Use ErrKernelModuleMissing when missing kernel module string is detected

cmd/gluetun/main.go

@@ -227,7 +227,7 @@ func _main(ctx context.Context, buildInfo models.BuildInformation,
 		firewallLogger.Patch(log.SetLevel(log.LevelDebug))
 	}
 	firewallConf, err := firewall.NewConfig(ctx, firewallLogger, cmder,
-		defaultRoutes, localNetworks)
+		netLinker, defaultRoutes, localNetworks)
 	if err != nil {
 		return err
 	}
@@ -237,10 +237,6 @@ func _main(ctx context.Context, buildInfo models.BuildInformation,
 		if err != nil {
 			return err
 		}
-		err = netLinker.FlushConntrack()
-		if err != nil {
-			logger.Warnf("flushing conntrack failed: %s", err)
-		}
 	}
 
 	// TODO run this in a loop or in openvpn to reload from file without restarting

internal/firewall/enable.go

@@ -69,6 +69,11 @@ func (c *Config) enable(ctx context.Context) (err error) {
 		return err
 	}
 
+	err = c.flushExistingConnections(ctx)
+	if err != nil {
+		return fmt.Errorf("flushing existing connections: %w", err)
+	}
+
 	if err = c.impl.AcceptEstablishedRelatedTraffic(ctx); err != nil {
 		return err
 	}

internal/firewall/firewall.go

@@ -13,6 +13,7 @@ import (
 
 type Config struct {
 	runner        CmdRunner
+	netlinker     Netlinker
 	logger        Logger
 	defaultRoutes []routing.DefaultRoute
 	localNetworks []routing.LocalNetwork
@@ -35,8 +36,8 @@ type Config struct {
 // NewConfig creates a new Config instance and returns an error
 // if no iptables implementation is available.
 func NewConfig(ctx context.Context, logger Logger,
-	runner CmdRunner, defaultRoutes []routing.DefaultRoute,
+	runner CmdRunner, netlinker Netlinker,
-	localNetworks []routing.LocalNetwork,
+	defaultRoutes []routing.DefaultRoute, localNetworks []routing.LocalNetwork,
 ) (config *Config, err error) {
 	impl, err := iptables.New(ctx, runner, logger)
 	if err != nil {
@@ -45,6 +46,7 @@ func NewConfig(ctx context.Context, logger Logger,
 
 	return &Config{
 		runner:            runner,
+		netlinker:         netlinker,
 		logger:            logger,
 		allowedInputPorts: make(map[uint16]map[string]struct{}),
 		// Obtained from routing

internal/firewall/flush.go

@@ -0,0 +1,74 @@
+package firewall
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/firewall/iptables"
+	"github.com/qdm12/gluetun/internal/netlink"
+)
+
+func (c *Config) flushExistingConnections(ctx context.Context) error {
+	tries := []struct {
+		name string
+		f    func(ctx context.Context) error
+	}{
+		{name: "flushing conntrack", f: func(_ context.Context) error {
+			return c.netlinker.FlushConntrack()
+		}},
+		{name: "marking and filtering unmarked packets", f: c.impl.AcceptOutputPublicOnlyNewTraffic},
+		{name: "rejecting connections for one second", f: c.rejectOutputTrafficTemporarily},
+		{name: "dropping connections for one second", f: c.dropOutputTrafficTemporarily},
+	}
+	errs := make([]error, 0, len(tries))
+	for i, try := range tries {
+		if i > 0 {
+			c.logger.Debugf("falling back to %s because %s failed: %s", try.name, tries[i-1].name, errs[i-1])
+		}
+		err := try.f(ctx)
+		if err == nil {
+			return nil
+		}
+		err = fmt.Errorf("%s: %w", try.name, err)
+		if !errors.Is(err, iptables.ErrKernelModuleMissing) && !errors.Is(err, netlink.ErrConntrackNetlinkNotSupported) {
+			return err
+		}
+		errs = append(errs, err)
+	}
+	return fmt.Errorf("all tries failed: %v", errs) //nolint:err113
+}
+
+func (c *Config) rejectOutputTrafficTemporarily(ctx context.Context) error {
+	return setupThenRevert(ctx, c.impl.RejectOutputPublicTraffic)
+}
+
+func (c *Config) dropOutputTrafficTemporarily(ctx context.Context) error {
+	return setupThenRevert(ctx, c.impl.DropOutputPublicTraffic)
+}
+
+// setupThenRevert is a helper function to run a setup function that takes a remove boolean argument,
+// and then run the same function with remove set to true after one second or when the context is canceled,
+// whichever comes first.
+func setupThenRevert(ctx context.Context, f func(ctx context.Context, remove bool) error) error {
+	remove := false
+	err := f(ctx, remove)
+	if err != nil {
+		return fmt.Errorf("setting up: %w", err)
+	}
+	timer := time.NewTimer(time.Second)
+	select {
+	case <-timer.C:
+	case <-ctx.Done():
+		timer.Stop()
+	}
+	remove = true
+	// Use [context.Background] to make sure this is removed, even if the context
+	// passed to this function is canceled.
+	err = f(context.Background(), remove)
+	if err != nil {
+		return fmt.Errorf("reverting: %w", err)
+	}
+	return nil
+}

internal/firewall/interfaces.go

@@ -14,15 +14,23 @@ type CmdRunner interface {
 
 type Logger interface {
 	Debug(s string)
+	Debugf(format string, args ...any)
 	Info(s string)
 	Warn(s string)
 	Error(s string)
 }
 
+type Netlinker interface {
+	FlushConntrack() error
+}
+
 type firewallImpl interface { //nolint:interfacebloat
 	SaveAndRestore(ctx context.Context) (restore func(context.Context), err error)
-	AcceptEstablishedRelatedTraffic(ctx context.Context) error
+	AcceptOutputPublicOnlyNewTraffic(ctx context.Context) error
+	RejectOutputPublicTraffic(ctx context.Context, remove bool) error
+	DropOutputPublicTraffic(ctx context.Context, remove bool) error
 	AcceptInputThroughInterface(ctx context.Context, intf string) error
+	AcceptEstablishedRelatedTraffic(ctx context.Context) error
 	AcceptInputToPort(ctx context.Context, intf string, port uint16, remove bool) error
 	AcceptInputToSubnet(ctx context.Context, intf string, subnet netip.Prefix) error
 	AcceptIpv6MulticastOutput(ctx context.Context, intf string) error

internal/firewall/iptables/firewall.go

@@ -2,9 +2,12 @@ package iptables
 
 import (
 	"context"
+	"errors"
 	"sync"
 )
 
+var ErrKernelModuleMissing = errors.New("kernel module is missing for this operation")
+
 type Config struct {
 	runner         CmdRunner
 	logger         Logger

internal/firewall/iptables/ip6tables.go

@@ -76,6 +76,9 @@ func (c *Config) runIP6tablesInstructionNoSave(ctx context.Context, instruction
 	cmd := exec.CommandContext(ctx, c.ip6Tables, flags...) // #nosec G204
 	c.logger.Debug(cmd.String())
 	if output, err := c.runner.Run(cmd); err != nil {
+		if strings.Contains(output, "missing kernel module") {
+			err = ErrKernelModuleMissing
+		}
 		return fmt.Errorf("command failed: \"%s %s\": %s: %w",
 			c.ip6Tables, instruction, output, err)
 	}

internal/firewall/iptables/iptables.go

@@ -92,6 +92,9 @@ func (c *Config) runIptablesInstructionNoSave(ctx context.Context, instruction s
 	cmd := exec.CommandContext(ctx, c.ipTables, flags...) // #nosec G204
 	c.logger.Debug(cmd.String())
 	if output, err := c.runner.Run(cmd); err != nil {
+		if strings.Contains(output, "missing kernel module") {
+			err = ErrKernelModuleMissing
+		}
 		return fmt.Errorf("command failed: \"%s %s\": %s: %w",
 			c.ipTables, instruction, output, err)
 	}
@@ -147,6 +150,136 @@ func (c *Config) AcceptEstablishedRelatedTraffic(ctx context.Context) error {
 	})
 }
 
+// AcceptOutputPublicOnlyNewTraffic adds rules to mark new output connections, and to accept
+// established or related packets with this mark only. This effectively forces
+// previously established or related traffic to be blocked.
+// If remove is true, the rules are removed instead of appended.
+// If the relevant kernel modules are not available, it returns an error indicating
+// which kernel module is missing.
+func (c *Config) AcceptOutputPublicOnlyNewTraffic(ctx context.Context) error {
+	ipv4Instructions, ipv6Instructions := makeCreatePublicIPChainInstructions()
+	appendToBoth := func(instruction string) {
+		ipv4Instructions = append(ipv4Instructions, instruction)
+		ipv6Instructions = append(ipv6Instructions, instruction)
+	}
+
+	// Mark new connections with mark 0x567
+	appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate NEW -j CONNMARK --set-mark 0x567")
+	// Drop related/established connections that made it through; marked connections would
+	// be directly accepted by the first rule in the OUTPUT chain (see below)
+	appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j DROP")
+	// Set the PUBLIC_ONLY chain as the second rule in the OUTPUT chain, so that it is evaluated
+	// after the accept rule below, for performance reasons.
+	appendToBoth("-I OUTPUT -j PUBLIC_ONLY")
+	appendToBoth("-I OUTPUT -m conntrack --ctstate RELATED,ESTABLISHED -m connmark --mark 0x567 -j ACCEPT")
+
+	c.iptablesMutex.Lock()
+	c.ip6tablesMutex.Lock()
+	defer c.iptablesMutex.Unlock()
+	defer c.ip6tablesMutex.Unlock()
+
+	restore, err := c.saveAndRestore(ctx)
+	if err != nil {
+		return err
+	}
+
+	err = c.runIptablesInstructionsNoSave(ctx, ipv4Instructions)
+	if err != nil {
+		restore(ctx)
+		return err
+	}
+	err = c.runIP6tablesInstructionsNoSave(ctx, ipv6Instructions)
+	if err != nil {
+		restore(ctx)
+		return err
+	}
+
+	return nil
+}
+
+func (c *Config) RejectOutputPublicTraffic(ctx context.Context, remove bool) error {
+	return c.targetOutputPublicTraffic(ctx, "REJECT", remove)
+}
+
+func (c *Config) DropOutputPublicTraffic(ctx context.Context, remove bool) error {
+	return c.targetOutputPublicTraffic(ctx, "DROP", remove)
+}
+
+func (c *Config) targetOutputPublicTraffic(ctx context.Context, target string, remove bool) error {
+	removeInstructions := []string{
+		"-D OUTPUT -j PUBLIC_ONLY",
+		"-F PUBLIC_ONLY",
+		"-X PUBLIC_ONLY",
+	}
+	if remove {
+		return c.runMixedIptablesInstructions(ctx, removeInstructions)
+	}
+
+	ipv4Instructions, ipv6Instructions := makeCreatePublicIPChainInstructions()
+	appendToBoth := func(instruction string) {
+		ipv4Instructions = append(ipv4Instructions, instruction)
+		ipv6Instructions = append(ipv6Instructions, instruction)
+	}
+
+	if target == "REJECT" {
+		// Block TCP by sending back TCP RST packets.
+		appendToBoth("-A PUBLIC_ONLY -p tcp -m conntrack --ctstate RELATED,ESTABLISHED " +
+			"-j REJECT --reject-with tcp-reset")
+		// Block UDP and ICMP, sending back ICMP port unreachable.
+		appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j REJECT")
+	} else {
+		appendToBoth("-A PUBLIC_ONLY -m conntrack --ctstate RELATED,ESTABLISHED -j " + target)
+	}
+	appendToBoth("-I OUTPUT -j PUBLIC_ONLY")
+
+	err := c.runIptablesInstructions(ctx, ipv4Instructions)
+	if err != nil {
+		if strings.Contains(err.Error(), " support") {
+			return fmt.Errorf("%w: %w", ErrKernelModuleMissing, err)
+		}
+	}
+
+	err = c.runIP6tablesInstructions(ctx, ipv6Instructions)
+	if err != nil {
+		_ = c.runIptablesInstructions(ctx, removeInstructions)
+		if strings.Contains(err.Error(), " support") {
+			return fmt.Errorf("%w: %w", ErrKernelModuleMissing, err)
+		}
+		return err
+	}
+
+	return nil
+}
+
+func makeCreatePublicIPChainInstructions() (ipv4Instructions, ipv6Instructions []string) {
+	ipv4PrivatePrefixes := []netip.Prefix{
+		netip.MustParsePrefix("10.0.0.0/8"),
+		netip.MustParsePrefix("172.16.0.0/12"),
+		netip.MustParsePrefix("192.168.0.0/16"),
+		netip.MustParsePrefix("127.0.0.0/8"),
+	}
+	ipv6PrivatePrefixes := []netip.Prefix{
+		netip.MustParsePrefix("fc00::/7"),
+		netip.MustParsePrefix("fe80::/10"),
+		netip.MustParsePrefix("::1/128"),
+	}
+
+	ipv4Instructions = append(ipv4Instructions, "-N PUBLIC_ONLY")
+	ipv6Instructions = append(ipv6Instructions, "-N PUBLIC_ONLY")
+
+	for _, prefix := range ipv4PrivatePrefixes {
+		ipv4Instructions = append(ipv4Instructions, fmt.Sprintf(
+			"-A PUBLIC_ONLY -d %s -j RETURN", prefix))
+	}
+
+	for _, prefix := range ipv6PrivatePrefixes {
+		ipv6Instructions = append(ipv6Instructions, fmt.Sprintf(
+			"-A PUBLIC_ONLY -d %s -j RETURN", prefix))
+	}
+
+	return ipv4Instructions, ipv6Instructions
+}
+
 func (c *Config) AcceptOutputTrafficToVPN(ctx context.Context,
 	defaultInterface string, connection models.Connection, remove bool,
 ) error {

internal/firewall/iptables/list.go

@@ -33,6 +33,9 @@ type chainRule struct {
 	ctstate         []string     // for example ["RELATED","ESTABLISHED"]. Can be empty.
 	tcpFlags        tcpFlags
 	mark            mark
+	connMark        mark
+	setMark         uint
+	rejectWith      string // for example "tcp-reset", only used for REJECT targets
 }
 
 type mark struct {
@@ -219,10 +222,6 @@ func parseChainRuleField(fieldIndex int, field string, rule *chainRule) (err err
 			return fmt.Errorf("parsing bytes: %w", err)
 		}
 	case targetIndex:
-		err = checkTarget(field)
-		if err != nil {
-			return fmt.Errorf("checking target: %w", err)
-		}
 		rule.target = field
 	case protocolIndex:
 		rule.protocol, err = parseProtocol(field)
@@ -293,6 +292,33 @@ func parseChainRuleOptionalFields(optionalFields []string, rule *chainRule) (err
 			}
 			rule.mark = mark
 			i += consumed
+		case "reject-with":
+			i++
+			rule.rejectWith = optionalFields[i] // for example "tcp-reset"
+			i++
+		case "connmark":
+			i++
+			connMark, consumed, err := parseMark(optionalFields[i:])
+			if err != nil {
+				return fmt.Errorf("parsing connmark: %w", err)
+			}
+			rule.connMark = connMark
+			i += consumed
+		case "CONNMARK":
+			i++
+			switch optionalFields[i] {
+			case "set":
+				i++
+				value, err := parseAny32bNumber(optionalFields[i])
+				if err != nil {
+					return fmt.Errorf("parsing CONNMARK set value: %w", err)
+				}
+				rule.setMark = value
+				i++
+			default:
+				return fmt.Errorf("%w: unexpected %q after CONNMARK",
+					ErrChainRuleMalformed, optionalFields[i])
+			}
 		default:
 			return fmt.Errorf("%w: unexpected optional field: %s",
 				ErrChainRuleMalformed, optionalFields[i])
@@ -422,8 +448,6 @@ func parsePortsCSV(s string) (ports []uint16, err error) {
 	return ports, nil
 }
 
-var errMarkValueMalformed = errors.New("mark value is malformed")
-
 func parseMark(optionalFields []string) (m mark, consumed int, err error) {
 	switch optionalFields[consumed] {
 	case "match":
@@ -433,13 +457,11 @@ func parseMark(optionalFields []string) (m mark, consumed int, err error) {
 			consumed++
 		}
 
-		const base = 0 // auto-detect
+		value, err := parseAny32bNumber(optionalFields[consumed])
-		const bits = 32
-		value, err := strconv.ParseUint(optionalFields[consumed], base, bits)
 		if err != nil {
-			return mark{}, 0, fmt.Errorf("%w: %s", errMarkValueMalformed, optionalFields[consumed])
+			return mark{}, 0, fmt.Errorf("value malformed: %w", err)
 		}
-		m.value = uint(value)
+		m.value = value
 		consumed++
 	default:
 		return mark{}, 0, fmt.Errorf("%w: unexpected mark mode field: %s",

internal/firewall/iptables/parse.go

@@ -9,9 +9,19 @@ import (
 	"strings"
 )
 
+type operation uint8
+
+const (
+	opNone operation = iota
+	opAppend
+	opDelete
+	opInsert
+	opReplace
+)
+
 type iptablesInstruction struct {
 	table           string // defaults to "filter", and can be "nat" for example.
-	append          bool
+	operation       operation
 	chain           string       // for example INPUT, PREROUTING. Cannot be empty.
 	target          string       // for example ACCEPT. Can be empty.
 	protocol        string       // "tcp" or "udp" or "" for all protocols.
@@ -25,6 +35,9 @@ type iptablesInstruction struct {
 	ctstate         []string     // if empty, there is no ctstate
 	tcpFlags        tcpFlags
 	mark            mark
+	connMark        mark
+	setMark         uint   // only used for jump CONNMARK --set-mark
+	rejectWith      string // only used for REJECT targets
 }
 
 func (i *iptablesInstruction) setDefaults() {
@@ -65,6 +78,12 @@ func (i *iptablesInstruction) equalToRule(table, chain string, rule chainRule) (
 		return false
 	case i.mark != rule.mark:
 		return false
+	case i.connMark != rule.connMark:
+		return false
+	case i.setMark != rule.setMark:
+		return false
+	case i.rejectWith != rule.rejectWith:
+		return false
 	default:
 		return true
 	}
@@ -113,13 +132,20 @@ func parseInstructionFlag(fields []string, instruction *iptablesInstruction) (co
 	case "-t", "--table":
 		instruction.table = value
 	case "-D", "--delete":
-		instruction.append = false
+		instruction.operation = opDelete
 		instruction.chain = value
 	case "-A", "--append":
-		instruction.append = true
+		instruction.operation = opAppend
+		instruction.chain = value
+	case "-I", "--insert":
+		instruction.operation = opInsert
 		instruction.chain = value
 	case "-j", "--jump":
-		instruction.target = value
+		subConsumed, err := parseJumpFlag(fields[1:], instruction)
+		if err != nil {
+			return 0, fmt.Errorf("parsing jump flag: %w", err)
+		}
+		consumed += subConsumed
 	case "-p", "--protocol":
 		instruction.protocol = value
 	case "-m", "--match":
@@ -128,13 +154,11 @@ func parseInstructionFlag(fields []string, instruction *iptablesInstruction) (co
 			return 0, fmt.Errorf("parsing match module: %w", err)
 		}
 	case "--mark":
-		const base = 0 // auto-detect
+		n, err := parseAny32bNumber(value)
-		const bits = 32
-		value, err := strconv.ParseUint(value, base, bits)
 		if err != nil {
-			return 0, fmt.Errorf("parsing mark value %q: %w", fields[2], err)
+			return 0, fmt.Errorf("parsing mark value %q: %w", value, err)
 		}
-		instruction.mark.value = uint(value)
+		instruction.mark.value = n
 	case "-i", "--in-interface":
 		instruction.inputInterface = value
 	case "-o", "--out-interface":
@@ -172,6 +196,8 @@ func parseInstructionFlag(fields []string, instruction *iptablesInstruction) (co
 		if err != nil {
 			return 0, fmt.Errorf("parsing TCP flags: %w", err)
 		}
+	case "--reject-with":
+		instruction.rejectWith = value // for example "tcp-reset"
 	default:
 		return 0, fmt.Errorf("%w: unknown key %q", ErrIptablesCommandMalformed, flag)
 	}
@@ -182,7 +208,7 @@ func preCheckInstructionFields(fields []string) (consumed int, err error) {
 	flag := fields[0]
 	// All flags use one value after the flag, except the following:
 	switch flag {
-	case "--tcp-flags": // -m can have 1 or 2 values
+	case "--tcp-flags":
 		const expected = 3
 		if len(fields) < expected {
 			return 0, fmt.Errorf("%w: flag %q requires at least 2 values, but got %s",
@@ -199,6 +225,34 @@ func preCheckInstructionFields(fields []string) (consumed int, err error) {
 	}
 }
 
+func parseJumpFlag(fields []string, instruction *iptablesInstruction) (consumed int, err error) {
+	instruction.target = fields[0]
+	// consumed in the caller already takes fields[0] into account
+	if instruction.target != "CONNMARK" {
+		return consumed, nil
+	}
+	// consumed already accounts for the "CONNMARK" value
+	const expectedFields = 3
+	if len(fields) < expectedFields {
+		return 0, fmt.Errorf("%w: jump CONNMARK requires at least two additional values",
+			ErrIptablesCommandMalformed)
+	}
+	switch fields[1] {
+	case "--set-mark":
+		n, err := parseAny32bNumber(fields[2])
+		if err != nil {
+			return 0, fmt.Errorf("parsing connmark mark value %q: %w", fields[2], err)
+		}
+		consumed++
+		instruction.setMark = n
+	default:
+		return consumed, fmt.Errorf("%w: unsupported jump CONNMARK with value: %s",
+			ErrIptablesCommandMalformed, fields[1])
+	}
+	consumed++
+	return consumed, nil
+}
+
 func parseIPPrefix(value string) (prefix netip.Prefix, err error) {
 	slashIndex := strings.Index(value, "/")
 	if slashIndex >= 0 {
@@ -221,6 +275,13 @@ func parsePort(value string) (port uint16, err error) {
 	return uint16(portValue), nil
 }
 
+func parseAny32bNumber(mark string) (value uint, err error) {
+	const base = 0 // auto-detect
+	const bits = 32
+	n, err := strconv.ParseUint(mark, base, bits)
+	return uint(n), err
+}
+
 func parseMatchModule(fields []string, instruction *iptablesInstruction) (
 	consumed int, err error,
 ) {
@@ -234,14 +295,30 @@ func parseMatchModule(fields []string, instruction *iptablesInstruction) (
 		// parse it twice.
 	case "mark":
 		consumed++
-		switch fields[consumed] {
+		switch {
-		case "!":
+		case len(fields[consumed:]) == 0 || strings.HasPrefix(fields[consumed], "-"):
+			// end or another flag
+			return consumed, nil
+		case fields[consumed] == "!":
 			consumed++
 			instruction.mark.invert = true
 		default:
 			return consumed, fmt.Errorf("%w: unsupported match mark with value: %s",
 				ErrIptablesCommandMalformed, fields[2])
 		}
+	case "connmark":
+		consumed++
+		switch {
+		case len(fields[consumed:]) == 0 || strings.HasPrefix(fields[consumed], "-"):
+			// end or another flag
+			return consumed, nil
+		case fields[consumed] == "!":
+			consumed++
+			instruction.connMark.invert = true
+		default:
+			return consumed, fmt.Errorf("%w: unsupported match connmark with value: %s",
+				ErrIptablesCommandMalformed, fields[2])
+		}
 	default:
 		return 0, fmt.Errorf("%w: unknown match value: %s",
 			ErrIptablesCommandMalformed, fields[consumed])

internal/firewall/iptables/parse_test.go

@@ -33,9 +33,9 @@ func Test_parseIptablesInstruction(t *testing.T) {
 		"one_pair": {
 			s: "-A INPUT",
 			instruction: iptablesInstruction{
-				table:  "filter",
+				table:     "filter",
-				chain:  "INPUT",
+				chain:     "INPUT",
-				append: true,
+				operation: opAppend,
 			},
 		},
 		"instruction_A": {
@@ -43,7 +43,7 @@ func Test_parseIptablesInstruction(t *testing.T) {
 			instruction: iptablesInstruction{
 				table:           "filter",
 				chain:           "INPUT",
-				append:          true,
+				operation:       opAppend,
 				inputInterface:  "tun0",
 				protocol:        "tcp",
 				source:          netip.MustParsePrefix("1.2.3.4/32"),
@@ -57,7 +57,7 @@ func Test_parseIptablesInstruction(t *testing.T) {
 			instruction: iptablesInstruction{
 				table:           "nat",
 				chain:           "PREROUTING",
-				append:          false,
+				operation:       opDelete,
 				inputInterface:  "tun0",
 				protocol:        "tcp",
 				destinationPort: 43716,

internal/firewall/iptables/tcp.go

@@ -64,7 +64,7 @@ func parseTCPFlag(s string) (tcpFlag, error) {
 	return 0, fmt.Errorf("%w: %s", errTCPFlagUnknown, s)
 }
 
-var ErrMarkMatchModuleMissing = errors.New("kernel is missing the mark module libxt_mark.so")
+var ErrMarkMatchModuleMissing = errors.New("libxt_mark.so module is missing")
 
 // TempDropOutputTCPRST temporarily drops outgoing TCP RST packets to the specified address and port,
 // for any TCP packets not marked with the excludeMark given.

internal/netlink/conntrack_linux.go

@@ -1,38 +1,44 @@
 package netlink
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/mdlayher/netlink"
 	"github.com/ti-mo/netfilter"
+	"golang.org/x/sys/unix"
 )
 
+var ErrConntrackNetlinkNotSupported = errors.New("nf_conntrack_netlink is not supported by the kernel")
+
 func (n *NetLink) FlushConntrack() error {
 	conn, err := netfilter.Dial(nil)
 	if err != nil {
+		if !n.conntrackNetlink {
+			err = fmt.Errorf("%w: %w", err, ErrConntrackNetlinkNotSupported)
+		}
 		return fmt.Errorf("dialing netfilter: %w", err)
 	}
 	defer conn.Close()
 
-	families := [...]netfilter.ProtoFamily{netfilter.ProtoIPv4, netfilter.ProtoIPv6}
+	const ipCtnlMsgCtDelete = netfilter.MessageType(2)
-	for _, family := range families {
+	header := netfilter.Header{
-		const IPCtnlMsgCtDelete = 2
+		SubsystemID: netfilter.NFSubsysCTNetlink,
-		request, err := netfilter.MarshalNetlink(
+		MessageType: ipCtnlMsgCtDelete,
-			netfilter.Header{
+		Family:      unix.AF_UNSPEC,
-				SubsystemID: netfilter.NFSubsysCTNetlink,
+		Flags:       netlink.Request | netlink.Acknowledge,
-				MessageType: netfilter.MessageType(IPCtnlMsgCtDelete),
+	}
-				Family:      family,
+	request, err := netfilter.MarshalNetlink(header, nil)
-				Flags:       netlink.Request | netlink.Acknowledge,
+	if err != nil {
-			},
+		return fmt.Errorf("encoding netlink request: %w", err)
-			nil)
+	}
-		if err != nil {
-			return fmt.Errorf("encoding netlink request: %w", err)
-		}
 
-		_, err = conn.Query(request)
+	_, err = conn.Query(request)
-		if err != nil {
+	if err != nil {
-			return fmt.Errorf("querying netlink request: %w", err)
+		if !n.conntrackNetlink {
+			err = fmt.Errorf("%w: %w", err, ErrConntrackNetlinkNotSupported)
 		}
+		return fmt.Errorf("querying netlink request: %w", err)
 	}
 	return nil
 }

internal/netlink/conntrack_unspecified.go

@@ -2,6 +2,10 @@
 
 package netlink
 
+import "errors"
+
+var ErrConntrackNetlinkNotSupported = errors.New("error not implemented")
+
 func (n *NetLink) FlushConntrack() error {
 	panic("not implemented")
 }

internal/netlink/netlink.go

@@ -1,14 +1,22 @@
 package netlink
 
-import "github.com/qdm12/log"
+import (
+	"github.com/qdm12/gluetun/internal/mod"
+	"github.com/qdm12/log"
+)
 
 type NetLink struct {
 	debugLogger DebugLogger
+
+	// Fixed state
+	conntrackNetlink bool
 }
 
 func New(debugLogger DebugLogger) *NetLink {
+	conntrackNetlink := mod.Probe("nf_conntrack_netlink") == nil
 	return &NetLink{
-		debugLogger: debugLogger,
+		debugLogger:      debugLogger,
+		conntrackNetlink: conntrackNetlink,
 	}
 }
 

internal/pmtud/pmtud.go

@@ -74,7 +74,7 @@ func PathMTUDiscover(ctx context.Context, icmpAddrs []netip.Addr, tcpAddrs []net
 	}
 	mtu, err = tcp.PathMTUDiscover(ctx, tcpAddrs, minMTU, maxPossibleMTU, tryTimeout, fw, logger)
 	if err != nil {
-		if errors.Is(err, iptables.ErrMarkMatchModuleMissing) {
+		if errors.Is(err, iptables.ErrKernelModuleMissing) {
 			logger.Debugf("aborting TCP path MTU discovery: %s", err)
 			if icmpSuccess {
 				return maxPossibleMTU, nil // only rely on ICMP PMTUD results

internal/pmtud/tcp/helpers_test.go

@@ -35,7 +35,7 @@ func getFirewall(t *testing.T) *firewall.Config {
 		noopLogger := &noopLogger{}
 		cmder := command.New()
 		var err error
-		testFirewall, err = firewall.NewConfig(t.Context(), noopLogger, cmder, nil, nil)
+		testFirewall, err = firewall.NewConfig(t.Context(), noopLogger, cmder, nil, nil, nil)
 		if errors.Is(err, iptables.ErrNotSupported) {
 			t.Skip("iptables not installed, skipping TCP PMTUD tests")
 		}

internal/pmtud/tcp/mss.go

@@ -43,7 +43,7 @@ func findHighestMSSDestination(ctx context.Context, familyToFD map[int]fileDescr
 		if result.err != nil {
 			switch {
 			case err != nil: // error already occurred for another findMSS goroutine
-			case errors.Is(result.err, iptables.ErrMarkMatchModuleMissing):
+			case errors.Is(result.err, iptables.ErrKernelModuleMissing):
 				err = fmt.Errorf("finding MSS for %s: %w", result.dst, result.err)
 			case dst.Addr().Is6() && errors.Is(result.err, ip.ErrNetworkUnreachable):
 				// silently discard IPv6 network unreachable errors since they are common