LookupServer: Cache DNS answers for TTL seconds

We now keep DNS answers around in a cache for TTL seconds after getting
them the first time. The cache is capped at 256 responses for now.

Suggested by @zecke in #10.

Servers/LookupServer/DNSAnswer.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "DNSAnswer.h"
+#include <time.h>
+
+DNSAnswer::DNSAnswer(const String& name, u16 type, u16 class_code, u32 ttl, const String& record_data)
+    : m_name(name)
+    , m_type(type)
+    , m_class_code(class_code)
+    , m_ttl(ttl)
+    , m_record_data(record_data)
+{
+    auto now = time(nullptr);
+    m_expiration_time = now + m_ttl;
+    if (m_expiration_time < now)
+        m_expiration_time = 0;
+}
+
+bool DNSAnswer::has_expired() const
+{
+    return time(nullptr) >= m_expiration_time;
+}

Servers/LookupServer/DNSAnswer.h

@@ -31,14 +31,7 @@
 
 class DNSAnswer {
 public:
-    DNSAnswer(const String& name, u16 type, u16 class_code, u32 ttl, const String& record_data)
-        : m_name(name)
-        , m_type(type)
-        , m_class_code(class_code)
-        , m_ttl(ttl)
-        , m_record_data(record_data)
-    {
-    }
+    DNSAnswer(const String& name, u16 type, u16 class_code, u32 ttl, const String& record_data);
 
     const String& name() const { return m_name; }
     u16 type() const { return m_type; }
@@ -46,10 +39,13 @@ public:
     u32 ttl() const { return m_ttl; }
     const String& record_data() const { return m_record_data; }
 
+    bool has_expired() const;
+
 private:
     String m_name;
     u16 m_type { 0 };
     u16 m_class_code { 0 };
     u32 m_ttl { 0 };
+    time_t m_expiration_time { 0 };
     String m_record_data;
 };

Servers/LookupServer/LookupServer.cpp

@@ -157,8 +157,16 @@ Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, u
 {
     if (auto it = m_lookup_cache.find(hostname); it != m_lookup_cache.end()) {
         auto& cached_lookup = it->value;
-        if (cached_lookup.record_type == record_type && cached_lookup.timestamp < (time(nullptr) + 60)) {
-            return it->value.responses;
+        if (cached_lookup.question.record_type() == record_type) {
+            Vector<String> responses;
+            for (auto& cached_answer : cached_lookup.answers) {
+                dbg() << "Cache hit: " << hostname << " -> " << cached_answer.record_data() << ", expired: " << cached_answer.has_expired();
+                if (!cached_answer.has_expired()) {
+                    responses.append(cached_answer.record_data());
+                }
+            }
+            if (!responses.is_empty())
+                return responses;
         }
         m_lookup_cache.remove(it);
     }
@@ -232,11 +240,13 @@ Vector<String> LookupServer::lookup(const String& hostname, bool& did_timeout, u
         return {};
     }
 
-    Vector<String> addresses;
+    Vector<String> responses;
     for (auto& answer : response.answers()) {
-        addresses.append(answer.record_data());
+        responses.append(answer.record_data());
     }
 
-    m_lookup_cache.set(hostname, { time(nullptr), record_type, addresses });
-    return addresses;
+    if (m_lookup_cache.size() >= 256)
+        m_lookup_cache.remove(m_lookup_cache.begin());
+    m_lookup_cache.set(hostname, { request.questions()[0], response.answers() });
+    return responses;
 }

Servers/LookupServer/LookupServer.h

@@ -27,11 +27,13 @@
 #pragma once
 
 #include "DNSRequest.h"
+#include "DNSResponse.h"
 #include <AK/HashMap.h>
 #include <LibCore/CObject.h>
 
 class CLocalSocket;
 class CLocalServer;
+class DNSAnswer;
 
 class LookupServer final : public CObject {
     C_OBJECT(LookupServer)
@@ -45,9 +47,8 @@ private:
     Vector<String> lookup(const String& hostname, bool& did_timeout, unsigned short record_type, ShouldRandomizeCase = ShouldRandomizeCase::Yes);
 
     struct CachedLookup {
-        time_t timestamp { 0 };
-        unsigned short record_type { 0 };
-        Vector<String> responses;
+        DNSQuestion question;
+        Vector<DNSAnswer> answers;
     };
 
     RefPtr<CLocalServer> m_local_server;

Servers/LookupServer/Makefile

@@ -2,6 +2,7 @@ OBJS = \
     LookupServer.o \
     DNSRequest.o \
     DNSResponse.o \
+    DNSAnswer.o \
     main.o
 
 PROGRAM = LookupServer