util/linuxfw, wgengine: allow ingress to magicsock UDP port on Linux (#10370)

* util/linuxfw, wgengine: allow ingress to magicsock UDP port on Linux

Updates #9084.

Currently, we have to tell users to manually open UDP ports on Linux when
certain firewalls (like ufw) are enabled. This change automates the process of
adding and updating those firewall rules as magicsock changes what port it
listens on.

Signed-off-by: Naman Sood <mail@nsood.in>

util/linuxfw/iptables_runner.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"net/netip"
 	"os/exec"
+	"strconv"
 	"strings"
 
 	"github.com/coreos/go-iptables/iptables"
@@ -236,7 +237,7 @@ func (i *iptablesRunner) AddBase(tunname string) error {
 	return nil
 }
 
-// addBase4 adds some basic IPv6 processing rules to be
+// addBase4 adds some basic IPv4 processing rules to be
 // supplemented by later calls to other helpers.
 func (i *iptablesRunner) addBase4(tunname string) error {
 	// Only allow CGNAT range traffic to come from tailscale0. There
@@ -311,7 +312,7 @@ func (i *iptablesRunner) ClampMSSToPMTU(tun string, addr netip.Addr) error {
 	return table.Append("mangle", "FORWARD", "-o", tun, "-p", "tcp", "--tcp-flags", "SYN,RST", "SYN", "-j", "TCPMSS", "--clamp-mss-to-pmtu")
 }
 
-// addBase6 adds some basic IPv4 processing rules to be
+// addBase6 adds some basic IPv6 processing rules to be
 // supplemented by later calls to other helpers.
 func (i *iptablesRunner) addBase6(tunname string) error {
 	// TODO: only allow traffic from Tailscale's ULA range to come
@@ -437,6 +438,63 @@ func (i *iptablesRunner) DelSNATRule() error {
 	return nil
 }
 
+// buildMagicsockPortRule generates the string slice containing the arguments
+// to describe a rule accepting traffic on a particular port to iptables. It is
+// separated out here to avoid repetition in AddMagicsockPortRule and
+// RemoveMagicsockPortRule, since it is important that the same rule is passed
+// to Append() and Delete().
+func buildMagicsockPortRule(port uint16) []string {
+	return []string{"-p", "udp", "--dport", strconv.FormatUint(uint64(port), 10), "-j", "ACCEPT"}
+}
+
+// AddMagicsockPortRule adds a rule to iptables to allow incoming traffic on
+// the specified UDP port, so magicsock can accept incoming connections.
+// network must be either "udp4" or "udp6" - this determines whether the rule
+// is added for IPv4 or IPv6.
+func (i *iptablesRunner) AddMagicsockPortRule(port uint16, network string) error {
+	var ipt iptablesInterface
+	switch network {
+	case "udp4":
+		ipt = i.ipt4
+	case "udp6":
+		ipt = i.ipt6
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	args := buildMagicsockPortRule(port)
+
+	if err := ipt.Append("filter", "ts-input", args...); err != nil {
+		return fmt.Errorf("adding %v in filter/ts-input: %w", args, err)
+	}
+
+	return nil
+}
+
+// DelMagicsockPortRule removes a rule added by AddMagicsockPortRule to accept
+// incoming traffic on a particular UDP port.
+// network must be either "udp4" or "udp6" - this determines whether the rule
+// is removed for IPv4 or IPv6.
+func (i *iptablesRunner) DelMagicsockPortRule(port uint16, network string) error {
+	var ipt iptablesInterface
+	switch network {
+	case "udp4":
+		ipt = i.ipt4
+	case "udp6":
+		ipt = i.ipt6
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	args := buildMagicsockPortRule(port)
+
+	if err := ipt.Delete("filter", "ts-input", args...); err != nil {
+		return fmt.Errorf("removing %v in filter/ts-input: %w", args, err)
+	}
+
+	return nil
+}
+
 // IPTablesCleanup removes all Tailscale added iptables rules.
 // Any errors that occur are logged to the provided logf.
 func IPTablesCleanup(logf logger.Logf) {

util/linuxfw/nftables_runner.go

@@ -509,6 +509,15 @@ type NetfilterRunner interface {
 	// ClampMSSToPMTU adds a rule to the mangle/FORWARD chain to clamp MSS for
 	// traffic destined for the provided tun interface.
 	ClampMSSToPMTU(tun string, addr netip.Addr) error
+
+	// AddMagicsockPortRule adds a rule to the ts-input chain to accept
+	// incoming traffic on the specified port, to allow magicsock to
+	// communicate.
+	AddMagicsockPortRule(port uint16, network string) error
+
+	// DelMagicsockPortRule removes the rule created by AddMagicsockPortRule,
+	// if it exists.
+	DelMagicsockPortRule(port uint16, network string) error
 }
 
 // New creates a NetfilterRunner, auto-detecting whether to use
@@ -584,6 +593,17 @@ func newLoadSaddrExpr(proto nftables.TableFamily, destReg uint32) (expr.Any, err
 	}
 }
 
+// newLoadDportExpr creates a new nftables express that loads the desination port
+// of a TCP/UDP packet into the given register.
+func newLoadDportExpr(destReg uint32) expr.Any {
+	return &expr.Payload{
+		DestRegister: destReg,
+		Base:         expr.PayloadBaseTransportHeader,
+		Offset:       2,
+		Len:          2,
+	}
+}
+
 // HasIPV6 reports true if the system supports IPv6.
 func (n *nftablesRunner) HasIPV6() bool {
 	return n.v6Available
@@ -1267,6 +1287,125 @@ func addAcceptOutgoingPacketRule(conn *nftables.Conn, table *nftables.Table, cha
 	return nil
 }
 
+// createAcceptOnPortRule creates a rule to accept incoming packets to
+// a given destination UDP port.
+func createAcceptOnPortRule(table *nftables.Table, chain *nftables.Chain, port uint16) *nftables.Rule {
+	portBytes := make([]byte, 2)
+	binary.BigEndian.PutUint16(portBytes, port)
+	return &nftables.Rule{
+		Table: table,
+		Chain: chain,
+		Exprs: []expr.Any{
+			&expr.Meta{
+				Key:      expr.MetaKeyL4PROTO,
+				Register: 1,
+			},
+			&expr.Cmp{
+				Op:       expr.CmpOpEq,
+				Register: 1,
+				Data:     []byte{unix.IPPROTO_UDP},
+			},
+			newLoadDportExpr(1),
+			&expr.Cmp{
+				Op:       expr.CmpOpEq,
+				Register: 1,
+				Data:     portBytes,
+			},
+			&expr.Counter{},
+			&expr.Verdict{
+				Kind: expr.VerdictAccept,
+			},
+		},
+	}
+}
+
+// addAcceptOnPortRule adds a rule to accept incoming packets to
+// a given destination UDP port.
+func addAcceptOnPortRule(conn *nftables.Conn, table *nftables.Table, chain *nftables.Chain, port uint16) error {
+	rule := createAcceptOnPortRule(table, chain, port)
+	_ = conn.AddRule(rule)
+
+	if err := conn.Flush(); err != nil {
+		return fmt.Errorf("flush add rule: %w", err)
+	}
+
+	return nil
+}
+
+// addAcceptOnPortRule removes a rule to accept incoming packets to
+// a given destination UDP port.
+func removeAcceptOnPortRule(conn *nftables.Conn, table *nftables.Table, chain *nftables.Chain, port uint16) error {
+	rule := createAcceptOnPortRule(table, chain, port)
+	rule, err := findRule(conn, rule)
+	if err != nil {
+		return fmt.Errorf("find rule: %v", err)
+	}
+
+	_ = conn.DelRule(rule)
+
+	if err := conn.Flush(); err != nil {
+		return fmt.Errorf("flush del rule: %w", err)
+	}
+
+	return nil
+}
+
+// AddMagicsockPortRule adds a rule to nftables to allow incoming traffic on
+// the specified UDP port, so magicsock can accept incoming connections.
+// network must be either "udp4" or "udp6" - this determines whether the rule
+// is added for IPv4 or IPv6.
+func (n *nftablesRunner) AddMagicsockPortRule(port uint16, network string) error {
+	var filterTable *nftables.Table
+	switch network {
+	case "udp4":
+		filterTable = n.nft4.Filter
+	case "udp6":
+		filterTable = n.nft6.Filter
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	inputChain, err := getChainFromTable(n.conn, filterTable, chainNameInput)
+	if err != nil {
+		return fmt.Errorf("get input chain: %v", err)
+	}
+
+	err = addAcceptOnPortRule(n.conn, filterTable, inputChain, port)
+	if err != nil {
+		return fmt.Errorf("add accept on port rule: %v", err)
+	}
+
+	return nil
+}
+
+// DelMagicsockPortRule removes a rule added by AddMagicsockPortRule to accept
+// incoming traffic on a particular UDP port.
+// network must be either "udp4" or "udp6" - this determines whether the rule
+// is removed for IPv4 or IPv6.
+func (n *nftablesRunner) DelMagicsockPortRule(port uint16, network string) error {
+	var filterTable *nftables.Table
+	switch network {
+	case "udp4":
+		filterTable = n.nft4.Filter
+	case "udp6":
+		filterTable = n.nft6.Filter
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	inputChain, err := getChainFromTable(n.conn, filterTable, chainNameInput)
+	if err != nil {
+		return fmt.Errorf("get input chain: %v", err)
+	}
+
+	err = removeAcceptOnPortRule(n.conn, filterTable, inputChain, port)
+	if err != nil {
+		return fmt.Errorf("add accept on port rule: %v", err)
+	}
+
+	return nil
+}
+
 // createAcceptIncomingPacketRule creates a rule to accept incoming packets to
 // the given interface.
 func createAcceptIncomingPacketRule(table *nftables.Table, chain *nftables.Chain, tunname string) *nftables.Rule {

wgengine/magicsock/magicsock.go

@@ -290,6 +290,10 @@ type Conn struct {
 
 	// wgPinger is the WireGuard only pinger used for latency measurements.
 	wgPinger lazy.SyncValue[*ping.Pinger]
+
+	// onPortUpdate is called with the new port when magicsock rebinds to
+	// a new port.
+	onPortUpdate func(port uint16, network string)
 }
 
 // SetDebugLoggingEnabled controls whether spammy debug logging is enabled.
@@ -355,6 +359,10 @@ type Options struct {
 	// ControlKnobs are the set of control knobs to use.
 	// If nil, they're ignored and not updated.
 	ControlKnobs *controlknobs.Knobs
+
+	// OnPortUpdate is called with the new port when magicsock rebinds to
+	// a new port.
+	OnPortUpdate func(port uint16, network string)
 }
 
 func (o *Options) logf() logger.Logf {
@@ -427,6 +435,7 @@ func NewConn(opts Options) (*Conn, error) {
 		c.portMapper.SetGatewayLookupFunc(opts.NetMon.GatewayAndSelfIP)
 	}
 	c.netMon = opts.NetMon
+	c.onPortUpdate = opts.OnPortUpdate
 
 	if err := c.rebind(keepCurrentPort); err != nil {
 		return nil, err
@@ -2342,6 +2351,19 @@ func (c *Conn) bindSocket(ruc *RebindingUDPConn, network string, curPortFate cur
 			c.logf("magicsock: unable to bind %v port %d: %v", network, port, err)
 			continue
 		}
+		if c.onPortUpdate != nil {
+			_, gotPortStr, err := net.SplitHostPort(pconn.LocalAddr().String())
+			if err != nil {
+				c.logf("could not parse port from %s: %w", pconn.LocalAddr().String(), err)
+			} else {
+				gotPort, err := strconv.ParseUint(gotPortStr, 10, 16)
+				if err != nil {
+					c.logf("could not parse port from %s: %w", gotPort, err)
+				} else {
+					c.onPortUpdate(uint16(gotPort), network)
+				}
+			}
+		}
 		trySetSocketBuffer(pconn, c.logf)
 
 		// Success.

wgengine/router/router.go

@@ -27,6 +27,14 @@ type Router interface {
 	// implementation should handle gracefully.
 	Set(*Config) error
 
+	// UpdateMagicsockPort tells the OS network stack what port magicsock
+	// is currently listening on, so it can be threaded through firewalls
+	// and such. This is distinct from Set() since magicsock may rebind
+	// ports independently from the Config changing.
+	//
+	// network should be either "udp4" or "udp6".
+	UpdateMagicsockPort(port uint16, network string) error
+
 	// Close closes the router.
 	Close() error
 }

wgengine/router/router_fake.go

@@ -27,6 +27,11 @@ func (r fakeRouter) Set(cfg *Config) error {
 	return nil
 }
 
+func (r fakeRouter) UpdateMagicsockPort(_ uint16, _ string) error {
+	r.logf("[v1] warning: fakeRouter.UpdateMagicsockPort: not implemented.")
+	return nil
+}
+
 func (r fakeRouter) Close() error {
 	r.logf("[v1] warning: fakeRouter.Close: not implemented.")
 	return nil

wgengine/router/router_linux.go

@@ -63,6 +63,9 @@ type linuxRouter struct {
 
 	cmd commandRunner
 	nfr linuxfw.NetfilterRunner
+
+	magicsockPortV4 uint16
+	magicsockPortV6 uint16
 }
 
 func newUserspaceRouter(logf logger.Logf, tunDev tun.Device, netMon *netmon.Monitor) (Router, error) {
@@ -400,6 +403,53 @@ func (r *linuxRouter) Set(cfg *Config) error {
 	return multierr.New(errs...)
 }
 
+// UpdateMagicsockPort implements the Router interface.
+func (r *linuxRouter) UpdateMagicsockPort(port uint16, network string) error {
+	if r.nfr == nil {
+		if err := r.setupNetfilter(r.netfilterKind); err != nil {
+			return fmt.Errorf("could not setup netfilter: %w", err)
+		}
+	}
+
+	var magicsockPort *uint16
+	switch network {
+	case "udp4":
+		magicsockPort = &r.magicsockPortV4
+	case "udp6":
+		if !r.nfr.HasIPV6() {
+			return nil
+		}
+		magicsockPort = &r.magicsockPortV6
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	// set the port, we'll make the firewall rule when netfilter turns back on
+	if r.netfilterMode == netfilterOff {
+		*magicsockPort = port
+		return nil
+	}
+
+	if *magicsockPort == port {
+		return nil
+	}
+
+	if *magicsockPort != 0 {
+		if err := r.nfr.DelMagicsockPortRule(*magicsockPort, network); err != nil {
+			return fmt.Errorf("del magicsock port rule: %w", err)
+		}
+	}
+
+	if port != 0 {
+		if err := r.nfr.AddMagicsockPortRule(*magicsockPort, network); err != nil {
+			return fmt.Errorf("add magicsock port rule: %w", err)
+		}
+	}
+
+	*magicsockPort = port
+	return nil
+}
+
 // setNetfilterMode switches the router to the given netfilter
 // mode. Netfilter state is created or deleted appropriately to
 // reflect the new mode, and r.snatSubnetRoutes is updated to reflect
@@ -468,6 +518,16 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 			if err := r.nfr.AddBase(r.tunname); err != nil {
 				return err
 			}
+			if r.magicsockPortV4 != 0 {
+				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV4, "udp4"); err != nil {
+					return fmt.Errorf("could not add magicsock port rule v4: %w", err)
+				}
+			}
+			if r.magicsockPortV6 != 0 && r.nfr.HasIPV6() {
+				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV6, "udp6"); err != nil {
+					return fmt.Errorf("could not add magicsock port rule v6: %w", err)
+				}
+			}
 			r.snatSubnetRoutes = false
 		case netfilterOn:
 			if err := r.nfr.DelHooks(r.logf); err != nil {
@@ -498,6 +558,16 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 			if err := r.nfr.AddBase(r.tunname); err != nil {
 				return err
 			}
+			if r.magicsockPortV4 != 0 {
+				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV4, "udp4"); err != nil {
+					return fmt.Errorf("could not add magicsock port rule v4: %w", err)
+				}
+			}
+			if r.magicsockPortV6 != 0 && r.nfr.HasIPV6() {
+				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV6, "udp6"); err != nil {
+					return fmt.Errorf("could not add magicsock port rule v6: %w", err)
+				}
+			}
 			r.snatSubnetRoutes = false
 		case netfilterNoDivert:
 			reprocess = true

wgengine/router/router_linux_test.go

@@ -607,6 +607,58 @@ func (n *fakeIPTablesRunner) DelSNATRule() error {
 	return nil
 }
 
+// buildMagicsockPortRule builds a fake rule to use in AddMagicsockPortRule and
+// DelMagicsockPortRule below.
+func buildMagicsockPortRule(port uint16) string {
+	return fmt.Sprintf("-p udp --dport %v -j ACCEPT", port)
+}
+
+// AddMagicsockPortRule implements the NetfilterRunner interface, but stores
+// rules in fakeIPTablesRunner's internal maps rather than actually calling out
+// to iptables. This is mainly to test the linux router implementation.
+func (n *fakeIPTablesRunner) AddMagicsockPortRule(port uint16, network string) error {
+	var ipt map[string][]string
+	switch network {
+	case "udp4":
+		ipt = n.ipt4
+	case "udp6":
+		ipt = n.ipt6
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	rule := buildMagicsockPortRule(port)
+
+	if err := appendRule(n, ipt, "filter/ts-input", rule); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// DelMagicsockPortRule implements the NetfilterRunner interface, but removes
+// rules from fakeIPTablesRunner's internal maps rather than actually calling
+// out to iptables. This is mainly to test the linux router implementation.
+func (n *fakeIPTablesRunner) DelMagicsockPortRule(port uint16, network string) error {
+	var ipt map[string][]string
+	switch network {
+	case "udp4":
+		ipt = n.ipt4
+	case "udp6":
+		ipt = n.ipt6
+	default:
+		return fmt.Errorf("unsupported network %s", network)
+	}
+
+	rule := buildMagicsockPortRule(port)
+
+	if err := deleteRule(n, ipt, "filter/ts-input", rule); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func (n *fakeIPTablesRunner) HasIPV6() bool    { return true }
 func (n *fakeIPTablesRunner) HasIPV6NAT() bool { return true }
 

wgengine/router/router_openbsd.go

@@ -228,6 +228,13 @@ func (r *openbsdRouter) Set(cfg *Config) error {
 	return errq
 }
 
+// UpdateMagicsockPort implements the Router interface. This implementation
+// does nothing and returns nil because this router does not currently need
+// to know what the magicsock UDP port is.
+func (r *openbsdRouter) UpdateMagicsockPort(_ uint16, _ string) error {
+	return nil
+}
+
 func (r *openbsdRouter) Close() error {
 	cleanup(r.logf, r.tunname)
 	return nil

wgengine/router/router_userspace_bsd.go

@@ -196,6 +196,13 @@ func (r *userspaceBSDRouter) Set(cfg *Config) (reterr error) {
 	return reterr
 }
 
+// UpdateMagicsockPort implements the Router interface. This implementation
+// does nothing and returns nil because this router does not currently need
+// to know what the magicsock UDP port is.
+func (r *userspaceBSDRouter) UpdateMagicsockPort(_ uint16, _ string) error {
+	return nil
+}
+
 func (r *userspaceBSDRouter) Close() error {
 	return nil
 }

wgengine/router/router_windows.go

@@ -102,6 +102,13 @@ func hasDefaultRoute(routes []netip.Prefix) bool {
 	return false
 }
 
+// UpdateMagicsockPort implements the Router interface. This implementation
+// does nothing and returns nil because this router does not currently need
+// to know what the magicsock UDP port is.
+func (r *winRouter) UpdateMagicsockPort(_ uint16, _ string) error {
+	return nil
+}
+
 func (r *winRouter) Close() error {
 	r.firewall.clear()
 

wgengine/userspace.go

@@ -324,6 +324,13 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 
 		e.RequestStatus()
 	}
+	onPortUpdate := func(port uint16, network string) {
+		e.logf("onPortUpdate(port=%v, network=%s)", port, network)
+
+		if err := e.router.UpdateMagicsockPort(port, network); err != nil {
+			e.logf("UpdateMagicsockPort(port=%v, network=%s) failed: %w", port, network, err)
+		}
+	}
 	magicsockOpts := magicsock.Options{
 		Logf:             logf,
 		Port:             conf.ListenPort,
@@ -333,6 +340,7 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		NoteRecvActivity: e.noteRecvActivity,
 		NetMon:           e.netMon,
 		ControlKnobs:     conf.ControlKnobs,
+		OnPortUpdate:     onPortUpdate,
 	}
 
 	var err error