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/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)
+}