feat(pmtud/tcp): use the TCP server with highest MSS to run MTU tests

internal/pmtud/tcp/mss_test.go

@@ -0,0 +1,59 @@
+//go:build linux
+
+package tcp
+
+import (
+	"context"
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/qdm12/gluetun/internal/netlink"
+	"github.com/qdm12/gluetun/internal/pmtud/constants"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_findHighestMSSDestination(t *testing.T) {
+	t.Parallel()
+
+	netlinker := netlink.New(&noopLogger{})
+	defaultIPv4MTU, err := findDefaultIPv4RouteMTU(netlinker)
+	require.NoError(t, err, "finding default IPv4 route MTU")
+
+	ctx, cancel := context.WithCancel(t.Context())
+
+	const family = constants.AF_INET
+	fd, stop, err := startRawSocket(family, excludeMark)
+	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)
+	})
+
+	dsts := []netip.AddrPort{
+		netip.AddrPortFrom(netip.AddrFrom4([4]byte{1, 1, 1, 1}), 443),
+		netip.AddrPortFrom(netip.AddrFrom4([4]byte{8, 8, 8, 8}), 443),
+	}
+	const timeout = time.Second
+	fw := getFirewall(t)
+	logger := &noopLogger{}
+
+	dst, mss, err := findHighestMSSDestination(t.Context(), fd, dsts,
+		excludeMark, defaultIPv4MTU, timeout, tracker, fw, logger)
+	require.NoError(t, err, "finding highest MSS destination")
+	assert.Contains(t, dsts, dst, "destination should be in the provided list")
+	assert.Greater(t, mss, uint32(1000), "MSS should be greater than 1000")
+	assert.LessOrEqual(t, mss, constants.MaxEthernetFrameSize,
+		"MSS should be less than or equal to the maximum Ethernet frame size	")
+}