Path MTU discovery fixes and improvements (#3109)

- Existing option `WIREGUARD_MTU` , if set, disables PMTUD and is used
- New option `PMTUD_ICMP_ADDRESSES=1.1.1.1,8.8.8.8` and `PMTUD_TCP_ADDRESSES=1.1.1.1:443,8.8.8.8:443`
- ICMP PMTUD now targets external-by-default IP addresses
- New TCP PMTUD (binary search only) as a second MTU confirmation and fallback mechanism.
- Force set TCP MSS to MTU - IP header - TCP base header - "magic 20 bytes" 🎆
- Fix #3108

.devcontainer/Dockerfile

@@ -1,2 +1,2 @@
 FROM ghcr.io/qdm12/godevcontainer:v0.21-alpine
-RUN apk add wireguard-tools htop openssl
+RUN apk add wireguard-tools htop openssl tcpdump

.golangci.yml

@@ -22,6 +22,7 @@ linters:
         - "^disabled$"
         # Firewall and routing strings
         - "^(ACCEPT|DROP)$"
+        - "^--append$"
         - "^--delete$"
         - "^all$"
         - "^(tcp|udp)$"

Dockerfile

@@ -110,8 +110,11 @@ ENV VPN_SERVICE_PROVIDER=pia \
     WIREGUARD_PERSISTENT_KEEPALIVE_INTERVAL=0 \
     WIREGUARD_ADDRESSES= \
     WIREGUARD_ADDRESSES_SECRETFILE=/run/secrets/wireguard_addresses \
-    WIREGUARD_MTU=1320 \
+    WIREGUARD_MTU= \
     WIREGUARD_IMPLEMENTATION=auto \
+    # PMTUD
+    PMTUD_ICMP_ADDRESSES=1.1.1.1,8.8.8.8 \
+    PMTUD_TCP_ADDRESSES=1.1.1.1:443,8.8.8.8:443 \
     # VPN server filtering
     SERVER_REGIONS= \
     SERVER_COUNTRIES= \

internal/configuration/settings/pmtud.go

@@ -0,0 +1,108 @@
+package settings
+
+import (
+	"errors"
+	"fmt"
+	"net/netip"
+	"strings"
+
+	"github.com/qdm12/gosettings"
+	"github.com/qdm12/gosettings/reader"
+	"github.com/qdm12/gotree"
+)
+
+// PMTUD contains settings to configure Path MTU Discovery.
+type PMTUD struct {
+	// ICMPAddresses is the redundancy list of addresses to use
+	// for ICMP path MTU discovery. Each address MUST handle ICMP
+	// packets for PMTUD to work.
+	// It cannot be nil in the internal state.
+	ICMPAddresses []netip.Addr `json:"icmp_addresses"`
+	// TCPAddresses is the redundancy list of addresses to use
+	// for TCP path MTU discovery. Each address MUST have a listening
+	// TCP server on the port specified.
+	// It cannot be nil in the internal state.
+	TCPAddresses []netip.AddrPort `json:"tcp_addresses"`
+}
+
+var (
+	ErrPMTUDICMPAddressNotValid = errors.New("PMTUD ICMP address is not valid")
+	ErrPMTUDTCPAddressNotValid  = errors.New("PMTUD TCP address is not valid")
+)
+
+// Validate validates PMTUD settings.
+func (p PMTUD) validate() (err error) {
+	for i, addr := range p.ICMPAddresses {
+		if !addr.IsValid() {
+			return fmt.Errorf("%w: at index %d", ErrPMTUDICMPAddressNotValid, i)
+		}
+	}
+	for i, addr := range p.TCPAddresses {
+		if !addr.IsValid() {
+			return fmt.Errorf("%w: at index %d", ErrPMTUDTCPAddressNotValid, i)
+		}
+	}
+	return nil
+}
+
+func (p *PMTUD) copy() (copied PMTUD) {
+	return PMTUD{
+		ICMPAddresses: gosettings.CopySlice(p.ICMPAddresses),
+		TCPAddresses:  gosettings.CopySlice(p.TCPAddresses),
+	}
+}
+
+func (p *PMTUD) overrideWith(other PMTUD) {
+	p.ICMPAddresses = gosettings.OverrideWithSlice(p.ICMPAddresses, other.ICMPAddresses)
+	p.TCPAddresses = gosettings.OverrideWithSlice(p.TCPAddresses, other.TCPAddresses)
+}
+
+func (p *PMTUD) setDefaults() {
+	defaultICMPAddresses := []netip.Addr{
+		netip.AddrFrom4([4]byte{1, 1, 1, 1}),
+		netip.AddrFrom4([4]byte{8, 8, 8, 8}),
+	}
+	p.ICMPAddresses = gosettings.DefaultSlice(p.ICMPAddresses, defaultICMPAddresses)
+
+	const tlsPort = 443
+	defaultTCPAddresses := []netip.AddrPort{
+		netip.AddrPortFrom(netip.AddrFrom4([4]byte{1, 1, 1, 1}), tlsPort),
+		netip.AddrPortFrom(netip.AddrFrom4([4]byte{8, 8, 8, 8}), tlsPort),
+	}
+	p.TCPAddresses = gosettings.DefaultSlice(p.TCPAddresses, defaultTCPAddresses)
+}
+
+func (p PMTUD) String() string {
+	return p.toLinesNode().String()
+}
+
+func (p PMTUD) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("Path MTU discovery:")
+
+	addrs := make([]string, len(p.ICMPAddresses))
+	for i, addr := range p.ICMPAddresses {
+		addrs[i] = addr.String()
+	}
+	node.Appendf("ICMP addresses: %s", strings.Join(addrs, ", "))
+
+	addrs = make([]string, len(p.TCPAddresses))
+	for i, addr := range p.TCPAddresses {
+		addrs[i] = addr.String()
+	}
+	node.Appendf("TCP addresses: %s", strings.Join(addrs, ", "))
+	return node
+}
+
+func (p *PMTUD) read(r *reader.Reader) (err error) {
+	p.ICMPAddresses, err = r.CSVNetipAddresses("PMTUD_ICMP_ADDRESSES")
+	if err != nil {
+		return err
+	}
+
+	p.TCPAddresses, err = r.CSVNetipAddrPorts("PMTUD_TCP_ADDRESSES")
+	if err != nil {
+		return err
+	}
+
+	return nil
+}

internal/configuration/settings/settings_test.go

@@ -29,14 +29,17 @@ func Test_Settings_String(t *testing.T) {
 |   |       └── OpenVPN server selection settings:
 |   |           ├── Protocol: UDP
 |   |           └── Private Internet Access encryption preset: strong
-|   └── OpenVPN settings:
-|       ├── OpenVPN version: 2.6
-|       ├── User: [not set]
-|       ├── Password: [not set]
-|       ├── Private Internet Access encryption preset: strong
-|       ├── Network interface: tun0
-|       ├── Run OpenVPN as: root
-|       └── Verbosity level: 1
+|   ├── OpenVPN settings:
+|   |   ├── OpenVPN version: 2.6
+|   |   ├── User: [not set]
+|   |   ├── Password: [not set]
+|   |   ├── Private Internet Access encryption preset: strong
+|   |   ├── Network interface: tun0
+|   |   ├── Run OpenVPN as: root
+|   |   └── Verbosity level: 1
+|   └── Path MTU discovery:
+|       ├── ICMP addresses: 1.1.1.1, 8.8.8.8
+|       └── TCP addresses: 1.1.1.1:443, 8.8.8.8:443
 ├── DNS settings:
 |   ├── Keep existing nameserver(s): no
 |   ├── DNS server address to use: 127.0.0.1

internal/configuration/settings/vpn.go

@@ -18,6 +18,7 @@ type VPN struct {
 	Provider  Provider  `json:"provider"`
 	OpenVPN   OpenVPN   `json:"openvpn"`
 	Wireguard Wireguard `json:"wireguard"`
+	PMTUD     PMTUD     `json:"pmtud"`
 }
 
 // TODO v4 remove pointer for receiver (because of Surfshark).
@@ -45,6 +46,11 @@ func (v *VPN) Validate(filterChoicesGetter FilterChoicesGetter, ipv6Supported bo
 		}
 	}
 
+	err = v.PMTUD.validate()
+	if err != nil {
+		return fmt.Errorf("PMTUD settings: %w", err)
+	}
+
 	return nil
 }
 
@@ -54,6 +60,7 @@ func (v *VPN) Copy() (copied VPN) {
 		Provider:  v.Provider.copy(),
 		OpenVPN:   v.OpenVPN.copy(),
 		Wireguard: v.Wireguard.copy(),
+		PMTUD:     v.PMTUD.copy(),
 	}
 }
 
@@ -62,6 +69,7 @@ func (v *VPN) OverrideWith(other VPN) {
 	v.Provider.overrideWith(other.Provider)
 	v.OpenVPN.overrideWith(other.OpenVPN)
 	v.Wireguard.overrideWith(other.Wireguard)
+	v.PMTUD.overrideWith(other.PMTUD)
 }
 
 func (v *VPN) setDefaults() {
@@ -69,6 +77,7 @@ func (v *VPN) setDefaults() {
 	v.Provider.setDefaults()
 	v.OpenVPN.setDefaults(v.Provider.Name)
 	v.Wireguard.setDefaults(v.Provider.Name)
+	v.PMTUD.setDefaults()
 }
 
 func (v VPN) String() string {
@@ -85,6 +94,7 @@ func (v VPN) toLinesNode() (node *gotree.Node) {
 	} else {
 		node.AppendNode(v.Wireguard.toLinesNode())
 	}
+	node.AppendNode(v.PMTUD.toLinesNode())
 
 	return node
 }
@@ -107,5 +117,10 @@ func (v *VPN) read(r *reader.Reader) (err error) {
 		return fmt.Errorf("wireguard: %w", err)
 	}
 
+	err = v.PMTUD.read(r)
+	if err != nil {
+		return fmt.Errorf("PMTUD: %w", err)
+	}
+
 	return nil
 }

internal/configuration/settings/wireguard.go

@@ -38,15 +38,9 @@ type Wireguard struct {
 	Interface                   string         `json:"interface"`
 	PersistentKeepaliveInterval *time.Duration `json:"persistent_keep_alive_interval"`
 	// Maximum Transmission Unit (MTU) of the Wireguard interface.
-	// It cannot be zero in the internal state, and defaults to
-	// 1320. Note it is not the wireguard-go MTU default of 1420
-	// because this impacts bandwidth a lot on some VPN providers,
-	// see https://github.com/qdm12/gluetun/issues/1650.
-	// It has been lowered to 1320 following quite a bit of
-	// investigation in the issue:
-	// https://github.com/qdm12/gluetun/issues/2533.
-	// Note this should now be replaced with the PMTUD feature.
-	MTU uint32 `json:"mtu"`
+	// It cannot be nil in the internal state, and defaults to
+	// 0 indicating to use PMTUD.
+	MTU *uint32 `json:"mtu"`
 	// Implementation is the Wireguard implementation to use.
 	// It can be "auto", "userspace" or "kernelspace".
 	// It defaults to "auto" and cannot be the empty string
@@ -195,8 +189,7 @@ func (w *Wireguard) setDefaults(vpnProvider string) {
 	w.AllowedIPs = gosettings.DefaultSlice(w.AllowedIPs, defaultAllowedIPs)
 	w.PersistentKeepaliveInterval = gosettings.DefaultPointer(w.PersistentKeepaliveInterval, 0)
 	w.Interface = gosettings.DefaultComparable(w.Interface, "wg0")
-	const defaultMTU = 1320
-	w.MTU = gosettings.DefaultComparable(w.MTU, defaultMTU)
+	w.MTU = gosettings.DefaultPointer(w.MTU, 0)
 	w.Implementation = gosettings.DefaultComparable(w.Implementation, "auto")
 }
 
@@ -232,7 +225,11 @@ func (w Wireguard) toLinesNode() (node *gotree.Node) {
 	}
 
 	interfaceNode := node.Appendf("Network interface: %s", w.Interface)
-	interfaceNode.Appendf("MTU: %d", w.MTU)
+	if *w.MTU == 0 {
+		interfaceNode.Append("MTU: use path MTU discovery")
+	} else {
+		interfaceNode.Appendf("MTU: %d", *w.MTU)
+	}
 
 	if w.Implementation != "auto" {
 		node.Appendf("Implementation: %s", w.Implementation)
@@ -273,11 +270,9 @@ func (w *Wireguard) read(r *reader.Reader) (err error) {
 		return err
 	}
 
-	mtuPtr, err := r.Uint32Ptr("WIREGUARD_MTU")
+	w.MTU, err = r.Uint32Ptr("WIREGUARD_MTU")
 	if err != nil {
 		return err
-	} else if mtuPtr != nil {
-		w.MTU = *mtuPtr
 	}
 	return nil
 }

internal/firewall/iptables.go

@@ -29,17 +29,16 @@ func appendOrDelete(remove bool) string {
 // flipRule changes an append rule in a delete rule or a delete rule into an
 // append rule.
 func flipRule(rule string) string {
-	switch {
-	case strings.HasPrefix(rule, "-A"):
-		return strings.Replace(rule, "-A", "-D", 1)
-	case strings.HasPrefix(rule, "--append"):
-		return strings.Replace(rule, "--append", "-D", 1)
-	case strings.HasPrefix(rule, "-D"):
-		return strings.Replace(rule, "-D", "-A", 1)
-	case strings.HasPrefix(rule, "--delete"):
-		return strings.Replace(rule, "--delete", "-A", 1)
+	fields := strings.Fields(rule)
+	for i, field := range fields {
+		switch field {
+		case "-A", "--append":
+			fields[i] = "--delete"
+		case "-D", "--delete":
+			fields[i] = "--append"
 		}
-	return rule
+	}
+	return strings.Join(fields, " ")
 }
 
 // Version obtains the version of the installed iptables.
@@ -86,11 +85,15 @@ func (c *Config) runIptablesInstruction(ctx context.Context, instruction string)
 }
 
 func (c *Config) clearAllRules(ctx context.Context) error {
+	tables := []string{"filter"}
+	for _, table := range tables {
 		return c.runMixedIptablesInstructions(ctx, []string{
-		"--flush",        // flush all chains
-		"--delete-chain", // delete all chains
+			"-t " + table + " --flush",        // flush all chains
+			"-t " + table + " --delete-chain", // delete all chains
 		})
 	}
+	return nil
+}
 
 func (c *Config) setIPv4AllPolicies(ctx context.Context, policy string) error {
 	switch policy {

internal/netlink/route.go

@@ -18,6 +18,7 @@ type Route struct {
 	Type      uint8
 	Scope     uint8
 	Proto     uint8
+	AdvMSS    uint32
 }
 
 func (r *Route) fromMessage(message rtnetlink.RouteMessage) {
@@ -35,6 +36,9 @@ func (r *Route) fromMessage(message rtnetlink.RouteMessage) {
 	r.Type = message.Type
 	r.Scope = message.Scope
 	r.Proto = message.Protocol
+	if metrics := message.Attributes.Metrics; metrics != nil {
+		r.AdvMSS = metrics.AdvMSS
+	}
 }
 
 func (r Route) message() *rtnetlink.RouteMessage {
@@ -58,7 +62,6 @@ func (r Route) message() *rtnetlink.RouteMessage {
 		Protocol:  r.Proto,
 		Attributes: rtnetlink.RouteAttributes{
 			OutIface: r.LinkIndex,
-			Dst:      *dst, // there should always be a dst for routes
 			Gateway:  netipAddrToNetIP(r.Gw),
 			Priority: r.Priority,
 			Table:    extendedTable,
@@ -67,6 +70,15 @@ func (r Route) message() *rtnetlink.RouteMessage {
 	if src != nil { // src is optional
 		message.Attributes.Src = *src
 	}
+	if dst != nil {
+		message.Attributes.Dst = *dst
+	}
+	if r.AdvMSS != 0 {
+		if message.Attributes.Metrics == nil {
+			message.Attributes.Metrics = &rtnetlink.RouteMetrics{}
+		}
+		message.Attributes.Metrics.AdvMSS = r.AdvMSS
+	}
 	return message
 }
 

internal/pmtud/constants/lengths.go

@@ -0,0 +1,24 @@
+package constants
+
+const (
+	MaxEthernetFrameSize uint32 = 1500
+	// MinIPv4MTU is defined according to
+	// https://en.wikipedia.org/wiki/Maximum_transmission_unit#MTUs_for_common_media
+	MinIPv4MTU uint32 = 68
+	MinIPv6MTU uint32 = 1280
+
+	IPv4HeaderLength uint32 = 20
+	IPv6HeaderLength uint32 = 40
+	UDPHeaderLength  uint32 = 8
+	// BaseTCPHeaderLength is the TCP header length without options,
+	// which is the minimum TCP header length.
+	BaseTCPHeaderLength uint32 = 20
+	// MaxTCPHeaderLength is the TCP header length with the maximum options length of 40 bytes.
+	// Note this is a hard maximum because of the 4-bit data offset field in the TCP header (15x4=60).
+	MaxTCPHeaderLength     uint32 = 60
+	WireguardHeaderLength  uint32 = 32
+	OpenVPNHeaderMaxLength uint32 = 1 + // opcode
+		8 + // session id
+		4 + // packet id
+		28 // max possible auth tag/iv
+)

internal/pmtud/errors.go

@@ -1,29 +0,0 @@
-package pmtud
-
-import (
-	"context"
-	"errors"
-	"fmt"
-	"net"
-	"strings"
-	"time"
-)
-
-var (
-	ErrICMPNotPermitted                            = errors.New("ICMP not permitted")
-	ErrICMPDestinationUnreachable                  = errors.New("ICMP destination unreachable")
-	ErrICMPCommunicationAdministrativelyProhibited = errors.New("communication administratively prohibited")
-	ErrICMPBodyUnsupported                         = errors.New("ICMP body type is not supported")
-)
-
-func wrapConnErr(err error, timedCtx context.Context, pingTimeout time.Duration) error { //nolint:revive
-	switch {
-	case strings.HasSuffix(err.Error(), "sendto: operation not permitted"):
-		err = fmt.Errorf("%w", ErrICMPNotPermitted)
-	case errors.Is(timedCtx.Err(), context.DeadlineExceeded):
-		err = fmt.Errorf("%w (timed out after %s)", net.ErrClosed, pingTimeout)
-	case timedCtx.Err() != nil:
-		err = timedCtx.Err()
-	}
-	return err
-}

internal/pmtud/icmp/apple_ipv4.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	"net"

internal/pmtud/icmp/check.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	"bytes"
@@ -9,17 +9,17 @@ import (
 )
 
 var (
-	ErrICMPNextHopMTUTooLow  = errors.New("ICMP Next Hop MTU is too low")
-	ErrICMPNextHopMTUTooHigh = errors.New("ICMP Next Hop MTU is too high")
+	ErrNextHopMTUTooLow  = errors.New("ICMP Next Hop MTU is too low")
+	ErrNextHopMTUTooHigh = errors.New("ICMP Next Hop MTU is too high")
 )
 
 func checkMTU(mtu, minMTU, physicalLinkMTU uint32) (err error) {
 	switch {
 	case mtu < minMTU:
-		return fmt.Errorf("%w: %d", ErrICMPNextHopMTUTooLow, mtu)
+		return fmt.Errorf("%w: %d", ErrNextHopMTUTooLow, mtu)
 	case mtu > physicalLinkMTU:
 		return fmt.Errorf("%w: %d is larger than physical link MTU %d",
-			ErrICMPNextHopMTUTooHigh, mtu, physicalLinkMTU)
+			ErrNextHopMTUTooHigh, mtu, physicalLinkMTU)
 	default:
 		return nil
 	}
@@ -34,13 +34,13 @@ func checkInvokingReplyIDMatch(icmpProtocol int, received []byte,
 	}
 	inboundBody, ok := inboundMessage.Body.(*icmp.Echo)
 	if !ok {
-		return false, fmt.Errorf("%w: %T", ErrICMPBodyUnsupported, inboundMessage.Body)
+		return false, fmt.Errorf("%w: %T", ErrBodyUnsupported, inboundMessage.Body)
 	}
 	outboundBody := outboundMessage.Body.(*icmp.Echo) //nolint:forcetypeassert
 	return inboundBody.ID == outboundBody.ID, nil
 }
 
-var ErrICMPIDMismatch = errors.New("ICMP id mismatch")
+var ErrIDMismatch = errors.New("ICMP id mismatch")
 
 func checkEchoReply(icmpProtocol int, received []byte,
 	outboundMessage *icmp.Message, truncatedBody bool,
@@ -51,12 +51,12 @@ func checkEchoReply(icmpProtocol int, received []byte,
 	}
 	inboundBody, ok := inboundMessage.Body.(*icmp.Echo)
 	if !ok {
-		return fmt.Errorf("%w: %T", ErrICMPBodyUnsupported, inboundMessage.Body)
+		return fmt.Errorf("%w: %T", ErrBodyUnsupported, inboundMessage.Body)
 	}
 	outboundBody := outboundMessage.Body.(*icmp.Echo) //nolint:forcetypeassert
 	if inboundBody.ID != outboundBody.ID {
 		return fmt.Errorf("%w: sent id %d and received id %d",
-			ErrICMPIDMismatch, outboundBody.ID, inboundBody.ID)
+			ErrIDMismatch, outboundBody.ID, inboundBody.ID)
 	}
 	err = checkEchoBodies(outboundBody.Data, inboundBody.Data, truncatedBody)
 	if err != nil {
@@ -65,19 +65,19 @@ func checkEchoReply(icmpProtocol int, received []byte,
 	return nil
 }
 
-var ErrICMPEchoDataMismatch = errors.New("ICMP data mismatch")
+var ErrEchoDataMismatch = errors.New("ICMP data mismatch")
 
 func checkEchoBodies(sent, received []byte, receivedTruncated bool) (err error) {
 	if len(received) > len(sent) {
 		return fmt.Errorf("%w: sent %d bytes and received %d bytes",
-			ErrICMPEchoDataMismatch, len(sent), len(received))
+			ErrEchoDataMismatch, len(sent), len(received))
 	}
 	if receivedTruncated {
 		sent = sent[:len(received)]
 	}
 	if !bytes.Equal(received, sent) {
 		return fmt.Errorf("%w: sent %x and received %x",
-			ErrICMPEchoDataMismatch, sent, received)
+			ErrEchoDataMismatch, sent, received)
 	}
 	return nil
 }

internal/pmtud/icmp/df.go

@@ -1,6 +1,6 @@
 //go:build !linux && !windows
 
-package pmtud
+package icmp
 
 // setDontFragment for platforms other than Linux and Windows
 // is not implemented, so we just return assuming the don't

internal/pmtud/icmp/df_linux.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	"syscall"

internal/pmtud/icmp/df_windows.go

@@ -1,6 +1,4 @@
-//go:build windows
-
-package pmtud
+package icmp
 
 import (
 	"syscall"

internal/pmtud/icmp/errors.go

@@ -0,0 +1,30 @@
+package icmp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"strings"
+	"time"
+)
+
+var (
+	ErrNotPermitted                            = errors.New("ICMP not permitted")
+	ErrDestinationUnreachable                  = errors.New("ICMP destination unreachable")
+	ErrCommunicationAdministrativelyProhibited = errors.New("communication administratively prohibited")
+	ErrBodyUnsupported                         = errors.New("ICMP body type is not supported")
+	ErrMTUNotFound                             = errors.New("MTU not found")
+)
+
+func wrapConnErr(err error, timedCtx context.Context, pingTimeout time.Duration) error { //nolint:revive
+	switch {
+	case strings.HasSuffix(err.Error(), "sendto: operation not permitted"):
+		err = fmt.Errorf("%w", ErrNotPermitted)
+	case errors.Is(timedCtx.Err(), context.DeadlineExceeded):
+		err = fmt.Errorf("%w (timed out after %s)", net.ErrClosed, pingTimeout)
+	case timedCtx.Err() != nil:
+		err = timedCtx.Err()
+	}
+	return err
+}

internal/pmtud/icmp/icmp.go

@@ -0,0 +1,53 @@
+package icmp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/netip"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+// PathMTUDiscover discovers the path MTU to the given IP address
+// using ICMP.
+// It first tries to get the next hop MTU using ICMP messages.
+// If that fails, it falls back to sending echo requests with
+// different packet sizes to find the maximum MTU.
+// The function returns [ErrMTUNotFound] if the MTU could not be determined.
+func PathMTUDiscover(ctx context.Context, ip netip.Addr,
+	physicalLinkMTU uint32, timeout time.Duration, logger Logger,
+) (mtu uint32, err error) {
+	if ip.Is4() {
+		logger.Debug("finding IPv4 next hop MTU")
+		mtu, err = findIPv4NextHopMTU(ctx, ip, physicalLinkMTU, timeout, logger)
+		switch {
+		case err == nil:
+			return mtu, nil
+		case errors.Is(err, net.ErrClosed) || errors.Is(err, ErrCommunicationAdministrativelyProhibited): // blackhole
+		default:
+			return 0, fmt.Errorf("finding IPv4 next hop MTU: %w", err)
+		}
+	} else {
+		logger.Debug("requesting IPv6 ICMP packet-too-big reply")
+		mtu, err = getIPv6PacketTooBig(ctx, ip, physicalLinkMTU, timeout, logger)
+		switch {
+		case err == nil:
+			return mtu, nil
+		case errors.Is(err, net.ErrClosed): // blackhole
+		default:
+			return 0, fmt.Errorf("getting IPv6 packet-too-big message: %w", err)
+		}
+	}
+
+	// Fall back method: send echo requests with different packet
+	// sizes and check which ones succeed to find the maximum MTU.
+	logger.Debug("falling back to sending different sized echo packets")
+	minMTU := constants.MinIPv4MTU
+	if ip.Is6() {
+		minMTU = constants.MinIPv6MTU
+	}
+	return pmtudMultiSizes(ctx, ip, minMTU, physicalLinkMTU, timeout, logger)
+}

internal/pmtud/icmp/interfaces.go

@@ -0,0 +1,7 @@
+package icmp
+
+type Logger interface {
+	Debug(msg string)
+	Debugf(msg string, args ...any)
+	Warnf(msg string, args ...any)
+}

internal/pmtud/icmp/ipv4.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	"context"
@@ -11,14 +11,13 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
 	"golang.org/x/net/icmp"
 	"golang.org/x/net/ipv4"
 )
 
 const (
-	// see https://en.wikipedia.org/wiki/Maximum_transmission_unit#MTUs_for_common_media
-	minIPv4MTU     uint32 = 68
-	icmpv4Protocol int    = 1
+	icmpv4Protocol = 1
 )
 
 func listenICMPv4(ctx context.Context) (conn net.PacketConn, err error) {
@@ -38,7 +37,7 @@ func listenICMPv4(ctx context.Context) (conn net.PacketConn, err error) {
 	packetConn, err := listenConfig.ListenPacket(ctx, "ip4:icmp", listenAddress)
 	if err != nil {
 		if strings.HasSuffix(err.Error(), "socket: operation not permitted") {
-			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrICMPNotPermitted)
+			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrNotPermitted)
 		}
 		return nil, err
 	}
@@ -83,7 +82,9 @@ func findIPv4NextHopMTU(ctx context.Context, ip netip.Addr,
 
 	buffer := make([]byte, physicalLinkMTU)
 
-	for { // for loop in case we read an echo reply for another ICMP request
+	// for loop in case we read an ICMP message from another ICMP request
+	// or TCP/UDP traffic triggering an ICMP response.
+	for {
 		// Note we need to read the whole packet in one call to ReadFrom, so the buffer
 		// must be large enough to read the entire reply packet. See:
 		// https://groups.google.com/g/golang-nuts/c/5dy2Q4nPs08/m/KmuSQAGEtG4J
@@ -108,24 +109,27 @@ func findIPv4NextHopMTU(ctx context.Context, ip netip.Addr,
 		switch typedBody := inboundMessage.Body.(type) {
 		case *icmp.DstUnreach:
 			const fragmentationRequiredAndDFFlagSetCode = 4
+			const portUnreachable = 3
 			const communicationAdministrativelyProhibitedCode = 13
 			switch inboundMessage.Code {
 			case fragmentationRequiredAndDFFlagSetCode:
+			case portUnreachable: // triggered by TCP or UDP from applications
+				continue // ignore and wait for the next message
 			case communicationAdministrativelyProhibitedCode:
 				return 0, fmt.Errorf("%w: %w (code %d)",
-					ErrICMPDestinationUnreachable,
-					ErrICMPCommunicationAdministrativelyProhibited,
+					ErrDestinationUnreachable,
+					ErrCommunicationAdministrativelyProhibited,
 					inboundMessage.Code)
 			default:
 				return 0, fmt.Errorf("%w: code %d",
-					ErrICMPDestinationUnreachable, inboundMessage.Code)
+					ErrDestinationUnreachable, inboundMessage.Code)
 			}
 
 			// See https://datatracker.ietf.org/doc/html/rfc1191#section-4
 			// Note: the go library does not handle this NextHopMTU section.
 			nextHopMTU := packetBytes[6:8]
 			mtu = uint32(binary.BigEndian.Uint16(nextHopMTU))
-			err = checkMTU(mtu, minIPv4MTU, physicalLinkMTU)
+			err = checkMTU(mtu, constants.MinIPv4MTU, physicalLinkMTU)
 			if err != nil {
 				return 0, fmt.Errorf("checking next-hop-mtu found: %w", err)
 			}
@@ -153,7 +157,7 @@ func findIPv4NextHopMTU(ctx context.Context, ip netip.Addr,
 				inboundID, outboundID)
 			continue
 		default:
-			return 0, fmt.Errorf("%w: %T", ErrICMPBodyUnsupported, typedBody)
+			return 0, fmt.Errorf("%w: %T", ErrBodyUnsupported, typedBody)
 		}
 	}
 }

internal/pmtud/icmp/ipv6.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	"context"
@@ -8,12 +8,12 @@ import (
 	"strings"
 	"time"
 
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
 	"golang.org/x/net/icmp"
 	"golang.org/x/net/ipv6"
 )
 
 const (
-	minIPv6MTU     = 1280
 	icmpv6Protocol = 58
 )
 
@@ -23,7 +23,7 @@ func listenICMPv6(ctx context.Context) (conn net.PacketConn, err error) {
 	packetConn, err := listenConfig.ListenPacket(ctx, "ip6:ipv6-icmp", listenAddress)
 	if err != nil {
 		if strings.HasSuffix(err.Error(), "socket: operation not permitted") {
-			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrICMPNotPermitted)
+			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrNotPermitted)
 		}
 		return nil, err
 	}
@@ -85,7 +85,7 @@ func getIPv6PacketTooBig(ctx context.Context, ip netip.Addr,
 		case *icmp.PacketTooBig:
 			// https://datatracker.ietf.org/doc/html/rfc1885#section-3.2
 			mtu = uint32(typedBody.MTU) //nolint:gosec
-			err = checkMTU(mtu, minIPv6MTU, physicalLinkMTU)
+			err = checkMTU(mtu, constants.MinIPv6MTU, physicalLinkMTU)
 			if err != nil {
 				return 0, fmt.Errorf("checking MTU: %w", err)
 			}
@@ -103,7 +103,7 @@ func getIPv6PacketTooBig(ctx context.Context, ip netip.Addr,
 			if err != nil {
 				return 0, fmt.Errorf("checking invoking message id: %w", err)
 			} else if idMatch {
-				return 0, fmt.Errorf("%w", ErrICMPDestinationUnreachable)
+				return 0, fmt.Errorf("%w", ErrDestinationUnreachable)
 			}
 			logger.Debug("discarding received ICMP destination unreachable reply with an unknown id")
 			continue
@@ -116,7 +116,7 @@ func getIPv6PacketTooBig(ctx context.Context, ip netip.Addr,
 				inboundID, outboundID)
 			continue
 		default:
-			return 0, fmt.Errorf("%w: %T", ErrICMPBodyUnsupported, typedBody)
+			return 0, fmt.Errorf("%w: %T", ErrBodyUnsupported, typedBody)
 		}
 	}
 }

internal/pmtud/icmp/message.go

@@ -1,4 +1,4 @@
-package pmtud
+package icmp
 
 import (
 	cryptorand "crypto/rand"

internal/pmtud/icmp/multi.go

@@ -0,0 +1,187 @@
+package icmp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/netip"
+	"strings"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/pmtud/test"
+	"golang.org/x/net/icmp"
+)
+
+type icmpTestUnit struct {
+	mtu       uint32
+	echoID    uint16
+	sentBytes int
+	ok        bool
+}
+
+func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
+	minMTU, maxPossibleMTU uint32, pingTimeout time.Duration,
+	logger Logger,
+) (maxMTU uint32, err error) {
+	var ipVersion string
+	var conn net.PacketConn
+	if ip.Is4() {
+		ipVersion = "v4"
+		conn, err = listenICMPv4(ctx)
+	} else {
+		ipVersion = "v6"
+		conn, err = listenICMPv6(ctx)
+	}
+	if err != nil {
+		if strings.HasSuffix(err.Error(), "socket: operation not permitted") {
+			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrNotPermitted)
+		}
+		return 0, fmt.Errorf("listening for ICMP packets: %w", err)
+	}
+
+	mtusToTest := test.MakeMTUsToTest(minMTU, maxPossibleMTU)
+	if len(mtusToTest) == 1 { // only minMTU because minMTU == maxPossibleMTU
+		return minMTU, nil
+	}
+	logger.Debugf("ICMP testing the following MTUs: %v", mtusToTest)
+
+	tests := make([]icmpTestUnit, len(mtusToTest))
+	for i := range mtusToTest {
+		tests[i] = icmpTestUnit{mtu: mtusToTest[i]}
+	}
+
+	timedCtx, cancel := context.WithTimeout(ctx, pingTimeout)
+	defer cancel()
+	go func() {
+		<-timedCtx.Done()
+		conn.Close()
+	}()
+
+	for i := range tests {
+		id, message := buildMessageToSend(ipVersion, tests[i].mtu)
+		tests[i].echoID = id
+
+		encodedMessage, err := message.Marshal(nil)
+		if err != nil {
+			return 0, fmt.Errorf("encoding ICMP message: %w", err)
+		}
+		tests[i].sentBytes = len(encodedMessage)
+
+		_, err = conn.WriteTo(encodedMessage, &net.IPAddr{IP: ip.AsSlice()})
+		if err != nil {
+			if strings.HasSuffix(err.Error(), "sendto: operation not permitted") {
+				err = fmt.Errorf("%w", ErrNotPermitted)
+			}
+			return 0, fmt.Errorf("writing ICMP message: %w", err)
+		}
+	}
+
+	err = collectReplies(conn, ipVersion, tests, logger)
+	switch {
+	case err == nil: // max possible MTU is working
+		return tests[len(tests)-1].mtu, nil
+	case err != nil && errors.Is(err, net.ErrClosed):
+		// we have timeouts (IPv4 testing or IPv6 PMTUD blackholes)
+		// so find the highest MTU which worked.
+		// Note we start from index len(tests) - 2 since the max MTU
+		// cannot be working if we had a timeout.
+		for i := len(tests) - 2; i >= 0; i-- { //nolint:mnd
+			if tests[i].ok {
+				return pmtudMultiSizes(ctx, ip, tests[i].mtu, tests[i+1].mtu-1,
+					pingTimeout, logger)
+			}
+		}
+
+		// All MTUs failed.
+		return 0, fmt.Errorf("%w: ICMP might be blocked", ErrMTUNotFound)
+	case err != nil:
+		return 0, fmt.Errorf("collecting ICMP echo replies: %w", err)
+	default:
+		panic("unreachable")
+	}
+}
+
+// The theoretical limit is 4GiB for IPv6 MTU path discovery jumbograms, but that would
+// create huge buffers which we don't really want to support anyway.
+// The standard frame maximum MTU is 1500 bytes, and there are Jumbo frames with
+// a conventional maximum of 9000 bytes. However, some manufacturers support up
+// 9216-20 = 9196 bytes for the maximum MTU. We thus use buffers of size 9196 to
+// match eventual Jumbo frames. More information at:
+// https://en.wikipedia.org/wiki/Maximum_transmission_unit#MTUs_for_common_media
+const maxPossibleMTU = 9196
+
+func collectReplies(conn net.PacketConn, ipVersion string,
+	tests []icmpTestUnit, logger Logger,
+) (err error) {
+	echoIDToTestIndex := make(map[uint16]int, len(tests))
+	for i, test := range tests {
+		echoIDToTestIndex[test.echoID] = i
+	}
+
+	buffer := make([]byte, maxPossibleMTU)
+
+	idsFound := 0
+	for idsFound < len(tests) {
+		// Note we need to read the whole packet in one call to ReadFrom, so the buffer
+		// must be large enough to read the entire reply packet. See:
+		// https://groups.google.com/g/golang-nuts/c/5dy2Q4nPs08/m/KmuSQAGEtG4J
+		bytesRead, _, err := conn.ReadFrom(buffer)
+		if err != nil {
+			return fmt.Errorf("reading from ICMP connection: %w", err)
+		}
+		packetBytes := buffer[:bytesRead]
+
+		ipPacketLength := len(packetBytes)
+
+		var icmpProtocol int
+		switch ipVersion {
+		case "v4":
+			icmpProtocol = icmpv4Protocol
+		case "v6":
+			icmpProtocol = icmpv6Protocol
+		default:
+			panic(fmt.Sprintf("unknown IP version: %s", ipVersion))
+		}
+
+		// Parse the ICMP message
+		// Note: this parsing works for a truncated 556 bytes ICMP reply packet.
+		message, err := icmp.ParseMessage(icmpProtocol, packetBytes)
+		if err != nil {
+			return fmt.Errorf("parsing message: %w", err)
+		}
+
+		echoBody, ok := message.Body.(*icmp.Echo)
+		if !ok {
+			return fmt.Errorf("%w: %T", ErrBodyUnsupported, message.Body)
+		}
+
+		id := uint16(echoBody.ID) //nolint:gosec
+		testIndex, testing := echoIDToTestIndex[id]
+		if !testing { // not an id we expected so ignore it
+			logger.Warnf("ignoring ICMP reply with unexpected ID %d (type: %d, code: %d, length: %d)",
+				echoBody.ID, message.Type, message.Code, ipPacketLength)
+			continue
+		}
+		idsFound++
+		sentBytes := tests[testIndex].sentBytes
+
+		// echo reply should be at most the number of bytes sent,
+		// and can be lower, more precisely 556 bytes, in case
+		// the host we are reaching wants to stay out of trouble
+		// and ensure its echo reply goes through without
+		// fragmentation, see the following page:
+		// https://datatracker.ietf.org/doc/html/rfc1122#page-59
+		const conservativeReplyLength = 556
+		truncated := ipPacketLength < sentBytes &&
+			ipPacketLength == conservativeReplyLength
+		// Check the packet size is the same if the reply is not truncated
+		if !truncated && sentBytes != ipPacketLength {
+			return fmt.Errorf("%w: sent %dB and received %dB",
+				ErrEchoDataMismatch, sentBytes, ipPacketLength)
+		}
+		// Truncated reply or matching reply size
+		tests[testIndex].ok = true
+	}
+	return nil
+}

internal/pmtud/ip/ipheader.go

@@ -0,0 +1,73 @@
+package ip
+
+import (
+	"encoding/binary"
+	"net/netip"
+	"syscall"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+func HeaderV4(srcIP, dstIP netip.Addr, payloadLength uint32) []byte {
+	ipHeader := make([]byte, constants.IPv4HeaderLength)
+	const version byte = 4
+	const headerLength byte = 20 / 4                                          // in 32-bit words
+	ipHeader[0] = (version << 4) | headerLength                               //nolint:mnd
+	ipHeader[1] = 0                                                           // type of Service
+	putUint16(ipHeader[2:], uint16(constants.IPv4HeaderLength+payloadLength)) //nolint:gosec
+	ipHeader[4], ipHeader[5] = 0, 0                                           // identification
+	const flagsAndOffset uint16 = 0x4000                                      // DF bit set
+	putUint16(ipHeader[6:], flagsAndOffset)
+	ipHeader[8] = 64 // ttl
+	ipHeader[9] = syscall.IPPROTO_TCP
+	srcIPBytes := srcIP.As4()
+	copy(ipHeader[12:16], srcIPBytes[:])
+	dstIPBytes := dstIP.As4()
+	copy(ipHeader[16:20], dstIPBytes[:])
+
+	checksum := ipChecksum(ipHeader)
+	ipHeader[10] = byte(checksum >> 8)   //nolint:mnd
+	ipHeader[11] = byte(checksum & 0xff) //nolint:mnd
+
+	return ipHeader
+}
+
+// ipChecksum calculates the checksum for the IP header.
+//
+//nolint:mnd
+func ipChecksum(header []byte) uint16 {
+	sum := uint32(0)
+	for i := 0; i < len(header)-1; i += 2 {
+		sum += uint32(header[i])<<8 + uint32(header[i+1])
+	}
+	if len(header)%2 != 0 {
+		sum += uint32(header[len(header)-1]) << 8
+	}
+	for (sum >> 16) > 0 {
+		sum = (sum & 0xFFFF) + (sum >> 16)
+	}
+	return ^uint16(sum) //nolint:gosec
+}
+
+// HeaderV6 makes an IPv6 header.
+// payloadLen is the length of the payload following the header.
+// nextHeader can be byte([syscall.IPPROTO_TCP]) for example.
+func HeaderV6(srcIP, dstIP netip.Addr,
+	payloadLen uint16, nextHeader byte,
+) []byte {
+	ipv6Header := make([]byte, constants.IPv6HeaderLength)
+	ipv6Header[0] = 0x60 // version (4 bits) | traffic Class (4 bits)
+	ipv6Header[1] = 0x00 // traffic Class (4 bits) | flow label (4 bits)
+
+	// Flow Label (remaining 16 bits)
+	ipv6Header[2] = 0x00
+	ipv6Header[3] = 0x00
+
+	binary.BigEndian.PutUint16(ipv6Header[4:], payloadLen)
+	ipv6Header[6] = nextHeader
+	const hopLimit = 64
+	ipv6Header[7] = hopLimit
+	copy(ipv6Header[8:24], srcIP.AsSlice())
+	copy(ipv6Header[24:40], dstIP.AsSlice())
+	return ipv6Header
+}

internal/pmtud/ip/ipheader_darwin.go

@@ -0,0 +1,9 @@
+package ip
+
+import (
+	"encoding/binary"
+)
+
+func putUint16(b []byte, v uint16) {
+	binary.NativeEndian.PutUint16(b, v)
+}

internal/pmtud/ip/ipheader_unspecified.go

@@ -0,0 +1,9 @@
+//go:build !darwin
+
+package ip
+
+import "encoding/binary"
+
+func putUint16(b []byte, v uint16) {
+	binary.BigEndian.PutUint16(b, v)
+}

internal/pmtud/ip/ipv4_unix.go

@@ -0,0 +1,9 @@
+//go:build linux || darwin
+
+package ip
+
+import "syscall"
+
+func SetIPv4HeaderIncluded(fd int) error {
+	return syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_HDRINCL, 1)
+}

internal/pmtud/ip/ipv4_unspecified.go

@@ -0,0 +1,7 @@
+//go:build !linux && !windows && !darwin
+
+package ip
+
+func SetIPv4HeaderIncluded(fd int) error {
+	panic("not implemented")
+}

internal/pmtud/ip/ipv4_windows.go

@@ -0,0 +1,12 @@
+package ip
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+func SetIPv4HeaderIncluded(handle syscall.Handle) error {
+	const ipHdrIncluded = windows.IP_HDRINCL
+	return syscall.SetsockoptInt(handle, syscall.IPPROTO_IP, ipHdrIncluded, 1)
+}

internal/pmtud/ip/ipv6_darwin.go

@@ -0,0 +1,5 @@
+package ip
+
+func SetIPv6HeaderIncluded(fd int) error {
+	panic("darwin does not allow an application to build IPv6 headers")
+}

internal/pmtud/ip/ipv6_linux.go

@@ -0,0 +1,8 @@
+package ip
+
+import "syscall"
+
+func SetIPv6HeaderIncluded(fd int) error {
+	const ipv6HdrIncluded = 36 // IPV6_HDRINCL
+	return syscall.SetsockoptInt(fd, syscall.IPPROTO_IPV6, ipv6HdrIncluded, 1)
+}

internal/pmtud/ip/ipv6_unspecified.go

@@ -0,0 +1,7 @@
+//go:build !linux && !windows && !darwin
+
+package ip
+
+func SetIPv6HeaderIncluded(fd int) error {
+	panic("not implemented")
+}

internal/pmtud/ip/ipv6_windows.go

@@ -0,0 +1,7 @@
+package ip
+
+import "syscall"
+
+func SetIPv6HeaderIncluded(fd syscall.Handle) error {
+	panic("windows does not allow an application to build IPv6 headers")
+}

internal/pmtud/ip/source.go

@@ -0,0 +1,123 @@
+package ip
+
+import (
+	"fmt"
+	"net/netip"
+	"syscall"
+
+	"github.com/jsimonetti/rtnetlink"
+)
+
+// SrcAddr determines the appropriate source IP address to use when sending a packet to the
+// specified destination. It also reserves an ephemeral source port for the specified protocol
+// to ensure that the port is not used by other processes. The cleanup function returned should
+// be called to release the reserved port when done.
+func SrcAddr(dst netip.AddrPort, proto int) (src netip.AddrPort, cleanup func(), err error) {
+	srcAddr, err := srcIP(dst.Addr())
+	if err != nil {
+		return netip.AddrPort{}, nil, fmt.Errorf("finding source IP: %w", err)
+	}
+
+	srcPort, cleanup, err := srcPort(srcAddr, proto)
+	if err != nil {
+		return netip.AddrPort{}, nil, fmt.Errorf("reserving source port: %w", err)
+	}
+
+	return netip.AddrPortFrom(srcAddr, srcPort), cleanup, nil
+}
+
+var errNoRoute = fmt.Errorf("no route to destination")
+
+func srcIP(dst netip.Addr) (netip.Addr, error) {
+	conn, err := rtnetlink.Dial(nil)
+	if err != nil {
+		return netip.Addr{}, err
+	}
+	defer conn.Close()
+
+	family := uint8(syscall.AF_INET)
+	if dst.Is6() {
+		family = syscall.AF_INET6
+	}
+
+	// Request route to destination
+	requestMessage := &rtnetlink.RouteMessage{
+		Family: family,
+		Attributes: rtnetlink.RouteAttributes{
+			Dst: dst.AsSlice(),
+		},
+	}
+	messages, err := conn.Route.Get(requestMessage)
+	if err != nil {
+		return netip.Addr{}, fmt.Errorf("getting routes to %s: %w", dst, err)
+	}
+
+	for _, message := range messages {
+		if message.Attributes.Src == nil {
+			continue
+		}
+		ipv6 := message.Attributes.Src.To4() == nil
+		if ipv6 {
+			return netip.AddrFrom16([16]byte(message.Attributes.Src)), nil
+		}
+		return netip.AddrFrom4([4]byte(message.Attributes.Src)), nil
+	}
+
+	return netip.Addr{}, fmt.Errorf("%w: in %d route(s)", errNoRoute, len(messages))
+}
+
+// srcPort reserves an ephemeral source port by opening a socket for the
+// protocol specified and binds it to the provided source address.
+// It doesn't actually listen on the port.
+// The cleanup function returned should be called to release the port when done.
+func srcPort(srcAddr netip.Addr, proto int) (srcPort uint16, cleanup func(), err error) {
+	family := syscall.AF_INET
+	if srcAddr.Is6() {
+		family = syscall.AF_INET6
+	}
+
+	fd, err := syscall.Socket(family, syscall.SOCK_STREAM, proto)
+	if err != nil {
+		return 0, nil, fmt.Errorf("creating reservation socket: %w", err)
+	}
+	cleanup = func() {
+		_ = syscall.Close(fd)
+	}
+
+	// Bind to port 0 to get an ephemeral port
+	const port = 0
+	var bindAddr syscall.Sockaddr
+	if srcAddr.Is4() {
+		bindAddr = &syscall.SockaddrInet4{
+			Port: port,
+			Addr: srcAddr.As4(),
+		}
+	} else {
+		bindAddr = &syscall.SockaddrInet6{
+			Port: port,
+			Addr: srcAddr.As16(),
+		}
+	}
+
+	err = syscall.Bind(fd, bindAddr)
+	if err != nil {
+		cleanup()
+		return 0, nil, fmt.Errorf("binding reservation socket: %w", err)
+	}
+
+	sockAddr, err := syscall.Getsockname(fd)
+	if err != nil {
+		cleanup()
+		return 0, nil, fmt.Errorf("getting bound socket name: %w", err)
+	}
+
+	switch typedSockAddr := sockAddr.(type) {
+	case *syscall.SockaddrInet4:
+		srcPort = uint16(typedSockAddr.Port) //nolint:gosec
+	case *syscall.SockaddrInet6:
+		srcPort = uint16(typedSockAddr.Port) //nolint:gosec
+	default:
+		panic(fmt.Sprintf("unexpected sockaddr type: %T", typedSockAddr))
+	}
+	return srcPort, cleanup, nil
+}

internal/pmtud/pmtud.go

@@ -4,268 +4,73 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"math"
-	"net"
 	"net/netip"
-	"strings"
 	"time"
 
-	"golang.org/x/net/icmp"
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+	"github.com/qdm12/gluetun/internal/pmtud/icmp"
+	"github.com/qdm12/gluetun/internal/pmtud/tcp"
 )
 
-var ErrMTUNotFound = errors.New("path MTU discovery failed to find MTU")
-
-// PathMTUDiscover discovers the maximum MTU for the path to the given ip address.
+// PathMTUDiscover discovers the maximum MTU using both ICMP and TCP.
+// Multiple ICMP addresses and TCP addresses can be specified for redundancy.
+// ICMP PMTUD is run first. If successful, the range of possible MTU values to
+// check for TCP PMTUD is reduced to [maxMTU-150, maxMTU] where maxMTU is the
+// maximum MTU found with ICMP PMTUD. Otherwise, TCP PMTUD is run with the
+// whole range of possible MTU values up to the physical link MTU to check.
 // If the physicalLinkMTU is zero, it defaults to 1500 which is the ethernet standard MTU.
 // If the pingTimeout is zero, it defaults to 1 second.
 // If the logger is nil, a no-op logger is used.
 // It returns [ErrMTUNotFound] if the MTU could not be determined.
-func PathMTUDiscover(ctx context.Context, ip netip.Addr,
-	physicalLinkMTU uint32, pingTimeout time.Duration, logger Logger) (
+func PathMTUDiscover(ctx context.Context, icmpAddrs []netip.Addr, tcpAddrs []netip.AddrPort,
+	physicalLinkMTU uint32, tryTimeout time.Duration, logger Logger) (
 	mtu uint32, err error,
 ) {
 	if physicalLinkMTU == 0 {
 		const ethernetStandardMTU = 1500
 		physicalLinkMTU = ethernetStandardMTU
 	}
-	if pingTimeout == 0 {
-		pingTimeout = time.Second
+	if tryTimeout == 0 {
+		tryTimeout = time.Second
 	}
 	if logger == nil {
 		logger = &noopLogger{}
 	}
 
-	if ip.Is4() {
-		logger.Debug("finding IPv4 next hop MTU")
-		mtu, err = findIPv4NextHopMTU(ctx, ip, physicalLinkMTU, pingTimeout, logger)
+	// Try finding the MTU using ICMP
+	maxPossibleMTU := physicalLinkMTU
+	icmpSuccess := false
+	for _, icmpIP := range icmpAddrs {
+		mtu, err := icmp.PathMTUDiscover(ctx, icmpIP, physicalLinkMTU,
+			tryTimeout, logger)
 		switch {
 		case err == nil:
-			return mtu, nil
-		case errors.Is(err, net.ErrClosed) || errors.Is(err, ErrICMPCommunicationAdministrativelyProhibited): // blackhole
+			logger.Debugf("ICMP path MTU discovery against %s found maximum valid MTU %d", icmpIP, mtu)
+			icmpSuccess = true
+			maxPossibleMTU = mtu
+		case errors.Is(err, icmp.ErrNotPermitted), errors.Is(err, icmp.ErrMTUNotFound):
+			logger.Debugf("ICMP path MTU discovery failed: %s", err)
 		default:
-			return 0, fmt.Errorf("finding IPv4 next hop MTU: %w", err)
-		}
-	} else {
-		logger.Debug("requesting IPv6 ICMP packet-too-big reply")
-		mtu, err = getIPv6PacketTooBig(ctx, ip, physicalLinkMTU, pingTimeout, logger)
-		switch {
-		case err == nil:
-			return mtu, nil
-		case errors.Is(err, net.ErrClosed): // blackhole
-		default:
-			return 0, fmt.Errorf("getting IPv6 packet-too-big message: %w", err)
+			return 0, fmt.Errorf("ICMP path MTU discovery: %w", err)
 		}
 	}
 
-	// Fall back method: send echo requests with different packet
-	// sizes and check which ones succeed to find the maximum MTU.
-	logger.Debug("falling back to sending different sized echo packets")
-	minMTU := minIPv4MTU
-	if ip.Is6() {
-		minMTU = minIPv6MTU
+	for _, addrPort := range tcpAddrs {
+		minMTU := constants.MinIPv4MTU
+		if addrPort.Addr().Is6() {
+			minMTU = constants.MinIPv6MTU
 		}
-	return pmtudMultiSizes(ctx, ip, minMTU, physicalLinkMTU, pingTimeout, logger)
-}
-
-type pmtudTestUnit struct {
-	mtu       uint32
-	echoID    uint16
-	sentBytes int
-	ok        bool
-}
-
-func pmtudMultiSizes(ctx context.Context, ip netip.Addr,
-	minMTU, maxPossibleMTU uint32, pingTimeout time.Duration,
-	logger Logger,
-) (maxMTU uint32, err error) {
-	var ipVersion string
-	var conn net.PacketConn
-	if ip.Is4() {
-		ipVersion = "v4"
-		conn, err = listenICMPv4(ctx)
-	} else {
-		ipVersion = "v6"
-		conn, err = listenICMPv6(ctx)
+		if icmpSuccess {
+			const mtuMargin = 150
+			minMTU = max(maxPossibleMTU-mtuMargin, minMTU)
 		}
+		mtu, err = tcp.PathMTUDiscover(ctx, addrPort, minMTU, maxPossibleMTU, logger)
 		if err != nil {
-		if strings.HasSuffix(err.Error(), "socket: operation not permitted") {
-			err = fmt.Errorf("%w: you can try adding NET_RAW capability to resolve this", ErrICMPNotPermitted)
-		}
-		return 0, fmt.Errorf("listening for ICMP packets: %w", err)
-	}
-
-	mtusToTest := makeMTUsToTest(minMTU, maxPossibleMTU)
-	if len(mtusToTest) == 1 { // only minMTU because minMTU == maxPossibleMTU
-		return minMTU, nil
-	}
-	logger.Debugf("testing the following MTUs: %v", mtusToTest)
-
-	tests := make([]pmtudTestUnit, len(mtusToTest))
-	for i := range mtusToTest {
-		tests[i] = pmtudTestUnit{mtu: mtusToTest[i]}
-	}
-
-	timedCtx, cancel := context.WithTimeout(ctx, pingTimeout)
-	defer cancel()
-	go func() {
-		<-timedCtx.Done()
-		conn.Close()
-	}()
-
-	for i := range tests {
-		id, message := buildMessageToSend(ipVersion, tests[i].mtu)
-		tests[i].echoID = id
-
-		encodedMessage, err := message.Marshal(nil)
-		if err != nil {
-			return 0, fmt.Errorf("encoding ICMP message: %w", err)
-		}
-		tests[i].sentBytes = len(encodedMessage)
-
-		_, err = conn.WriteTo(encodedMessage, &net.IPAddr{IP: ip.AsSlice()})
-		if err != nil {
-			if strings.HasSuffix(err.Error(), "sendto: operation not permitted") {
-				err = fmt.Errorf("%w", ErrICMPNotPermitted)
-			}
-			return 0, fmt.Errorf("writing ICMP message: %w", err)
-		}
-	}
-
-	err = collectReplies(conn, ipVersion, tests, logger)
-	switch {
-	case err == nil: // max possible MTU is working
-		return tests[len(tests)-1].mtu, nil
-	case err != nil && errors.Is(err, net.ErrClosed):
-		// we have timeouts (IPv4 testing or IPv6 PMTUD blackholes)
-		// so find the highest MTU which worked.
-		// Note we start from index len(tests) - 2 since the max MTU
-		// cannot be working if we had a timeout.
-		for i := len(tests) - 2; i >= 0; i-- { //nolint:mnd
-			if tests[i].ok {
-				return pmtudMultiSizes(ctx, ip, tests[i].mtu, tests[i+1].mtu-1,
-					pingTimeout, logger)
-			}
-		}
-
-		// All MTUs failed.
-		return 0, fmt.Errorf("%w: ICMP might be blocked", ErrMTUNotFound)
-	case err != nil:
-		return 0, fmt.Errorf("collecting ICMP echo replies: %w", err)
-	default:
-		panic("unreachable")
-	}
-}
-
-// Create the MTU slice of length 11 such that:
-// - the first element is the minMTU
-// - the last element is the maxMTU
-// - elements in-between are separated as close to each other
-// The number 11 is chosen to find the final MTU in 3 searches,
-// with a total search space of 1728 MTUs which is enough;
-// to find it in 2 searches requires 37 parallel queries which
-// could be blocked by firewalls.
-func makeMTUsToTest(minMTU, maxMTU uint32) (mtus []uint32) {
-	const mtusLength = 11 // find the final MTU in 3 searches
-	diff := maxMTU - minMTU
-	switch {
-	case minMTU > maxMTU:
-		panic("minMTU > maxMTU")
-	case diff <= mtusLength:
-		mtus = make([]uint32, 0, diff)
-		for mtu := minMTU; mtu <= maxMTU; mtu++ {
-			mtus = append(mtus, mtu)
-		}
-	default:
-		step := float64(diff) / float64(mtusLength-1)
-		mtus = make([]uint32, 0, mtusLength)
-		for mtu := float64(minMTU); len(mtus) < mtusLength-1; mtu += step {
-			mtus = append(mtus, uint32(math.Round(mtu)))
-		}
-		mtus = append(mtus, maxMTU) // last element is the maxMTU
-	}
-
-	return mtus
-}
-
-func collectReplies(conn net.PacketConn, ipVersion string,
-	tests []pmtudTestUnit, logger Logger,
-) (err error) {
-	echoIDToTestIndex := make(map[uint16]int, len(tests))
-	for i, test := range tests {
-		echoIDToTestIndex[test.echoID] = i
-	}
-
-	// The theoretical limit is 4GiB for IPv6 MTU path discovery jumbograms, but that would
-	// create huge buffers which we don't really want to support anyway.
-	// The standard frame maximum MTU is 1500 bytes, and there are Jumbo frames with
-	// a conventional maximum of 9000 bytes. However, some manufacturers support up
-	// 9216-20 = 9196 bytes for the maximum MTU. We thus use buffers of size 9196 to
-	// match eventual Jumbo frames. More information at:
-	// https://en.wikipedia.org/wiki/Maximum_transmission_unit#MTUs_for_common_media
-	const maxPossibleMTU = 9196
-	buffer := make([]byte, maxPossibleMTU)
-
-	idsFound := 0
-	for idsFound < len(tests) {
-		// Note we need to read the whole packet in one call to ReadFrom, so the buffer
-		// must be large enough to read the entire reply packet. See:
-		// https://groups.google.com/g/golang-nuts/c/5dy2Q4nPs08/m/KmuSQAGEtG4J
-		bytesRead, _, err := conn.ReadFrom(buffer)
-		if err != nil {
-			return fmt.Errorf("reading from ICMP connection: %w", err)
-		}
-		packetBytes := buffer[:bytesRead]
-
-		ipPacketLength := len(packetBytes)
-
-		var icmpProtocol int
-		switch ipVersion {
-		case "v4":
-			icmpProtocol = icmpv4Protocol
-		case "v6":
-			icmpProtocol = icmpv6Protocol
-		default:
-			panic(fmt.Sprintf("unknown IP version: %s", ipVersion))
-		}
-
-		// Parse the ICMP message
-		// Note: this parsing works for a truncated 556 bytes ICMP reply packet.
-		message, err := icmp.ParseMessage(icmpProtocol, packetBytes)
-		if err != nil {
-			return fmt.Errorf("parsing message: %w", err)
-		}
-
-		echoBody, ok := message.Body.(*icmp.Echo)
-		if !ok {
-			return fmt.Errorf("%w: %T", ErrICMPBodyUnsupported, message.Body)
-		}
-
-		id := uint16(echoBody.ID) //nolint:gosec
-		testIndex, testing := echoIDToTestIndex[id]
-		if !testing { // not an id we expected so ignore it
-			logger.Warnf("ignoring ICMP reply with unexpected ID %d (type: %d, code: %d, length: %d)",
-				echoBody.ID, message.Type, message.Code, ipPacketLength)
+			logger.Debugf("TCP path MTU discovery to %s failed: %s", addrPort, err)
 			continue
 		}
-		idsFound++
-		sentBytes := tests[testIndex].sentBytes
-
-		// echo reply should be at most the number of bytes sent,
-		// and can be lower, more precisely 556 bytes, in case
-		// the host we are reaching wants to stay out of trouble
-		// and ensure its echo reply goes through without
-		// fragmentation, see the following page:
-		// https://datatracker.ietf.org/doc/html/rfc1122#page-59
-		const conservativeReplyLength = 556
-		truncated := ipPacketLength < sentBytes &&
-			ipPacketLength == conservativeReplyLength
-		// Check the packet size is the same if the reply is not truncated
-		if !truncated && sentBytes != ipPacketLength {
-			return fmt.Errorf("%w: sent %dB and received %dB",
-				ErrICMPEchoDataMismatch, sentBytes, ipPacketLength)
+		logger.Debugf("TCP path MTU discovery to %s found maximum valid MTU %d", addrPort, mtu)
+		return mtu, nil
 	}
-		// Truncated reply or matching reply size
-		tests[testIndex].ok = true
-	}
-	return nil
+	return 0, fmt.Errorf("TCP path MTU discovery: last error: %w", err)
 }

internal/pmtud/tcp/interfaces.go

@@ -0,0 +1,7 @@
+package tcp
+
+type Logger interface {
+	Debug(msg string)
+	Debugf(msg string, args ...any)
+	Warnf(msg string, args ...any)
+}

internal/pmtud/tcp/mocks_generate_test.go

@@ -0,0 +1,3 @@
+package tcp
+
+//go:generate mockgen -destination=mocks_test.go -package=$GOPACKAGE . Logger

internal/pmtud/tcp/mocks_test.go

@@ -0,0 +1,80 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/qdm12/gluetun/internal/pmtud/tcp (interfaces: Logger)
+
+// Package tcp is a generated GoMock package.
+package tcp
+
+import (
+	reflect "reflect"
+
+	gomock "github.com/golang/mock/gomock"
+)
+
+// MockLogger is a mock of Logger interface.
+type MockLogger struct {
+	ctrl     *gomock.Controller
+	recorder *MockLoggerMockRecorder
+}
+
+// MockLoggerMockRecorder is the mock recorder for MockLogger.
+type MockLoggerMockRecorder struct {
+	mock *MockLogger
+}
+
+// NewMockLogger creates a new mock instance.
+func NewMockLogger(ctrl *gomock.Controller) *MockLogger {
+	mock := &MockLogger{ctrl: ctrl}
+	mock.recorder = &MockLoggerMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockLogger) EXPECT() *MockLoggerMockRecorder {
+	return m.recorder
+}
+
+// Debug mocks base method.
+func (m *MockLogger) Debug(arg0 string) {
+	m.ctrl.T.Helper()
+	m.ctrl.Call(m, "Debug", arg0)
+}
+
+// Debug indicates an expected call of Debug.
+func (mr *MockLoggerMockRecorder) Debug(arg0 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Debug", reflect.TypeOf((*MockLogger)(nil).Debug), arg0)
+}
+
+// Debugf mocks base method.
+func (m *MockLogger) Debugf(arg0 string, arg1 ...interface{}) {
+	m.ctrl.T.Helper()
+	varargs := []interface{}{arg0}
+	for _, a := range arg1 {
+		varargs = append(varargs, a)
+	}
+	m.ctrl.Call(m, "Debugf", varargs...)
+}
+
+// Debugf indicates an expected call of Debugf.
+func (mr *MockLoggerMockRecorder) Debugf(arg0 interface{}, arg1 ...interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	varargs := append([]interface{}{arg0}, arg1...)
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Debugf", reflect.TypeOf((*MockLogger)(nil).Debugf), varargs...)
+}
+
+// Warnf mocks base method.
+func (m *MockLogger) Warnf(arg0 string, arg1 ...interface{}) {
+	m.ctrl.T.Helper()
+	varargs := []interface{}{arg0}
+	for _, a := range arg1 {
+		varargs = append(varargs, a)
+	}
+	m.ctrl.Call(m, "Warnf", varargs...)
+}
+
+// Warnf indicates an expected call of Warnf.
+func (mr *MockLoggerMockRecorder) Warnf(arg0 interface{}, arg1 ...interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	varargs := append([]interface{}{arg0}, arg1...)
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Warnf", reflect.TypeOf((*MockLogger)(nil).Warnf), varargs...)
+}

internal/pmtud/tcp/multi.go

@@ -0,0 +1,89 @@
+package tcp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/netip"
+	"syscall"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/pmtud/test"
+)
+
+var ErrMTUNotFound = errors.New("MTU not found")
+
+type testUnit struct {
+	mtu uint32
+	ok  bool
+}
+
+func PathMTUDiscover(ctx context.Context, addrPort netip.AddrPort,
+	minMTU, maxPossibleMTU uint32, logger Logger,
+) (mtu uint32, err error) {
+	mtusToTest := test.MakeMTUsToTest(minMTU, maxPossibleMTU)
+	if len(mtusToTest) == 1 { // only minMTU because minMTU == maxPossibleMTU
+		return minMTU, nil
+	}
+	logger.Debugf("TCP testing the following MTUs: %v", mtusToTest)
+
+	tests := make([]testUnit, len(mtusToTest))
+	for i := range mtusToTest {
+		tests[i] = testUnit{mtu: mtusToTest[i]}
+	}
+
+	family := syscall.AF_INET
+	if addrPort.Addr().Is6() {
+		family = syscall.AF_INET6
+	}
+	fd, stop, err := startRawSocket(family)
+	if err != nil {
+		return 0, fmt.Errorf("starting raw socket: %w", err)
+	}
+	defer stop()
+
+	tracker := newTracker(fd, addrPort.Addr().Is4())
+
+	const timeout = time.Second
+	runCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+	errCh := make(chan error)
+	go func() {
+		errCh <- tracker.listen(runCtx)
+	}()
+
+	doneCh := make(chan struct{})
+	for i := range tests {
+		go func(i int) {
+			err := runTest(runCtx, fd, tracker, addrPort, tests[i].mtu)
+			tests[i].ok = err == nil
+			doneCh <- struct{}{}
+		}(i)
+	}
+
+	for range tests {
+		select {
+		case <-doneCh:
+		case err := <-errCh:
+			if err == nil { // timeout
+				break
+			}
+			return 0, fmt.Errorf("listening for TCP replies: %w", err)
+		}
+	}
+
+	if tests[len(tests)-1].ok {
+		return tests[len(tests)-1].mtu, nil
+	}
+
+	for i := len(tests) - 2; i >= 0; i-- { //nolint:mnd
+		if tests[i].ok {
+			stop()
+			cancel()
+			return PathMTUDiscover(ctx, addrPort,
+				tests[i].mtu, tests[i+1].mtu-1, logger)
+		}
+	}
+
+	return 0, fmt.Errorf("%w: your connection might not be working at all", ErrMTUNotFound)
+}

internal/pmtud/tcp/packet.go

@@ -0,0 +1,89 @@
+package tcp
+
+import (
+	"math/rand/v2"
+	"net/netip"
+	"syscall"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+	"github.com/qdm12/gluetun/internal/pmtud/ip"
+)
+
+// createSYNPacket creates a TCP SYN packet for initiating a handshake.
+// SYN packets have normally no data payload, so you SHOULD set mtu to 0.
+// However, in some cases where the server closes the connection with RST immediately,
+// it can be useful to add some data payload to a SYN packet and check if the server still
+// replies. Only set mtu to a non zero value if you know what you are doing.
+func createSYNPacket(src, dst netip.AddrPort, mtu uint32) (packet []byte, seq uint32) {
+	seq = rand.Uint32()                            //nolint:gosec
+	const ack = 0                                  // SYN has no ACK number
+	payloadLength := constants.BaseTCPHeaderLength // no data payload
+	if mtu > 0 {
+		payloadLength = getPayloadLength(mtu, dst)
+	}
+	return createPacket(src, dst, seq, ack, payloadLength, synFlag), seq
+}
+
+// createACKPacket creates a TCP ACK packet.
+// If the mtu is set to 0, no payload is sent.
+// Otherwise, the payload is calculated to test the MTU given.
+func createACKPacket(src, dst netip.AddrPort, seq, ack uint32, mtu uint32) []byte {
+	payloadLength := constants.BaseTCPHeaderLength // no data payload
+	if mtu > 0 {
+		payloadLength = getPayloadLength(mtu, dst)
+	}
+	const flags = ackFlag | pshFlag
+	return createPacket(src, dst, seq, ack, payloadLength, flags)
+}
+
+func createRSTPacket(src, dst netip.AddrPort, seq, ack uint32) []byte {
+	const payloadLength = constants.BaseTCPHeaderLength // no data payload
+	return createPacket(src, dst, seq, ack, payloadLength, rstFlag)
+}
+
+func getPayloadLength(mtu uint32, dst netip.AddrPort) uint32 {
+	var ipHeaderLength uint32
+	if dst.Addr().Is4() {
+		ipHeaderLength = constants.IPv4HeaderLength
+	} else {
+		ipHeaderLength = constants.IPv6HeaderLength
+	}
+	if mtu < ipHeaderLength+constants.BaseTCPHeaderLength {
+		panic("MTU too small to hold IP and TCP headers")
+	}
+	return mtu - ipHeaderLength
+}
+
+func createPacket(src, dst netip.AddrPort,
+	seq, ack, payloadLength uint32, flags byte,
+) []byte {
+	if payloadLength < constants.BaseTCPHeaderLength {
+		panic("payload length is too small to hold TCP header")
+	}
+
+	var ipHeader []byte
+	if dst.Addr().Is4() {
+		ipHeader = ip.HeaderV4(src.Addr(), dst.Addr(), payloadLength)
+	} else {
+		ipHeader = ip.HeaderV6(src.Addr(), dst.Addr(),
+			uint16(payloadLength), byte(syscall.IPPROTO_TCP)) //nolint:gosec
+	}
+
+	tcpHeader := makeTCPHeader(src.Port(), dst.Port(), seq, ack, flags)
+
+	// data is just zeroes
+	dataLength := int(payloadLength) - int(constants.BaseTCPHeaderLength)
+	var data []byte
+	if dataLength > 0 {
+		data = make([]byte, dataLength)
+	}
+	checksum := tcpChecksum(ipHeader, tcpHeader, data)
+	tcpHeader[16] = byte(checksum >> 8)   //nolint:mnd
+	tcpHeader[17] = byte(checksum & 0xff) //nolint:mnd
+
+	packet := make([]byte, len(ipHeader)+int(constants.BaseTCPHeaderLength)+dataLength)
+	copy(packet, ipHeader)
+	copy(packet[len(ipHeader):], tcpHeader)
+	copy(packet[len(ipHeader)+int(constants.BaseTCPHeaderLength):], data)
+	return packet
+}

internal/pmtud/tcp/tcp.go

@@ -0,0 +1,196 @@
+package tcp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/netip"
+	"syscall"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+	"github.com/qdm12/gluetun/internal/pmtud/ip"
+)
+
+func startRawSocket(family int) (fd fileDescriptor, stop func(), err error) {
+	fdPlatform, err := syscall.Socket(family, syscall.SOCK_RAW, syscall.IPPROTO_TCP)
+	if err != nil {
+		return 0, nil, fmt.Errorf("creating raw socket: %w", err)
+	}
+
+	if family == syscall.AF_INET {
+		err = ip.SetIPv4HeaderIncluded(fdPlatform)
+	} else {
+		err = ip.SetIPv6HeaderIncluded(fdPlatform)
+	}
+	if err != nil {
+		_ = syscall.Close(fdPlatform)
+		return 0, nil, fmt.Errorf("setting header option on raw socket: %w", err)
+	}
+
+	// Allow sending packets larger than cached PMTU (for PMTUD probing)
+	err = setMTUDiscovery(fdPlatform)
+	if err != nil {
+		_ = syscall.Close(fdPlatform)
+		return 0, nil, fmt.Errorf("setting IP_MTU_DISCOVER: %w", err)
+	}
+
+	// use polling because some Linux systems do not cancel
+	// blocking syscalls such as recvfrom when the socket is closed,
+	// which would cause things to hang indefinitely.
+	err = setNonBlock(fdPlatform)
+	if err != nil {
+		_ = syscall.Close(fdPlatform)
+		return 0, nil, fmt.Errorf("setting non-blocking mode: %w", err)
+	}
+
+	stop = func() {
+		_ = syscall.Close(fdPlatform)
+	}
+	return fileDescriptor(fdPlatform), stop, nil
+}
+
+var (
+	errTCPPacketNotSynAck        = errors.New("TCP packet is not a SYN-ACK")
+	errTCPSynAckAckMismatch      = errors.New("TCP SYN-ACK ACK number does not match expected value")
+	errFinalPacketTypeUnexpected = errors.New("final TCP packet type is unexpected")
+)
+
+// Craft and send a raw TCP packet to test the MTU.
+// It expects either an RST reply (if no server is listening)
+// or a SYN-ACK/ACK reply (if a server is listening).
+func runTest(ctx context.Context, fd fileDescriptor,
+	tracker *tracker, dst netip.AddrPort, mtu uint32,
+) error {
+	const proto = syscall.IPPROTO_TCP
+	src, cleanup, err := ip.SrcAddr(dst, proto)
+	if err != nil {
+		return fmt.Errorf("getting source address: %w", err)
+	}
+	defer cleanup()
+
+	ch := make(chan []byte)
+	abort := make(chan struct{})
+	defer close(abort)
+	tracker.register(src.Port(), dst.Port(), ch, abort)
+	defer tracker.unregister(src.Port(), dst.Port())
+
+	dstSockAddr := makeSockAddr(dst)
+
+	synPacket, synSeq := createSYNPacket(src, dst, 0)
+	const sendToFlags = 0
+	err = sendTo(fd, synPacket, sendToFlags, dstSockAddr)
+	if err != nil {
+		return fmt.Errorf("sending SYN packet: %w", err)
+	}
+
+	var reply []byte
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case reply = <-ch:
+	}
+
+	packetType, synAckSeq, synAckAck, err := parseTCPHeader(reply[:constants.BaseTCPHeaderLength])
+	switch {
+	case err != nil:
+		return fmt.Errorf("parsing first reply TCP header: %w", err)
+	case packetType == packetTypeRST:
+		// server actively closed the connection, try sending a SYN with data
+		return handleRSTReply(ctx, fd, ch, src, dst, mtu)
+	case packetType != packetTypeSYNACK:
+		return fmt.Errorf("%w: unexpected packet type %s", errTCPPacketNotSynAck, packetType)
+	case synAckAck != synSeq+1:
+		return fmt.Errorf("%w: expected %d, got %d", errTCPSynAckAckMismatch, synSeq+1, synAckAck)
+	}
+
+	// Send a no-data ACK packet to finish the 3-way handshake.
+	const ackMTU = 0 // no data payload initially
+	ackPacket := createACKPacket(src, dst, synAckAck, synAckSeq+1, ackMTU)
+	err = sendTo(fd, ackPacket, sendToFlags, dstSockAddr)
+	if err != nil {
+		return fmt.Errorf("sending ACK-without-data packet: %w", err)
+	}
+
+	// Send a data ACK packet to test the MTU given.
+	ackPacket = createACKPacket(src, dst, synAckAck, synAckSeq+1, mtu)
+	err = sendTo(fd, ackPacket, sendToFlags, dstSockAddr)
+	if err != nil {
+		return fmt.Errorf("sending ACK-with-data packet: %w", err)
+	}
+
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case reply = <-ch:
+	}
+
+	packetType, _, ack, err := parseTCPHeader(reply[:constants.BaseTCPHeaderLength])
+	if err != nil {
+		return fmt.Errorf("parsing second reply TCP header: %w", err)
+	}
+
+	switch packetType { //nolint:exhaustive
+	case packetTypeRST:
+		return nil
+	case packetTypeACK:
+		err = sendRST(fd, src, dst, ack)
+		if err != nil {
+			return fmt.Errorf("sending RST packet: %w", err)
+		}
+		return nil
+	default:
+		_ = sendRST(fd, src, dst, ack)
+		return fmt.Errorf("%w: %s", errFinalPacketTypeUnexpected, packetType)
+	}
+}
+
+func makeSockAddr(addr netip.AddrPort) syscall.Sockaddr {
+	if addr.Addr().Is4() {
+		return &syscall.SockaddrInet4{
+			Port: int(addr.Port()),
+			Addr: addr.Addr().As4(),
+		}
+	}
+	return &syscall.SockaddrInet6{
+		Port: int(addr.Port()),
+		Addr: addr.Addr().As16(),
+	}
+}
+
+var errTCPPacketNotRST = errors.New("TCP packet is not an RST")
+
+func handleRSTReply(ctx context.Context, fd fileDescriptor, ch <-chan []byte,
+	src, dst netip.AddrPort, mtu uint32,
+) error {
+	packet, _ := createSYNPacket(src, dst, mtu)
+	const sendToFlags = 0
+	err := sendTo(fd, packet, sendToFlags, makeSockAddr(dst))
+	if err != nil {
+		return fmt.Errorf("sending SYN MTU-test packet: %w", err)
+	}
+
+	var reply []byte
+	select {
+	case <-ctx.Done():
+		return ctx.Err() // timeout: the MTU test SYN packet was too big
+	case reply = <-ch:
+	}
+
+	packetType, _, _, err := parseTCPHeader(reply[:constants.BaseTCPHeaderLength])
+	if err != nil {
+		return fmt.Errorf("parsing reply TCP header: %w", err)
+	} else if packetType != packetTypeRST {
+		return fmt.Errorf("%w: %s", errTCPPacketNotRST, packetType)
+	}
+	return nil
+}
+
+func sendRST(fd fileDescriptor, src, dst netip.AddrPort,
+	previousACK uint32,
+) error {
+	seq := previousACK
+	const ack = 0
+	rstPacket := createRSTPacket(src, dst, seq, ack)
+	const sendToFlags = 0
+	return sendTo(fd, rstPacket, sendToFlags, makeSockAddr(dst))
+}

internal/pmtud/tcp/tcp_darwin.go

@@ -0,0 +1,5 @@
+package tcp
+
+func stripIPv4Header(reply []byte) (result []byte, ok bool) {
+	return reply, true
+}

internal/pmtud/tcp/tcp_linux.go

@@ -0,0 +1,7 @@
+package tcp
+
+import "syscall"
+
+func setMTUDiscovery(fd int) error {
+	return syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_PROBE)
+}

internal/pmtud/tcp/tcp_notdarwin.go

@@ -0,0 +1,30 @@
+//go:build !darwin
+
+package tcp
+
+import (
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+func stripIPv4Header(reply []byte) (result []byte, ok bool) {
+	if len(reply) < int(constants.IPv4HeaderLength) {
+		return nil, false // not an IPv4 packet
+	}
+
+	version := reply[0] >> 4 //nolint:mnd
+	const ipv4Version = 4
+	if version != ipv4Version {
+		return nil, false
+	}
+	// For IPv4 we need to skip the IP header, which is at least
+	// 20B and can be up to 60B.
+	// The Internet Header Length is the lower 4 bits of the first byte and
+	// represents the number of 32-bit words of the header length.
+	const ihlMask byte = 0x0F
+	const bytesInWord = 4
+	headerLength := int((reply[0] & ihlMask)) * bytesInWord
+	if len(reply) < headerLength {
+		return nil, false // not enough data for full IPv4 header
+	}
+	return reply[headerLength:], true
+}

internal/pmtud/tcp/tcp_test.go

@@ -0,0 +1,199 @@
+package tcp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/netip"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/netlink"
+	"github.com/qdm12/gluetun/internal/routing"
+	"github.com/qdm12/log"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_runTest(t *testing.T) {
+	t.Parallel()
+
+	noopLogger := &noopLogger{}
+	netlinker := netlink.New(noopLogger)
+	loopbackMTU, err := findLoopbackMTU(netlinker)
+	require.NoError(t, err, "finding loopback IPv4 MTU")
+	defaultIPv4MTU, err := findDefaultIPv4RouteMTU(netlinker)
+	require.NoError(t, err, "finding default IPv4 route MTU")
+
+	ctx, cancel := context.WithCancel(t.Context())
+
+	const family = syscall.AF_INET
+	fd, stop, err := startRawSocket(family)
+	require.NoError(t, err)
+
+	const ipv4 = true
+	tracker := newTracker(fd, ipv4)
+	trackerCh := make(chan error)
+	go func() {
+		trackerCh <- tracker.listen(ctx)
+	}()
+
+	t.Cleanup(func() {
+		stop()
+		cancel() // stop listening
+		err = <-trackerCh
+		require.NoError(t, err)
+	})
+
+	testCases := map[string]struct {
+		timeout time.Duration
+		dst     func(t *testing.T) netip.AddrPort
+		mtu     uint32
+		success bool
+	}{
+		"local_not_listening": {
+			timeout: time.Hour,
+			dst: func(t *testing.T) netip.AddrPort {
+				t.Helper()
+				port := reserveClosedPort(t)
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte{127, 0, 0, 1}), port)
+			},
+			mtu:     loopbackMTU,
+			success: true,
+		},
+		"remote_not_listening": {
+			timeout: 50 * time.Millisecond,
+			dst: func(_ *testing.T) netip.AddrPort {
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte{1, 1, 1, 1}), 12345)
+			},
+			mtu: defaultIPv4MTU,
+		},
+		"1.1.1.1:443": {
+			timeout: time.Second,
+			dst: func(_ *testing.T) netip.AddrPort {
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte{1, 1, 1, 1}), 443)
+			},
+			mtu:     defaultIPv4MTU,
+			success: true,
+		},
+		"1.1.1.1:80": {
+			timeout: time.Second,
+			dst: func(_ *testing.T) netip.AddrPort {
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte{1, 1, 1, 1}), 80)
+			},
+			mtu:     defaultIPv4MTU,
+			success: true,
+		},
+		"8.8.8.8:443": {
+			timeout: time.Second,
+			dst: func(_ *testing.T) netip.AddrPort {
+				return netip.AddrPortFrom(netip.AddrFrom4([4]byte{8, 8, 8, 8}), 443)
+			},
+			mtu:     defaultIPv4MTU,
+			success: true,
+		},
+	}
+
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			ctx, cancel := context.WithTimeout(t.Context(), testCase.timeout)
+			defer cancel()
+			dst := testCase.dst(t)
+			err := runTest(ctx, fd, tracker, dst, testCase.mtu)
+			if testCase.success {
+				require.NoError(t, err)
+			} else {
+				require.Error(t, err)
+			}
+		})
+	}
+}
+
+var errRouteNotFound = errors.New("route not found")
+
+func findLoopbackMTU(netlinker *netlink.NetLink) (mtu uint32, err error) {
+	routes, err := netlinker.RouteList(netlink.FamilyV4)
+	if err != nil {
+		return 0, fmt.Errorf("getting routes list: %w", err)
+	}
+	for _, route := range routes {
+		if route.Dst.IsValid() && route.Dst.Addr().IsLoopback() {
+			link, err := netlinker.LinkByIndex(route.LinkIndex)
+			if err != nil {
+				return 0, fmt.Errorf("getting link by index: %w", err)
+			}
+			// Quirk: make sure it is maximum 65535, and not i.e. 65536
+			// or the IP header 16 bits will fail to fit that packet length value.
+			const maxMTU = 65535
+			return min(link.MTU, maxMTU), nil
+		}
+	}
+	return 0, fmt.Errorf("%w: no loopback route found", errRouteNotFound)
+}
+
+func findDefaultIPv4RouteMTU(netlinker *netlink.NetLink) (mtu uint32, err error) {
+	noopLogger := &noopLogger{}
+	routing := routing.New(netlinker, noopLogger)
+	defaultRoutes, err := routing.DefaultRoutes()
+	if err != nil {
+		return 0, fmt.Errorf("getting default routes: %w", err)
+	}
+	for _, route := range defaultRoutes {
+		if route.Family != netlink.FamilyV4 {
+			continue
+		}
+		link, err := netlinker.LinkByName(defaultRoutes[0].NetInterface)
+		if err != nil {
+			return 0, fmt.Errorf("getting link by name: %w", err)
+		}
+		return link.MTU, nil
+	}
+	return 0, fmt.Errorf("%w: no default route found", errRouteNotFound)
+}
+
+func reserveClosedPort(t *testing.T) (port uint16) {
+	t.Helper()
+
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, syscall.IPPROTO_TCP)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		err := syscall.Close(fd)
+		assert.NoError(t, err)
+	})
+
+	addr := &syscall.SockaddrInet4{
+		Port: 0,
+		Addr: [4]byte{127, 0, 0, 1},
+	}
+
+	err = syscall.Bind(fd, addr)
+	if err != nil {
+		_ = syscall.Close(fd)
+		t.Fatal(err)
+	}
+
+	sockAddr, err := syscall.Getsockname(fd)
+	if err != nil {
+		_ = syscall.Close(fd)
+		t.Fatal(err)
+	}
+
+	sockAddr4, ok := sockAddr.(*syscall.SockaddrInet4)
+	if !ok {
+		_ = syscall.Close(fd)
+		t.Fatal("not an IPv4 address")
+	}
+
+	return uint16(sockAddr4.Port) //nolint:gosec
+}
+
+type noopLogger struct{}
+
+func (l *noopLogger) Patch(_ ...log.Option)     {}
+func (l *noopLogger) Debug(_ string)            {}
+func (l *noopLogger) Debugf(_ string, _ ...any) {}
+func (l *noopLogger) Info(_ string)             {}
+func (l *noopLogger) Warn(_ string)             {}
+func (l *noopLogger) Error(_ string)            {}

internal/pmtud/tcp/tcp_unix.go

@@ -0,0 +1,28 @@
+//go:build linux || darwin
+
+package tcp
+
+import (
+	"syscall"
+	"time"
+)
+
+// fileDescriptor is a platform-independent type for socket file descriptors.
+type fileDescriptor int
+
+func sendTo(fd fileDescriptor, p []byte, flags int, to syscall.Sockaddr) (err error) {
+	return syscall.Sendto(int(fd), p, flags, to)
+}
+
+func setSocketTimeout(fd fileDescriptor, timeout time.Duration) (err error) {
+	timeval := syscall.NsecToTimeval(timeout.Nanoseconds())
+	return syscall.SetsockoptTimeval(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVTIMEO, &timeval)
+}
+
+func recvFrom(fd fileDescriptor, p []byte, flags int) (n int, from syscall.Sockaddr, err error) {
+	return syscall.Recvfrom(int(fd), p, flags)
+}
+
+func setNonBlock(fd int) error {
+	return syscall.SetNonblock(fd, true)
+}

internal/pmtud/tcp/tcp_unspecified.go

@@ -0,0 +1,7 @@
+//go:build !linux && !windows
+
+package tcp
+
+func setMTUDiscovery(fd int) error {
+	panic("not implemented")
+}

internal/pmtud/tcp/tcp_windows.go

@@ -0,0 +1,37 @@
+package tcp
+
+import (
+	"syscall"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+type fileDescriptor syscall.Handle
+
+func sendTo(fd fileDescriptor, p []byte, flags int, to syscall.Sockaddr) (err error) {
+	return syscall.Sendto(syscall.Handle(fd), p, flags, to)
+}
+
+func setSocketTimeout(fd fileDescriptor, timeout time.Duration) (err error) {
+	timeval := int(timeout.Milliseconds())
+	return syscall.SetsockoptInt(syscall.Handle(fd), syscall.SOL_SOCKET, windows.SO_RCVTIMEO, timeval)
+}
+
+func recvFrom(fd fileDescriptor, p []byte, flags int) (n int, from syscall.Sockaddr, err error) {
+	return syscall.Recvfrom(syscall.Handle(fd), p, flags)
+}
+
+func setMTUDiscovery(fd syscall.Handle) error {
+	panic("not implemented")
+}
+
+func setNonBlock(fd syscall.Handle) error {
+	// Windows: Use ioctlsocket with FIONBIO
+	var arg uint32 = 1 // 1 to enable non-blocking mode
+	var bytesReturned uint32
+	const FIONBIO = 0x8004667e
+	return syscall.WSAIoctl(fd, FIONBIO, (*byte)(unsafe.Pointer(&arg)),
+		uint32(unsafe.Sizeof(arg)), nil, 0, &bytesReturned, nil, 0)
+}

internal/pmtud/tcp/tcpheader.go

@@ -0,0 +1,124 @@
+package tcp
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+// For SYN, ack is 0.
+// For SYN-ACK, ack is the sequence number + 1 of the SYN.
+func makeTCPHeader(srcPort, dstPort uint16, seq, ack uint32, flags byte) []byte {
+	header := make([]byte, constants.BaseTCPHeaderLength)
+	binary.BigEndian.PutUint16(header[0:], srcPort)
+	binary.BigEndian.PutUint16(header[2:], dstPort)
+	binary.BigEndian.PutUint32(header[4:], seq)
+	binary.BigEndian.PutUint32(header[8:], ack)
+	//nolint:mnd
+	header[12] = byte(constants.BaseTCPHeaderLength) << 2 // data offset
+	header[13] = flags
+	// windowSize can be left to 5840 even for IPv6, it doesn't matter.
+	const windowSize = 5840
+	binary.BigEndian.PutUint16(header[14:], windowSize)
+	// header[16:17] is the checksum, set later
+	// header[18:19] is urgent pointer, not needed for our use case
+	return header
+}
+
+//nolint:mnd
+func tcpChecksum(ipHeader, tcpHeader, payload []byte) uint16 {
+	var pseudoHeader []byte
+	isIPv6 := len(ipHeader) >= 40 && (ipHeader[0]>>4) == 6
+	if isIPv6 {
+		pseudoHeader = make([]byte, 40)
+		copy(pseudoHeader[0:16], ipHeader[8:24])             // Source Address
+		copy(pseudoHeader[16:32], ipHeader[24:40])           // Destination Address
+		totalLength := uint32(len(tcpHeader) + len(payload)) //nolint:gosec
+		binary.BigEndian.PutUint32(pseudoHeader[32:], totalLength)
+		pseudoHeader[39] = 6 // Next Header (TCP)
+	} else {
+		pseudoHeader = make([]byte, 12)
+		copy(pseudoHeader[0:4], ipHeader[12:16])
+		copy(pseudoHeader[4:8], ipHeader[16:20])
+		pseudoHeader[9] = 6
+		totalLength := uint16(len(tcpHeader) + len(payload)) //nolint:gosec
+		binary.BigEndian.PutUint16(pseudoHeader[10:], totalLength)
+	}
+
+	sum := uint32(0)
+	for _, slice := range [][]byte{pseudoHeader, tcpHeader, payload} {
+		for i := 0; i < len(slice)-1; i += 2 {
+			sum += uint32(binary.BigEndian.Uint16(slice[i : i+2]))
+		}
+		if len(slice)%2 != 0 {
+			sum += uint32(slice[len(slice)-1]) << 8
+		}
+	}
+	for (sum >> 16) > 0 {
+		sum = (sum & 0xFFFF) + (sum >> 16)
+	}
+	return ^uint16(sum) //nolint:gosec
+}
+
+const (
+	tcpFlagsOffset      = 13
+	rstFlag        byte = 0x04
+	synFlag        byte = 0x02
+	ackFlag        byte = 0x10
+	pshFlag        byte = 0x08
+)
+
+type packetType uint8
+
+const (
+	packetTypeSYN packetType = iota + 1
+	packetTypeSYNACK
+	packetTypeACK
+	packetTypeRST
+)
+
+func (p packetType) String() string {
+	switch p {
+	case packetTypeSYN:
+		return "SYN"
+	case packetTypeSYNACK:
+		return "SYN-ACK"
+	case packetTypeACK:
+		return "ACK"
+	case packetTypeRST:
+		return "RST"
+	default:
+		panic("unknown packet type")
+	}
+}
+
+var (
+	errTCPHeaderTooShort    = errors.New("TCP header is too short")
+	errTCPPacketTypeUnknown = errors.New("TCP packet type is unknown")
+)
+
+// parseTCPHeader parses some elements from the TCP header.
+func parseTCPHeader(header []byte) (packetType packetType, seq, ack uint32, err error) {
+	if len(header) < int(constants.BaseTCPHeaderLength) {
+		return 0, 0, 0, fmt.Errorf("%w: %d bytes", errTCPHeaderTooShort, len(header))
+	}
+	flags := header[tcpFlagsOffset]
+	switch {
+	case (flags&synFlag) != 0 && (flags&ackFlag) == 0:
+		packetType = packetTypeSYN
+	case (flags&synFlag) != 0 && (flags&ackFlag) != 0:
+		packetType = packetTypeSYNACK
+	case (flags & rstFlag) != 0:
+		packetType = packetTypeRST
+	case (flags & ackFlag) != 0:
+		packetType = packetTypeACK
+	default:
+		return 0, 0, 0, fmt.Errorf("%w: flags are 0x%02x", errTCPPacketTypeUnknown, flags)
+	}
+
+	seq = binary.BigEndian.Uint32(header[4:8])
+	ack = binary.BigEndian.Uint32(header[8:12])
+	return packetType, seq, ack, nil
+}

internal/pmtud/tcp/tracker.go

@@ -0,0 +1,134 @@
+package tcp
+
+import (
+	"context"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"sync"
+	"syscall"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+type tracker struct {
+	fd              fileDescriptor
+	ipv4            bool
+	mutex           sync.RWMutex
+	portsToDispatch map[uint32]dispatch
+}
+
+type dispatch struct {
+	replyCh chan<- []byte
+	abort   <-chan struct{}
+}
+
+func newTracker(fd fileDescriptor, ipv4 bool) *tracker {
+	return &tracker{
+		fd:              fd,
+		ipv4:            ipv4,
+		portsToDispatch: make(map[uint32]dispatch),
+	}
+}
+
+func (t *tracker) constructKey(localPort, remotePort uint16) uint32 {
+	buf := make([]byte, 4) //nolint:mnd
+	binary.BigEndian.PutUint16(buf[0:2], localPort)
+	binary.BigEndian.PutUint16(buf[2:4], remotePort)
+	return binary.BigEndian.Uint32(buf)
+}
+
+func (t *tracker) register(localPort, remotePort uint16,
+	ch chan<- []byte, abort <-chan struct{},
+) {
+	key := t.constructKey(localPort, remotePort)
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	t.portsToDispatch[key] = dispatch{
+		replyCh: ch,
+		abort:   abort,
+	}
+}
+
+func (t *tracker) unregister(localPort, remotePort uint16) {
+	key := t.constructKey(localPort, remotePort)
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+	delete(t.portsToDispatch, key)
+}
+
+// listen listens for incoming TCP packets and dispatches them to the
+// correct channel based on the source and destination port.
+// If the context has a deadline associated, this one is used on the socket.
+// Note it returns a nil error on context cancellation.
+func (t *tracker) listen(ctx context.Context) error {
+	deadline, hasDeadline := ctx.Deadline()
+	for ctx.Err() == nil {
+		if hasDeadline {
+			remaining := time.Until(deadline)
+			if remaining <= 0 {
+				return nil
+			}
+			err := setSocketTimeout(t.fd, remaining)
+			if err != nil {
+				return fmt.Errorf("setting socket receive timeout: %w", err)
+			}
+		}
+
+		reply := make([]byte, constants.MaxEthernetFrameSize)
+		n, _, err := recvFrom(t.fd, reply, 0)
+		if err != nil {
+			switch {
+			case errors.Is(err, syscall.EAGAIN),
+				errors.Is(err, syscall.EWOULDBLOCK):
+				pollSleep(ctx)
+				continue
+			case ctx.Err() != nil:
+				// context canceled, stop listening so exit cleanly with no error
+				return nil //nolint:nilerr
+			default:
+				return fmt.Errorf("receiving on socket: %w", err)
+			}
+		}
+		reply = reply[:n]
+
+		if t.ipv4 {
+			var ok bool
+			reply, ok = stripIPv4Header(reply)
+			if !ok {
+				continue // not an IPv4 packet
+			}
+		}
+
+		const minTCPHeaderLength = 20
+		if len(reply) < minTCPHeaderLength {
+			continue
+		}
+
+		srcPort := binary.BigEndian.Uint16(reply[0:2])
+		dstPort := binary.BigEndian.Uint16(reply[2:4])
+		key := t.constructKey(dstPort, srcPort)
+		t.mutex.RLock()
+		dispatch, exists := t.portsToDispatch[key]
+		t.mutex.RUnlock()
+		if !exists {
+			continue
+		}
+		select {
+		case dispatch.replyCh <- reply:
+		case <-dispatch.abort:
+		}
+	}
+	return nil
+}
+
+func pollSleep(ctx context.Context) {
+	const sleepBetweenPolls = 10 * time.Millisecond
+	timer := time.NewTimer(sleepBetweenPolls)
+	select {
+	case <-ctx.Done():
+		timer.Stop()
+	case <-timer.C:
+	}
+}

internal/pmtud/test/mtu.go

@@ -0,0 +1,36 @@
+package test
+
+import "math"
+
+// MakeMTUsToTest determines a slice of MTU values to test
+// between minMTU and maxMTU inclusive. It creates an MTU
+// slice of length up to 11 MTUs such that:
+// - the first element is the minMTU
+// - the last element is the maxMTU
+// - elements in-between are separated as close to each other
+// The number 11 is chosen to find the final MTU in 3 searches,
+// with a total search space of 1728 MTUs which is enough;
+// to find it in 2 searches requires 37 parallel queries which
+// could be blocked by firewalls.
+func MakeMTUsToTest(minMTU, maxMTU uint32) (mtus []uint32) {
+	const mtusLength = 11 // find the final MTU in 3 searches
+	diff := maxMTU - minMTU
+	switch {
+	case minMTU > maxMTU:
+		panic("minMTU > maxMTU")
+	case diff <= mtusLength:
+		mtus = make([]uint32, 0, diff)
+		for mtu := minMTU; mtu <= maxMTU; mtu++ {
+			mtus = append(mtus, mtu)
+		}
+	default:
+		step := float64(diff) / float64(mtusLength-1)
+		mtus = make([]uint32, 0, mtusLength)
+		for mtu := float64(minMTU); len(mtus) < mtusLength-1; mtu += step {
+			mtus = append(mtus, uint32(math.Round(mtu)))
+		}
+		mtus = append(mtus, maxMTU) // last element is the maxMTU
+	}
+
+	return mtus
+}

internal/pmtud/test/mtu_test.go

@@ -1,4 +1,4 @@
-package pmtud
+package test
 
 import (
 	"testing"
@@ -6,7 +6,7 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func Test_makeMTUsToTest(t *testing.T) {
+func Test_MakeMTUsToTest(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]struct {
@@ -48,7 +48,7 @@ func Test_makeMTUsToTest(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			t.Parallel()
 
-			mtus := makeMTUsToTest(testCase.minMTU, testCase.maxMTU)
+			mtus := MakeMTUsToTest(testCase.minMTU, testCase.maxMTU)
 			assert.Equal(t, testCase.mtus, mtus)
 		})
 	}

internal/pmtud/vpn.go

@@ -0,0 +1,40 @@
+package pmtud
+
+import (
+	"net/netip"
+
+	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/constants/vpn"
+	pconstants "github.com/qdm12/gluetun/internal/pmtud/constants"
+)
+
+// MaxTheoreticalVPNMTU returns the theoretical maximum MTU for a VPN tunnel
+// given the VPN type, network protocol, and VPN gateway IP address.
+// This is notably useful to skip testing MTU values higher than this value.
+// The function panics if the network or VPN type is unknown.
+func MaxTheoreticalVPNMTU(vpnType, network string, vpnGateway netip.Addr) uint32 {
+	const physicalLinkMTU = pconstants.MaxEthernetFrameSize
+	vpnLinkMTU := physicalLinkMTU
+	if vpnGateway.Is4() {
+		vpnLinkMTU -= pconstants.IPv4HeaderLength
+	} else {
+		vpnLinkMTU -= pconstants.IPv6HeaderLength
+	}
+	switch network {
+	case constants.TCP:
+		vpnLinkMTU -= pconstants.BaseTCPHeaderLength
+	case constants.UDP:
+		vpnLinkMTU -= pconstants.UDPHeaderLength
+	default:
+		panic("unknown network protocol: " + network)
+	}
+	switch vpnType {
+	case vpn.Wireguard:
+		vpnLinkMTU -= pconstants.WireguardHeaderLength
+	case vpn.OpenVPN:
+		vpnLinkMTU -= pconstants.OpenVPNHeaderMaxLength
+	default:
+		panic("unknown VPN type: " + vpnType)
+	}
+	return vpnLinkMTU
+}

internal/provider/utils/wireguard.go

@@ -16,7 +16,17 @@ func BuildWireguardSettings(connection models.Connection,
 	settings.PreSharedKey = *userSettings.PreSharedKey
 	settings.InterfaceName = userSettings.Interface
 	settings.Implementation = userSettings.Implementation
-	settings.MTU = userSettings.MTU
+	if *userSettings.MTU > 0 {
+		settings.MTU = *userSettings.MTU
+	} else {
+		// The default is 1320 which is NOT the wireguard-go default
+		// of 1420 because this impacts bandwidth a lot on some
+		// VPN providers, see https://github.com/qdm12/gluetun/issues/1650.
+		// It has been lowered to 1320 following quite a bit of
+		// investigation in the issue: https://github.com/qdm12/gluetun/issues/2533.
+		const defaultMTU = 1320
+		settings.MTU = defaultMTU
+	}
 	settings.IPv6 = &ipv6Supported
 
 	const rulePriority = 101 // 100 is to receive external connections

internal/provider/utils/wireguard_test.go

@@ -22,7 +22,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 		ipv6Supported bool
 		settings      wireguard.Settings
 	}{
-		"some settings": {
+		"some_settings": {
 			connection: models.Connection{
 				IP:     netip.AddrFrom4([4]byte{1, 2, 3, 4}),
 				Port:   51821,
@@ -41,6 +41,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 				},
 				PersistentKeepaliveInterval: ptrTo(time.Hour),
 				Interface:                   "wg1",
+				MTU:                         ptrTo(uint32(1000)),
 			},
 			ipv6Supported: false,
 			settings: wireguard.Settings{
@@ -58,6 +59,7 @@ func Test_BuildWireguardSettings(t *testing.T) {
 				PersistentKeepaliveInterval: time.Hour,
 				RulePriority:                101,
 				IPv6:                        boolPtr(false),
+				MTU:                         1000,
 			},
 		},
 	}

internal/routing/vpn.go

@@ -47,3 +47,26 @@ func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (ip netip.Addr, err error) {
 	}
 	return ip, fmt.Errorf("%w: in %d routes", ErrVPNLocalGatewayIPNotFound, len(routes))
 }
+
+var ErrVPNRouteNotFound = errors.New("VPN route not found")
+
+func (r *Routing) VPNRoute(vpnIntf string) (route netlink.Route, err error) {
+	vpnLink, err := r.netLinker.LinkByName(vpnIntf)
+	if err != nil {
+		return route, fmt.Errorf("finding link %s: %w", vpnIntf, err)
+	}
+	vpnLinkIndex := vpnLink.Index
+
+	routes, err := r.netLinker.RouteList(netlink.FamilyAll)
+	if err != nil {
+		return route, fmt.Errorf("listing routes: %w", err)
+	}
+	for _, route := range routes {
+		if route.LinkIndex == vpnLinkIndex &&
+			!route.Dst.IsValid() {
+			return route, nil
+		}
+	}
+	return route, fmt.Errorf("%w: for interface %s in %d routes",
+		ErrVPNRouteNotFound, vpnIntf, len(routes))
+}

internal/vpn/interfaces.go

@@ -21,6 +21,7 @@ type Firewall interface {
 
 type Routing interface {
 	VPNLocalGatewayIP(vpnInterface string) (gateway netip.Addr, err error)
+	VPNRoute(vpnIntf string) (route netlink.Route, err error)
 }
 
 type PortForward interface {
@@ -67,6 +68,7 @@ type NetLinker interface {
 type Router interface {
 	RouteList(family uint8) (routes []netlink.Route, err error)
 	RouteAdd(route netlink.Route) error
+	RouteReplace(route netlink.Route) error
 }
 
 type Ruler interface {

internal/vpn/run.go

@@ -47,7 +47,13 @@ func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 			continue
 		}
 		tunnelUpData := tunnelUpData{
+			pmtud: tunnelUpPMTUDData{
+				enabled:   settings.Type != vpn.Wireguard || *settings.Wireguard.MTU == 0,
 				vpnType:   settings.Type,
+				network:   connection.Protocol,
+				icmpAddrs: settings.PMTUD.ICMPAddresses,
+				tcpAddrs:  settings.PMTUD.TCPAddresses,
+			},
 			serverIP:       connection.IP,
 			serverName:     connection.ServerName,
 			canPortForward: connection.PortForward,

internal/vpn/tunnelup.go

@@ -2,7 +2,6 @@ package vpn
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"net/netip"
 	"time"
@@ -10,6 +9,7 @@ import (
 	"github.com/qdm12/dns/v2/pkg/check"
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/pmtud"
+	pconstants "github.com/qdm12/gluetun/internal/pmtud/constants"
 	"github.com/qdm12/gluetun/internal/version"
 	"github.com/qdm12/log"
 )
@@ -17,9 +17,7 @@ import (
 type tunnelUpData struct {
 	// Healthcheck
 	serverIP netip.Addr
-	// vpnType is used for path MTU discovery to find the protocol overhead.
-	// It can be "wireguard" or "openvpn".
-	vpnType string
+	pmtud    tunnelUpPMTUDData
 	// Port forwarding
 	vpnIntf        string
 	serverName     string // used for PIA
@@ -29,6 +27,23 @@ type tunnelUpData struct {
 	portForwarder  PortForwarder
 }
 
+type tunnelUpPMTUDData struct {
+	// enabled is notably false if the user specifies a custom MTU.
+	enabled bool
+	// vpnType is used to find the maximum VPN header overhead.
+	// It can be [vpn.Wireguard] or [vpn.OpenVPN].
+	vpnType string
+	// network is used to find the network level header overhead.
+	// It can be [constants.UDP] or [constants.TCP].
+	network string
+	// icmpAddrs is the list of addresses to use for ICMP path MTU discovery.
+	// Each address should handle ICMP packets for PMTUD to work.
+	icmpAddrs []netip.Addr
+	// tcpAddrs is the list of addresses to use for TCP path MTU discovery.
+	// Each address should have a listening TCP server on the port specified.
+	tcpAddrs []netip.AddrPort
+}
+
 func (l *Loop) onTunnelUp(ctx, loopCtx context.Context, data tunnelUpData) {
 	l.client.CloseIdleConnections()
 
@@ -39,12 +54,15 @@ func (l *Loop) onTunnelUp(ctx, loopCtx context.Context, data tunnelUpData) {
 		}
 	}
 
+	if data.pmtud.enabled {
 		mtuLogger := l.logger.New(log.SetComponent("MTU discovery"))
-	err := updateToMaxMTU(ctx, data.vpnIntf, data.vpnType,
+		err := updateToMaxMTU(ctx, data.vpnIntf, data.pmtud.vpnType,
+			data.pmtud.network, data.pmtud.icmpAddrs, data.pmtud.tcpAddrs,
 			l.netLinker, l.routing, mtuLogger)
 		if err != nil {
 			mtuLogger.Error(err.Error())
 		}
+	}
 
 	icmpTargetIPs := l.healthSettings.ICMPTargetIPs
 	if len(icmpTargetIPs) == 1 && icmpTargetIPs[0].IsUnspecified() {
@@ -136,12 +154,11 @@ func (l *Loop) restartVPN(ctx context.Context, healthErr error) {
 	_, _ = l.ApplyStatus(ctx, constants.Running)
 }
 
-var errVPNTypeUnknown = errors.New("unknown VPN type")
-
 func updateToMaxMTU(ctx context.Context, vpnInterface string,
-	vpnType string, netlinker NetLinker, routing Routing, logger *log.Logger,
+	vpnType, network string, icmpAddrs []netip.Addr, tcpAddrs []netip.AddrPort,
+	netlinker NetLinker, routing Routing, logger *log.Logger,
 ) error {
-	logger.Info("finding maximum MTU, this can take up to 4 seconds")
+	logger.Info("finding maximum MTU, this can take up to 6 seconds")
 
 	vpnGatewayIP, err := routing.VPNLocalGatewayIP(vpnInterface)
 	if err != nil {
@@ -155,18 +172,7 @@ func updateToMaxMTU(ctx context.Context, vpnInterface string,
 
 	originalMTU := link.MTU
 
-	// Note: no point testing for an MTU of 1500, it will never work due to the VPN
-	// protocol overhead, so start lower than 1500 according to the protocol used.
-	const physicalLinkMTU uint32 = 1500
-	vpnLinkMTU := physicalLinkMTU
-	switch vpnType {
-	case "wireguard":
-		vpnLinkMTU -= 60 // Wireguard overhead
-	case "openvpn":
-		vpnLinkMTU -= 41 // OpenVPN overhead
-	default:
-		return fmt.Errorf("%w: %q", errVPNTypeUnknown, vpnType)
-	}
+	vpnLinkMTU := pmtud.MaxTheoreticalVPNMTU(vpnType, network, vpnGatewayIP)
 
 	// Setting the VPN link MTU to 1500 might interrupt the connection until
 	// the new MTU is set again, but this is necessary to find the highest valid MTU.
@@ -178,16 +184,14 @@ func updateToMaxMTU(ctx context.Context, vpnInterface string,
 	}
 
 	const pingTimeout = time.Second
-	vpnLinkMTU, err = pmtud.PathMTUDiscover(ctx, vpnGatewayIP, vpnLinkMTU, pingTimeout, logger)
-	switch {
-	case err == nil:
-		logger.Infof("setting VPN interface %s MTU to maximum valid MTU %d", vpnInterface, vpnLinkMTU)
-	case errors.Is(err, pmtud.ErrMTUNotFound) || errors.Is(err, pmtud.ErrICMPNotPermitted):
+	vpnLinkMTU, err = pmtud.PathMTUDiscover(ctx, icmpAddrs, tcpAddrs,
+		vpnLinkMTU, pingTimeout, logger)
+	if err != nil {
 		vpnLinkMTU = originalMTU
 		logger.Infof("reverting VPN interface %s MTU to %d (due to: %s)",
 			vpnInterface, originalMTU, err)
-	default:
-		return fmt.Errorf("path MTU discovering: %w", err)
+	} else {
+		logger.Infof("setting VPN interface %s MTU to maximum valid MTU %d", vpnInterface, vpnLinkMTU)
 	}
 
 	err = netlinker.LinkSetMTU(link.Index, vpnLinkMTU)
@@ -195,5 +199,33 @@ func updateToMaxMTU(ctx context.Context, vpnInterface string,
 		return fmt.Errorf("setting VPN interface %s MTU to %d: %w", vpnInterface, vpnLinkMTU, err)
 	}
 
+	err = setTCPMSSOnVPNRoute(vpnInterface, vpnLinkMTU, routing, netlinker)
+	if err != nil {
+		return fmt.Errorf("setting safe TCP MSS for MTU %d: %w", vpnLinkMTU, err)
+	}
+
+	return nil
+}
+
+func setTCPMSSOnVPNRoute(vpnIntf string, mtu uint32,
+	routing Routing, netlinker NetLinker,
+) error {
+	route, err := routing.VPNRoute(vpnIntf)
+	if err != nil {
+		return fmt.Errorf("getting VPN route: %w", err)
+	}
+
+	ipHeaderLength := pconstants.IPv4HeaderLength
+	if route.Dst.Addr().Is6() {
+		ipHeaderLength = pconstants.IPv6HeaderLength
+	}
+	const mysteriousOverhead = 20 // most likely TCP options, such as the 12B of timestamps
+	overhead := ipHeaderLength + pconstants.BaseTCPHeaderLength + mysteriousOverhead
+	mss := mtu - overhead
+	route.AdvMSS = mss
+	err = netlinker.RouteReplace(route)
+	if err != nil {
+		return fmt.Errorf("replacing VPN route with MSS changed to %d: %w", mss, err)
+	}
 	return nil
 }