network: add auto-detection of optimal MTU for WireGuard interface, --wg-mtu flag, set max_mtu for Corrosion to 1232

internal/machine/network/mtu.go

@@ -0,0 +1,22 @@
+package network
+
+import "log/slog"
+
+// DetectMTU returns the optimal MTU for the WireGuard interface based on the machine's egress network.
+// The egress MTU is capped at MaxWireGuardMTU to not overestimate the path MTU between machines which can go over
+// the public internet. If the egress MTU cannot be detected, it falls back to MaxWireGuardMTU.
+func DetectMTU() int {
+	egressMTU, err := detectEgressMTU()
+	if err != nil {
+		slog.Warn("Failed to detect egress network MTU, falling back to the default WireGuard MTU.",
+			"mtu", MaxWireGuardMTU, "err", err)
+		return MaxWireGuardMTU
+	}
+
+	mtu := egressMTU - wireGuardEncapOverhead
+	// Clamp the computed MTU to the range [MinWireGuardMTU, MaxWireGuardMTU].
+	mtu = min(max(mtu, MinWireGuardMTU), MaxWireGuardMTU)
+	slog.Info("Detected optimal WireGuard MTU from the egress network.", "mtu", mtu, "egress_mtu", egressMTU)
+
+	return mtu
+}