule: %s", descr)
	}

	// Remove
	if err := programChainRule(rule, descr, false); err != nil {
		t.Fatalf("Failed to remove iptable rule %s: %s", descr, err.Error())
	}
	if rule.Exists() {
		t.Fatalf("Failed to effectively remove iptable rule: %s", descr)
	}
}

// Assert function which create chains.
func assertChainConfig(d *driver, t *testing.T) {
	var err error

	d.natChain, d.filterChain, d.isolationChain1, d.isolationChain2, err = setupIPChains(d.config, iptables.IPv4)
	if err != nil {
		t.Fatal(err)
	}
	if d.config.EnableIP6Tables {
		d.natChainV6, d.filterChainV6, d.isolationChain1V6, d.isolationChain2V6, err = setupIPChains(d.config, iptables.IPv6)
		if err != nil {
			t.Fatal(err)
		}
	}
}

// Assert function which pushes chains based on bridge config parameters.
func assertBridgeConfig(config *networkConfiguration, br *bridgeInterface, d *driver, t *testing.T) {
	nw := bridgeNetwork{
		portMapper:   portmapper.New(),
		portMapperV6: portmapper.New(),
		config:       config,
	}
	nw.driver = d

	// Attempt programming of ip tables.
	err := nw.setupIP4Tables(config, br)
	if err != nil {
		t.Fatalf("%v", err)
	}
	if d.config.EnableIP6Tables {
		if err := nw.setupIP6Tables(config, br); err != nil {
			t.Fatalf("%v", err)
		}
	}
}

// Regression test for https://github.com/moby/moby/issues/46445
func TestSetupIP6TablesWithHostIPv4(t *testing.T) {
	defer netnsutils.SetupTestOSContext(t)()
	d := newDriver()
	dc := &configuration{
		EnableIPTables:  true,
		EnableIP6Tables: true,
	}
	if err := d.configure(map[string]interface{}{netlabel.GenericData: dc}); err != nil {
		t.Fatal(err)
	}
	nc := &networkConfiguration{
		BridgeName:         DefaultBridgeName,
		AddressIPv4:        &net.IPNet{IP: net.ParseIP(iptablesTestBridgeIP), Mask: net.CIDRMask(16, 32)},
		EnableIPMasquerade: true,
		EnableIPv6:         true,
		AddressIPv6:        &net.IPNet{IP: net.ParseIP("2001:db8::1"), Mask: net.CIDRMask(64, 128)},
		HostIPv4:           net.ParseIP("192.0.2.2"),
	}
	nh, err := netlink.NewHandle()
	if err != nil {
		t.Fatal(err)
	}
	br := &bridgeInterface{nlh: nh}
	createTestBridge(nc, br, t)
	assertBridgeConfig(nc, br, d, t)
}

func TestOutgoingNATRules(t *testing.T) {
	br := "br-nattest"
	brIPv4 := &net.IPNet{IP: net.ParseIP(iptablesTestBridgeIP), Mask: net.CIDRMask(16, 32)}
	brIPv6 := &net.IPNet{IP: net.ParseIP("2001:db8::1"), Mask: net.CIDRMask(64, 128)}
	maskedBrIPv4 := &net.IPNet{IP: brIPv4.IP.Mask(brIPv4.Mask), Mask: brIPv4.Mask}
	maskedBrIPv6 := &net.IPNet{IP: brIPv6.IP.Mask(brIPv6.Mask), Mask: brIPv6.Mask}
	hostIPv4 := net.ParseIP("192.0.2.2")
	for _, tc := range []struct {
		desc               string
		enableIPTables     bool
		enableIP6Tables    bool
		enableIPv6         bool
		enableIPMasquerade bool
		hostIPv4           net.IP
		// Hairpin NAT rules are not tested here because they are orthogonal to outgoing NAT.  They
		// exist to support the port forwarding DNAT rules: without any port forwarding there would be
		// no need for any hairpin NAT rules, and when there is port forwarding then hairpin NAT rules
		// are needed even if outgoing NAT is disabled.  Hairpin NAT tests belong with the port
		// forwarding DNAT tests.
		wantIPv4Masq bool
		wantIPv4Snat bool
		wantIPv6Masq bool
	}{
		{
			desc: "everything disabled",
		},
		{
			desc:               "iptables/ip6tables disabled",
			enableIPv6:         true,
			enableIPMasquerade: true,
		},
		{
			desc:               "host IP with iptables/ip6tables disabled",
			enableIPv6:         true,
			enableIPMasquerade: true,
			hostIPv4:           hostIPv4,
		},
		{
			desc:            "masquerade disabled, no host IP",
			enableIPTables:  true,
			enableIP6Tables: true,
			enableIPv6:      true,
		},
		{
			desc:            "masquerade disabled, with host IP",
			enableIPTables:  true,
			enableIP6Tables: true,
			enableIPv6:      true,
			hostIPv4:        hostIPv4,
		},
		{
			desc:               "IPv4 masquerade, IPv6 disabled",
			enableIPTables:     true,
			enableIPMasquerade: true,
			wantIPv4Masq:       true,
		},
		{
			desc:               "IPv4 SNAT, IPv6 disabled",
			enableIPTables:     true,
			enableIPMasquerade: true,
			hostIPv4:           hostIPv4,
			wantIPv4Snat:       true,
		},
		{
			desc:               "IPv4 masquerade, IPv6 masquerade",
			enableIPTables:     true,
			enableIP6Tables:    true,
			enableIPv6:         true,
			enableIPMasquerade: true,
			wantIPv4Masq:       true,
			wantIPv6Masq:       true,
		},
		{
			desc:               "IPv4 SNAT, IPv6 masquerade",
			enableIPTables:     true,
			enableIP6Tables:    true,
			enableIPv6:         true,
			enableIPMasquerade: true,
			hostIPv4:           hostIPv4,
			wantIPv4Snat:       true,
			wantIPv6Masq:       true,
		},
	} {
		t.Run(tc.desc, func(t *testing.T) {
			defer netnsutils.SetupTestOSContext(t)()
			dc := &configuration{
				EnableIPTables:  tc.enableIPTables,
				EnableIP6Tables: tc.enableIP6Tables,
			}
			r := &testRegisterer{t: t}
			if err := Register(r, map[string]interface{}{netlabel.GenericData: dc}); err != nil {
				t.Fatal(err)
			}
			if r.d == nil {
				t.Fatal("testRegisterer.RegisterDriver never called")
			}
			nc := &networkConfiguration{
				BridgeName:         br,
				AddressIPv4:        brIPv4,
				AddressIPv6:        brIPv6,
				EnableIPv6:         tc.enableIPv6,
				EnableIPMasquerade: tc.enableIPMasquerade,
				HostIPv4:           tc.hostIPv4,
			}
			ipv4Data := []driverapi.IPAMData{{Pool: maskedBrIPv4, Gateway: brIPv4}}
			ipv6Data := []driverapi.IPAMData{{Pool: maskedBrIPv6, Gateway: brIPv6}}
			if !nc.EnableIPv6 {
				nc.AddressIPv6 = nil
				ipv6Data = nil
			}
			if err := r.d.CreateNetwork("nattest", map[string]interface{}{netlabel.GenericData: nc}, nil, ipv4Data, ipv6Data); err != nil {
				t.Fatal(err)
			}
			defer func() {
				if err := r.d.DeleteNetwork("nattest"); err != nil {
					t.Fatal(err)
				}
			}()
			// Log the contents of all chains to aid troubleshooting.
			for _, ipv := range []iptables.IPVersion{iptables.IPv4, iptables.IPv6} {
				ipt := iptables.GetIptable(ipv)
				for _, table := range []iptables.Table{iptables.Nat, iptables.Filter, iptables.Mangle} {
					out, err := ipt.Raw("-t", string(table), "-S")
					if err != nil {
						t.Error(err)
					}
					t.Logf("%s: %s %s table rules:\n%s", tc.desc, ipv, table, string(out))
				}
			}
			for _, rc := range []struct {
				want bool
				rule iptRule
			}{
				// Rule order doesn't matter: At most one of the following IPv4 rules will exist, and the
				// same goes for the IPv6 rules.
				{tc.wantIPv4Masq, iptRule{iptables.IPv4, iptables.Nat, "POSTROUTING", []string{"-s", maskedBrIPv4.String(), "!", "-o", br, "-j", "MASQUERADE"}}},
				{tc.wantIPv4Snat, iptRule{iptables.IPv4, iptables.Nat, "POSTROUTING", []string{"-s", maskedBrIPv4.String(), "!", "-o", br, "-j", "SNAT", "--to-source", hostIPv4.String()}}},
				{tc.wantIPv6Masq, iptRule{iptables.IPv6, iptables.Nat, "POSTROUTING", []string{"-s", maskedBrIPv6.String(), "!", "-o", br, "-j", "MASQUERADE"}}},
			} {
				assert.Equal(t, rc.rule.Exists(), rc.want)
			}
		})
	}
}

State is change on line 318, which is fine because netnsutils.SetupTestOSContext is called on line 292. The only reason I brought it to your attention earlier was to let you know that you could safely assert on the contents of iptables chains verbatim.
```suggestion
```
