1 files changed, 220 insertions, 0 deletions
diff --git a/MdnsOffloadManagerService/src/com/android/tv/mdnsoffloadmanager/MdnsPacketParser.java b/MdnsOffloadManagerService/src/com/android/tv/mdnsoffloadmanager/MdnsPacketParser.java
new file mode 100644
index 0000000..cd930dc
--- /dev/null
+++ b/MdnsOffloadManagerService/src/com/android/tv/mdnsoffloadmanager/MdnsPacketParser.java
@@ -0,0 +1,220 @@
+/*
+ * Copyright (C) 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.tv.mdnsoffloadmanager;
+
+import androidx.annotation.NonNull;
+
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+import device.google.atv.mdns_offload.IMdnsOffload.MdnsProtocolData.MatchCriteria;
+
+/**
+ * Tool class to help read mdns data from a fully formed mDNS response packet.
+ */
+public final class MdnsPacketParser {
+
+    private static final int OFFSET_QUERIES_COUNT = 4;
+    private static final int OFFSET_ANSWERS_COUNT = 6;
+    private static final int OFFSET_AUTHORITY_COUNT = 8;
+    private static final int OFFSET_ADDITIONAL_COUNT = 10;
+    private static final int OFFSET_DATA_SECTION_START = 12;
+
+    private final byte[] mMdnsData;
+    private int mCursorIndex;
+
+    private MdnsPacketParser(@NonNull byte[] mDNSData) {
+        this.mMdnsData = mDNSData;
+    }
+
+    /**
+     * Extracts a label starting at offset and then follows RFC1035-4.1.4 The offset should start
+     * either at a data length value or at a pointer value.
+     */
+    public static String extractFullName(@NonNull byte[] array, int offset) {
+        MdnsPacketParser parser = new MdnsPacketParser(array);
+        parser.setCursor(offset);
+        StringBuilder builder = new StringBuilder();
+
+        while (!parser.isCursorOnRootLabel()) {
+            if (parser.isCursorOnPointer()) {
+                parser.setCursor(parser.pollPointerOffset());
+            } else if (parser.isCursorOnLabel()) {
+                builder.append(parser.pollLabel());
+                builder.append('.');
+            } else {
+                throw new IllegalArgumentException("mDNS response packet is badly formed.");
+            }
+        }
+        return builder.toString();
+    }
+
+    /**
+     * Finds all the RRNAMEs ans RRTYPEs in the mdns response packet provided. Expects a packet only
+     * with responses.
+     */
+    public static List<MatchCriteria> extractMatchCriteria(@NonNull byte[] mdnsResponsePacket) {
+        Objects.requireNonNull(mdnsResponsePacket);
+
+        // Parse MdnsPacket and read labels and find
+        List<MatchCriteria> criteriaList = new ArrayList<>();
+        MdnsPacketParser parser = new MdnsPacketParser(mdnsResponsePacket);
+
+        if (parser.getQueriesCount() != 0
+                || parser.getAuthorityCount() != 0
+                || parser.getAdditionalCount() != 0) {
+            throw new IllegalArgumentException(
+                    "mDNS response packet contains data that is not answers");
+        }
+        int answersToRead = parser.getAnswersCount();
+
+        parser.moveToDataSection();
+        while (answersToRead > 0) {
+            // Each record starts with the RRNAME, so the offset is correct for the criteria.
+            MatchCriteria criteria = new MatchCriteria();
+            criteria.nameOffset = parser.getCursorOffset();
+
+            /// Skip labels first
+            while (parser.isCursorOnLabel()) {
+                parser.pollLabel();
+            }
+            // We can be on a root label or on a pointer. Skip both.
+            if (parser.isCursorOnRootLabel()) {
+                parser.skipBytes(1);
+            } else if (parser.isCursorOnPointer()) {
+                parser.pollPointerOffset();
+            }
+
+            // The cursor must be on the RRTYPE.
+            criteria.type = parser.pollUint16();
+
+            // The next 6 bytes point to cache flush, rrclass, and ttl
+            parser.skipBytes(6);
+
+            // Now the index points to the data length on 2 bytes
+            int dataLength = parser.pollUint16();
+
+            // Then we can skip those data bytes.
+            parser.skipBytes(dataLength);
+
+            // Criteria is complete, it can be added.
+            criteriaList.add(criteria);
+            answersToRead--;
+        }
+        if (parser.hasContent()) {
+            // The packet is badly formed. All answers where read successfully, but data remains
+            // available.
+            throw new IllegalArgumentException(
+                    "mDNS response packet is badly formed. Too much data.");
+        }
+
+        return criteriaList;
+    }
+
+    private boolean hasContent() {
+        return mCursorIndex < mMdnsData.length;
+    }
+
+    private void setCursor(int position) {
+        if (position < 0) {
+            throw new IllegalArgumentException("Setting cursor on negative offset is not allowed.");
+        }
+        mCursorIndex = position;
+    }
+
+    private int getCursorOffset() {
+        return mCursorIndex;
+    }
+
+    private void skipBytes(int bytesToSkip) {
+        mCursorIndex += bytesToSkip;
+    }
+
+    private int getQueriesCount() {
+        return readUint16(OFFSET_QUERIES_COUNT);
+    }
+
+    private int getAnswersCount() {
+        return readUint16(OFFSET_ANSWERS_COUNT);
+    }
+
+    private int getAuthorityCount() {
+        return readUint16(OFFSET_AUTHORITY_COUNT);
+    }
+
+    private int getAdditionalCount() {
+        return readUint16(OFFSET_ADDITIONAL_COUNT);
+    }
+
+    private void moveToDataSection() {
+        mCursorIndex = OFFSET_DATA_SECTION_START;
+    }
+
+    private String pollLabel() {
+        int labelSize = readUint8(mCursorIndex);
+        mCursorIndex++;
+        if (mCursorIndex + labelSize > mMdnsData.length) {
+            throw new IllegalArgumentException(
+                    "mDNS response packet is badly formed. Not enough data.");
+        }
+        String value = new String(mMdnsData, mCursorIndex, labelSize, StandardCharsets.UTF_8);
+        mCursorIndex += labelSize;
+        return value;
+    }
+
+    private boolean isCursorOnLabel() {
+        return !isCursorOnRootLabel() && (readUint8(mCursorIndex) & 0b11000000) == 0b00000000;
+    }
+
+    private boolean isCursorOnPointer() {
+        return (readUint8(mCursorIndex) & 0b11000000) == 0b11000000;
+    }
+
+    private boolean isCursorOnRootLabel() {
+        return readUint8(mCursorIndex) == 0;
+    }
+
+    private int pollPointerOffset() {
+        int value = readUint16(mCursorIndex) & 0b0011111111111111;
+        mCursorIndex += 2;
+        return value;
+    }
+
+    private int readUint8(int offset) {
+        if (offset >= mMdnsData.length) {
+            throw new IllegalArgumentException(
+                    "mDNS response packet is badly formed. Not enough data.");
+        }
+        return ((int) mMdnsData[offset]) & 0xff;
+    }
+
+    private int readUint16(int offset) {
+        if (offset + 1 >= mMdnsData.length) {
+            throw new IllegalArgumentException(
+                    "mDNS response packet is badly formed. Not enough data.");
+        }
+        return (readUint8(offset) << 8) + readUint8(offset + 1);
+    }
+
+    private int pollUint16() {
+        int value = readUint16(mCursorIndex);
+        mCursorIndex += 2;
+        return value;
+    }
+}