[client] Clamp MSS on outbound traffic (#4735)

client/firewall/create.go

@@ -15,13 +15,13 @@ import (
 )
 
 // NewFirewall creates a firewall manager instance
-func NewFirewall(iface IFaceMapper, _ *statemanager.Manager, flowLogger nftypes.FlowLogger, disableServerRoutes bool) (firewall.Manager, error) {
+func NewFirewall(iface IFaceMapper, _ *statemanager.Manager, flowLogger nftypes.FlowLogger, disableServerRoutes bool, mtu uint16) (firewall.Manager, error) {
 	if !iface.IsUserspaceBind() {
 		return nil, fmt.Errorf("not implemented for this OS: %s", runtime.GOOS)
 	}
 
 	// use userspace packet filtering firewall
-	fm, err := uspfilter.Create(iface, disableServerRoutes, flowLogger)
+	fm, err := uspfilter.Create(iface, disableServerRoutes, flowLogger, mtu)
 	if err != nil {
 		return nil, err
 	}

client/firewall/create_linux.go

@@ -34,12 +34,12 @@ const SKIP_NFTABLES_ENV = "NB_SKIP_NFTABLES_CHECK"
 // FWType is the type for the firewall type
 type FWType int
 
-func NewFirewall(iface IFaceMapper, stateManager *statemanager.Manager, flowLogger nftypes.FlowLogger, disableServerRoutes bool) (firewall.Manager, error) {
+func NewFirewall(iface IFaceMapper, stateManager *statemanager.Manager, flowLogger nftypes.FlowLogger, disableServerRoutes bool, mtu uint16) (firewall.Manager, error) {
 	// on the linux system we try to user nftables or iptables
 	// in any case, because we need to allow netbird interface traffic
 	// so we use AllowNetbird traffic from these firewall managers
 	// for the userspace packet filtering firewall
-	fm, err := createNativeFirewall(iface, stateManager, disableServerRoutes)
+	fm, err := createNativeFirewall(iface, stateManager, disableServerRoutes, mtu)
 
 	if !iface.IsUserspaceBind() {
 		return fm, err
@@ -48,11 +48,11 @@ func NewFirewall(iface IFaceMapper, stateManager *statemanager.Manager, flowLogg
 	if err != nil {
 		log.Warnf("failed to create native firewall: %v. Proceeding with userspace", err)
 	}
-	return createUserspaceFirewall(iface, fm, disableServerRoutes, flowLogger)
+	return createUserspaceFirewall(iface, fm, disableServerRoutes, flowLogger, mtu)
 }
 
-func createNativeFirewall(iface IFaceMapper, stateManager *statemanager.Manager, routes bool) (firewall.Manager, error) {
+func createNativeFirewall(iface IFaceMapper, stateManager *statemanager.Manager, routes bool, mtu uint16) (firewall.Manager, error) {
-	fm, err := createFW(iface)
+	fm, err := createFW(iface, mtu)
 	if err != nil {
 		return nil, fmt.Errorf("create firewall: %s", err)
 	}
@@ -64,26 +64,26 @@ func createNativeFirewall(iface IFaceMapper, stateManager *statemanager.Manager,
 	return fm, nil
 }
 
-func createFW(iface IFaceMapper) (firewall.Manager, error) {
+func createFW(iface IFaceMapper, mtu uint16) (firewall.Manager, error) {
 	switch check() {
 	case IPTABLES:
 		log.Info("creating an iptables firewall manager")
-		return nbiptables.Create(iface)
+		return nbiptables.Create(iface, mtu)
 	case NFTABLES:
 		log.Info("creating an nftables firewall manager")
-		return nbnftables.Create(iface)
+		return nbnftables.Create(iface, mtu)
 	default:
 		log.Info("no firewall manager found, trying to use userspace packet filtering firewall")
 		return nil, errors.New("no firewall manager found")
 	}
 }
 
-func createUserspaceFirewall(iface IFaceMapper, fm firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger) (firewall.Manager, error) {
+func createUserspaceFirewall(iface IFaceMapper, fm firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger, mtu uint16) (firewall.Manager, error) {
 	var errUsp error
 	if fm != nil {
-		fm, errUsp = uspfilter.CreateWithNativeFirewall(iface, fm, disableServerRoutes, flowLogger)
+		fm, errUsp = uspfilter.CreateWithNativeFirewall(iface, fm, disableServerRoutes, flowLogger, mtu)
 	} else {
-		fm, errUsp = uspfilter.Create(iface, disableServerRoutes, flowLogger)
+		fm, errUsp = uspfilter.Create(iface, disableServerRoutes, flowLogger, mtu)
 	}
 
 	if errUsp != nil {

client/firewall/iptables/manager_linux.go

@@ -36,7 +36,7 @@ type iFaceMapper interface {
 }
 
 // Create iptables firewall manager
-func Create(wgIface iFaceMapper) (*Manager, error) {
+func Create(wgIface iFaceMapper, mtu uint16) (*Manager, error) {
 	iptablesClient, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 	if err != nil {
 		return nil, fmt.Errorf("init iptables: %w", err)
@@ -47,7 +47,7 @@ func Create(wgIface iFaceMapper) (*Manager, error) {
 		ipv4Client: iptablesClient,
 	}
 
-	m.router, err = newRouter(iptablesClient, wgIface)
+	m.router, err = newRouter(iptablesClient, wgIface, mtu)
 	if err != nil {
 		return nil, fmt.Errorf("create router: %w", err)
 	}
@@ -66,6 +66,7 @@ func (m *Manager) Init(stateManager *statemanager.Manager) error {
 			NameStr:       m.wgIface.Name(),
 			WGAddress:     m.wgIface.Address(),
 			UserspaceBind: m.wgIface.IsUserspaceBind(),
+			MTU:           m.router.mtu,
 		},
 	}
 	stateManager.RegisterState(state)

client/firewall/iptables/manager_linux_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 )
 
@@ -53,7 +54,7 @@ func TestIptablesManager(t *testing.T) {
 	require.NoError(t, err)
 
 	// just check on the local interface
-	manager, err := Create(ifaceMock)
+	manager, err := Create(ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NoError(t, manager.Init(nil))
 
@@ -114,7 +115,7 @@ func TestIptablesManagerDenyRules(t *testing.T) {
 	ipv4Client, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 	require.NoError(t, err)
 
-	manager, err := Create(ifaceMock)
+	manager, err := Create(ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NoError(t, manager.Init(nil))
 
@@ -198,7 +199,7 @@ func TestIptablesManagerIPSet(t *testing.T) {
 	}
 
 	// just check on the local interface
-	manager, err := Create(mock)
+	manager, err := Create(mock, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NoError(t, manager.Init(nil))
 
@@ -264,7 +265,7 @@ func TestIptablesCreatePerformance(t *testing.T) {
 	for _, testMax := range []int{10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000} {
 		t.Run(fmt.Sprintf("Testing %d rules", testMax), func(t *testing.T) {
 			// just check on the local interface
-			manager, err := Create(mock)
+			manager, err := Create(mock, iface.DefaultMTU)
 			require.NoError(t, err)
 			require.NoError(t, manager.Init(nil))
 			time.Sleep(time.Second)

client/firewall/iptables/router_linux.go

@@ -30,17 +30,20 @@ const (
 
 	chainPOSTROUTING        = "POSTROUTING"
 	chainPREROUTING         = "PREROUTING"
+	chainFORWARD            = "FORWARD"
 	chainRTNAT              = "NETBIRD-RT-NAT"
 	chainRTFWDIN            = "NETBIRD-RT-FWD-IN"
 	chainRTFWDOUT           = "NETBIRD-RT-FWD-OUT"
 	chainRTPRE              = "NETBIRD-RT-PRE"
 	chainRTRDR              = "NETBIRD-RT-RDR"
+	chainRTMSSCLAMP         = "NETBIRD-RT-MSSCLAMP"
 	routingFinalForwardJump = "ACCEPT"
 	routingFinalNatJump     = "MASQUERADE"
 
 	jumpManglePre  = "jump-mangle-pre"
 	jumpNatPre     = "jump-nat-pre"
 	jumpNatPost    = "jump-nat-post"
+	jumpMSSClamp   = "jump-mss-clamp"
 	markManglePre  = "mark-mangle-pre"
 	markManglePost = "mark-mangle-post"
 	matchSet       = "--match-set"
@@ -48,6 +51,9 @@ const (
 	dnatSuffix = "_dnat"
 	snatSuffix = "_snat"
 	fwdSuffix  = "_fwd"
+
+	// ipTCPHeaderMinSize represents minimum IP (20) + TCP (20) header size for MSS calculation
+	ipTCPHeaderMinSize = 40
 )
 
 type ruleInfo struct {
@@ -77,16 +83,18 @@ type router struct {
 	ipsetCounter     *ipsetCounter
 	wgIface          iFaceMapper
 	legacyManagement bool
+	mtu              uint16
 
 	stateManager *statemanager.Manager
 	ipFwdState   *ipfwdstate.IPForwardingState
 }
 
-func newRouter(iptablesClient *iptables.IPTables, wgIface iFaceMapper) (*router, error) {
+func newRouter(iptablesClient *iptables.IPTables, wgIface iFaceMapper, mtu uint16) (*router, error) {
 	r := &router{
 		iptablesClient: iptablesClient,
 		rules:          make(map[string][]string),
 		wgIface:        wgIface,
+		mtu:            mtu,
 		ipFwdState:     ipfwdstate.NewIPForwardingState(),
 	}
 
@@ -392,6 +400,7 @@ func (r *router) cleanUpDefaultForwardRules() error {
 		{chainRTPRE, tableMangle},
 		{chainRTNAT, tableNat},
 		{chainRTRDR, tableNat},
+		{chainRTMSSCLAMP, tableMangle},
 	} {
 		ok, err := r.iptablesClient.ChainExists(chainInfo.table, chainInfo.chain)
 		if err != nil {
@@ -416,6 +425,7 @@ func (r *router) createContainers() error {
 		{chainRTPRE, tableMangle},
 		{chainRTNAT, tableNat},
 		{chainRTRDR, tableNat},
+		{chainRTMSSCLAMP, tableMangle},
 	} {
 		if err := r.iptablesClient.NewChain(chainInfo.table, chainInfo.chain); err != nil {
 			return fmt.Errorf("create chain %s in table %s: %w", chainInfo.chain, chainInfo.table, err)
@@ -438,6 +448,10 @@ func (r *router) createContainers() error {
 		return fmt.Errorf("add jump rules: %w", err)
 	}
 
+	if err := r.addMSSClampingRules(); err != nil {
+		log.Errorf("failed to add MSS clamping rules: %s", err)
+	}
+
 	return nil
 }
 
@@ -518,6 +532,35 @@ func (r *router) addPostroutingRules() error {
 	return nil
 }
 
+// addMSSClampingRules adds MSS clamping rules to prevent fragmentation for forwarded traffic.
+// TODO: Add IPv6 support
+func (r *router) addMSSClampingRules() error {
+	mss := r.mtu - ipTCPHeaderMinSize
+
+	// Add jump rule from FORWARD chain in mangle table to our custom chain
+	jumpRule := []string{
+		"-j", chainRTMSSCLAMP,
+	}
+	if err := r.iptablesClient.Insert(tableMangle, chainFORWARD, 1, jumpRule...); err != nil {
+		return fmt.Errorf("add jump to MSS clamp chain: %w", err)
+	}
+	r.rules[jumpMSSClamp] = jumpRule
+
+	ruleOut := []string{
+		"-o", r.wgIface.Name(),
+		"-p", "tcp",
+		"--tcp-flags", "SYN,RST", "SYN",
+		"-j", "TCPMSS",
+		"--set-mss", fmt.Sprintf("%d", mss),
+	}
+	if err := r.iptablesClient.Append(tableMangle, chainRTMSSCLAMP, ruleOut...); err != nil {
+		return fmt.Errorf("add outbound MSS clamp rule: %w", err)
+	}
+	r.rules["mss-clamp-out"] = ruleOut
+
+	return nil
+}
+
 func (r *router) insertEstablishedRule(chain string) error {
 	establishedRule := getConntrackEstablished()
 
@@ -558,7 +601,7 @@ func (r *router) addJumpRules() error {
 }
 
 func (r *router) cleanJumpRules() error {
-	for _, ruleKey := range []string{jumpNatPost, jumpManglePre, jumpNatPre} {
+	for _, ruleKey := range []string{jumpNatPost, jumpManglePre, jumpNatPre, jumpMSSClamp} {
 		if rule, exists := r.rules[ruleKey]; exists {
 			var table, chain string
 			switch ruleKey {
@@ -571,6 +614,9 @@ func (r *router) cleanJumpRules() error {
 			case jumpNatPre:
 				table = tableNat
 				chain = chainPREROUTING
+			case jumpMSSClamp:
+				table = tableMangle
+				chain = chainFORWARD
 			default:
 				return fmt.Errorf("unknown jump rule: %s", ruleKey)
 			}

client/firewall/iptables/router_linux_test.go

@@ -14,6 +14,7 @@ import (
 
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/firewall/test"
+	"github.com/netbirdio/netbird/client/iface"
 	nbnet "github.com/netbirdio/netbird/client/net"
 )
 
@@ -30,7 +31,7 @@ func TestIptablesManager_RestoreOrCreateContainers(t *testing.T) {
 	iptablesClient, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 	require.NoError(t, err, "failed to init iptables client")
 
-	manager, err := newRouter(iptablesClient, ifaceMock)
+	manager, err := newRouter(iptablesClient, ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err, "should return a valid iptables manager")
 	require.NoError(t, manager.init(nil))
 
@@ -38,7 +39,6 @@ func TestIptablesManager_RestoreOrCreateContainers(t *testing.T) {
 		assert.NoError(t, manager.Reset(), "shouldn't return error")
 	}()
 
-	// Now 5 rules:
 	// 1. established rule forward in
 	// 2. estbalished rule forward out
 	// 3. jump rule to POST nat chain
@@ -48,7 +48,9 @@ func TestIptablesManager_RestoreOrCreateContainers(t *testing.T) {
 	// 7. static return masquerade rule
 	// 8. mangle prerouting mark rule
 	// 9. mangle postrouting mark rule
-	require.Len(t, manager.rules, 9, "should have created rules map")
+	// 10. jump rule to MSS clamping chain
+	// 11. MSS clamping rule for outbound traffic
+	require.Len(t, manager.rules, 11, "should have created rules map")
 
 	exists, err := manager.iptablesClient.Exists(tableNat, chainPOSTROUTING, "-j", chainRTNAT)
 	require.NoError(t, err, "should be able to query the iptables %s table and %s chain", tableNat, chainPOSTROUTING)
@@ -82,7 +84,7 @@ func TestIptablesManager_AddNatRule(t *testing.T) {
 			iptablesClient, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 			require.NoError(t, err, "failed to init iptables client")
 
-			manager, err := newRouter(iptablesClient, ifaceMock)
+			manager, err := newRouter(iptablesClient, ifaceMock, iface.DefaultMTU)
 			require.NoError(t, err, "shouldn't return error")
 			require.NoError(t, manager.init(nil))
 
@@ -155,7 +157,7 @@ func TestIptablesManager_RemoveNatRule(t *testing.T) {
 		t.Run(testCase.Name, func(t *testing.T) {
 			iptablesClient, _ := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 
-			manager, err := newRouter(iptablesClient, ifaceMock)
+			manager, err := newRouter(iptablesClient, ifaceMock, iface.DefaultMTU)
 			require.NoError(t, err, "shouldn't return error")
 			require.NoError(t, manager.init(nil))
 			defer func() {
@@ -217,7 +219,7 @@ func TestRouter_AddRouteFiltering(t *testing.T) {
 	iptablesClient, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 	require.NoError(t, err, "Failed to create iptables client")
 
-	r, err := newRouter(iptablesClient, ifaceMock)
+	r, err := newRouter(iptablesClient, ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err, "Failed to create router manager")
 	require.NoError(t, r.init(nil))
 

client/firewall/iptables/state_linux.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 )
 
@@ -11,6 +12,7 @@ type InterfaceState struct {
 	NameStr       string         `json:"name"`
 	WGAddress     wgaddr.Address `json:"wg_address"`
 	UserspaceBind bool           `json:"userspace_bind"`
+	MTU           uint16         `json:"mtu"`
 }
 
 func (i *InterfaceState) Name() string {
@@ -42,7 +44,11 @@ func (s *ShutdownState) Name() string {
 }
 
 func (s *ShutdownState) Cleanup() error {
-	ipt, err := Create(s.InterfaceState)
+	mtu := s.InterfaceState.MTU
+	if mtu == 0 {
+		mtu = iface.DefaultMTU
+	}
+	ipt, err := Create(s.InterfaceState, mtu)
 	if err != nil {
 		return fmt.Errorf("create iptables manager: %w", err)
 	}

client/firewall/nftables/manager_linux.go

@@ -44,7 +44,7 @@ type Manager struct {
 }
 
 // Create nftables firewall manager
-func Create(wgIface iFaceMapper) (*Manager, error) {
+func Create(wgIface iFaceMapper, mtu uint16) (*Manager, error) {
 	m := &Manager{
 		rConn:   &nftables.Conn{},
 		wgIface: wgIface,
@@ -53,7 +53,7 @@ func Create(wgIface iFaceMapper) (*Manager, error) {
 	workTable := &nftables.Table{Name: tableNameNetbird, Family: nftables.TableFamilyIPv4}
 
 	var err error
-	m.router, err = newRouter(workTable, wgIface)
+	m.router, err = newRouter(workTable, wgIface, mtu)
 	if err != nil {
 		return nil, fmt.Errorf("create router: %w", err)
 	}
@@ -93,6 +93,7 @@ func (m *Manager) Init(stateManager *statemanager.Manager) error {
 			NameStr:       m.wgIface.Name(),
 			WGAddress:     m.wgIface.Address(),
 			UserspaceBind: m.wgIface.IsUserspaceBind(),
+			MTU:           m.router.mtu,
 		},
 	}); err != nil {
 		log.Errorf("failed to update state: %v", err)

client/firewall/nftables/manager_linux_test.go

@@ -16,6 +16,7 @@ import (
 	"golang.org/x/sys/unix"
 
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 )
 
@@ -56,7 +57,7 @@ func (i *iFaceMock) IsUserspaceBind() bool { return false }
 func TestNftablesManager(t *testing.T) {
 
 	// just check on the local interface
-	manager, err := Create(ifaceMock)
+	manager, err := Create(ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NoError(t, manager.Init(nil))
 	time.Sleep(time.Second * 3)
@@ -168,7 +169,7 @@ func TestNftablesManager(t *testing.T) {
 func TestNftablesManagerRuleOrder(t *testing.T) {
 	// This test verifies rule insertion order in nftables peer ACLs
 	// We add accept rule first, then deny rule to test ordering behavior
-	manager, err := Create(ifaceMock)
+	manager, err := Create(ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NoError(t, manager.Init(nil))
 
@@ -261,7 +262,7 @@ func TestNFtablesCreatePerformance(t *testing.T) {
 	for _, testMax := range []int{10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000} {
 		t.Run(fmt.Sprintf("Testing %d rules", testMax), func(t *testing.T) {
 			// just check on the local interface
-			manager, err := Create(mock)
+			manager, err := Create(mock, iface.DefaultMTU)
 			require.NoError(t, err)
 			require.NoError(t, manager.Init(nil))
 			time.Sleep(time.Second * 3)
@@ -345,7 +346,7 @@ func TestNftablesManagerCompatibilityWithIptables(t *testing.T) {
 	stdout, stderr := runIptablesSave(t)
 	verifyIptablesOutput(t, stdout, stderr)
 
-	manager, err := Create(ifaceMock)
+	manager, err := Create(ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err, "failed to create manager")
 	require.NoError(t, manager.Init(nil))
 

client/firewall/nftables/router_linux.go

@@ -16,6 +16,7 @@ import (
 	"github.com/google/nftables/xt"
 	"github.com/hashicorp/go-multierror"
 	log "github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 
 	nberrors "github.com/netbirdio/netbird/client/errors"
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
@@ -32,12 +33,16 @@ const (
 	chainNameRoutingNat    = "netbird-rt-postrouting"
 	chainNameRoutingRdr    = "netbird-rt-redirect"
 	chainNameForward       = "FORWARD"
+	chainNameMangleForward = "netbird-mangle-forward"
 
 	userDataAcceptForwardRuleIif = "frwacceptiif"
 	userDataAcceptForwardRuleOif = "frwacceptoif"
 
 	dnatSuffix = "_dnat"
 	snatSuffix = "_snat"
+
+	// ipTCPHeaderMinSize represents minimum IP (20) + TCP (20) header size for MSS calculation
+	ipTCPHeaderMinSize = 40
 )
 
 const refreshRulesMapError = "refresh rules map: %w"
@@ -63,9 +68,10 @@ type router struct {
 	wgIface          iFaceMapper
 	ipFwdState       *ipfwdstate.IPForwardingState
 	legacyManagement bool
+	mtu              uint16
 }
 
-func newRouter(workTable *nftables.Table, wgIface iFaceMapper) (*router, error) {
+func newRouter(workTable *nftables.Table, wgIface iFaceMapper, mtu uint16) (*router, error) {
 	r := &router{
 		conn:       &nftables.Conn{},
 		workTable:  workTable,
@@ -73,6 +79,7 @@ func newRouter(workTable *nftables.Table, wgIface iFaceMapper) (*router, error)
 		rules:      make(map[string]*nftables.Rule),
 		wgIface:    wgIface,
 		ipFwdState: ipfwdstate.NewIPForwardingState(),
+		mtu:        mtu,
 	}
 
 	r.ipsetCounter = refcounter.New(
@@ -220,11 +227,23 @@ func (r *router) createContainers() error {
 		Type:     nftables.ChainTypeFilter,
 	})
 
+	r.chains[chainNameMangleForward] = r.conn.AddChain(&nftables.Chain{
+		Name:     chainNameMangleForward,
+		Table:    r.workTable,
+		Hooknum:  nftables.ChainHookForward,
+		Priority: nftables.ChainPriorityMangle,
+		Type:     nftables.ChainTypeFilter,
+	})
+
 	// Add the single NAT rule that matches on mark
 	if err := r.addPostroutingRules(); err != nil {
 		return fmt.Errorf("add single nat rule: %v", err)
 	}
 
+	if err := r.addMSSClampingRules(); err != nil {
+		log.Errorf("failed to add MSS clamping rules: %s", err)
+	}
+
 	if err := r.acceptForwardRules(); err != nil {
 		log.Errorf("failed to add accept rules for the forward chain: %s", err)
 	}
@@ -745,6 +764,83 @@ func (r *router) addPostroutingRules() error {
 	return nil
 }
 
+// addMSSClampingRules adds MSS clamping rules to prevent fragmentation for forwarded traffic.
+// TODO: Add IPv6 support
+func (r *router) addMSSClampingRules() error {
+	mss := r.mtu - ipTCPHeaderMinSize
+
+	exprsOut := []expr.Any{
+		&expr.Meta{
+			Key:      expr.MetaKeyOIFNAME,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     ifname(r.wgIface.Name()),
+		},
+		&expr.Meta{
+			Key:      expr.MetaKeyL4PROTO,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     []byte{unix.IPPROTO_TCP},
+		},
+		&expr.Payload{
+			DestRegister: 1,
+			Base:         expr.PayloadBaseTransportHeader,
+			Offset:       13,
+			Len:          1,
+		},
+		&expr.Bitwise{
+			DestRegister:   1,
+			SourceRegister: 1,
+			Len:            1,
+			Mask:           []byte{0x02},
+			Xor:            []byte{0x00},
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpNeq,
+			Register: 1,
+			Data:     []byte{0x00},
+		},
+		&expr.Counter{},
+		&expr.Exthdr{
+			DestRegister: 1,
+			Type:         2,
+			Offset:       2,
+			Len:          2,
+			Op:           expr.ExthdrOpTcpopt,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpGt,
+			Register: 1,
+			Data:     binaryutil.BigEndian.PutUint16(uint16(mss)),
+		},
+		&expr.Immediate{
+			Register: 1,
+			Data:     binaryutil.BigEndian.PutUint16(uint16(mss)),
+		},
+		&expr.Exthdr{
+			SourceRegister: 1,
+			Type:           2,
+			Offset:         2,
+			Len:            2,
+			Op:             expr.ExthdrOpTcpopt,
+		},
+	}
+
+	r.conn.AddRule(&nftables.Rule{
+		Table: r.workTable,
+		Chain: r.chains[chainNameMangleForward],
+		Exprs: exprsOut,
+	})
+
+	return nil
+}
+
 // addLegacyRouteRule adds a legacy routing rule for mgmt servers pre route acls
 func (r *router) addLegacyRouteRule(pair firewall.RouterPair) error {
 	sourceExp, err := r.applyNetwork(pair.Source, nil, true)

client/firewall/nftables/router_linux_test.go

@@ -17,6 +17,7 @@ import (
 
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/firewall/test"
+	"github.com/netbirdio/netbird/client/iface"
 )
 
 const (
@@ -36,7 +37,7 @@ func TestNftablesManager_AddNatRule(t *testing.T) {
 	for _, testCase := range test.InsertRuleTestCases {
 		t.Run(testCase.Name, func(t *testing.T) {
 			// need fw manager to init both acl mgr and router for all chains to be present
-			manager, err := Create(ifaceMock)
+			manager, err := Create(ifaceMock, iface.DefaultMTU)
 			t.Cleanup(func() {
 				require.NoError(t, manager.Close(nil))
 			})
@@ -125,7 +126,7 @@ func TestNftablesManager_RemoveNatRule(t *testing.T) {
 
 	for _, testCase := range test.RemoveRuleTestCases {
 		t.Run(testCase.Name, func(t *testing.T) {
-			manager, err := Create(ifaceMock)
+			manager, err := Create(ifaceMock, iface.DefaultMTU)
 			t.Cleanup(func() {
 				require.NoError(t, manager.Close(nil))
 			})
@@ -197,7 +198,7 @@ func TestRouter_AddRouteFiltering(t *testing.T) {
 
 	defer deleteWorkTable()
 
-	r, err := newRouter(workTable, ifaceMock)
+	r, err := newRouter(workTable, ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err, "Failed to create router")
 	require.NoError(t, r.init(workTable))
 
@@ -364,7 +365,7 @@ func TestNftablesCreateIpSet(t *testing.T) {
 
 	defer deleteWorkTable()
 
-	r, err := newRouter(workTable, ifaceMock)
+	r, err := newRouter(workTable, ifaceMock, iface.DefaultMTU)
 	require.NoError(t, err, "Failed to create router")
 	require.NoError(t, r.init(workTable))
 

client/firewall/nftables/state_linux.go

@@ -3,6 +3,7 @@ package nftables
 import (
 	"fmt"
 
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 )
 
@@ -10,6 +11,7 @@ type InterfaceState struct {
 	NameStr       string         `json:"name"`
 	WGAddress     wgaddr.Address `json:"wg_address"`
 	UserspaceBind bool           `json:"userspace_bind"`
+	MTU           uint16         `json:"mtu"`
 }
 
 func (i *InterfaceState) Name() string {
@@ -33,7 +35,11 @@ func (s *ShutdownState) Name() string {
 }
 
 func (s *ShutdownState) Cleanup() error {
-	nft, err := Create(s.InterfaceState)
+	mtu := s.InterfaceState.MTU
+	if mtu == 0 {
+		mtu = iface.DefaultMTU
+	}
+	nft, err := Create(s.InterfaceState, mtu)
 	if err != nil {
 		return fmt.Errorf("create nftables manager: %w", err)
 	}

client/firewall/uspfilter/filter.go

@@ -1,6 +1,7 @@
 package uspfilter
 
 import (
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"net"
@@ -27,7 +28,12 @@ import (
 	"github.com/netbirdio/netbird/client/internal/statemanager"
 )
 
-const layerTypeAll = 0
+const (
+	layerTypeAll = 0
+
+	// ipTCPHeaderMinSize represents minimum IP (20) + TCP (20) header size for MSS calculation
+	ipTCPHeaderMinSize = 40
+)
 
 const (
 	// EnvDisableConntrack disables the stateful filter, replies to outbound traffic won't be allowed.
@@ -36,6 +42,9 @@ const (
 	// EnvDisableUserspaceRouting disables userspace routing, to-be-routed packets will be dropped.
 	EnvDisableUserspaceRouting = "NB_DISABLE_USERSPACE_ROUTING"
 
+	// EnvDisableMSSClamping disables TCP MSS clamping for forwarded traffic.
+	EnvDisableMSSClamping = "NB_DISABLE_MSS_CLAMPING"
+
 	// EnvForceUserspaceRouter forces userspace routing even if native routing is available.
 	EnvForceUserspaceRouter = "NB_FORCE_USERSPACE_ROUTER"
 
@@ -122,6 +131,10 @@ type Manager struct {
 
 	netstackServices     map[serviceKey]struct{}
 	netstackServiceMutex sync.RWMutex
+
+	mtu             uint16
+	mssClampValue   uint16
+	mssClampEnabled bool
 }
 
 // decoder for packages
@@ -140,16 +153,16 @@ type decoder struct {
 }
 
 // Create userspace firewall manager constructor
-func Create(iface common.IFaceMapper, disableServerRoutes bool, flowLogger nftypes.FlowLogger) (*Manager, error) {
+func Create(iface common.IFaceMapper, disableServerRoutes bool, flowLogger nftypes.FlowLogger, mtu uint16) (*Manager, error) {
-	return create(iface, nil, disableServerRoutes, flowLogger)
+	return create(iface, nil, disableServerRoutes, flowLogger, mtu)
 }
 
-func CreateWithNativeFirewall(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger) (*Manager, error) {
+func CreateWithNativeFirewall(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger, mtu uint16) (*Manager, error) {
 	if nativeFirewall == nil {
 		return nil, errors.New("native firewall is nil")
 	}
 
-	mgr, err := create(iface, nativeFirewall, disableServerRoutes, flowLogger)
+	mgr, err := create(iface, nativeFirewall, disableServerRoutes, flowLogger, mtu)
 	if err != nil {
 		return nil, err
 	}
@@ -157,8 +170,8 @@ func CreateWithNativeFirewall(iface common.IFaceMapper, nativeFirewall firewall.
 	return mgr, nil
 }
 
-func parseCreateEnv() (bool, bool) {
+func parseCreateEnv() (bool, bool, bool) {
-	var disableConntrack, enableLocalForwarding bool
+	var disableConntrack, enableLocalForwarding, disableMSSClamping bool
 	var err error
 	if val := os.Getenv(EnvDisableConntrack); val != "" {
 		disableConntrack, err = strconv.ParseBool(val)
@@ -177,12 +190,18 @@ func parseCreateEnv() (bool, bool) {
 			log.Warnf("failed to parse %s: %v", EnvEnableLocalForwarding, err)
 		}
 	}
+	if val := os.Getenv(EnvDisableMSSClamping); val != "" {
+		disableMSSClamping, err = strconv.ParseBool(val)
+		if err != nil {
+			log.Warnf("failed to parse %s: %v", EnvDisableMSSClamping, err)
+		}
+	}
 
-	return disableConntrack, enableLocalForwarding
+	return disableConntrack, enableLocalForwarding, disableMSSClamping
 }
 
-func create(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger) (*Manager, error) {
+func create(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableServerRoutes bool, flowLogger nftypes.FlowLogger, mtu uint16) (*Manager, error) {
-	disableConntrack, enableLocalForwarding := parseCreateEnv()
+	disableConntrack, enableLocalForwarding, disableMSSClamping := parseCreateEnv()
 
 	m := &Manager{
 		decoders: sync.Pool{
@@ -213,13 +232,17 @@ func create(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableSe
 		dnatMappings:        make(map[netip.Addr]netip.Addr),
 		portDNATRules:       []portDNATRule{},
 		netstackServices:    make(map[serviceKey]struct{}),
+		mtu:                 mtu,
 	}
 	m.routingEnabled.Store(false)
 
+	if !disableMSSClamping {
+		m.mssClampEnabled = true
+		m.mssClampValue = mtu - ipTCPHeaderMinSize
+	}
 	if err := m.localipmanager.UpdateLocalIPs(iface); err != nil {
 		return nil, fmt.Errorf("update local IPs: %w", err)
 	}
-
 	if disableConntrack {
 		log.Info("conntrack is disabled")
 	} else {
@@ -227,14 +250,11 @@ func create(iface common.IFaceMapper, nativeFirewall firewall.Manager, disableSe
 		m.icmpTracker = conntrack.NewICMPTracker(conntrack.DefaultICMPTimeout, m.logger, flowLogger)
 		m.tcpTracker = conntrack.NewTCPTracker(conntrack.DefaultTCPTimeout, m.logger, flowLogger)
 	}
-
-	// netstack needs the forwarder for local traffic
 	if m.netstack && m.localForwarding {
 		if err := m.initForwarder(); err != nil {
 			log.Errorf("failed to initialize forwarder: %v", err)
 		}
 	}
-
 	if err := iface.SetFilter(m); err != nil {
 		return nil, fmt.Errorf("set filter: %w", err)
 	}
@@ -337,7 +357,7 @@ func (m *Manager) initForwarder() error {
 		return errors.New("forwarding not supported")
 	}
 
-	forwarder, err := forwarder.New(m.wgIface, m.logger, m.flowLogger, m.netstack)
+	forwarder, err := forwarder.New(m.wgIface, m.logger, m.flowLogger, m.netstack, m.mtu)
 	if err != nil {
 		m.routingEnabled.Store(false)
 		return fmt.Errorf("create forwarder: %w", err)
@@ -643,8 +663,17 @@ func (m *Manager) filterOutbound(packetData []byte, size int) bool {
 		return false
 	}
 
-	if d.decoded[1] == layers.LayerTypeUDP && m.udpHooksDrop(uint16(d.udp.DstPort), dstIP, packetData) {
+	switch d.decoded[1] {
-		return true
+	case layers.LayerTypeUDP:
+		if m.udpHooksDrop(uint16(d.udp.DstPort), dstIP, packetData) {
+			return true
+		}
+	case layers.LayerTypeTCP:
+		// Clamp MSS on all TCP SYN packets, including those from local IPs.
+		// SNATed routed traffic may appear as local IP but still requires clamping.
+		if m.mssClampEnabled {
+			m.clampTCPMSS(packetData, d)
+		}
 	}
 
 	m.trackOutbound(d, srcIP, dstIP, packetData, size)
@@ -691,6 +720,97 @@ func getTCPFlags(tcp *layers.TCP) uint8 {
 	return flags
 }
 
+// clampTCPMSS clamps the TCP MSS option in SYN and SYN-ACK packets to prevent fragmentation.
+// Both sides advertise their MSS during connection establishment, so we need to clamp both.
+func (m *Manager) clampTCPMSS(packetData []byte, d *decoder) bool {
+	if !d.tcp.SYN {
+		return false
+	}
+	if len(d.tcp.Options) == 0 {
+		return false
+	}
+
+	mssOptionIndex := -1
+	var currentMSS uint16
+	for i, opt := range d.tcp.Options {
+		if opt.OptionType == layers.TCPOptionKindMSS && len(opt.OptionData) == 2 {
+			currentMSS = binary.BigEndian.Uint16(opt.OptionData)
+			if currentMSS > m.mssClampValue {
+				mssOptionIndex = i
+				break
+			}
+		}
+	}
+
+	if mssOptionIndex == -1 {
+		return false
+	}
+
+	ipHeaderSize := int(d.ip4.IHL) * 4
+	if ipHeaderSize < 20 {
+		return false
+	}
+
+	if !m.updateMSSOption(packetData, d, mssOptionIndex, ipHeaderSize) {
+		return false
+	}
+
+	m.logger.Trace2("Clamped TCP MSS from %d to %d", currentMSS, m.mssClampValue)
+	return true
+}
+
+func (m *Manager) updateMSSOption(packetData []byte, d *decoder, mssOptionIndex, ipHeaderSize int) bool {
+	tcpHeaderStart := ipHeaderSize
+	tcpOptionsStart := tcpHeaderStart + 20
+
+	optOffset := tcpOptionsStart
+	for j := 0; j < mssOptionIndex; j++ {
+		switch d.tcp.Options[j].OptionType {
+		case layers.TCPOptionKindEndList, layers.TCPOptionKindNop:
+			optOffset++
+		default:
+			optOffset += 2 + len(d.tcp.Options[j].OptionData)
+		}
+	}
+
+	mssValueOffset := optOffset + 2
+	binary.BigEndian.PutUint16(packetData[mssValueOffset:mssValueOffset+2], m.mssClampValue)
+
+	m.recalculateTCPChecksum(packetData, d, tcpHeaderStart)
+	return true
+}
+
+func (m *Manager) recalculateTCPChecksum(packetData []byte, d *decoder, tcpHeaderStart int) {
+	tcpLayer := packetData[tcpHeaderStart:]
+	tcpLength := len(packetData) - tcpHeaderStart
+
+	tcpLayer[16] = 0
+	tcpLayer[17] = 0
+
+	var pseudoSum uint32
+	pseudoSum += uint32(d.ip4.SrcIP[0])<<8 | uint32(d.ip4.SrcIP[1])
+	pseudoSum += uint32(d.ip4.SrcIP[2])<<8 | uint32(d.ip4.SrcIP[3])
+	pseudoSum += uint32(d.ip4.DstIP[0])<<8 | uint32(d.ip4.DstIP[1])
+	pseudoSum += uint32(d.ip4.DstIP[2])<<8 | uint32(d.ip4.DstIP[3])
+	pseudoSum += uint32(d.ip4.Protocol)
+	pseudoSum += uint32(tcpLength)
+
+	var sum uint32 = pseudoSum
+	for i := 0; i < tcpLength-1; i += 2 {
+		sum += uint32(tcpLayer[i])<<8 | uint32(tcpLayer[i+1])
+	}
+	if tcpLength%2 == 1 {
+		sum += uint32(tcpLayer[tcpLength-1]) << 8
+	}
+
+	for sum > 0xFFFF {
+		sum = (sum & 0xFFFF) + (sum >> 16)
+	}
+
+	checksum := ^uint16(sum)
+	binary.BigEndian.PutUint16(tcpLayer[16:18], checksum)
+}
+
 func (m *Manager) trackOutbound(d *decoder, srcIP, dstIP netip.Addr, packetData []byte, size int) {
 	transport := d.decoded[1]
 	switch transport {

client/firewall/uspfilter/filter_bench_test.go

@@ -17,6 +17,7 @@ import (
 
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/firewall/uspfilter/conntrack"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 )
 
@@ -169,7 +170,7 @@ func BenchmarkCoreFiltering(b *testing.B) {
 				// Create manager and basic setup
 				manager, _ := Create(&IFaceMock{
 					SetFilterFunc: func(device.PacketFilter) error { return nil },
-				}, false, flowLogger)
+				}, false, flowLogger, iface.DefaultMTU)
 				defer b.Cleanup(func() {
 					require.NoError(b, manager.Close(nil))
 				})
@@ -209,7 +210,7 @@ func BenchmarkStateScaling(b *testing.B) {
 		b.Run(fmt.Sprintf("conns_%d", count), func(b *testing.B) {
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -252,7 +253,7 @@ func BenchmarkEstablishmentOverhead(b *testing.B) {
 		b.Run(sc.name, func(b *testing.B) {
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -410,7 +411,7 @@ func BenchmarkRoutedNetworkReturn(b *testing.B) {
 		b.Run(sc.name, func(b *testing.B) {
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -537,7 +538,7 @@ func BenchmarkLongLivedConnections(b *testing.B) {
 
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			defer b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -620,7 +621,7 @@ func BenchmarkShortLivedConnections(b *testing.B) {
 
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			defer b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -731,7 +732,7 @@ func BenchmarkParallelLongLivedConnections(b *testing.B) {
 
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			defer b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -811,7 +812,7 @@ func BenchmarkParallelShortLivedConnections(b *testing.B) {
 
 			manager, _ := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			defer b.Cleanup(func() {
 				require.NoError(b, manager.Close(nil))
 			})
@@ -896,38 +897,6 @@ func BenchmarkParallelShortLivedConnections(b *testing.B) {
 	}
 }
 
-// generateTCPPacketWithFlags creates a TCP packet with specific flags
-func generateTCPPacketWithFlags(b *testing.B, srcIP, dstIP net.IP, srcPort, dstPort, flags uint16) []byte {
-	b.Helper()
-
-	ipv4 := &layers.IPv4{
-		TTL:      64,
-		Version:  4,
-		SrcIP:    srcIP,
-		DstIP:    dstIP,
-		Protocol: layers.IPProtocolTCP,
-	}
-
-	tcp := &layers.TCP{
-		SrcPort: layers.TCPPort(srcPort),
-		DstPort: layers.TCPPort(dstPort),
-	}
-
-	// Set TCP flags
-	tcp.SYN = (flags & uint16(conntrack.TCPSyn)) != 0
-	tcp.ACK = (flags & uint16(conntrack.TCPAck)) != 0
-	tcp.PSH = (flags & uint16(conntrack.TCPPush)) != 0
-	tcp.RST = (flags & uint16(conntrack.TCPRst)) != 0
-	tcp.FIN = (flags & uint16(conntrack.TCPFin)) != 0
-
-	require.NoError(b, tcp.SetNetworkLayerForChecksum(ipv4))
-
-	buf := gopacket.NewSerializeBuffer()
-	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
-	require.NoError(b, gopacket.SerializeLayers(buf, opts, ipv4, tcp, gopacket.Payload("test")))
-	return buf.Bytes()
-}
-
 func BenchmarkRouteACLs(b *testing.B) {
 	manager := setupRoutedManager(b, "10.10.0.100/16")
 
@@ -990,3 +959,231 @@ func BenchmarkRouteACLs(b *testing.B) {
 		}
 	}
 }
+
+// BenchmarkMSSClamping benchmarks the MSS clamping impact on filterOutbound.
+// This shows the overhead difference between the common case (non-SYN packets, fast path)
+// and the rare case (SYN packets that need clamping, expensive path).
+func BenchmarkMSSClamping(b *testing.B) {
+	scenarios := []struct {
+		name        string
+		description string
+		genPacket   func(*testing.B, net.IP, net.IP) []byte
+		frequency   string
+	}{
+		{
+			name:        "syn_needs_clamp",
+			description: "SYN packet needing MSS clamping",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateSYNPacketWithMSS(b, src, dst, 12345, 80, 1460)
+			},
+			frequency: "~0.1% of traffic - EXPENSIVE",
+		},
+		{
+			name:        "syn_no_clamp_needed",
+			description: "SYN packet with already-small MSS",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateSYNPacketWithMSS(b, src, dst, 12345, 80, 1200)
+			},
+			frequency: "~0.05% of traffic",
+		},
+		{
+			name:        "tcp_ack",
+			description: "Non-SYN TCP packet (ACK, data transfer)",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateTCPPacketWithFlags(b, src, dst, 12345, 80, uint16(conntrack.TCPAck))
+			},
+			frequency: "~60-70% of traffic - FAST PATH",
+		},
+		{
+			name:        "tcp_psh_ack",
+			description: "TCP data packet (PSH+ACK)",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateTCPPacketWithFlags(b, src, dst, 12345, 80, uint16(conntrack.TCPPush|conntrack.TCPAck))
+			},
+			frequency: "~10-20% of traffic - FAST PATH",
+		},
+		{
+			name:        "udp",
+			description: "UDP packet",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generatePacket(b, src, dst, 12345, 80, layers.IPProtocolUDP)
+			},
+			frequency: "~20-30% of traffic - FAST PATH",
+		},
+	}
+
+	for _, sc := range scenarios {
+		b.Run(sc.name, func(b *testing.B) {
+			manager, err := Create(&IFaceMock{
+				SetFilterFunc: func(device.PacketFilter) error { return nil },
+			}, false, flowLogger, iface.DefaultMTU)
+			require.NoError(b, err)
+			defer func() {
+				require.NoError(b, manager.Close(nil))
+			}()
+
+			manager.mssClampEnabled = true
+			manager.mssClampValue = 1240
+
+			srcIP := net.ParseIP("100.64.0.2")
+			dstIP := net.ParseIP("8.8.8.8")
+			packet := sc.genPacket(b, srcIP, dstIP)
+
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				manager.filterOutbound(packet, len(packet))
+			}
+		})
+	}
+}
+
+// BenchmarkMSSClampingOverhead compares overhead of MSS clamping enabled vs disabled
+// for the common case (non-SYN TCP packets).
+func BenchmarkMSSClampingOverhead(b *testing.B) {
+	scenarios := []struct {
+		name      string
+		enabled   bool
+		genPacket func(*testing.B, net.IP, net.IP) []byte
+	}{
+		{
+			name:    "disabled_tcp_ack",
+			enabled: false,
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateTCPPacketWithFlags(b, src, dst, 12345, 80, uint16(conntrack.TCPAck))
+			},
+		},
+		{
+			name:    "enabled_tcp_ack",
+			enabled: true,
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateTCPPacketWithFlags(b, src, dst, 12345, 80, uint16(conntrack.TCPAck))
+			},
+		},
+		{
+			name:    "disabled_syn_needs_clamp",
+			enabled: false,
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateSYNPacketWithMSS(b, src, dst, 12345, 80, 1460)
+			},
+		},
+		{
+			name:    "enabled_syn_needs_clamp",
+			enabled: true,
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateSYNPacketWithMSS(b, src, dst, 12345, 80, 1460)
+			},
+		},
+	}
+
+	for _, sc := range scenarios {
+		b.Run(sc.name, func(b *testing.B) {
+			manager, err := Create(&IFaceMock{
+				SetFilterFunc: func(device.PacketFilter) error { return nil },
+			}, false, flowLogger, iface.DefaultMTU)
+			require.NoError(b, err)
+			defer func() {
+				require.NoError(b, manager.Close(nil))
+			}()
+
+			manager.mssClampEnabled = sc.enabled
+			if sc.enabled {
+				manager.mssClampValue = 1240
+			}
+
+			srcIP := net.ParseIP("100.64.0.2")
+			dstIP := net.ParseIP("8.8.8.8")
+			packet := sc.genPacket(b, srcIP, dstIP)
+
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				manager.filterOutbound(packet, len(packet))
+			}
+		})
+	}
+}
+
+// BenchmarkMSSClampingMemory measures memory allocations for common vs rare cases
+func BenchmarkMSSClampingMemory(b *testing.B) {
+	scenarios := []struct {
+		name      string
+		genPacket func(*testing.B, net.IP, net.IP) []byte
+	}{
+		{
+			name: "tcp_ack_fast_path",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateTCPPacketWithFlags(b, src, dst, 12345, 80, uint16(conntrack.TCPAck))
+			},
+		},
+		{
+			name: "syn_needs_clamp",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generateSYNPacketWithMSS(b, src, dst, 12345, 80, 1460)
+			},
+		},
+		{
+			name: "udp_fast_path",
+			genPacket: func(b *testing.B, src, dst net.IP) []byte {
+				return generatePacket(b, src, dst, 12345, 80, layers.IPProtocolUDP)
+			},
+		},
+	}
+
+	for _, sc := range scenarios {
+		b.Run(sc.name, func(b *testing.B) {
+			manager, err := Create(&IFaceMock{
+				SetFilterFunc: func(device.PacketFilter) error { return nil },
+			}, false, flowLogger, iface.DefaultMTU)
+			require.NoError(b, err)
+			defer func() {
+				require.NoError(b, manager.Close(nil))
+			}()
+
+			manager.mssClampEnabled = true
+			manager.mssClampValue = 1240
+
+			srcIP := net.ParseIP("100.64.0.2")
+			dstIP := net.ParseIP("8.8.8.8")
+			packet := sc.genPacket(b, srcIP, dstIP)
+
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				manager.filterOutbound(packet, len(packet))
+			}
+		})
+	}
+}
+
+func generateSYNPacketNoMSS(b *testing.B, srcIP, dstIP net.IP, srcPort, dstPort uint16) []byte {
+	b.Helper()
+
+	ip := &layers.IPv4{
+		Version:  4,
+		IHL:      5,
+		TTL:      64,
+		Protocol: layers.IPProtocolTCP,
+		SrcIP:    srcIP,
+		DstIP:    dstIP,
+	}
+
+	tcp := &layers.TCP{
+		SrcPort: layers.TCPPort(srcPort),
+		DstPort: layers.TCPPort(dstPort),
+		SYN:     true,
+		Seq:     1000,
+		Window:  65535,
+	}
+
+	require.NoError(b, tcp.SetNetworkLayerForChecksum(ip))
+
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{
+		FixLengths:       true,
+		ComputeChecksums: true,
+	}
+
+	require.NoError(b, gopacket.SerializeLayers(buf, opts, ip, tcp, gopacket.Payload([]byte{})))
+	return buf.Bytes()
+}

client/firewall/uspfilter/filter_filter_test.go

@@ -12,6 +12,7 @@ import (
 	wgdevice "golang.zx2c4.com/wireguard/device"
 
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 	"github.com/netbirdio/netbird/client/iface/mocks"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
@@ -31,7 +32,7 @@ func TestPeerACLFiltering(t *testing.T) {
 		},
 	}
 
-	manager, err := Create(ifaceMock, false, flowLogger)
+	manager, err := Create(ifaceMock, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	require.NotNil(t, manager)
 
@@ -616,7 +617,7 @@ func setupRoutedManager(tb testing.TB, network string) *Manager {
 		},
 	}
 
-	manager, err := Create(ifaceMock, false, flowLogger)
+	manager, err := Create(ifaceMock, false, flowLogger, iface.DefaultMTU)
 	require.NoError(tb, err)
 	require.NoError(tb, manager.EnableRouting())
 	require.NotNil(tb, manager)
@@ -1462,7 +1463,7 @@ func TestRouteACLSet(t *testing.T) {
 		},
 	}
 
-	manager, err := Create(ifaceMock, false, flowLogger)
+	manager, err := Create(ifaceMock, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	t.Cleanup(func() {
 		require.NoError(t, manager.Close(nil))

client/firewall/uspfilter/filter_test.go

@@ -1,6 +1,7 @@
 package uspfilter
 
 import (
+	"encoding/binary"
 	"fmt"
 	"net"
 	"net/netip"
@@ -17,6 +18,7 @@ import (
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/firewall/uspfilter/conntrack"
 	"github.com/netbirdio/netbird/client/firewall/uspfilter/log"
+	nbiface "github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 	"github.com/netbirdio/netbird/client/internal/netflow"
@@ -66,7 +68,7 @@ func TestManagerCreate(t *testing.T) {
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
 	}
 
-	m, err := Create(ifaceMock, false, flowLogger)
+	m, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Errorf("failed to create Manager: %v", err)
 		return
@@ -86,7 +88,7 @@ func TestManagerAddPeerFiltering(t *testing.T) {
 		},
 	}
 
-	m, err := Create(ifaceMock, false, flowLogger)
+	m, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Errorf("failed to create Manager: %v", err)
 		return
@@ -119,7 +121,7 @@ func TestManagerDeleteRule(t *testing.T) {
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
 	}
 
-	m, err := Create(ifaceMock, false, flowLogger)
+	m, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Errorf("failed to create Manager: %v", err)
 		return
@@ -215,7 +217,7 @@ func TestAddUDPPacketHook(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			manager, err := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, nbiface.DefaultMTU)
 			require.NoError(t, err)
 
 			manager.AddUDPPacketHook(tt.in, tt.ip, tt.dPort, tt.hook)
@@ -265,7 +267,7 @@ func TestManagerReset(t *testing.T) {
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
 	}
 
-	m, err := Create(ifaceMock, false, flowLogger)
+	m, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Errorf("failed to create Manager: %v", err)
 		return
@@ -304,7 +306,7 @@ func TestNotMatchByIP(t *testing.T) {
 		},
 	}
 
-	m, err := Create(ifaceMock, false, flowLogger)
+	m, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Errorf("failed to create Manager: %v", err)
 		return
@@ -367,7 +369,7 @@ func TestRemovePacketHook(t *testing.T) {
 	}
 
 	// creating manager instance
-	manager, err := Create(iface, false, flowLogger)
+	manager, err := Create(iface, false, flowLogger, nbiface.DefaultMTU)
 	if err != nil {
 		t.Fatalf("Failed to create Manager: %s", err)
 	}
@@ -413,7 +415,7 @@ func TestRemovePacketHook(t *testing.T) {
 func TestProcessOutgoingHooks(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, nbiface.DefaultMTU)
 	require.NoError(t, err)
 
 	manager.udpTracker.Close()
@@ -495,7 +497,7 @@ func TestUSPFilterCreatePerformance(t *testing.T) {
 			ifaceMock := &IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
 			}
-			manager, err := Create(ifaceMock, false, flowLogger)
+			manager, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 			require.NoError(t, err)
 			time.Sleep(time.Second)
 
@@ -522,7 +524,7 @@ func TestUSPFilterCreatePerformance(t *testing.T) {
 func TestStatefulFirewall_UDPTracking(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, nbiface.DefaultMTU)
 	require.NoError(t, err)
 
 	manager.udpTracker.Close() // Close the existing tracker
@@ -729,7 +731,7 @@ func TestUpdateSetMerge(t *testing.T) {
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
 	}
 
-	manager, err := Create(ifaceMock, false, flowLogger)
+	manager, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	require.NoError(t, err)
 	t.Cleanup(func() {
 		require.NoError(t, manager.Close(nil))
@@ -815,7 +817,7 @@ func TestUpdateSetDeduplication(t *testing.T) {
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
 	}
 
-	manager, err := Create(ifaceMock, false, flowLogger)
+	manager, err := Create(ifaceMock, false, flowLogger, nbiface.DefaultMTU)
 	require.NoError(t, err)
 	t.Cleanup(func() {
 		require.NoError(t, manager.Close(nil))
@@ -923,3 +925,192 @@ func TestUpdateSetDeduplication(t *testing.T) {
 		require.Equal(t, tc.expected, isAllowed, tc.desc)
 	}
 }
+
+func TestMSSClamping(t *testing.T) {
+	ifaceMock := &IFaceMock{
+		SetFilterFunc: func(device.PacketFilter) error { return nil },
+		AddressFunc: func() wgaddr.Address {
+			return wgaddr.Address{
+				IP:      netip.MustParseAddr("100.10.0.100"),
+				Network: netip.MustParsePrefix("100.10.0.0/16"),
+			}
+		},
+	}
+
+	manager, err := Create(ifaceMock, false, flowLogger, 1280)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, manager.Close(nil))
+	}()
+
+	require.True(t, manager.mssClampEnabled, "MSS clamping should be enabled by default")
+	expectedMSSValue := uint16(1280 - ipTCPHeaderMinSize)
+	require.Equal(t, expectedMSSValue, manager.mssClampValue, "MSS clamp value should be MTU - 40")
+
+	err = manager.UpdateLocalIPs()
+	require.NoError(t, err)
+
+	srcIP := net.ParseIP("100.10.0.2")
+	dstIP := net.ParseIP("8.8.8.8")
+
+	t.Run("SYN packet with high MSS gets clamped", func(t *testing.T) {
+		highMSS := uint16(1460)
+		packet := generateSYNPacketWithMSS(t, srcIP, dstIP, 12345, 80, highMSS)
+
+		manager.filterOutbound(packet, len(packet))
+
+		d := parsePacket(t, packet)
+		require.Len(t, d.tcp.Options, 1, "Should have MSS option")
+		require.Equal(t, uint8(layers.TCPOptionKindMSS), uint8(d.tcp.Options[0].OptionType))
+		actualMSS := binary.BigEndian.Uint16(d.tcp.Options[0].OptionData)
+		require.Equal(t, expectedMSSValue, actualMSS, "MSS should be clamped to MTU - 40")
+	})
+
+	t.Run("SYN packet with low MSS unchanged", func(t *testing.T) {
+		lowMSS := uint16(1200)
+		packet := generateSYNPacketWithMSS(t, srcIP, dstIP, 12345, 80, lowMSS)
+
+		manager.filterOutbound(packet, len(packet))
+
+		d := parsePacket(t, packet)
+		require.Len(t, d.tcp.Options, 1, "Should have MSS option")
+		actualMSS := binary.BigEndian.Uint16(d.tcp.Options[0].OptionData)
+		require.Equal(t, lowMSS, actualMSS, "Low MSS should not be modified")
+	})
+
+	t.Run("SYN-ACK packet gets clamped", func(t *testing.T) {
+		highMSS := uint16(1460)
+		packet := generateSYNACKPacketWithMSS(t, srcIP, dstIP, 12345, 80, highMSS)
+
+		manager.filterOutbound(packet, len(packet))
+
+		d := parsePacket(t, packet)
+		require.Len(t, d.tcp.Options, 1, "Should have MSS option")
+		actualMSS := binary.BigEndian.Uint16(d.tcp.Options[0].OptionData)
+		require.Equal(t, expectedMSSValue, actualMSS, "MSS in SYN-ACK should be clamped")
+	})
+
+	t.Run("Non-SYN packet unchanged", func(t *testing.T) {
+		packet := generateTCPPacketWithFlags(t, srcIP, dstIP, 12345, 80, uint16(conntrack.TCPAck))
+
+		manager.filterOutbound(packet, len(packet))
+
+		d := parsePacket(t, packet)
+		require.Empty(t, d.tcp.Options, "ACK packet should have no options")
+	})
+}
+
+func generateSYNPacketWithMSS(tb testing.TB, srcIP, dstIP net.IP, srcPort, dstPort uint16, mss uint16) []byte {
+	tb.Helper()
+
+	ipLayer := &layers.IPv4{
+		Version:  4,
+		TTL:      64,
+		Protocol: layers.IPProtocolTCP,
+		SrcIP:    srcIP,
+		DstIP:    dstIP,
+	}
+
+	tcpLayer := &layers.TCP{
+		SrcPort: layers.TCPPort(srcPort),
+		DstPort: layers.TCPPort(dstPort),
+		SYN:     true,
+		Window:  65535,
+		Options: []layers.TCPOption{
+			{
+				OptionType:   layers.TCPOptionKindMSS,
+				OptionLength: 4,
+				OptionData:   binary.BigEndian.AppendUint16(nil, mss),
+			},
+		},
+	}
+	err := tcpLayer.SetNetworkLayerForChecksum(ipLayer)
+	require.NoError(tb, err)
+
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
+	err = gopacket.SerializeLayers(buf, opts, ipLayer, tcpLayer, gopacket.Payload([]byte{}))
+	require.NoError(tb, err)
+
+	return buf.Bytes()
+}
+
+func generateSYNACKPacketWithMSS(tb testing.TB, srcIP, dstIP net.IP, srcPort, dstPort uint16, mss uint16) []byte {
+	tb.Helper()
+
+	ipLayer := &layers.IPv4{
+		Version:  4,
+		TTL:      64,
+		Protocol: layers.IPProtocolTCP,
+		SrcIP:    srcIP,
+		DstIP:    dstIP,
+	}
+
+	tcpLayer := &layers.TCP{
+		SrcPort: layers.TCPPort(srcPort),
+		DstPort: layers.TCPPort(dstPort),
+		SYN:     true,
+		ACK:     true,
+		Window:  65535,
+		Options: []layers.TCPOption{
+			{
+				OptionType:   layers.TCPOptionKindMSS,
+				OptionLength: 4,
+				OptionData:   binary.BigEndian.AppendUint16(nil, mss),
+			},
+		},
+	}
+	err := tcpLayer.SetNetworkLayerForChecksum(ipLayer)
+	require.NoError(tb, err)
+
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
+	err = gopacket.SerializeLayers(buf, opts, ipLayer, tcpLayer, gopacket.Payload([]byte{}))
+	require.NoError(tb, err)
+
+	return buf.Bytes()
+}
+
+func generateTCPPacketWithFlags(tb testing.TB, srcIP, dstIP net.IP, srcPort, dstPort uint16, flags uint16) []byte {
+	tb.Helper()
+
+	ipLayer := &layers.IPv4{
+		Version:  4,
+		TTL:      64,
+		Protocol: layers.IPProtocolTCP,
+		SrcIP:    srcIP,
+		DstIP:    dstIP,
+	}
+
+	tcpLayer := &layers.TCP{
+		SrcPort: layers.TCPPort(srcPort),
+		DstPort: layers.TCPPort(dstPort),
+		Window:  65535,
+	}
+
+	if flags&uint16(conntrack.TCPSyn) != 0 {
+		tcpLayer.SYN = true
+	}
+	if flags&uint16(conntrack.TCPAck) != 0 {
+		tcpLayer.ACK = true
+	}
+	if flags&uint16(conntrack.TCPFin) != 0 {
+		tcpLayer.FIN = true
+	}
+	if flags&uint16(conntrack.TCPRst) != 0 {
+		tcpLayer.RST = true
+	}
+	if flags&uint16(conntrack.TCPPush) != 0 {
+		tcpLayer.PSH = true
+	}
+
+	err := tcpLayer.SetNetworkLayerForChecksum(ipLayer)
+	require.NoError(tb, err)
+
+	buf := gopacket.NewSerializeBuffer()
+	opts := gopacket.SerializeOptions{ComputeChecksums: true, FixLengths: true}
+	err = gopacket.SerializeLayers(buf, opts, ipLayer, tcpLayer, gopacket.Payload([]byte{}))
+	require.NoError(tb, err)
+
+	return buf.Bytes()
+}

client/firewall/uspfilter/forwarder/forwarder.go

@@ -45,7 +45,7 @@ type Forwarder struct {
 	netstack     bool
 }
 
-func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.FlowLogger, netstack bool) (*Forwarder, error) {
+func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.FlowLogger, netstack bool, mtu uint16) (*Forwarder, error) {
 	s := stack.New(stack.Options{
 		NetworkProtocols: []stack.NetworkProtocolFactory{ipv4.NewProtocol},
 		TransportProtocols: []stack.TransportProtocolFactory{
@@ -56,10 +56,6 @@ func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.Flow
 		HandleLocal: false,
 	})
 
-	mtu, err := iface.GetDevice().MTU()
-	if err != nil {
-		return nil, fmt.Errorf("get MTU: %w", err)
-	}
 	nicID := tcpip.NICID(1)
 	endpoint := &endpoint{
 		logger: logger,
@@ -68,7 +64,7 @@ func New(iface common.IFaceMapper, logger *nblog.Logger, flowLogger nftypes.Flow
 	}
 
 	if err := s.CreateNIC(nicID, endpoint); err != nil {
-		return nil, fmt.Errorf("failed to create NIC: %v", err)
+		return nil, fmt.Errorf("create NIC: %v", err)
 	}
 
 	protoAddr := tcpip.ProtocolAddress{

client/firewall/uspfilter/forwarder/udp.go

@@ -49,7 +49,7 @@ type idleConn struct {
 	conn *udpPacketConn
 }
 
-func newUDPForwarder(mtu int, logger *nblog.Logger, flowLogger nftypes.FlowLogger) *udpForwarder {
+func newUDPForwarder(mtu uint16, logger *nblog.Logger, flowLogger nftypes.FlowLogger) *udpForwarder {
 	ctx, cancel := context.WithCancel(context.Background())
 	f := &udpForwarder{
 		logger:     logger,

client/firewall/uspfilter/nat_bench_test.go

@@ -12,6 +12,7 @@ import (
 
 	log "github.com/sirupsen/logrus"
 
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 )
 
@@ -65,7 +66,7 @@ func BenchmarkDNATTranslation(b *testing.B) {
 		b.Run(sc.name, func(b *testing.B) {
 			manager, err := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			require.NoError(b, err)
 			defer func() {
 				require.NoError(b, manager.Close(nil))
@@ -125,7 +126,7 @@ func BenchmarkDNATTranslation(b *testing.B) {
 func BenchmarkDNATConcurrency(b *testing.B) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(b, err)
 	defer func() {
 		require.NoError(b, manager.Close(nil))
@@ -197,7 +198,7 @@ func BenchmarkDNATScaling(b *testing.B) {
 		b.Run(fmt.Sprintf("mappings_%d", count), func(b *testing.B) {
 			manager, err := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			require.NoError(b, err)
 			defer func() {
 				require.NoError(b, manager.Close(nil))
@@ -309,7 +310,7 @@ func BenchmarkChecksumUpdate(b *testing.B) {
 func BenchmarkDNATMemoryAllocations(b *testing.B) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(b, err)
 	defer func() {
 		require.NoError(b, manager.Close(nil))
@@ -472,7 +473,7 @@ func BenchmarkPortDNAT(b *testing.B) {
 		b.Run(sc.name, func(b *testing.B) {
 			manager, err := Create(&IFaceMock{
 				SetFilterFunc: func(device.PacketFilter) error { return nil },
-			}, false, flowLogger)
+			}, false, flowLogger, iface.DefaultMTU)
 			require.NoError(b, err)
 			defer func() {
 				require.NoError(b, manager.Close(nil))

client/firewall/uspfilter/nat_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	firewall "github.com/netbirdio/netbird/client/firewall/manager"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 )
 
@@ -16,7 +17,7 @@ import (
 func TestDNATTranslationCorrectness(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		require.NoError(t, manager.Close(nil))
@@ -100,7 +101,7 @@ func parsePacket(t testing.TB, packetData []byte) *decoder {
 func TestDNATMappingManagement(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		require.NoError(t, manager.Close(nil))
@@ -148,7 +149,7 @@ func TestDNATMappingManagement(t *testing.T) {
 func TestInboundPortDNAT(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		require.NoError(t, manager.Close(nil))
@@ -198,7 +199,7 @@ func TestInboundPortDNAT(t *testing.T) {
 func TestInboundPortDNATNegative(t *testing.T) {
 	manager, err := Create(&IFaceMock{
 		SetFilterFunc: func(device.PacketFilter) error { return nil },
-	}, false, flowLogger)
+	}, false, flowLogger, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		require.NoError(t, manager.Close(nil))

client/firewall/uspfilter/tracer_test.go

@@ -10,6 +10,7 @@ import (
 	fw "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/firewall/uspfilter/conntrack"
 	"github.com/netbirdio/netbird/client/firewall/uspfilter/forwarder"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 )
@@ -44,7 +45,7 @@ func TestTracePacket(t *testing.T) {
 			},
 		}
 
-		m, err := Create(ifaceMock, false, flowLogger)
+		m, err := Create(ifaceMock, false, flowLogger, iface.DefaultMTU)
 		require.NoError(t, err)
 
 		if !statefulMode {

client/internal/acl/manager_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/netbirdio/netbird/client/firewall"
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/wgaddr"
 	"github.com/netbirdio/netbird/client/internal/acl/mocks"
 	"github.com/netbirdio/netbird/client/internal/netflow"
@@ -52,7 +53,7 @@ func TestDefaultManager(t *testing.T) {
 	}).AnyTimes()
 	ifaceMock.EXPECT().GetWGDevice().Return(nil).AnyTimes()
 
-	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false)
+	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		err = fw.Close(nil)
@@ -170,7 +171,7 @@ func TestDefaultManagerStateless(t *testing.T) {
 	}).AnyTimes()
 	ifaceMock.EXPECT().GetWGDevice().Return(nil).AnyTimes()
 
-	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false)
+	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		err = fw.Close(nil)
@@ -321,7 +322,7 @@ func TestDefaultManagerEnableSSHRules(t *testing.T) {
 	}).AnyTimes()
 	ifaceMock.EXPECT().GetWGDevice().Return(nil).AnyTimes()
 
-	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false)
+	fw, err := firewall.NewFirewall(ifaceMock, nil, flowLogger, false, iface.DefaultMTU)
 	require.NoError(t, err)
 	defer func() {
 		err = fw.Close(nil)

client/internal/dns/server_test.go

@@ -944,7 +944,7 @@ func createWgInterfaceWithBind(t *testing.T) (*iface.WGIface, error) {
 		return nil, err
 	}
 
-	pf, err := uspfilter.Create(wgIface, false, flowLogger)
+	pf, err := uspfilter.Create(wgIface, false, flowLogger, iface.DefaultMTU)
 	if err != nil {
 		t.Fatalf("failed to create uspfilter: %v", err)
 		return nil, err

client/internal/engine.go

@@ -506,7 +506,7 @@ func (e *Engine) createFirewall() error {
 	}
 
 	var err error
-	e.firewall, err = firewall.NewFirewall(e.wgInterface, e.stateManager, e.flowManager.GetLogger(), e.config.DisableServerRoutes)
+	e.firewall, err = firewall.NewFirewall(e.wgInterface, e.stateManager, e.flowManager.GetLogger(), e.config.DisableServerRoutes, e.config.MTU)
 	if err != nil || e.firewall == nil {
 		log.Errorf("failed creating firewall manager: %s", err)
 		return nil