Don't report errors for ICMP type 3 packets

These are noisy and annoying, and although it's unclear where they
come from, there's no sensible way to handle them other than simply
dropping them, since we can't actually speak ICMP upstream.

Author: pimterry

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

@@ -241,6 +241,10 @@ class ProxyVpnService : VpnService(), IProtectSocket {
 
         SocketProtector.getInstance().setProtector(this)
 
+
+
+        // TODO: Should we support *?
+
         vpnRunnable = ProxyVpnRunnable(
             vpnInterface,
             proxyConfig.ip,

app/src/main/java/tech/httptoolkit/android/vpn/SessionHandler.java

@@ -424,6 +424,18 @@ private void handleICMPPacket(
 		final ICMPPacket requestPacket = ICMPPacketFactory.parseICMPPacket(clientPacketData);
 		Log.d(TAG, "Got an ICMP ping packet, type " + requestPacket.toString());
 
+		if (requestPacket.type == ICMPPacket.DESTINATION_UNREACHABLE_TYPE) {
+			// This is a packet from the phone, telling somebody that a destination is unreachable.
+			// Might be caused by issues on our end, but it's unclear what kind of issues. Regardless,
+			// we can't send ICMP messages ourselves or react usefully, so we drop these silently.
+			return;
+		} else if (requestPacket.type != ICMPPacket.ECHO_REQUEST_TYPE) {
+			// We only actually support outgoing ping packets. Loudly drop anything else:
+			throw new PacketHeaderException(
+				"Unknown ICMP type (" + requestPacket.type + "). Only echo requests are supported"
+			);
+		}
+
 		pingThreadpool.execute(new Runnable() {
 			@Override
 			public void run() {

app/src/main/java/tech/httptoolkit/android/vpn/transport/icmp/ICMPPacket.java

@@ -3,10 +3,16 @@
 import tech.httptoolkit.android.vpn.transport.PacketHeaderException;
 
 public class ICMPPacket {
+    // Two ICMP packets we can handle: simple ping & pong
     public static final byte ECHO_REQUEST_TYPE = 8;
     public static final byte ECHO_SUCCESS_TYPE = 0;
 
-    final byte type;
+    // One very common packet we ignore: connection rejection. Unclear why this happens,
+    // random incoming connections that the phone tries to reply to? Nothing we can do though,
+    // as we can't forward ICMP onwards, and we can't usefully respond or react.
+    public static final byte DESTINATION_UNREACHABLE_TYPE = 3;
+
+    public final byte type;
     final byte code; // 0 for request, 0 for success, 0 - 15 for error subtypes
 
     final int checksum;
@@ -23,10 +29,6 @@ public class ICMPPacket {
         int sequenceNumber,
         byte[] data
     ) throws PacketHeaderException {
-        if (type != ECHO_REQUEST_TYPE && type != ECHO_SUCCESS_TYPE) {
-            throw new PacketHeaderException("ICMP packet with id must be request or response");
-        }
-
         this.type = (byte) type;
         this.code = (byte) code;
         this.checksum = checksum;

app/src/main/java/tech/httptoolkit/android/vpn/transport/icmp/ICMPPacketFactory.java

@@ -24,11 +24,7 @@ public static ICMPPacket parseICMPPacket(@NonNull ByteBuffer stream) throws Pack
         final byte[] data = new byte[stream.remaining()];
         stream.get(data);
 
-        if (type == 8) {
-            return new ICMPPacket(type, code, checksum, identifier, sequenceNumber, data);
-        } else {
-            throw new PacketHeaderException("Unknown ICMP type (" + type + "). Only echo requests are supported");
-        }
+        return new ICMPPacket(type, code, checksum, identifier, sequenceNumber, data);
     }
 
     public static ICMPPacket buildSuccessPacket(ICMPPacket requestPacket) throws PacketHeaderException {