tailscale/util/linuxfw/nftables_runner.go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

//go:build linux

package linuxfw

import (
	"encoding/binary"
	"encoding/hex"
	"errors"
	"fmt"
	"net"
	"net/netip"
	"reflect"

	"github.com/google/nftables"
	"github.com/google/nftables/expr"
	"tailscale.com/net/tsaddr"
	"tailscale.com/types/logger"
)

const (
	chainNameForward     = "ts-forward"
	chainNameInput       = "ts-input"
	chainNamePostrouting = "ts-postrouting"
)

type chainInfo struct {
	table         *nftables.Table
	name          string
	chainType     nftables.ChainType
	chainHook     *nftables.ChainHook
	chainPriority *nftables.ChainPriority
}

type nftable struct {
	Proto  nftables.TableFamily
	Filter *nftables.Table
	Nat    *nftables.Table
}

type nftablesRunner struct {
	conn *nftables.Conn
	nft4 *nftable
	nft6 *nftable

	v6Available    bool
	v6NATAvailable bool
}

// createTableIfNotExist creates a nftables table via connection c if it does not exist within the given family.
func createTableIfNotExist(c *nftables.Conn, family nftables.TableFamily, name string) (*nftables.Table, error) {
	tables, err := c.ListTables()
	if err != nil {
		return nil, fmt.Errorf("get tables: %w", err)
	}
	for _, table := range tables {
		if table.Name == name && table.Family == family {
			return table, nil
		}
	}

	t := c.AddTable(&nftables.Table{
		Family: family,
		Name:   name,
	})
	if err := c.Flush(); err != nil {
		return nil, fmt.Errorf("add table: %w", err)
	}
	return t, nil
}

type errorChainNotFound struct {
	chainName string
	tableName string
}

func (e errorChainNotFound) Error() string {
	return fmt.Sprintf("chain %s not found in table %s", e.chainName, e.tableName)
}

// getChainFromTable returns the chain with the given name from the given table.
// Note that a chain name is unique within a table.
func getChainFromTable(c *nftables.Conn, table *nftables.Table, name string) (*nftables.Chain, error) {
	chains, err := c.ListChainsOfTableFamily(table.Family)
	if err != nil {
		return nil, fmt.Errorf("list chains: %w", err)
	}

	for _, chain := range chains {
		// Table family is already checked so table name is unique
		if chain.Table.Name == table.Name && chain.Name == name {
			return chain, nil
		}
	}

	return nil, errorChainNotFound{table.Name, name}
}

// getChainsFromTable returns all chains from the given table.
func getChainsFromTable(c *nftables.Conn, table *nftables.Table) ([]*nftables.Chain, error) {
	chains, err := c.ListChainsOfTableFamily(table.Family)
	if err != nil {
		return nil, fmt.Errorf("list chains: %w", err)
	}

	var ret []*nftables.Chain
	for _, chain := range chains {
		// Table family is already checked so table name is unique
		if chain.Table.Name == table.Name {
			ret = append(ret, chain)
		}
	}

	return ret, nil
}

// createChainIfNotExist creates a chain with the given name in the given table
// if it does not exist.
func createChainIfNotExist(c *nftables.Conn, cinfo chainInfo) error {
	chain, err := getChainFromTable(c, cinfo.table, cinfo.name)
	if err != nil && !errors.Is(err, errorChainNotFound{cinfo.table.Name, cinfo.name}) {
		return fmt.Errorf("get chain: %w", err)
	} else if err == nil {
		// Chain already exists
		if chain.Type != cinfo.chainType || chain.Hooknum != cinfo.chainHook || chain.Priority != cinfo.chainPriority {
			return fmt.Errorf("chain %s already exists with different type/hook/priority", cinfo.name)
		}
		return nil
	}

	_ = c.AddChain(&nftables.Chain{
		Name:     cinfo.name,
		Table:    cinfo.table,
		Type:     cinfo.chainType,
		Hooknum:  cinfo.chainHook,
		Priority: cinfo.chainPriority,
	})

	if err := c.Flush(); err != nil {
		return fmt.Errorf("add chain: %w", err)
	}

	return nil
}

// NewNfTablesRunner creates a new nftablesRunner without guaranteeing
// the existence of the tables and chains.
func NewNfTablesRunner(logf logger.Logf) (*nftablesRunner, error) {
	conn, err := nftables.New()
	if err != nil {
		return nil, fmt.Errorf("nftables connection: %w", err)
	}
	nft4 := &nftable{Proto: nftables.TableFamilyIPv4}

	v6err := checkIPv6(logf)
	if v6err != nil {
		logf("disabling tunneled IPv6 due to system IPv6 config: %w", v6err)
	}
	supportsV6 := v6err == nil
	supportsV6NAT := supportsV6 && checkSupportsV6NAT()

	var nft6 *nftable
	if supportsV6 {
		logf("v6nat availability: %v", supportsV6NAT)
		nft6 = &nftable{Proto: nftables.TableFamilyIPv6}
	}

	// TODO(KevinLiang10): convert iptables rule to nftable rules if they exist in the iptables

	return &nftablesRunner{
		conn:           conn,
		nft4:           nft4,
		nft6:           nft6,
		v6Available:    supportsV6,
		v6NATAvailable: supportsV6NAT,
	}, nil
}

// newLoadSaddrExpr creates a new nftables expression that loads the source
// address of the packet into the given register.
func newLoadSaddrExpr(proto nftables.TableFamily, destReg uint32) (expr.Any, error) {
	switch proto {
	case nftables.TableFamilyIPv4:
		return &expr.Payload{
			DestRegister: destReg,
			Base:         expr.PayloadBaseNetworkHeader,
			Offset:       12,
			Len:          4,
		}, nil
	case nftables.TableFamilyIPv6:
		return &expr.Payload{
			DestRegister: destReg,
			Base:         expr.PayloadBaseNetworkHeader,
			Offset:       8,
			Len:          16,
		}, nil
	default:
		return nil, fmt.Errorf("table family %v is neither IPv4 nor IPv6", proto)
	}
}

// HasIPV6 returns true if the system supports IPv6.
func (n *nftablesRunner) HasIPV6() bool {
	return n.v6Available
}

// HasIPV6NAT returns true if the system supports IPv6 NAT.
func (n *nftablesRunner) HasIPV6NAT() bool {
	return n.v6NATAvailable
}

// findRule iterates through the rules to find the rule with matching expressions.
func findRule(conn *nftables.Conn, rule *nftables.Rule) (*nftables.Rule, error) {
	rules, err := conn.GetRules(rule.Table, rule.Chain)
	if err != nil {
		return nil, fmt.Errorf("get nftables rules: %w", err)
	}
	if len(rules) == 0 {
		return nil, nil
	}

ruleLoop:
	for _, r := range rules {
		if len(r.Exprs) != len(rule.Exprs) {
			continue
		}

		for i, e := range r.Exprs {
			if !reflect.DeepEqual(e, rule.Exprs[i]) {
				continue ruleLoop
			}
		}
		return r, nil
	}

	return nil, nil
}

func createLoopbackRule(
	proto nftables.TableFamily,
	table *nftables.Table,
	chain *nftables.Chain,
	addr netip.Addr,
) (*nftables.Rule, error) {
	saddrExpr, err := newLoadSaddrExpr(proto, 1)
	if err != nil {
		return nil, fmt.Errorf("newLoadSaddrExpr: %w", err)
	}
	loopBackRule := &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: []expr.Any{
			&expr.Meta{
				Key:      expr.MetaKeyIIFNAME,
				Register: 1,
			},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     []byte("lo"),
			},
			saddrExpr,
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     addr.AsSlice(),
			},
			&expr.Counter{},
			&expr.Verdict{
				Kind: expr.VerdictAccept,
			},
		},
	}
	return loopBackRule, nil
}

// insertLoopbackRule inserts the TS loop back rule into
// the given chain as the first rule if it does not exist.
func insertLoopbackRule(
	conn *nftables.Conn, proto nftables.TableFamily,
	table *nftables.Table, chain *nftables.Chain, addr netip.Addr) error {

	loopBackRule, err := createLoopbackRule(proto, table, chain, addr)
	if err != nil {
		return fmt.Errorf("create loopback rule: %w", err)
	}

	// If TestDial is set, we are running in test mode and we should not
	// find rule because header will mismatch.
	if conn.TestDial == nil {
		// Check if the rule already exists.
		rule, err := findRule(conn, loopBackRule)
		if err != nil {
			return fmt.Errorf("find rule: %w", err)
		}
		if rule != nil {
			// Rule already exists, no need to insert.
			return nil
		}
	}

	// This inserts the rule to the top of the chain
	_ = conn.InsertRule(loopBackRule)

	if err = conn.Flush(); err != nil {
		return fmt.Errorf("insert rule: %w", err)
	}
	return nil
}

// getNFTByAddr returns the nftables with correct IP family
// that we will be using for the given address.
func (n *nftablesRunner) getNFTByAddr(addr netip.Addr) *nftable {
	if addr.Is6() {
		return n.nft6
	}
	return n.nft4
}

// AddLoopbackRule adds an nftables rule to permit loopback traffic to
// a local Tailscale IP. This rule is added only if it does not already exist.
func (n *nftablesRunner) AddLoopbackRule(addr netip.Addr) error {
	nf := n.getNFTByAddr(addr)

	inputChain, err := getChainFromTable(n.conn, nf.Filter, chainNameInput)
	if err != nil {
		return fmt.Errorf("get input chain: %w", err)
	}

	if err := insertLoopbackRule(n.conn, nf.Proto, nf.Filter, inputChain, addr); err != nil {
		return fmt.Errorf("add loopback rule: %w", err)
	}

	return nil
}

// DelLoopbackRule removes the nftables rule permitting loopback
// traffic to a Tailscale IP.
func (n *nftablesRunner) DelLoopbackRule(addr netip.Addr) error {
	nf := n.getNFTByAddr(addr)

	inputChain, err := getChainFromTable(n.conn, nf.Filter, chainNameInput)
	if err != nil {
		return fmt.Errorf("get input chain: %w", err)
	}

	loopBackRule, err := createLoopbackRule(nf.Proto, nf.Filter, inputChain, addr)
	if err != nil {
		return fmt.Errorf("create loopback rule: %w", err)
	}

	existingLoopBackRule, err := findRule(n.conn, loopBackRule)
	if err != nil {
		return fmt.Errorf("find loop back rule: %w", err)
	}
	if existingLoopBackRule == nil {
		// Rule does not exist, no need to delete.
		return nil
	}

	if err := n.conn.DelRule(existingLoopBackRule); err != nil {
		return fmt.Errorf("delete rule: %w", err)
	}

	return n.conn.Flush()
}

// getTables gets the available nftable in nftables runner.
func (n *nftablesRunner) getTables() []*nftable {
	if n.v6Available {
		return []*nftable{n.nft4, n.nft6}
	}
	return []*nftable{n.nft4}
}

// getNATTables gets the available nftable in nftables runner.
// If the system does not support IPv6 NAT, only the IPv4 nftable
// will be returned.
func (n *nftablesRunner) getNATTables() []*nftable {
	if n.v6NATAvailable {
		return n.getTables()
	}
	return []*nftable{n.nft4}
}

// AddChains creates custom Tailscale chains in netfilter via nftables
// if the ts-chain doesn't already exist.
func (n *nftablesRunner) AddChains() error {
	for _, table := range n.getTables() {
		filter, err := createTableIfNotExist(n.conn, table.Proto, "ts-filter")
		if err != nil {
			return fmt.Errorf("create table: %w", err)
		}
		table.Filter = filter
		if err = createChainIfNotExist(n.conn, chainInfo{filter, chainNameForward, nftables.ChainTypeFilter, nftables.ChainHookForward, nftables.ChainPriorityRef(-1)}); err != nil {
			return fmt.Errorf("create forward chain: %w", err)
		}
		if err = createChainIfNotExist(n.conn, chainInfo{filter, chainNameInput, nftables.ChainTypeFilter, nftables.ChainHookInput, nftables.ChainPriorityRef(-1)}); err != nil {
			return fmt.Errorf("create input chain: %w", err)
		}
	}

	for _, table := range n.getNATTables() {
		nat, err := createTableIfNotExist(n.conn, table.Proto, "ts-nat")
		if err != nil {
			return fmt.Errorf("create table: %w", err)
		}
		table.Nat = nat
		if err = createChainIfNotExist(n.conn, chainInfo{nat, chainNamePostrouting, nftables.ChainTypeNAT, nftables.ChainHookPostrouting, nftables.ChainPriorityNATDest}); err != nil {
			return fmt.Errorf("create postrouting chain: %w", err)
		}
	}

	return n.conn.Flush()
}

// deleteChainIfExists deletes a chain if it exists.
func deleteChainIfExists(c *nftables.Conn, table *nftables.Table, name string) error {
	chain, err := getChainFromTable(c, table, name)
	if err != nil && !errors.Is(err, errorChainNotFound{table.Name, name}) {
		return fmt.Errorf("get chain: %w", err)
	} else if err != nil {
		// If the chain doesn't exist, we don't need to delete it.
		return nil
	}

	c.FlushChain(chain)
	c.DelChain(chain)

	if err := c.Flush(); err != nil {
		return fmt.Errorf("flush and delete chain: %w", err)
	}

	return nil
}

// DelChains removes the custom Tailscale chains from netfilter via nftables.
func (n *nftablesRunner) DelChains() error {
	for _, table := range n.getTables() {
		if err := deleteChainIfExists(n.conn, table.Filter, chainNameForward); err != nil {
			return fmt.Errorf("delete chain: %w", err)
		}
		if err := deleteChainIfExists(n.conn, table.Filter, chainNameInput); err != nil {
			return fmt.Errorf("delete chain: %w", err)
		}
		n.conn.DelTable(table.Filter)
	}

	if err := deleteChainIfExists(n.conn, n.nft4.Nat, chainNamePostrouting); err != nil {
		return fmt.Errorf("delete chain: %w", err)
	}
	n.conn.DelTable(n.nft4.Nat)

	if n.v6NATAvailable {
		if err := deleteChainIfExists(n.conn, n.nft6.Nat, chainNamePostrouting); err != nil {
			return fmt.Errorf("delete chain: %w", err)
		}
		n.conn.DelTable(n.nft6.Nat)
	}

	if err := n.conn.Flush(); err != nil {
		return fmt.Errorf("flush: %w", err)
	}

	return nil
}

// AddHooks is defined to satisfy the interface. NfTables does not require
// AddHooks, since we don't have any default tables or chains in nftables.
func (n *nftablesRunner) AddHooks() error {
	return nil
}

// DelHooks is defined to satisfy the interface. NfTables does not require
// DelHooks, since we don't have any default tables or chains in nftables.
func (n *nftablesRunner) DelHooks(logf logger.Logf) error {
	return nil
}

// maskof returns the mask of the given prefix in big endian bytes.
func maskof(pfx netip.Prefix) []byte {
	mask := make([]byte, 4)
	binary.BigEndian.PutUint32(mask, ^(uint32(0xffff_ffff) >> pfx.Bits()))
	return mask
}

// createRangeRule creates a rule that matches packets with source IP from the give
// range (like CGNAT range or ChromeOSVM range) and the interface is not the tunname,
// and makes the given decision. Only IPv4 is supported.
func createRangeRule(
	table *nftables.Table, chain *nftables.Chain,
	tunname string, rng netip.Prefix, decision expr.VerdictKind,
) (*nftables.Rule, error) {
	if rng.Addr().Is6() {
		return nil, errors.New("IPv6 is not supported")
	}
	saddrExpr, err := newLoadSaddrExpr(nftables.TableFamilyIPv4, 1)
	if err != nil {
		return nil, fmt.Errorf("newLoadSaddrExpr: %w", err)
	}
	netip := rng.Addr().AsSlice()
	mask := maskof(rng)
	rule := &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: []expr.Any{
			&expr.Meta{Key: expr.MetaKeyIIFNAME, Register: 1},
			&expr.Cmp{
				Op:       expr.CmpOpNeq,
				Register: 1,
				Data:     []byte(tunname),
			},
			saddrExpr,
			&expr.Bitwise{
				SourceRegister: 1,
				DestRegister:   1,
				Len:            4,
				Mask:           mask,
				Xor:            []byte{0x00, 0x00, 0x00, 0x00},
			},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     netip,
			},
			&expr.Counter{},
			&expr.Verdict{
				Kind: decision,
			},
		},
	}
	return rule, nil

}

// addReturnChromeOSVMRangeRule adds a rule to return if the source IP
// is in the ChromeOS VM range.
func addReturnChromeOSVMRangeRule(c *nftables.Conn, table *nftables.Table, chain *nftables.Chain, tunname string) error {
	rule, err := createRangeRule(table, chain, tunname, tsaddr.ChromeOSVMRange(), expr.VerdictReturn)
	if err != nil {
		return fmt.Errorf("create rule: %w", err)
	}
	_ = c.AddRule(rule)
	if err = c.Flush(); err != nil {
		return fmt.Errorf("add rule: %w", err)
	}
	return nil
}

// addDropCGNATRangeRule adds a rule to drop if the source IP is in the
// CGNAT range.
func addDropCGNATRangeRule(c *nftables.Conn, table *nftables.Table, chain *nftables.Chain, tunname string) error {
	rule, err := createRangeRule(table, chain, tunname, tsaddr.CGNATRange(), expr.VerdictDrop)
	if err != nil {
		return fmt.Errorf("create rule: %w", err)
	}
	_ = c.AddRule(rule)
	if err = c.Flush(); err != nil {
		return fmt.Errorf("add rule: %w", err)
	}
	return nil
}

// createSetSubnetRouteMarkRule creates a rule to set the subnet route
// mark if the packet is from the given interface.
func createSetSubnetRouteMarkRule(table *nftables.Table, chain *nftables.Chain, tunname string) (*nftables.Rule, error) {
	hexTsFwmarkMaskNeg := getTailscaleFwmarkMaskNeg()
	hexTSSubnetRouteMark := getTailscaleSubnetRouteMark()

	rule := &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: []expr.Any{
			&expr.Meta{Key: expr.MetaKeyIIFNAME, Register: 1},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     []byte(tunname),
			},
			&expr.Counter{},
			&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
			&expr.Bitwise{
				SourceRegister: 1,
				DestRegister:   1,
				Len:            4,
				Mask:           hexTsFwmarkMaskNeg,
				Xor:            hexTSSubnetRouteMark,
			},
			&expr.Meta{
				Key:            expr.MetaKeyMARK,
				SourceRegister: true,
				Register:       1,
			},
		},
	}
	return rule, nil
}

// addSetSubnetRouteMarkRule adds a rule to set the subnet route mark
// if the packet is from the given interface.
func addSetSubnetRouteMarkRule(c *nftables.Conn, table *nftables.Table, chain *nftables.Chain, tunname string) error {
	rule, err := createSetSubnetRouteMarkRule(table, chain, tunname)
	if err != nil {
		return fmt.Errorf("create rule: %w", err)
	}
	_ = c.AddRule(rule)

	if err := c.Flush(); err != nil {
		return fmt.Errorf("add rule: %w", err)
	}

	return nil
}

// createDropOutgoingPacketFromCGNATRangeRuleWithTunname creates a rule to drop
// outgoing packets from the CGNAT range.
func createDropOutgoingPacketFromCGNATRangeRuleWithTunname(table *nftables.Table, chain *nftables.Chain, tunname string) (*nftables.Rule, error) {
	_, ipNet, err := net.ParseCIDR(tsaddr.CGNATRange().String())
	if err != nil {
		return nil, fmt.Errorf("parse cidr: %v", err)
	}
	mask, err := hex.DecodeString(ipNet.Mask.String())
	if err != nil {
		return nil, fmt.Errorf("decode mask: %v", err)
	}
	netip := ipNet.IP.Mask(ipNet.Mask).To4()
	saddrExpr, err := newLoadSaddrExpr(nftables.TableFamilyIPv4, 1)
	if err != nil {
		return nil, fmt.Errorf("newLoadSaddrExpr: %v", err)
	}
	rule := &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: []expr.Any{
			&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     []byte(tunname),
			},
			saddrExpr,
			&expr.Bitwise{
				SourceRegister: 1,
				DestRegister:   1,
				Len:            4,
				Mask:           mask,
				Xor:            []byte{0x00, 0x00, 0x00, 0x00},
			},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     netip,
			},
			&expr.Counter{},
			&expr.Verdict{
				Kind: expr.VerdictDrop,
			},
		},
	}
	return rule, nil
}

// addDropOutgoingPacketFromCGNATRangeRuleWithTunname adds a rule to drop
// outgoing packets from the CGNAT range.
func addDropOutgoingPacketFromCGNATRangeRuleWithTunname(conn *nftables.Conn, table *nftables.Table, chain *nftables.Chain, tunname string) error {
	rule, err := createDropOutgoingPacketFromCGNATRangeRuleWithTunname(table, chain, tunname)
	if err != nil {
		return fmt.Errorf("create rule: %w", err)
	}
	_ = conn.AddRule(rule)

	if err := conn.Flush(); err != nil {
		return fmt.Errorf("add rule: %w", err)
	}
	return nil
}

// createAcceptOutgoingPacketRule creates a rule to accept outgoing packets
// from the given interface.
func createAcceptOutgoingPacketRule(table *nftables.Table, chain *nftables.Chain, tunname string) *nftables.Rule {
	return &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: []expr.Any{
			&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
			&expr.Cmp{
				Op:       expr.CmpOpEq,
				Register: 1,
				Data:     []byte(tunname),
			},
			&expr.Counter{},
			&expr.Verdict{
				Kind: expr.VerdictAccept,
			},
		},
	}
}

// addAcceptOutgoingPacketRule adds a rule to accept outgoing packets
// from the given interface.
func addAcceptOutgoingPacketRule(conn *nftables.Conn, table *nftables.Table, chain *nftables.Chain, tunname string) error {
	rule := createAcceptOutgoingPacketRule(table, chain, tunname)
	_ = conn.AddRule(rule)

	if err := conn.Flush(); err != nil {
		return fmt.Errorf("flush add rule: %w", err)
	}

	return nil
}

// AddBase adds some basic processing rules.
func (n *nftablesRunner) AddBase(tunname string) error {
	if err := n.addBase4(tunname); err != nil {
		return fmt.Errorf("add base v4: %w", err)
	}
	if n.HasIPV6() {
		if err := n.addBase6(tunname); err != nil {
			return fmt.Errorf("add base v6: %w", err)
		}
	}
	return nil
}

// addBase4 adds some basic IPv4 processing rules.
func (n *nftablesRunner) addBase4(tunname string) error {
	conn := n.conn

	inputChain, err := getChainFromTable(conn, n.nft4.Filter, chainNameInput)
	if err != nil {
		return fmt.Errorf("get input chain v4: %v", err)
	}
	if err = addReturnChromeOSVMRangeRule(conn, n.nft4.Filter, inputChain, tunname); err != nil {
		return fmt.Errorf("add return chromeos vm range rule v4: %w", err)
	}
	if err = addDropCGNATRangeRule(conn, n.nft4.Filter, inputChain, tunname); err != nil {
		return fmt.Errorf("add drop cgnat range rule v4: %w", err)
	}

	forwardChain, err := getChainFromTable(conn, n.nft4.Filter, chainNameForward)
	if err != nil {
		return fmt.Errorf("get forward chain v4: %v", err)
	}

	if err = addSetSubnetRouteMarkRule(conn, n.nft4.Filter, forwardChain, tunname); err != nil {
		return fmt.Errorf("add set subnet route mark rule v4: %w", err)
	}

	if err = addMatchSubnetRouteMarkRule(conn, n.nft4.Filter, forwardChain, Accept); err != nil {
		return fmt.Errorf("add match subnet route mark rule v4: %w", err)
	}

	if err = addDropOutgoingPacketFromCGNATRangeRuleWithTunname(conn, n.nft4.Filter, forwardChain, tunname); err != nil {
		return fmt.Errorf("add drop outgoing packet from cgnat range rule v4: %w", err)
	}

	if err = addAcceptOutgoingPacketRule(conn, n.nft4.Filter, forwardChain, tunname); err != nil {
		return fmt.Errorf("add accept outgoing packet rule v4: %w", err)
	}

	if err = conn.Flush(); err != nil {
		return fmt.Errorf("flush base v4: %w", err)
	}

	return nil
}

// addBase6 adds some basic IPv6 processing rules.
func (n *nftablesRunner) addBase6(tunname string) error {
	conn := n.conn

	forwardChain, err := getChainFromTable(conn, n.nft6.Filter, chainNameForward)
	if err != nil {
		return fmt.Errorf("get forward chain v6: %w", err)
	}

	if err = addSetSubnetRouteMarkRule(conn, n.nft6.Filter, forwardChain, tunname); err != nil {
		return fmt.Errorf("add set subnet route mark rule v6: %w", err)
	}

	if err = addMatchSubnetRouteMarkRule(conn, n.nft6.Filter, forwardChain, Accept); err != nil {
		return fmt.Errorf("add match subnet route mark rule v6: %w", err)
	}

	if err = addAcceptOutgoingPacketRule(conn, n.nft6.Filter, forwardChain, tunname); err != nil {
		return fmt.Errorf("add accept outgoing packet rule v6: %w", err)
	}

	if err = conn.Flush(); err != nil {
		return fmt.Errorf("flush base v6: %w", err)
	}

	return nil
}

// DelBase empties, but does not remove, custom Tailscale chains from
// netfilter via iptables.
func (n *nftablesRunner) DelBase() error {
	conn := n.conn

	for _, table := range n.getTables() {
		inputChain, err := getChainFromTable(conn, table.Filter, chainNameInput)
		if err != nil {
			return fmt.Errorf("get input chain: %v", err)
		}
		conn.FlushChain(inputChain)
		forwardChain, err := getChainFromTable(conn, table.Filter, chainNameForward)
		if err != nil {
			return fmt.Errorf("get forward chain: %v", err)
		}
		conn.FlushChain(forwardChain)
	}

	for _, table := range n.getNATTables() {
		postrouteChain, err := getChainFromTable(conn, table.Nat, chainNamePostrouting)
		if err != nil {
			return fmt.Errorf("get postrouting chain v4: %v", err)
		}
		conn.FlushChain(postrouteChain)
	}

	return conn.Flush()
}

// createMatchSubnetRouteMarkRule creates a rule that matches packets
// with the subnet route mark and takes the specified action.
func createMatchSubnetRouteMarkRule(table *nftables.Table, chain *nftables.Chain, action MatchDecision) (*nftables.Rule, error) {
	hexTSFwmarkMask := getTailscaleFwmarkMask()
	hexTSSubnetRouteMark := getTailscaleSubnetRouteMark()

	var endAction expr.Any
	endAction = &expr.Verdict{Kind: expr.VerdictAccept}
	if action == Masq {
		endAction = &expr.Masq{}
	}

	exprs := []expr.Any{
		&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
		&expr.Bitwise{
			SourceRegister: 1,
			DestRegister:   1,
			Len:            4,
			Mask:           hexTSFwmarkMask,
			Xor:            []byte{0x00, 0x00, 0x00, 0x00},
		},
		&expr.Cmp{
			Op:       expr.CmpOpEq,
			Register: 1,
			Data:     hexTSSubnetRouteMark,
		},
		&expr.Counter{},
		endAction,
	}

	rule := &nftables.Rule{
		Table: table,
		Chain: chain,
		Exprs: exprs,
	}
	return rule, nil
}

// addMatchSubnetRouteMarkRule adds a rule that matches packets with
// the subnet route mark and takes the specified action.
func addMatchSubnetRouteMarkRule(conn *nftables.Conn, table *nftables.Table, chain *nftables.Chain, action MatchDecision) error {
	rule, err := createMatchSubnetRouteMarkRule(table, chain, action)
	if err != nil {
		return fmt.Errorf("create match subnet route mark rule: %w", err)
	}
	_ = conn.AddRule(rule)

	if err := conn.Flush(); err != nil {
		return fmt.Errorf("flush add rule: %w", err)
	}

	return nil
}

// AddSNATRule adds a netfilter rule to SNAT traffic destined for
// local subnets.
func (n *nftablesRunner) AddSNATRule() error {
	conn := n.conn

	for _, table := range n.getNATTables() {
		chain, err := getChainFromTable(conn, table.Nat, chainNamePostrouting)
		if err != nil {
			return fmt.Errorf("get postrouting chain v4: %w", err)
		}

		if err = addMatchSubnetRouteMarkRule(conn, table.Nat, chain, Masq); err != nil {
			return fmt.Errorf("add match subnet route mark rule v4: %w", err)
		}
	}

	if err := conn.Flush(); err != nil {
		return fmt.Errorf("flush add SNAT rule: %w", err)
	}

	return nil
}

// DelSNATRule removes the netfilter rule to SNAT traffic destined for
// local subnets. An error is returned if the rule does not exist.
func (n *nftablesRunner) DelSNATRule() error {
	conn := n.conn

	hexTSFwmarkMask := getTailscaleFwmarkMask()
	hexTSSubnetRouteMark := getTailscaleSubnetRouteMark()

	exprs := []expr.Any{
		&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
		&expr.Bitwise{
			SourceRegister: 1,
			DestRegister:   1,
			Len:            4,
			Mask:           hexTSFwmarkMask,
		},
		&expr.Cmp{
			Op:       expr.CmpOpEq,
			Register: 1,
			Data:     hexTSSubnetRouteMark,
		},
		&expr.Counter{},
		&expr.Masq{},
	}

	for _, table := range n.getNATTables() {
		chain, err := getChainFromTable(conn, table.Nat, chainNamePostrouting)
		if err != nil {
			return fmt.Errorf("get postrouting chain v4: %w", err)
		}

		rule := &nftables.Rule{
			Table: table.Nat,
			Chain: chain,
			Exprs: exprs,
		}

		SNATRule, err := findRule(conn, rule)
		if err != nil {
			return fmt.Errorf("find SNAT rule v4: %w", err)
		}

		_ = conn.DelRule(SNATRule)
	}

	if err := conn.Flush(); err != nil {
		return fmt.Errorf("flush del SNAT rule: %w", err)
	}

	return nil
}

// NfTablesCleanUp removes all Tailscale added nftables rules.
// Any errors that occur are logged to the provided logf.
func NfTablesCleanUp(logf logger.Logf) {
	conn, err := nftables.New()
	if err != nil {
		logf("ERROR: nftables connection: %w", err)
	}

	tables, err := conn.ListTables() // both v4 and v6
	if err != nil {
		logf("ERROR: list tables: %w", err)
	}

	for _, table := range tables {
		if table.Name == "ts-filter" || table.Name == "ts-nat" {
			conn.DelTable(table)
			if err := conn.Flush(); err != nil {
				logf("ERROR: flush table %s: %w", table.Name, err)
			}
		}
	}
}