LookupServer: Implement basic mDNS support :^)

The implementation is extremely basic, and is far from fully conforming
to the spec. Among other things, it does not really work in case there
are multiple network adapters.

Nevertheless, it works quite well for the simple case! You can now do
this on your host machine:

$ ping courage.local

and same on your Serenity box:

$ ping host-machine-name.local

Userland/Services/LookupServer/MulticastDNS.cpp

@@ -0,0 +1,191 @@
+/*
+ * Copyright (c) 2021, Sergey Bugaev <bugaevc@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "MulticastDNS.h"
+#include "DNSPacket.h"
+#include <AK/IPv4Address.h>
+#include <AK/JsonArray.h>
+#include <AK/JsonObject.h>
+#include <AK/JsonValue.h>
+#include <AK/String.h>
+#include <LibCore/File.h>
+#include <poll.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+namespace LookupServer {
+
+MulticastDNS::MulticastDNS(Object* parent)
+    : Core::UDPServer(parent)
+    , m_hostname("courage.local")
+{
+    char buffer[HOST_NAME_MAX];
+    int rc = gethostname(buffer, sizeof(buffer));
+    if (rc < 0) {
+        perror("gethostname");
+    } else {
+        m_hostname = String::formatted("{}.local", buffer);
+    }
+
+    u8 zero = 0;
+    rc = setsockopt(fd(), IPPROTO_IP, IP_MULTICAST_LOOP, &zero, 1);
+    if (rc < 0)
+        perror("setsockopt(IP_MULTICAST_LOOP)");
+    ip_mreq mreq = {
+        mdns_addr.sin_addr,
+        { htonl(INADDR_ANY) },
+    };
+    rc = setsockopt(fd(), IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
+    if (rc < 0)
+        perror("setsockopt(IP_ADD_MEMBESHIP)");
+
+    bind(IPv4Address(), 5353);
+
+    on_ready_to_receive = [this]() {
+        handle_packet();
+    };
+
+    // TODO: Announce on startup. We cannot just call announce() here,
+    // because it races with the network interfaces getting configured.
+}
+
+void MulticastDNS::handle_packet()
+{
+    auto buffer = receive(1024);
+    auto optional_packet = DNSPacket::from_raw_packet(buffer.data(), buffer.size());
+    if (!optional_packet.has_value()) {
+        dbgln("Got an invalid mDNS packet");
+        return;
+    }
+    auto& packet = optional_packet.value();
+
+    if (packet.is_query())
+        handle_query(packet);
+}
+
+void MulticastDNS::handle_query(const DNSPacket& packet)
+{
+    bool should_reply = false;
+
+    for (auto& question : packet.questions())
+        if (question.name() == m_hostname)
+            should_reply = true;
+
+    if (!should_reply)
+        return;
+
+    announce();
+}
+
+void MulticastDNS::announce()
+{
+    DNSPacket response;
+    response.set_is_response();
+    response.set_code(DNSPacket::Code::NOERROR);
+
+    for (auto& address : local_addresses()) {
+        auto raw_addr = address.to_in_addr_t();
+        DNSAnswer answer {
+            m_hostname,
+            T_A,
+            C_IN | 0x8000,
+            120,
+            String { (const char*)&raw_addr, sizeof(raw_addr) }
+        };
+        response.add_answer(answer);
+    }
+
+    int rc = emit_packet(response);
+    if (rc < 0)
+        perror("Failed to emit response packet");
+}
+
+ssize_t MulticastDNS::emit_packet(const DNSPacket& packet, const sockaddr_in* destination)
+{
+    auto buffer = packet.to_byte_buffer();
+    if (!destination)
+        destination = &mdns_addr;
+    return sendto(fd(), buffer.data(), buffer.size(), 0, (const sockaddr*)destination, sizeof(*destination));
+}
+
+Vector<IPv4Address> MulticastDNS::local_addresses() const
+{
+    auto file = Core::File::construct("/proc/net/adapters");
+    if (!file->open(Core::IODevice::ReadOnly)) {
+        dbgln("Failed to open /proc/net/adapters: {}", file->error_string());
+        return {};
+    }
+
+    auto file_contents = file->read_all();
+    auto json = JsonValue::from_string(file_contents);
+    VERIFY(json.has_value());
+
+    Vector<IPv4Address> addresses;
+
+    json.value().as_array().for_each([&addresses](auto& value) {
+        auto if_object = value.as_object();
+        auto address = if_object.get("ipv4_address").to_string();
+        auto ipv4_address = IPv4Address::from_string(address);
+        // Skip unconfigured interfaces.
+        if (!ipv4_address.has_value())
+            return;
+        // Skip loopback adapters.
+        if (ipv4_address.value()[0] == IN_LOOPBACKNET)
+            return;
+        addresses.append(ipv4_address.value());
+    });
+
+    return addresses;
+}
+
+Vector<DNSAnswer> MulticastDNS::lookup(const DNSName& name, unsigned short record_type)
+{
+    DNSPacket request;
+    request.set_is_query();
+    request.add_question({ name, record_type, C_IN });
+
+    int rc = emit_packet(request);
+    if (rc < 0) {
+        perror("failed to emit request packet");
+        return {};
+    }
+
+    Vector<DNSAnswer> answers;
+
+    // FIXME: It would be better not to block
+    // the main loop while we wait for a response.
+    while (true) {
+        pollfd pfd { fd(), POLLIN, 0 };
+        rc = poll(&pfd, 1, 1000);
+        if (rc < 0) {
+            perror("poll");
+        } else if (rc == 0) {
+            // Timed out.
+            return {};
+        }
+
+        auto buffer = receive(1024);
+        if (!buffer)
+            return {};
+        auto optional_packet = DNSPacket::from_raw_packet(buffer.data(), buffer.size());
+        if (!optional_packet.has_value()) {
+            dbgln("Got an invalid mDNS packet");
+            continue;
+        }
+        auto& packet = optional_packet.value();
+
+        if (packet.is_query())
+            continue;
+
+        for (auto& answer : packet.answers())
+            if (answer.name() == name && answer.type() == record_type)
+                answers.append(answer);
+        if (!answers.is_empty())
+            return answers;
+    }
+}
+
+}