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

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)
+}