wip

internal/firewall/nftables/atomic.go

@@ -0,0 +1,99 @@
+package nftables
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/nftables"
+)
+
+// SaveAndRestore saves the current nftables tree and returns a restore function that
+// can be called to restore the saved tree.
+func (f *Firewall) SaveAndRestore(_ context.Context) (restore func(context.Context), err error) {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return nil, fmt.Errorf("creating nftables connection: %w", err)
+	}
+	tables, err := saveTables(conn)
+	if err != nil {
+		return nil, fmt.Errorf("saving nftables state: %w", err)
+	}
+	return func(_ context.Context) {
+		conn, err := nftables.New()
+		if err != nil {
+			f.logger.Warnf("creating nftables connection for restore: %s", err)
+			return
+		}
+		err = restoreTables(conn, tables)
+		if err != nil {
+			f.logger.Warnf("restoring nftables state: %s", err)
+		}
+	}, nil
+}
+
+type savedTable struct {
+	table  *nftables.Table
+	chains []savedChain
+}
+
+type savedChain struct {
+	chain *nftables.Chain
+	rules []*nftables.Rule
+}
+
+func saveTables(conn *nftables.Conn) ([]savedTable, error) {
+	tables, err := conn.ListTables()
+	if err != nil {
+		return nil, err
+	}
+
+	savedTables := make([]savedTable, len(tables))
+	for i, table := range tables {
+		savedTables[i].table = table
+
+		chains, err := conn.ListChains()
+		if err != nil {
+			return nil, err
+		}
+
+		for _, chain := range chains {
+			if chain.Table.Name != table.Name ||
+				chain.Table.Family != table.Family {
+				continue
+			}
+			rules, err := conn.GetRules(table, chain)
+			if err != nil {
+				return nil, fmt.Errorf("getting rules for chain %s in table %s: %w", chain.Name, table.Name, err)
+			}
+			savedChain := savedChain{chain: chain, rules: rules}
+			savedTables[i].chains = append(savedTables[i].chains, savedChain)
+		}
+	}
+
+	return savedTables, nil
+}
+
+func restoreTables(conn *nftables.Conn, savedTables []savedTable) error {
+	conn.FlushRuleset()
+
+	for _, savedTable := range savedTables {
+		table := conn.AddTable(savedTable.table)
+		for _, savedChain := range savedTable.chains {
+			// Make the [nftables.Chain.Table] points to the new [nftables.Table]
+			// created in this connection.
+			savedChain.chain.Table = table
+			savedChain.chain = conn.AddChain(savedChain.chain)
+
+			for _, rule := range savedChain.rules {
+				rule.Table = table
+				rule.Chain = savedChain.chain
+				conn.AddRule(rule)
+			}
+		}
+	}
+
+	return conn.Flush()
+}

internal/firewall/nftables/basechains.go

@@ -0,0 +1,50 @@
+package nftables
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/google/nftables"
+)
+
+var ErrPolicyUnknown = errors.New("unknown policy")
+
+// SetBaseChainsPolicy sets the policy of all the base chains (INPUT, FORWARD, or OUTPUT)
+// for the filter table to the given policy (accept or drop).
+func (f *Firewall) SetBaseChainsPolicy(_ context.Context, policy string) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	var chainPolicy nftables.ChainPolicy
+	switch strings.ToLower(policy) {
+	case "accept":
+		chainPolicy = nftables.ChainPolicyAccept
+	case "drop":
+		chainPolicy = nftables.ChainPolicyDrop
+	default:
+		return fmt.Errorf("%w: %s", ErrPolicyUnknown, policy)
+	}
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	_, inputChain, forwardChain, outputChain := setupFilterWithBaseChains(conn)
+	inputChain.Policy = &chainPolicy
+	forwardChain.Policy = &chainPolicy
+	outputChain.Policy = &chainPolicy
+
+	conn.AddChain(inputChain)
+	conn.AddChain(forwardChain)
+	conn.AddChain(outputChain)
+
+	err = conn.Flush()
+	if err != nil {
+		return fmt.Errorf("flushing nftables changes: %w", err)
+	}
+
+	return nil
+}

internal/firewall/nftables/conntrack.go

@@ -0,0 +1,61 @@
+package nftables
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/nftables"
+	"github.com/google/nftables/expr"
+)
+
+func (f *Firewall) AcceptEstablishedRelatedTraffic(_ context.Context) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	table, inputChain, _, outputChain := setupFilterWithBaseChains(conn)
+
+	ctStateExprs := []expr.Any{
+		&expr.Ct{
+			Key:      expr.CtKeySTATE,
+			Register: 1,
+		},
+		&expr.Bitwise{
+			SourceRegister: 1,
+			DestRegister:   1,
+			Len:            4, //nolint:mnd
+			Mask:           []byte{byte(expr.CtStateBitESTABLISHED | expr.CtStateBitRELATED), 0x00, 0x00, 0x00},
+			Xor:            []byte{0x00, 0x00, 0x00, 0x00},
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpNeq,
+			Register: 1,
+			Data:     []byte{0x00, 0x00, 0x00, 0x00},
+		},
+		&expr.Verdict{
+			Kind: expr.VerdictAccept,
+		},
+	}
+
+	conn.AddRule(&nftables.Rule{
+		Table: table,
+		Chain: inputChain,
+		Exprs: ctStateExprs,
+	})
+
+	conn.AddRule(&nftables.Rule{
+		Table: table,
+		Chain: outputChain,
+		Exprs: ctStateExprs,
+	})
+
+	if err := conn.Flush(); err != nil {
+		return fmt.Errorf("flushing: %w", err)
+	}
+
+	return nil
+}

internal/firewall/nftables/delete.go

@@ -0,0 +1,27 @@
+package nftables
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+
+	"github.com/google/nftables"
+)
+
+var errRuleToDeleteNotFound = errors.New("rule not found for removal")
+
+func (f *Firewall) deleteRule(conn *nftables.Conn, rule *nftables.Rule) error {
+	for i, existing := range f.rules {
+		if !reflect.DeepEqual(existing, rule) {
+			continue
+		}
+		err := conn.DelRule(existing)
+		if err != nil {
+			return fmt.Errorf("deleting rule: %w", err)
+		}
+		f.rules[i], f.rules[len(f.rules)-1] = f.rules[len(f.rules)-1], f.rules[i]
+		f.rules = f.rules[:len(f.rules)-1]
+		return nil
+	}
+	return fmt.Errorf("%w: %#v", errRuleToDeleteNotFound, rule)
+}

internal/firewall/nftables/filter.go

@@ -0,0 +1,38 @@
+package nftables
+
+import "github.com/google/nftables"
+
+func setupFilterWithBaseChains(conn *nftables.Conn) (table *nftables.Table,
+	inputChain, forwardChain, outputChain *nftables.Chain,
+) {
+	table = conn.AddTable(&nftables.Table{
+		Family: nftables.TableFamilyINet,
+		Name:   "filter",
+	})
+
+	inputChain = conn.AddChain(&nftables.Chain{
+		Name:     "input",
+		Table:    table,
+		Type:     nftables.ChainTypeFilter,
+		Hooknum:  nftables.ChainHookInput,
+		Priority: nftables.ChainPriorityFilter,
+	})
+
+	forwardChain = conn.AddChain(&nftables.Chain{
+		Name:     "forward",
+		Table:    table,
+		Type:     nftables.ChainTypeFilter,
+		Hooknum:  nftables.ChainHookForward,
+		Priority: nftables.ChainPriorityFilter,
+	})
+
+	outputChain = conn.AddChain(&nftables.Chain{
+		Name:     "output",
+		Table:    table,
+		Type:     nftables.ChainTypeFilter,
+		Hooknum:  nftables.ChainHookOutput,
+		Priority: nftables.ChainPriorityFilter,
+	})
+
+	return table, inputChain, forwardChain, outputChain
+}

internal/firewall/nftables/firewall.go

@@ -0,0 +1,22 @@
+package nftables
+
+import (
+	"sync"
+
+	"github.com/google/nftables"
+)
+
+type Firewall struct {
+	logger Logger
+
+	// rules are only rules added and tracked for later removal.
+	// Not all rules added are tracked for removal.
+	rules []*nftables.Rule
+	mutex sync.Mutex
+}
+
+func New(logger Logger) *Firewall {
+	return &Firewall{
+		logger: logger,
+	}
+}

internal/firewall/nftables/input.go

@@ -0,0 +1,170 @@
+package nftables
+
+import (
+	"context"
+	"fmt"
+	"net/netip"
+
+	"github.com/google/nftables"
+	"github.com/google/nftables/expr"
+)
+
+func (f *Firewall) AcceptInputThroughInterface(_ context.Context, intf string) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	table, inputChain, _, _ := setupFilterWithBaseChains(conn)
+
+	rule := &nftables.Rule{
+		Table: table,
+		Chain: inputChain,
+		Exprs: []expr.Any{
+			&expr.Meta{
+				Key:      expr.MetaKeyIIFNAME,
+				Register: 1,
+			},
+			&expr.Cmp{
+				Op:       expr.CmpOpEq,
+				Register: 1,
+				Data:     []byte(intf + "\x00"),
+			},
+			&expr.Verdict{
+				Kind: expr.VerdictAccept,
+			},
+		},
+	}
+
+	conn.AddRule(rule)
+
+	err = conn.Flush()
+	if err != nil {
+		return fmt.Errorf("flushing: %w", err)
+	}
+
+	return nil
+}
+
+// AcceptInputToPort accepts incoming traffic on the specified port, for both TCP and UDP
+// protocols, on the interface intf. If intf is empty or "*", the interface is not used as a filter.
+// If remove is true, the rule is removed instead of added. This is used for port forwarding, with
+// intf set to the VPN tunnel interface.
+func (f *Firewall) AcceptInputToPort(_ context.Context, intf string, port uint16, remove bool) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	table, inputChain, _, _ := setupFilterWithBaseChains(conn)
+	portBytes := []byte{byte(port >> 8), byte(port)} //nolint:mnd
+	const tcp, udp uint8 = 6, 17
+	protocols := []uint8{tcp, udp}
+
+	for _, protocol := range protocols {
+		const maxExprsLen = 7
+		exprs := make([]expr.Any, 0, maxExprsLen)
+		if intf != "" && intf != "*" {
+			exprs = append(exprs,
+				&expr.Meta{Key: expr.MetaKeyIIFNAME, Register: 1},
+				&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: []byte(intf + "\x00")},
+			)
+		}
+		exprs = append(exprs,
+			&expr.Payload{DestRegister: 1, Base: expr.PayloadBaseNetworkHeader, Offset: 9, Len: 1}, //nolint:mnd
+			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: []byte{protocol}},
+			&expr.Payload{DestRegister: 1, Base: expr.PayloadBaseTransportHeader, Offset: 2, Len: 2}, //nolint:mnd
+			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: portBytes},
+			&expr.Verdict{Kind: expr.VerdictAccept},
+		)
+
+		rule := &nftables.Rule{
+			Table: table,
+			Chain: inputChain,
+			Exprs: exprs,
+		}
+
+		if !remove {
+			conn.AddRule(rule)
+			f.rules = append(f.rules, rule)
+			continue
+		}
+		err = f.deleteRule(conn, rule)
+		if err != nil {
+			return fmt.Errorf("deleting rule: %w", err)
+		}
+	}
+
+	err = conn.Flush()
+	if err != nil {
+		f.rules = f.rules[:len(f.rules)-len(protocols)]
+		return fmt.Errorf("flushing: %w", err)
+	}
+
+	return nil
+}
+
+func (f *Firewall) AcceptInputToSubnet(_ context.Context, intf string, subnet netip.Prefix) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	table, inputChain, _, _ := setupFilterWithBaseChains(conn)
+
+	const maxExprsLen = 5
+	exprs := make([]expr.Any, 0, maxExprsLen)
+
+	if intf != "" && intf != "*" {
+		exprs = append(exprs,
+			&expr.Meta{Key: expr.MetaKeyIIFNAME, Register: 1},
+			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: []byte(intf + "\x00")},
+		)
+	}
+
+	var payloadOffset uint32
+	if subnet.Addr().Is4() {
+		payloadOffset = 16
+	} else {
+		payloadOffset = 24
+	}
+
+	exprs = append(exprs,
+		&expr.Payload{
+			DestRegister: 1,
+			Base:         expr.PayloadBaseNetworkHeader,
+			Offset:       payloadOffset,
+			Len:          uint32(len(subnet.Addr().AsSlice())), //nolint:gosec
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     subnet.Addr().AsSlice(),
+		},
+		&expr.Verdict{Kind: expr.VerdictAccept},
+	)
+
+	rule := &nftables.Rule{
+		Table: table,
+		Chain: inputChain,
+		Exprs: exprs,
+	}
+
+	conn.AddRule(rule)
+
+	err = conn.Flush()
+	if err != nil {
+		return fmt.Errorf("flushing: %w", err)
+	}
+
+	return nil
+}

internal/firewall/nftables/interfaces.go

@@ -0,0 +1,5 @@
+package nftables
+
+type Logger interface {
+	Warnf(format string, args ...any)
+}

internal/firewall/nftables/output.go

@@ -0,0 +1,78 @@
+package nftables
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/nftables"
+	"github.com/google/nftables/expr"
+)
+
+func (f *Firewall) AcceptIpv6MulticastOutput(_ context.Context, intf string) error {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+
+	conn, err := nftables.New()
+	if err != nil {
+		return fmt.Errorf("creating nftables connection: %w", err)
+	}
+
+	table, _, _, outputChain := setupFilterWithBaseChains(conn)
+
+	const maxExprsLen = 6
+	exprs := make([]expr.Any, 0, maxExprsLen)
+
+	if intf != "" && intf != "*" {
+		exprs = append(exprs,
+			&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
+			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: []byte(intf + "\x00")},
+		)
+	}
+
+	// ff02::1:ff00:0/104 mask is 13 bytes of 0xff
+	mask := []byte{
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00,
+	} //nolint:mnd
+	addr := []byte{
+		0xff, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x01, 0xff, 0x00, 0x00, 0x00,
+	} //nolint:mnd
+
+	exprs = append(exprs,
+		&expr.Payload{
+			DestRegister: 1,
+			Base:         expr.PayloadBaseNetworkHeader,
+			Offset:       24, // IPv6 Destination Address offset //nolint:mnd
+			Len:          16, //nolint:mnd
+		},
+		&expr.Bitwise{
+			SourceRegister: 1,
+			DestRegister:   1,
+			Len:            16, //nolint:mnd
+			Mask:           mask,
+			Xor:            []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, //nolint:mnd
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     addr,
+		},
+		&expr.Verdict{Kind: expr.VerdictAccept},
+	)
+
+	rule := &nftables.Rule{
+		Table: table,
+		Chain: outputChain,
+		Exprs: exprs,
+	}
+
+	conn.AddRule(rule)
+
+	err = conn.Flush()
+	if err != nil {
+		return fmt.Errorf("flushing: %w", err)
+	}
+
+	return nil
+}

internal/firewall/nftables/support.go

@@ -0,0 +1,12 @@
+package nftables
+
+import "github.com/google/nftables"
+
+func IsSupported() bool {
+	conn, err := nftables.New()
+	if err != nil {
+		return false
+	}
+	_, err = conn.ListTable("filter")
+	return err == nil
+}