LookupServer: Retry with 0x20 randomization turned off on EREFUSED

Apparently some authoritative servers don't handle 0x20 randomization
well and may send EREFUSED. Retry with randomization turned off then.

Reference: https://github.com/dns-violations/dns-violations/blob/master/2017/DVE-2017-0006.md

More work towards #10.

Servers/LookupServer/DNSRequest.cpp

@@ -13,22 +13,24 @@ DNSRequest::DNSRequest()
 {
 }
 
-void DNSRequest::add_question(const String& name, u16 record_type)
+void DNSRequest::add_question(const String& name, u16 record_type, ShouldRandomizeCase should_randomize_case)
 {
     ASSERT(m_questions.size() <= UINT16_MAX);
 
     if (name.is_empty())
         return;
 
-    // Randomize the 0x20 bit in every ASCII character.
     StringBuilder builder;
     for (size_t i = 0; i < name.length(); ++i) {
         u8 ch = name[i];
-        if (isalpha(ch)) {
-            if (arc4random_uniform(2))
-                ch |= 0x20;
-            else
-                ch &= ~0x20;
+        if (should_randomize_case == ShouldRandomizeCase::Yes) {
+            // Randomize the 0x20 bit in every ASCII character.
+            if (isalpha(ch)) {
+                if (arc4random_uniform(2))
+                    ch |= 0x20;
+                else
+                    ch &= ~0x20;
+            }
         }
         builder.append(ch);
     }

Servers/LookupServer/DNSRequest.h

@@ -12,11 +12,16 @@
 #define T_PTR 12
 #define T_MX 15
 
+enum class ShouldRandomizeCase {
+    No = 0,
+    Yes
+};
+
 class DNSRequest {
 public:
     DNSRequest();
 
-    void add_question(const String& name, u16 record_type);
+    void add_question(const String& name, u16 record_type, ShouldRandomizeCase);
 
     const Vector<DNSQuestion>& questions() const { return m_questions; }
 

Servers/LookupServer/DNSResponse.cpp

@@ -44,6 +44,10 @@ Optional<DNSResponse> DNSResponse::from_raw_response(const u8* raw_data, size_t
 
     DNSResponse response;
     response.m_id = response_header.id();
+    response.m_code = response_header.response_code();
+
+    if (response.code() != DNSResponse::Code::NOERROR)
+        return response;
 
     size_t offset = sizeof(DNSPacket);
 

Servers/LookupServer/DNSResponse.h

@@ -26,10 +26,26 @@ public:
         return m_answers.size();
     }
 
+    enum class Code : u8 {
+        NOERROR = 0,
+        FORMERR = 1,
+        SERVFAIL = 2,
+        NXDOMAIN = 3,
+        NOTIMP = 4,
+        REFUSED = 5,
+        YXDOMAIN = 6,
+        XRRSET = 7,
+        NOTAUTH = 8,
+        NOTZONE = 9,
+    };
+
+    Code code() const { return (Code)m_code; }
+
 private:
     DNSResponse() {}
 
     u16 m_id { 0 };
+    u8 m_code { 0 };
     Vector<DNSQuestion> m_questions;
     Vector<DNSAnswer> m_answers;
 };

Servers/LookupServer/LookupServer.cpp

@@ -161,7 +161,7 @@ void LookupServer::service_client(RefPtr<CLocalSocket> socket)
     }
 }
 
-Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, unsigned short record_type)
+Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, unsigned short record_type, ShouldRandomizeCase should_randomize_case)
 {
     if (auto it = m_lookup_cache.find(hostname); it != m_lookup_cache.end()) {
         auto& cached_lookup = it->value;
@@ -172,7 +172,7 @@ Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, u
     }
 
     DNSRequest request;
-    request.add_question(hostname, record_type);
+    request.add_question(hostname, record_type, should_randomize_case);
 
     auto buffer = request.to_byte_buffer();
 
@@ -216,6 +216,15 @@ 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.code() == DNSResponse::Code::REFUSED) {
+        if (should_randomize_case == ShouldRandomizeCase::Yes) {
+            // Retry with 0x20 case randomization turned off.
+            return lookup(hostname, did_timeout, record_type, ShouldRandomizeCase::No);
+        }
+        return {};
+    }
+
     if (response.question_count() != request.question_count()) {
         dbgprintf("LookupServer: Question count (%u vs %u) :(\n", response.question_count(), request.question_count());
         return {};

Servers/LookupServer/LookupServer.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include "DNSRequest.h"
 #include <AK/HashMap.h>
 #include <AK/RefPtr.h>
 #include <AK/String.h>
@@ -44,7 +45,7 @@ public:
 private:
     void load_etc_hosts();
     void service_client(RefPtr<CLocalSocket>);
-    Vector<String> lookup(const String& hostname, bool& did_timeout, unsigned short record_type);
+    Vector<String> lookup(const String& hostname, bool& did_timeout, unsigned short record_type, ShouldRandomizeCase = ShouldRandomizeCase::Yes);
 
     int make_dns_request_socket(sockaddr_in& dst_addr);