LookupServer: Verify that DNS response questions match the request

To protect against DNS spoof attacks, we now check that the questions
in incoming responses match the questions in the request we sent out.

Suggested by @zecke in SerenityOS#10.

Servers/LookupServer/DNSQuestion.h

@@ -16,6 +16,16 @@ class DNSQuestion {
  u16 class_code() const { return m_class_code; }
  const String& name() const { return m_name; }
 
+ bool operator==(const DNSQuestion& other) const
+ {
+ return m_name == other.m_name && m_record_type == other.m_record_type && m_class_code == other.m_class_code;
+ }
+
+ bool operator!=(const DNSQuestion& other) const
+ {
+ return !(*this == other);
+ }
+
 private:
  String m_name;
  u16 m_record_type { 0 };

Servers/LookupServer/DNSRequest.h

@@ -18,6 +18,8 @@ class DNSRequest {
 
  void add_question(const String& name, u16 record_type);
 
+ const Vector<DNSQuestion>& questions() const { return m_questions; }
+
  u16 question_count() const
  {
  ASSERT(m_questions.size() < UINT16_MAX);

Servers/LookupServer/DNSResponse.cpp

@@ -28,7 +28,6 @@ class [[gnu::packed]] DNSRecordWithoutName
 
 static_assert(sizeof(DNSRecordWithoutName) == 10);
 
-
 Optional<DNSResponse> DNSResponse::from_raw_response(const u8* raw_data, size_t raw_size)
 {
  if (raw_size < sizeof(DNSPacket)) {
@@ -79,7 +78,7 @@ Optional<DNSResponse> DNSResponse::from_raw_response(const u8* raw_data, size_t
  // FIXME: Parse some other record types perhaps?
  dbg() << " data=(unimplemented record type " << record.type() << ")";
  }
- dbg() << "Answer #" << i << ": type=" << record.type() << ", ttl=" << record.ttl() << ", length=" << record.data_length() << ", data=_" << data << "_";
+ dbg() << "Answer #" << i << ": name=_" << name << "_, type=" << record.type() << ", ttl=" << record.ttl() << ", length=" << record.data_length() << ", data=_" << data << "_";
  response.m_answers.empend(name, record.type(), record.record_class(), record.ttl(), data);
  offset += record.data_length();
  }

Servers/LookupServer/LookupServer.cpp

@@ -216,10 +216,22 @@ Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, u
  dbgprintf("LookupServer: ID mismatch (%u vs %u) :(\n", response.id(), request.id());
  return {};
  }
- if (response.question_count() != 1) {
+ if (response.question_count() != request.question_count()) {
  dbgprintf("LookupServer: Question count (%u vs %u) :(\n", response.question_count(), request.question_count());
  return {};
  }
+
+ for (size_t i = 0; i < request.question_count(); ++i) {
+ auto& request_question = request.questions()[i];
+ auto& response_question = response.questions()[i];
+ if (request_question != response_question) {
+ dbg() << "Request and response questions do not match";
+ dbg() << " Request: {_" << request_question.name() << "_, " << request_question.record_type() << ", " << request_question.class_code() << "}";
+ dbg() << " Response: {_" << response_question.name() << "_, " << response_question.record_type() << ", " << response_question.class_code() << "}";
+ return {};
+ }
+ }
+
  if (response.answer_count() < 1) {
  dbgprintf("LookupServer: Not enough answers (%u) :(\n", response.answer_count());
  return {};