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/CMakeLists.txt

@@ -10,6 +10,7 @@ set(SOURCES
     LookupServerEndpoint.h
     LookupClientEndpoint.h
     ClientConnection.cpp
+    MulticastDNS.cpp
     main.cpp
 )
 

Userland/Services/LookupServer/LookupServer.cpp

@@ -46,6 +46,7 @@ LookupServer::LookupServer()
         m_dns_server = DNSServer::construct(this);
         // TODO: drop root privileges here.
     }
+    m_mdns = MulticastDNS::construct(this);
 
     m_local_server = Core::LocalServer::construct(this);
     m_local_server->on_ready_to_accept = [this]() {
@@ -150,6 +151,14 @@ Vector<DNSAnswer> LookupServer::lookup(const DNSName& name, unsigned short recor
             return answers;
     }
 
+    // Look up .local names using mDNS instead of DNS nameservers.
+    if (name.as_string().ends_with(".local")) {
+        answers = m_mdns->lookup(name, record_type);
+        for (auto& answer : answers)
+            put_in_cache(answer);
+        return answers;
+    }
+
     // Third, ask the upstream nameservers.
     for (auto& nameserver : m_nameservers) {
         dbgln_if(LOOKUPSERVER_DEBUG, "Doing lookup using nameserver '{}'", nameserver);

Userland/Services/LookupServer/LookupServer.h

@@ -9,6 +9,7 @@
 #include "DNSName.h"
 #include "DNSPacket.h"
 #include "DNSServer.h"
+#include "MulticastDNS.h"
 #include <LibCore/Object.h>
 
 namespace LookupServer {
@@ -32,6 +33,7 @@ private:
 
     RefPtr<Core::LocalServer> m_local_server;
     RefPtr<DNSServer> m_dns_server;
+    RefPtr<MulticastDNS> m_mdns;
     Vector<String> m_nameservers;
     HashMap<DNSName, Vector<DNSAnswer>, DNSName::Traits> m_etc_hosts;
     HashMap<DNSName, Vector<DNSAnswer>, DNSName::Traits> m_lookup_cache;

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;
+    }
+}
+
+}

Userland/Services/LookupServer/MulticastDNS.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021, Sergey Bugaev <bugaevc@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "DNSAnswer.h"
+#include "DNSName.h"
+#include "DNSPacket.h"
+#include <AK/IPv4Address.h>
+#include <LibCore/UDPServer.h>
+#include <netinet/in.h>
+
+namespace LookupServer {
+
+class MulticastDNS : public Core::UDPServer {
+    C_OBJECT(MulticastDNS)
+public:
+    Vector<DNSAnswer> lookup(const DNSName&, unsigned short record_type);
+
+private:
+    explicit MulticastDNS(Object* parent = nullptr);
+
+    void announce();
+    ssize_t emit_packet(const DNSPacket&, const sockaddr_in* destination = nullptr);
+
+    void handle_packet();
+    void handle_query(const DNSPacket&);
+
+    Vector<IPv4Address> local_addresses() const;
+
+    DNSName m_hostname;
+
+    static constexpr sockaddr_in mdns_addr {
+        AF_INET,
+        // htons(5353)
+        0xe914,
+        // 224.0.0.251
+        { 0xfb0000e0 },
+        0
+    };
+};
+
+}

Userland/Services/LookupServer/main.cpp

@@ -20,11 +20,12 @@ int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
     Core::EventLoop event_loop;
     auto server = LookupServer::LookupServer::construct();
 
-    if (pledge("stdio accept inet", nullptr) < 0) {
+    if (pledge("stdio accept inet rpath", nullptr) < 0) {
         perror("pledge");
         return 1;
     }
 
+    unveil("/proc/net/adapters", "r");
     unveil(nullptr, nullptr);
 
     return event_loop.exec();