feat(dns): add DNS server with configurable port and IP via CLI

src/dns/dns_cli.cpp

@@ -0,0 +1,34 @@
+#include "dns_cli.hpp"
+
+// helper for default values
+constexpr auto withDefault = [](auto value) {
+    using T = decltype(value);
+    return boost::program_options::value<T>()->default_value(value);
+};
+
+DnsCli::DnsCli()
+    : CliBase("DNS Server Options")
+    , port_(5300)
+    , ip_("127.0.0.1")
+{
+}
+
+void DnsCli::setupOptions()
+{
+    // clang-format off
+    desc_.add_options()
+        ("help,h", "Show help message")
+        ("port,p", withDefault(5300), "UDP port to listen on")
+        ("ip,i", withDefault(std::string("127.0.0.1")), "IP address to return in A record");
+    // clang-format on
+}
+
+int DnsCli::getPort() const
+{
+    return vm_["port"].as<int>();
+}
+
+std::string DnsCli::getIp() const
+{
+    return vm_["ip"].as<std::string>();
+}

src/dns/dns_cli.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "cli_base.hpp"
+#include <string>
+
+class DnsCli : public CliBase
+{
+  public:
+    DnsCli();
+
+    int getPort() const;
+    std::string getIp() const;
+
+  protected:
+    void setupOptions() override;
+
+  private:
+    int port_;
+    std::string ip_;
+};

src/dns/dns_server.cpp

@@ -0,0 +1,57 @@
+#include "dns_server.hpp"
+#include "logger.hpp"
+
+DnsServer::DnsServer(IoContext &io, unsigned short port, std::string answerIp)
+    : UdpServerBase(io, port)
+    , answerIp_(std::move(answerIp))
+{
+}
+
+void DnsServer::handleRequest(
+    const std::string_view data, const UdpEndpoint &sender
+)
+{
+    if (data.size() < 12) {
+        Logger::warn("Invalid DNS packet");
+        return;
+    }
+
+    // response buffer
+    std::string resp;
+
+    // copy ID
+    resp.append(data.substr(0, 2));
+
+    // flags: QR=1 (response), AA=1
+    resp.push_back('\x81');
+    resp.push_back('\x80');
+
+    // QDCOUNT=1, ANCOUNT=1, NSCOUNT=0, ARCOUNT=0
+    resp.append("\0\1\0\1\0\0\0\0", 8);
+
+    // copy question (from offset 12 till end of query)
+    resp.append(data.substr(12));
+
+    // answer section
+    resp.append("\xc0\x0c");   // name pointer to question
+    resp.append("\0\1");       // TYPE A
+    resp.append("\0\1");       // CLASS IN
+    resp.append("\0\0\0\x3c"); // TTL = 60
+    resp.append("\0\x04");     // RDLENGTH = 4
+
+    // answer IP (from CLI or default 127.0.0.1)
+    unsigned int b1, b2, b3, b4;
+    if (sscanf(answerIp_.c_str(), "%u.%u.%u.%u", &b1, &b2, &b3, &b4) != 4) {
+        b1 = 127;
+        b2 = 0;
+        b3 = 0;
+        b4 = 1;
+    }
+    resp.push_back(static_cast<char>(b1));
+    resp.push_back(static_cast<char>(b2));
+    resp.push_back(static_cast<char>(b3));
+    resp.push_back(static_cast<char>(b4));
+
+    Logger::info("DNS query answered with A " + answerIp_);
+    doSend(resp, sender);
+}

src/dns/dns_server.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "udp_server_base.hpp"
+#include <string>
+
+class DnsServer : public UdpServerBase
+{
+  public:
+    DnsServer(IoContext &io, unsigned short port, std::string answerIp);
+
+  protected:
+    void handleRequest(
+        const std::string_view data, const UdpEndpoint &sender
+    ) override;
+
+  private:
+    std::string answerIp_;
+};

src/main/main_dns.cpp

@@ -0,0 +1,26 @@
+#include "dns_cli.hpp"
+#include "dns_server.hpp"
+#include "logger.hpp"
+
+int main(int argc, char *argv[])
+{
+    DnsCli cli;
+    cli.parse(argc, argv);
+
+    if (cli.isHelp()) {
+        cli.printHelp();
+        return 0;
+    }
+
+    Logger::init(true, "dns_server.log");
+    Logger::info(
+        "DNS server starting on port " + std::to_string(cli.getPort())
+        + ", answering with IP " + cli.getIp()
+    );
+
+    IoContext io;
+    DnsServer server(io, cli.getPort(), cli.getIp());
+    server.start();
+
+    io.run();
+}