LookupServer: Implement a DNS server :^)

LookupServer can now itself server as a DNS server! To service DNS clients, it
uses the exact same lookup logic as it does for LibIPC clients. Namely, it will
synthesize records for data from /etc/hosts on its own (you can use this to
configure host names for your domain!), and forward other questions to
configured upstream DNS servers. On top of that, it implements its own caching,
so once a DNS resource record has been obtained from an upstream server,
LookupServer will cache it locally for faster future lookups.

The DNS server part of LookupServer is disabled by default, because it requires
you to run it as root (for it to bind to the port 53) and on boot, and we don't
want either by default. If you want to try it, modify SystemServer.ini like so:

[LookupServer]
Socket=/tmp/portal/lookup
SocketPermissions=666
Priority=low
KeepAlive=1
User=root
BootModes=text,graphical

and enable server mode in LookupServer.ini like so:

[DNS]
Nameservers=...
EnableServer=1

If in the future we implement socket takeover for IP sockets, these limitations
may be lifted.

Base/etc/LookupServer.ini

@@ -1,2 +1,3 @@
 [DNS]
 Nameservers=1.1.1.1,1.0.0.1
+EnableServer=0

Userland/Services/LookupServer/CMakeLists.txt

@@ -5,6 +5,7 @@ set(SOURCES
     DNSAnswer.cpp
     DNSName.cpp
     DNSPacket.cpp
+    DNSServer.cpp
     LookupServer.cpp
     LookupServerEndpoint.h
     LookupClientEndpoint.h

Userland/Services/LookupServer/DNSPacket.cpp

@@ -44,6 +44,13 @@ void DNSPacket::add_question(const DNSQuestion& question)
     ASSERT(m_questions.size() <= UINT16_MAX);
 }
 
+void DNSPacket::add_answer(const DNSAnswer& answer)
+{
+    m_answers.empend(answer);
+
+    ASSERT(m_answers.size() <= UINT16_MAX);
+}
+
 ByteBuffer DNSPacket::to_byte_buffer() const
 {
     DNSPacketHeader header;
@@ -54,6 +61,7 @@ ByteBuffer DNSPacket::to_byte_buffer() const
         header.set_is_response();
     // FIXME: What should this be?
     header.set_opcode(0);
+    header.set_response_code(m_code);
     header.set_truncated(false); // hopefully...
     header.set_recursion_desired(true);
     // FIXME: what should the be for requests?

Userland/Services/LookupServer/DNSPacket.h

@@ -80,6 +80,7 @@ public:
     }
 
     void add_question(const DNSQuestion&);
+    void add_answer(const DNSAnswer&);
 
     enum class Code : u8 {
         NOERROR = 0,

Userland/Services/LookupServer/DNSServer.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2021, Sergey Bugaev <bugaevc@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 "DNSServer.h"
+#include "DNSPacket.h"
+#include "LookupServer.h"
+#include <AK/IPv4Address.h>
+
+namespace LookupServer {
+
+DNSServer::DNSServer(Object* parent)
+    : Core::UDPServer(parent)
+{
+    bind(IPv4Address(), 53);
+    on_ready_to_receive = [this]() {
+        handle_client();
+    };
+}
+
+void DNSServer::handle_client()
+{
+    sockaddr_in client_address;
+    auto buffer = receive(1024, client_address);
+    auto optional_request = DNSPacket::from_raw_packet(buffer.data(), buffer.size());
+    if (!optional_request.has_value()) {
+        dbgln("Got an invalid DNS packet");
+        return;
+    }
+    auto& request = optional_request.value();
+
+    if (!request.is_query()) {
+        dbgln("It's not a request");
+        return;
+    }
+
+    LookupServer& lookup_server = LookupServer::the();
+
+    DNSPacket response;
+    response.set_is_response();
+    response.set_id(request.id());
+
+    for (auto& question : request.questions()) {
+        if (question.class_code() != C_IN)
+            continue;
+        response.add_question(question);
+        auto answers = lookup_server.lookup(question.name(), question.record_type());
+        for (auto& answer : answers) {
+            response.add_answer(answer);
+        }
+    }
+
+    if (response.answer_count() == 0)
+        response.set_code(DNSPacket::Code::NXDOMAIN);
+    else
+        response.set_code(DNSPacket::Code::NOERROR);
+
+    buffer = response.to_byte_buffer();
+    sendto(fd(), buffer.data(), buffer.size(), 0, (const sockaddr*)&client_address, sizeof(client_address));
+}
+
+}

Userland/Services/LookupServer/DNSServer.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2021, Sergey Bugaev <bugaevc@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.
+ */
+
+#pragma once
+
+#include <LibCore/UDPServer.h>
+
+namespace LookupServer {
+
+class DNSServer : public Core::UDPServer {
+    C_OBJECT(DNSServer)
+
+private:
+    explicit DNSServer(Object* parent = nullptr);
+
+    void handle_client();
+};
+
+}

Userland/Services/LookupServer/LookupServer.cpp

@@ -62,6 +62,11 @@ LookupServer::LookupServer()
 
     load_etc_hosts();
 
+    if (config->read_bool_entry("DNS", "EnableServer")) {
+        m_dns_server = DNSServer::construct(this);
+        // TODO: drop root privileges here.
+    }
+
     m_local_server = Core::LocalServer::construct(this);
     m_local_server->on_ready_to_accept = [this]() {
         auto socket = m_local_server->accept();

Userland/Services/LookupServer/LookupServer.h

@@ -26,8 +26,9 @@
 
 #pragma once
 
-#include "DNSPacket.h"
 #include "DNSName.h"
+#include "DNSPacket.h"
+#include "DNSServer.h"
 #include <LibCore/Object.h>
 
 namespace LookupServer {
@@ -49,8 +50,8 @@ private:
 
     Vector<DNSAnswer> lookup(const DNSName& hostname, const String& nameserver, bool& did_get_response, unsigned short record_type, ShouldRandomizeCase = ShouldRandomizeCase::Yes);
 
-
     RefPtr<Core::LocalServer> m_local_server;
+    RefPtr<DNSServer> m_dns_server;
     Vector<String> m_nameservers;
     HashMap<DNSName, Vector<DNSAnswer>, DNSName::Traits> m_etc_hosts;
     HashMap<DNSName, Vector<DNSAnswer>, DNSName::Traits> m_lookup_cache;