Make the VPN MTU explicit

This doesn't solve an immediate issue, but removes the risk that
we overflow the VpnRunnable buffer, which assumes a default MTU
of 1500.

Author: pimterry

app/src/main/java/tech/httptoolkit/android/ProxyVpnRunnable.kt

@@ -13,8 +13,8 @@ import java.io.FileOutputStream
 import java.net.InetSocketAddress
 import java.nio.ByteBuffer
 
-// Taken from ToyShark - I suspect this is somewhat arbitrary
-private const val MAX_PACKET_LEN = 1500
+// Set on our VPN as the MTU, which should guarantee all packets fit this
+const val MAX_PACKET_LEN = 1500
 
 class ProxyVpnRunnable(
     vpnInterface: ParcelFileDescriptor,

app/src/main/java/tech/httptoolkit/android/ProxyVpnService.kt

@@ -148,7 +148,10 @@ class ProxyVpnService : VpnService(), IProtectSocket {
             vpnInterface = Builder()
                 .addAddress(VPN_IP_ADDRESS, 32)
                 .addRoute(ALL_ROUTES, 0)
-                .setBlocking(true) // We use a blocking loop to read this in ProxyVpnRunnable
+
+                .setMtu(MAX_PACKET_LEN) // Limit the packet size to the buffer used by ProxyVpnRunnable
+                .setBlocking(true) // We use a blocking loop to read in ProxyVpnRunnable
+
                 .apply {
                     // We exclude ourselves from interception, so we can still make network requests
                     // separately, primarily because otherwise pinging with isReachable is recursive.