util/linuxfw: decoupling IPTables logic from linux router

This change is introducing new netfilterRunner interface and moving iptables manipulation to a lower leveled iptables runner.

For #391

Signed-off-by: KevinLiang10 <kevinliang@tailscale.com>

wgengine/router/router_linux.go

@@ -4,7 +4,6 @@
 package router
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"net"
@@ -17,7 +16,6 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/coreos/go-iptables/iptables"
 	"github.com/tailscale/netlink"
 	"github.com/tailscale/wireguard-go/tun"
 	"go4.org/netipx"
@@ -25,9 +23,9 @@ import (
 	"golang.org/x/time/rate"
 	"tailscale.com/envknob"
 	"tailscale.com/net/netmon"
-	"tailscale.com/net/tsaddr"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/preftype"
+	"tailscale.com/util/linuxfw"
 	"tailscale.com/util/multierr"
 	"tailscale.com/version/distro"
 )
@@ -38,56 +36,34 @@ const (
 	netfilterOn       = preftype.NetfilterOn
 )
 
-// The following bits are added to packet marks for Tailscale use.
-//
-// We tried to pick bits sufficiently out of the way that it's
-// unlikely to collide with existing uses. We have 4 bytes of mark
-// bits to play with. We leave the lower byte alone on the assumption
-// that sysadmins would use those. Kubernetes uses a few bits in the
-// second byte, so we steer clear of that too.
-//
-// Empirically, most of the documentation on packet marks on the
-// internet gives the impression that the marks are 16 bits
-// wide. Based on this, we theorize that the upper two bytes are
-// relatively unused in the wild, and so we consume bits 16:23 (the
-// third byte).
-//
-// The constants are in the iptables/iproute2 string format for
-// matching and setting the bits, so they can be directly embedded in
-// commands.
-const (
-	// The mask for reading/writing the 'firewall mask' bits on a packet.
-	// See the comment on the const block on why we only use the third byte.
-	//
-	// We claim bits 16:23 entirely. For now we only use the lower four
-	// bits, leaving the higher 4 bits for future use.
-	tailscaleFwmarkMask    = "0xff0000"
-	tailscaleFwmarkMaskNum = 0xff0000
-
-	// Packet is from Tailscale and to a subnet route destination, so
-	// is allowed to be routed through this machine.
-	tailscaleSubnetRouteMark = "0x40000"
-
-	// Packet was originated by tailscaled itself, and must not be
-	// routed over the Tailscale network.
-	//
-	// Keep this in sync with tailscaleBypassMark in
-	// net/netns/netns_linux.go.
-	tailscaleBypassMark    = "0x80000"
-	tailscaleBypassMarkNum = 0x80000
-)
-
 // netfilterRunner abstracts helpers to run netfilter commands. It
 // exists purely to swap out go-iptables for a fake implementation in
 // tests.
 type netfilterRunner interface {
-	Insert(table, chain string, pos int, args ...string) error
-	Append(table, chain string, args ...string) error
-	Exists(table, chain string, args ...string) (bool, error)
-	Delete(table, chain string, args ...string) error
-	ClearChain(table, chain string) error
-	NewChain(table, chain string) error
-	DeleteChain(table, chain string) error
+	AddLoopbackRule(addr netip.Addr) error
+	DelLoopbackRule(addr netip.Addr) error
+	AddHooks() error
+	DelHooks(logf logger.Logf) error
+	AddChains() error
+	DelChains() error
+	AddBase(tunname string) error
+	DelBase() error
+	AddSNATRule() error
+	DelSNATRule() error
+
+	HasIPV6() bool
+	HasIPV6NAT() bool
+}
+
+func newNetfilterRunner(logf logger.Logf) (netfilterRunner, error) {
+	var nfr netfilterRunner
+	var err error
+	nfr, err = linuxfw.NewIPTablesRunner(logf)
+	if err != nil {
+		return nil, err
+	}
+
+	return nfr, nil
 }
 
 type linuxRouter struct {
@@ -109,16 +85,13 @@ type linuxRouter struct {
 
 	// Various feature checks for the network stack.
 	ipRuleAvailable bool // whether kernel was built with IP_MULTIPLE_TABLES
-	v6Available     bool
-	v6NATAvailable  bool
 	fwmaskWorks     bool // whether we can use 'ip rule...fwmark <mark>/<mask>'
 
 	// ipPolicyPrefBase is the base priority at which ip rules are installed.
 	ipPolicyPrefBase int
 
-	ipt4 netfilterRunner
-	ipt6 netfilterRunner
-	cmd  commandRunner
+	nfr netfilterRunner
+	cmd commandRunner
 }
 
 func newUserspaceRouter(logf logger.Logf, tunDev tun.Device, netMon *netmon.Monitor) (Router, error) {
@@ -127,51 +100,27 @@ func newUserspaceRouter(logf logger.Logf, tunDev tun.Device, netMon *netmon.Moni
 		return nil, err
 	}
 
-	ipt4, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
+	nfr, err := newNetfilterRunner(logf)
 	if err != nil {
 		return nil, err
 	}
 
-	v6err := checkIPv6(logf)
-	if v6err != nil {
-		logf("disabling tunneled IPv6 due to system IPv6 config: %v", v6err)
-	}
-	supportsV6 := v6err == nil
-	supportsV6NAT := supportsV6 && supportsV6NAT()
-	if supportsV6 {
-		logf("v6nat = %v", supportsV6NAT)
-	}
-
-	var ipt6 netfilterRunner
-	if supportsV6 {
-		// The iptables package probes for `ip6tables` and errors out
-		// if unavailable. We want that to be a non-fatal error.
-		ipt6, err = iptables.NewWithProtocol(iptables.ProtocolIPv6)
-		if err != nil {
-			return nil, err
-		}
-	}
-
 	cmd := osCommandRunner{
 		ambientCapNetAdmin: useAmbientCaps(),
 	}
 
-	return newUserspaceRouterAdvanced(logf, tunname, netMon, ipt4, ipt6, cmd, supportsV6, supportsV6NAT)
+	return newUserspaceRouterAdvanced(logf, tunname, netMon, nfr, cmd)
 }
 
-func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, netMon *netmon.Monitor, netfilter4, netfilter6 netfilterRunner, cmd commandRunner, supportsV6, supportsV6NAT bool) (Router, error) {
+func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, netMon *netmon.Monitor, nfr netfilterRunner, cmd commandRunner) (Router, error) {
 	r := &linuxRouter{
 		logf:          logf,
 		tunname:       tunname,
 		netfilterMode: netfilterOff,
 		netMon:        netMon,
 
-		v6Available:    supportsV6,
-		v6NATAvailable: supportsV6NAT,
-
-		ipt4: netfilter4,
-		ipt6: netfilter6,
-		cmd:  cmd,
+		nfr: nfr,
+		cmd: cmd,
 
 		ipRuleFixLimiter: rate.NewLimiter(rate.Every(5*time.Second), 10),
 		ipPolicyPrefBase: 5200,
@@ -484,23 +433,23 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 	case netfilterOff:
 		switch r.netfilterMode {
 		case netfilterNoDivert:
-			if err := r.delNetfilterBase(); err != nil {
+			if err := r.nfr.DelBase(); err != nil {
 				return err
 			}
-			if err := r.delNetfilterChains(); err != nil {
+			if err := r.nfr.DelChains(); err != nil {
 				r.logf("note: %v", err)
 				// harmless, continue.
 				// This can happen if someone left a ref to
 				// this table somewhere else.
 			}
 		case netfilterOn:
-			if err := r.delNetfilterHooks(); err != nil {
+			if err := r.nfr.DelHooks(r.logf); err != nil {
 				return err
 			}
-			if err := r.delNetfilterBase(); err != nil {
+			if err := r.nfr.DelBase(); err != nil {
 				return err
 			}
-			if err := r.delNetfilterChains(); err != nil {
+			if err := r.nfr.DelChains(); err != nil {
 				r.logf("note: %v", err)
 				// harmless, continue.
 				// This can happen if someone left a ref to
@@ -512,15 +461,15 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 		switch r.netfilterMode {
 		case netfilterOff:
 			reprocess = true
-			if err := r.addNetfilterChains(); err != nil {
+			if err := r.nfr.AddChains(); err != nil {
 				return err
 			}
-			if err := r.addNetfilterBase(); err != nil {
+			if err := r.nfr.AddBase(r.tunname); err != nil {
 				return err
 			}
 			r.snatSubnetRoutes = false
 		case netfilterOn:
-			if err := r.delNetfilterHooks(); err != nil {
+			if err := r.nfr.DelHooks(r.logf); err != nil {
 				return err
 			}
 		}
@@ -529,33 +478,33 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 		// we can't add a "-j ts-forward" rule to FORWARD
 		// while ts-forward contains an "-m mark" rule. But
 		// we can add the row *before* populating ts-forward.
-		// So we have to delNetFilterBase, then add the hooks,
-		// then re-addNetFilterBase, just in case.
+		// So we have to delBase, then add the hooks,
+		// then re-addBase, just in case.
 		switch r.netfilterMode {
 		case netfilterOff:
 			reprocess = true
-			if err := r.addNetfilterChains(); err != nil {
+			if err := r.nfr.AddChains(); err != nil {
 				return err
 			}
-			if err := r.delNetfilterBase(); err != nil {
+			if err := r.nfr.DelBase(); err != nil {
 				return err
 			}
-			if err := r.addNetfilterHooks(); err != nil {
+			if err := r.nfr.AddHooks(); err != nil {
 				return err
 			}
-			if err := r.addNetfilterBase(); err != nil {
+			if err := r.nfr.AddBase(r.tunname); err != nil {
 				return err
 			}
 			r.snatSubnetRoutes = false
 		case netfilterNoDivert:
 			reprocess = true
-			if err := r.delNetfilterBase(); err != nil {
+			if err := r.nfr.DelBase(); err != nil {
 				return err
 			}
-			if err := r.addNetfilterHooks(); err != nil {
+			if err := r.nfr.AddHooks(); err != nil {
 				return err
 			}
-			if err := r.addNetfilterBase(); err != nil {
+			if err := r.nfr.AddBase(r.tunname); err != nil {
 				return err
 			}
 			r.snatSubnetRoutes = false
@@ -579,11 +528,19 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 	return nil
 }
 
+func (r *linuxRouter) getV6Available() bool {
+	return r.nfr.HasIPV6()
+}
+
+func (r *linuxRouter) getV6NATAvailable() bool {
+	return r.nfr.HasIPV6NAT()
+}
+
 // addAddress adds an IP/mask to the tunnel interface. Fails if the
 // address is already assigned to the interface, or if the addition
 // fails.
 func (r *linuxRouter) addAddress(addr netip.Prefix) error {
-	if !r.v6Available && addr.Addr().Is6() {
+	if !r.getV6Available() && addr.Addr().Is6() {
 		return nil
 	}
 	if r.useIPCommand() {
@@ -609,7 +566,7 @@ func (r *linuxRouter) addAddress(addr netip.Prefix) error {
 // the address is not assigned to the interface, or if the removal
 // fails.
 func (r *linuxRouter) delAddress(addr netip.Prefix) error {
-	if !r.v6Available && addr.Addr().Is6() {
+	if !r.getV6Available() && addr.Addr().Is6() {
 		return nil
 	}
 	if err := r.delLoopbackRule(addr.Addr()); err != nil {
@@ -638,17 +595,8 @@ func (r *linuxRouter) addLoopbackRule(addr netip.Addr) error {
 		return nil
 	}
 
-	nf := r.ipt4
-	if addr.Is6() {
-		if !r.v6Available {
-			// IPv6 not available, ignore.
-			return nil
-		}
-		nf = r.ipt6
-	}
-
-	if err := nf.Insert("filter", "ts-input", 1, "-i", "lo", "-s", addr.String(), "-j", "ACCEPT"); err != nil {
-		return fmt.Errorf("adding loopback allow rule for %q: %w", addr, err)
+	if err := r.nfr.AddLoopbackRule(addr); err != nil {
+		return err
 	}
 	return nil
 }
@@ -660,17 +608,8 @@ func (r *linuxRouter) delLoopbackRule(addr netip.Addr) error {
 		return nil
 	}
 
-	nf := r.ipt4
-	if addr.Is6() {
-		if !r.v6Available {
-			// IPv6 not available, ignore.
-			return nil
-		}
-		nf = r.ipt6
-	}
-
-	if err := nf.Delete("filter", "ts-input", "-i", "lo", "-s", addr.String(), "-j", "ACCEPT"); err != nil {
-		return fmt.Errorf("deleting loopback allow rule for %q: %w", addr, err)
+	if err := r.nfr.DelLoopbackRule(addr); err != nil {
+		return err
 	}
 	return nil
 }
@@ -679,7 +618,7 @@ func (r *linuxRouter) delLoopbackRule(addr netip.Addr) error {
 // interface. Fails if the route already exists, or if adding the
 // route fails.
 func (r *linuxRouter) addRoute(cidr netip.Prefix) error {
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	if r.useIPCommand() {
@@ -704,7 +643,7 @@ func (r *linuxRouter) addThrowRoute(cidr netip.Prefix) error {
 	if !r.ipRuleAvailable {
 		return nil
 	}
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	if r.useIPCommand() {
@@ -712,7 +651,7 @@ func (r *linuxRouter) addThrowRoute(cidr netip.Prefix) error {
 	}
 	err := netlink.RouteReplace(&netlink.Route{
 		Dst:   netipx.PrefixIPNet(cidr.Masked()),
-		Table: tailscaleRouteTable.num,
+		Table: tailscaleRouteTable.Num,
 		Type:  unix.RTN_THROW,
 	})
 	if err != nil {
@@ -722,7 +661,7 @@ func (r *linuxRouter) addThrowRoute(cidr netip.Prefix) error {
 }
 
 func (r *linuxRouter) addRouteDef(routeDef []string, cidr netip.Prefix) error {
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	args := append([]string{"ip", "route", "add"}, routeDef...)
@@ -756,7 +695,7 @@ var (
 // interface. Fails if the route doesn't exist, or if removing the
 // route fails.
 func (r *linuxRouter) delRoute(cidr netip.Prefix) error {
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	if r.useIPCommand() {
@@ -784,7 +723,7 @@ func (r *linuxRouter) delThrowRoute(cidr netip.Prefix) error {
 	if !r.ipRuleAvailable {
 		return nil
 	}
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	if r.useIPCommand() {
@@ -803,7 +742,7 @@ func (r *linuxRouter) delThrowRoute(cidr netip.Prefix) error {
 }
 
 func (r *linuxRouter) delRouteDef(routeDef []string, cidr netip.Prefix) error {
-	if !r.v6Available && cidr.Addr().Is6() {
+	if !r.getV6Available() && cidr.Addr().Is6() {
 		return nil
 	}
 	args := append([]string{"ip", "route", "del"}, routeDef...)
@@ -865,7 +804,7 @@ func (r *linuxRouter) linkIndex() (int, error) {
 // routeTable returns the route table to use.
 func (r *linuxRouter) routeTable() int {
 	if r.ipRuleAvailable {
-		return tailscaleRouteTable.num
+		return tailscaleRouteTable.Num
 	}
 	return 0
 }
@@ -962,7 +901,7 @@ func (f addrFamily) netlinkInt() int {
 }
 
 func (r *linuxRouter) addrFamilies() []addrFamily {
-	if r.v6Available {
+	if r.getV6Available() {
 		return []addrFamily{v4, v6}
 	}
 	return []addrFamily{v4}
@@ -985,30 +924,34 @@ func (r *linuxRouter) addIPRules() error {
 	return r.justAddIPRules()
 }
 
-// routeTable is a Linux routing table: both its name and number.
+// RouteTable is a Linux routing table: both its name and number.
 // See /etc/iproute2/rt_tables.
-type routeTable struct {
-	name string
-	num  int
+type RouteTable struct {
+	Name string
+	Num  int
 }
 
-// ipCmdArg returns the string form of the table to pass to the "ip" command.
-func (rt routeTable) ipCmdArg() string {
-	if rt.num >= 253 {
-		return rt.name
+var routeTableByNumber = map[int]RouteTable{}
+
+// IpCmdArg returns the string form of the table to pass to the "ip" command.
+func (rt RouteTable) ipCmdArg() string {
+	if rt.Num >= 253 {
+		return rt.Name
 	}
-	return strconv.Itoa(rt.num)
+	return strconv.Itoa(rt.Num)
 }
 
-var routeTableByNumber = map[int]routeTable{}
-
-func newRouteTable(name string, num int) routeTable {
-	rt := routeTable{name, num}
+func newRouteTable(name string, num int) RouteTable {
+	rt := RouteTable{name, num}
 	routeTableByNumber[num] = rt
 	return rt
 }
 
-func mustRouteTable(num int) routeTable {
+// MustRouteTable returns the RouteTable with the given number key.
+// It panics if the number is unknown because this result is a part
+// of IP rule argument and we don't want to continue with an invalid
+// argument with table no exist.
+func mustRouteTable(num int) RouteTable {
 	rt, ok := routeTableByNumber[num]
 	if !ok {
 		panic(fmt.Sprintf("unknown route table %v", num))
@@ -1059,22 +1002,22 @@ var ipRules = []netlink.Rule{
 	// main routing table.
 	{
 		Priority: 10,
-		Mark:     tailscaleBypassMarkNum,
-		Table:    mainRouteTable.num,
+		Mark:     linuxfw.TailscaleBypassMarkNum,
+		Table:    mainRouteTable.Num,
 	},
 	// ...and then we try the 'default' table, for correctness,
 	// even though it's been empty on every Linux system I've ever seen.
 	{
 		Priority: 30,
-		Mark:     tailscaleBypassMarkNum,
-		Table:    defaultRouteTable.num,
+		Mark:     linuxfw.TailscaleBypassMarkNum,
+		Table:    defaultRouteTable.Num,
 	},
 	// If neither of those matched (no default route on this system?)
 	// then packets from us should be aborted rather than falling through
 	// to the tailscale routes, because that would create routing loops.
 	{
 		Priority: 50,
-		Mark:     tailscaleBypassMarkNum,
+		Mark:     linuxfw.TailscaleBypassMarkNum,
 		Type:     unix.RTN_UNREACHABLE,
 	},
 	// If we get to this point, capture all packets and send them
@@ -1084,7 +1027,7 @@ var ipRules = []netlink.Rule{
 	// beat non-VPN routes.
 	{
 		Priority: 70,
-		Table:    tailscaleRouteTable.num,
+		Table:    tailscaleRouteTable.Num,
 	},
 	// If that didn't match, then non-fwmark packets fall through to the
 	// usual rules (pref 32766 and 32767, ie. main and default).
@@ -1105,7 +1048,7 @@ func (r *linuxRouter) justAddIPRules() error {
 			// Note: r is a value type here; safe to mutate it.
 			ru.Family = family.netlinkInt()
 			if ru.Mark != 0 {
-				ru.Mask = tailscaleFwmarkMaskNum
+				ru.Mask = linuxfw.TailscaleFwmarkMaskNum
 			}
 			ru.Goto = -1
 			ru.SuppressIfgroup = -1
@@ -1138,7 +1081,7 @@ func (r *linuxRouter) addIPRulesWithIPCommand() error {
 			}
 			if rule.Mark != 0 {
 				if r.fwmaskWorks {
-					args = append(args, "fwmark", fmt.Sprintf("0x%x/%s", rule.Mark, tailscaleFwmarkMask))
+					args = append(args, "fwmark", fmt.Sprintf("0x%x/%s", rule.Mark, linuxfw.TailscaleFwmarkMask))
 				} else {
 					args = append(args, "fwmark", fmt.Sprintf("0x%x", rule.Mark))
 				}
@@ -1239,284 +1182,6 @@ func (r *linuxRouter) delIPRulesWithIPCommand() error {
 	return rg.ErrAcc
 }
 
-func (r *linuxRouter) netfilterFamilies() []netfilterRunner {
-	if r.v6Available {
-		return []netfilterRunner{r.ipt4, r.ipt6}
-	}
-	return []netfilterRunner{r.ipt4}
-}
-
-// addNetfilterChains creates custom Tailscale chains in netfilter.
-func (r *linuxRouter) addNetfilterChains() error {
-	create := func(ipt netfilterRunner, table, chain string) error {
-		err := ipt.ClearChain(table, chain)
-		if errCode(err) == 1 {
-			// nonexistent chain. let's create it!
-			return ipt.NewChain(table, chain)
-		}
-		if err != nil {
-			return fmt.Errorf("setting up %s/%s: %w", table, chain, err)
-		}
-		return nil
-	}
-
-	for _, ipt := range r.netfilterFamilies() {
-		if err := create(ipt, "filter", "ts-input"); err != nil {
-			return err
-		}
-		if err := create(ipt, "filter", "ts-forward"); err != nil {
-			return err
-		}
-	}
-	if err := create(r.ipt4, "nat", "ts-postrouting"); err != nil {
-		return err
-	}
-	if r.v6NATAvailable {
-		if err := create(r.ipt6, "nat", "ts-postrouting"); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// addNetfilterBase adds some basic processing rules to be
-// supplemented by later calls to other helpers.
-func (r *linuxRouter) addNetfilterBase() error {
-	if err := r.addNetfilterBase4(); err != nil {
-		return err
-	}
-	if r.v6Available {
-		if err := r.addNetfilterBase6(); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// addNetfilterBase4 adds some basic IPv4 processing rules to be
-// supplemented by later calls to other helpers.
-func (r *linuxRouter) addNetfilterBase4() error {
-	// Only allow CGNAT range traffic to come from tailscale0. There
-	// is an exception carved out for ranges used by ChromeOS, for
-	// which we fall out of the Tailscale chain.
-	//
-	// Note, this will definitely break nodes that end up using the
-	// CGNAT range for other purposes :(.
-	args := []string{"!", "-i", r.tunname, "-s", tsaddr.ChromeOSVMRange().String(), "-j", "RETURN"}
-	if err := r.ipt4.Append("filter", "ts-input", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-input: %w", args, err)
-	}
-	args = []string{"!", "-i", r.tunname, "-s", tsaddr.CGNATRange().String(), "-j", "DROP"}
-	if err := r.ipt4.Append("filter", "ts-input", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-input: %w", args, err)
-	}
-
-	// Forward all traffic from the Tailscale interface, and drop
-	// traffic to the tailscale interface by default. We use packet
-	// marks here so both filter/FORWARD and nat/POSTROUTING can match
-	// on these packets of interest.
-	//
-	// In particular, we only want to apply SNAT rules in
-	// nat/POSTROUTING to packets that originated from the Tailscale
-	// interface, but we can't match on the inbound interface in
-	// POSTROUTING. So instead, we match on the inbound interface in
-	// filter/FORWARD, and set a packet mark that nat/POSTROUTING can
-	// use to effectively run that same test again.
-	args = []string{"-i", r.tunname, "-j", "MARK", "--set-mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask}
-	if err := r.ipt4.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-forward: %w", args, err)
-	}
-	args = []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask, "-j", "ACCEPT"}
-	if err := r.ipt4.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-forward: %w", args, err)
-	}
-	args = []string{"-o", r.tunname, "-s", tsaddr.CGNATRange().String(), "-j", "DROP"}
-	if err := r.ipt4.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-forward: %w", args, err)
-	}
-	args = []string{"-o", r.tunname, "-j", "ACCEPT"}
-	if err := r.ipt4.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/filter/ts-forward: %w", args, err)
-	}
-
-	return nil
-}
-
-// addNetfilterBase4 adds some basic IPv6 processing rules to be
-// supplemented by later calls to other helpers.
-func (r *linuxRouter) addNetfilterBase6() error {
-	// TODO: only allow traffic from Tailscale's ULA range to come
-	// from tailscale0.
-
-	args := []string{"-i", r.tunname, "-j", "MARK", "--set-mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask}
-	if err := r.ipt6.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v6/filter/ts-forward: %w", args, err)
-	}
-	args = []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask, "-j", "ACCEPT"}
-	if err := r.ipt6.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v6/filter/ts-forward: %w", args, err)
-	}
-	// TODO: drop forwarded traffic to tailscale0 from tailscale's ULA
-	// (see corresponding IPv4 CGNAT rule).
-	args = []string{"-o", r.tunname, "-j", "ACCEPT"}
-	if err := r.ipt6.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v6/filter/ts-forward: %w", args, err)
-	}
-
-	return nil
-}
-
-// delNetfilterChains removes the custom Tailscale chains from netfilter.
-func (r *linuxRouter) delNetfilterChains() error {
-	del := func(ipt netfilterRunner, table, chain string) error {
-		if err := ipt.ClearChain(table, chain); err != nil {
-			if errCode(err) == 1 {
-				// nonexistent chain. That's fine, since it's
-				// the desired state anyway.
-				return nil
-			}
-			return fmt.Errorf("flushing %s/%s: %w", table, chain, err)
-		}
-		if err := ipt.DeleteChain(table, chain); err != nil {
-			// this shouldn't fail, because if the chain didn't
-			// exist, we would have returned after ClearChain.
-			return fmt.Errorf("deleting %s/%s: %v", table, chain, err)
-		}
-		return nil
-	}
-
-	for _, ipt := range r.netfilterFamilies() {
-		if err := del(ipt, "filter", "ts-input"); err != nil {
-			return err
-		}
-		if err := del(ipt, "filter", "ts-forward"); err != nil {
-			return err
-		}
-	}
-	if err := del(r.ipt4, "nat", "ts-postrouting"); err != nil {
-		return err
-	}
-	if r.v6NATAvailable {
-		if err := del(r.ipt6, "nat", "ts-postrouting"); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
-// delNetfilterBase empties but does not remove custom Tailscale chains from
-// netfilter.
-func (r *linuxRouter) delNetfilterBase() error {
-	del := func(ipt netfilterRunner, table, chain string) error {
-		if err := ipt.ClearChain(table, chain); err != nil {
-			if errCode(err) == 1 {
-				// nonexistent chain. That's fine, since it's
-				// the desired state anyway.
-				return nil
-			}
-			return fmt.Errorf("flushing %s/%s: %w", table, chain, err)
-		}
-		return nil
-	}
-
-	for _, ipt := range r.netfilterFamilies() {
-		if err := del(ipt, "filter", "ts-input"); err != nil {
-			return err
-		}
-		if err := del(ipt, "filter", "ts-forward"); err != nil {
-			return err
-		}
-	}
-	if err := del(r.ipt4, "nat", "ts-postrouting"); err != nil {
-		return err
-	}
-	if r.v6NATAvailable {
-		if err := del(r.ipt6, "nat", "ts-postrouting"); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
-// addNetfilterHooks inserts calls to tailscale's netfilter chains in
-// the relevant main netfilter chains. The tailscale chains must
-// already exist.
-func (r *linuxRouter) addNetfilterHooks() error {
-	divert := func(ipt netfilterRunner, table, chain string) error {
-		tsChain := tsChain(chain)
-
-		args := []string{"-j", tsChain}
-		exists, err := ipt.Exists(table, chain, args...)
-		if err != nil {
-			return fmt.Errorf("checking for %v in %s/%s: %w", args, table, chain, err)
-		}
-		if exists {
-			return nil
-		}
-		if err := ipt.Insert(table, chain, 1, args...); err != nil {
-			return fmt.Errorf("adding %v in %s/%s: %w", args, table, chain, err)
-		}
-		return nil
-	}
-
-	for _, ipt := range r.netfilterFamilies() {
-		if err := divert(ipt, "filter", "INPUT"); err != nil {
-			return err
-		}
-		if err := divert(ipt, "filter", "FORWARD"); err != nil {
-			return err
-		}
-	}
-	if err := divert(r.ipt4, "nat", "POSTROUTING"); err != nil {
-		return err
-	}
-	if r.v6NATAvailable {
-		if err := divert(r.ipt6, "nat", "POSTROUTING"); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// delNetfilterHooks deletes the calls to tailscale's netfilter chains
-// in the relevant main netfilter chains.
-func (r *linuxRouter) delNetfilterHooks() error {
-	del := func(ipt netfilterRunner, table, chain string) error {
-		tsChain := tsChain(chain)
-		args := []string{"-j", tsChain}
-		if err := ipt.Delete(table, chain, args...); err != nil {
-			// TODO(apenwarr): check for errCode(1) here.
-			// Unfortunately the error code from the iptables
-			// module resists unwrapping, unlike with other
-			// calls. So we have to assume if Delete fails,
-			// it's because there is no such rule.
-			r.logf("note: deleting %v in %s/%s: %w", args, table, chain, err)
-			return nil
-		}
-		return nil
-	}
-
-	for _, ipt := range r.netfilterFamilies() {
-		if err := del(ipt, "filter", "INPUT"); err != nil {
-			return err
-		}
-		if err := del(ipt, "filter", "FORWARD"); err != nil {
-			return err
-		}
-	}
-	if err := del(r.ipt4, "nat", "POSTROUTING"); err != nil {
-		return err
-	}
-	if r.v6NATAvailable {
-		if err := del(r.ipt6, "nat", "POSTROUTING"); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
 // addSNATRule adds a netfilter rule to SNAT traffic destined for
 // local subnets.
 func (r *linuxRouter) addSNATRule() error {
@@ -1524,14 +1189,8 @@ func (r *linuxRouter) addSNATRule() error {
 		return nil
 	}
 
-	args := []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask, "-j", "MASQUERADE"}
-	if err := r.ipt4.Append("nat", "ts-postrouting", args...); err != nil {
-		return fmt.Errorf("adding %v in v4/nat/ts-postrouting: %w", args, err)
-	}
-	if r.v6NATAvailable {
-		if err := r.ipt6.Append("nat", "ts-postrouting", args...); err != nil {
-			return fmt.Errorf("adding %v in v6/nat/ts-postrouting: %w", args, err)
-		}
+	if err := r.nfr.AddSNATRule(); err != nil {
+		return err
 	}
 	return nil
 }
@@ -1543,14 +1202,8 @@ func (r *linuxRouter) delSNATRule() error {
 		return nil
 	}
 
-	args := []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark + "/" + tailscaleFwmarkMask, "-j", "MASQUERADE"}
-	if err := r.ipt4.Delete("nat", "ts-postrouting", args...); err != nil {
-		return fmt.Errorf("deleting %v in v4/nat/ts-postrouting: %w", args, err)
-	}
-	if r.v6NATAvailable {
-		if err := r.ipt6.Delete("nat", "ts-postrouting", args...); err != nil {
-			return fmt.Errorf("deleting %v in v6/nat/ts-postrouting: %w", args, err)
-		}
+	if err := r.nfr.DelSNATRule(); err != nil {
+		return err
 	}
 	return nil
 }
@@ -1619,12 +1272,6 @@ func cidrDiff(kind string, old map[netip.Prefix]bool, new []netip.Prefix, add, d
 	return ret, nil
 }
 
-// tsChain returns the name of the tailscale sub-chain corresponding
-// to the given "parent" chain (e.g. INPUT, FORWARD, ...).
-func tsChain(chain string) string {
-	return "ts-" + strings.ToLower(chain)
-}
-
 // normalizeCIDR returns cidr as an ip/mask string, with the host bits
 // of the IP address zeroed out.
 func normalizeCIDR(cidr netip.Prefix) string {
@@ -1632,105 +1279,9 @@ func normalizeCIDR(cidr netip.Prefix) string {
 }
 
 func cleanup(logf logger.Logf, interfaceName string) {
-	// TODO(dmytro): clean up iptables.
-}
-
-// checkIPv6 checks whether the system appears to have a working IPv6
-// network stack. It returns an error explaining what looks wrong or
-// missing.  It does not check that IPv6 is currently functional or
-// that there's a global address, just that the system would support
-// IPv6 if it were on an IPv6 network.
-func checkIPv6(logf logger.Logf) error {
-	_, err := os.Stat("/proc/sys/net/ipv6")
-	if os.IsNotExist(err) {
-		return err
+	if interfaceName != "userspace-networking" {
+		linuxfw.IPTablesCleanup(logf)
 	}
-	bs, err := os.ReadFile("/proc/sys/net/ipv6/conf/all/disable_ipv6")
-	if err != nil {
-		// Be conservative if we can't find the IPv6 configuration knob.
-		return err
-	}
-	disabled, err := strconv.ParseBool(strings.TrimSpace(string(bs)))
-	if err != nil {
-		return errors.New("disable_ipv6 has invalid bool")
-	}
-	if disabled {
-		return errors.New("disable_ipv6 is set")
-	}
-
-	// Older kernels don't support IPv6 policy routing. Some kernels
-	// support policy routing but don't have this knob, so absence of
-	// the knob is not fatal.
-	bs, err = os.ReadFile("/proc/sys/net/ipv6/conf/all/disable_policy")
-	if err == nil {
-		disabled, err = strconv.ParseBool(strings.TrimSpace(string(bs)))
-		if err != nil {
-			return errors.New("disable_policy has invalid bool")
-		}
-		if disabled {
-			return errors.New("disable_policy is set")
-		}
-	}
-
-	if err := checkIPRuleSupportsV6(logf); err != nil {
-		return fmt.Errorf("kernel doesn't support IPv6 policy routing: %w", err)
-	}
-
-	// Some distros ship ip6tables separately from iptables.
-	if _, err := exec.LookPath("ip6tables"); err != nil {
-		return err
-	}
-
-	return nil
-}
-
-// supportsV6NAT returns whether the system has a "nat" table in the
-// IPv6 netfilter stack.
-//
-// The nat table was added after the initial release of ipv6
-// netfilter, so some older distros ship a kernel that can't NAT IPv6
-// traffic.
-func supportsV6NAT() bool {
-	bs, err := os.ReadFile("/proc/net/ip6_tables_names")
-	if err != nil {
-		// Can't read the file. Assume SNAT works.
-		return true
-	}
-	if bytes.Contains(bs, []byte("nat\n")) {
-		return true
-	}
-	// In nftables mode, that proc file will be empty. Try another thing:
-	if exec.Command("modprobe", "ip6table_nat").Run() == nil {
-		return true
-	}
-	return false
-}
-
-func checkIPRuleSupportsV6(logf logger.Logf) error {
-	// First try just a read-only operation to ideally avoid
-	// having to modify any state.
-	if rules, err := netlink.RuleList(netlink.FAMILY_V6); err != nil {
-		return fmt.Errorf("querying IPv6 policy routing rules: %w", err)
-	} else {
-		if len(rules) > 0 {
-			logf("[v1] kernel supports IPv6 policy routing (found %d rules)", len(rules))
-			return nil
-		}
-	}
-
-	// Try to actually create & delete one as a test.
-	rule := netlink.NewRule()
-	rule.Priority = 1234
-	rule.Mark = tailscaleBypassMarkNum
-	rule.Table = tailscaleRouteTable.num
-	rule.Family = netlink.FAMILY_V6
-	// First delete the rule unconditionally, and don't check for
-	// errors. This is just cleaning up anything that might be already
-	// there.
-	netlink.RuleDel(rule)
-	// And clean up on exit.
-	defer netlink.RuleDel(rule)
-	return netlink.RuleAdd(rule)
 }
 
 // Checks if the running openWRT system is using mwan3, based on the heuristic