init: arp lan scanner

.github/workflows/ci.yml

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+name: ci
+
+on:
+  push:
+    branches: [master]
+
+jobs:
+  build:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ilammy/msvc-dev-cmd@v1
+      - run: cmake -B build
+      - run: cmake --build build --config Release

.gitignore

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+build/
+.claude/
+*.obj
+*.exe
+*.pdb
+*.ilk

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.16)
+project(lanscan CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+add_executable(lanscan src/main.cpp)
+
+if(WIN32)
+    target_link_libraries(lanscan ws2_32 iphlpapi)
+endif()

README.md

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+# lanscan
+
+ARP-сканер локальной сети на C++. находит свою подсеть сам, шлёт ARP-запросы всем хостам параллельно, печатает кто ответил.
+
+```mermaid
+sequenceDiagram
+    lanscan->>OS: узнать свой IP + маску
+    loop каждый хост в подсети
+        lanscan->>Host: ARP-запрос (кто ты?)
+        Host-->>lanscan: MAC-адрес (если жив)
+    end
+```
+
+через `SendARP` из iphlpapi — без сырых сокетов и драйверов, работает без админа.
+
+## сборка
+
+```
+cmake -B build
+cmake --build build --config Release
+```
+
+## юзать
+
+```
+build\Release\lanscan.exe
+```
+
+сам находит подсеть по активному адаптеру и сканирует её целиком.

src/main.cpp

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+#include <winsock2.h>
+#include <ws2tcpip.h>
+#include <windows.h>
+#include <iphlpapi.h>
+#include <iostream>
+#include <vector>
+#include <thread>
+#include <mutex>
+#include <cstdio>
+
+#pragma comment(lib, "ws2_32.lib")
+#pragma comment(lib, "iphlpapi.lib")
+
+struct LocalNet {
+    uint32_t ip = 0;
+    uint32_t mask = 0;
+    bool found = false;
+};
+
+static LocalNet find_local_net() {
+    LocalNet net;
+    ULONG size = 0;
+    GetAdaptersAddresses(AF_INET, GAA_FLAG_INCLUDE_PREFIX, nullptr, nullptr, &size);
+    std::vector<uint8_t> buf(size);
+    IP_ADAPTER_ADDRESSES* addrs = reinterpret_cast<IP_ADAPTER_ADDRESSES*>(buf.data());
+    if (GetAdaptersAddresses(AF_INET, GAA_FLAG_INCLUDE_PREFIX, nullptr, addrs, &size) != NO_ERROR) {
+        return net;
+    }
+
+    for (IP_ADAPTER_ADDRESSES* a = addrs; a != nullptr; a = a->Next) {
+        if (a->OperStatus != IfOperStatusUp) continue;
+        if (a->IfType == IF_TYPE_SOFTWARE_LOOPBACK) continue;
+
+        for (IP_ADAPTER_UNICAST_ADDRESS* ua = a->FirstUnicastAddress; ua != nullptr; ua = ua->Next) {
+            sockaddr_in* sin = reinterpret_cast<sockaddr_in*>(ua->Address.lpSockaddr);
+            uint32_t ip = ntohl(sin->sin_addr.s_addr);
+            if ((ip >> 24) == 127) continue;
+
+            uint8_t prefix = ua->OnLinkPrefixLength;
+            if (prefix == 0 || prefix > 32) continue;
+
+            uint32_t mask = prefix == 32 ? 0xFFFFFFFFu : (~0u << (32 - prefix));
+            net.ip = ip;
+            net.mask = mask;
+            net.found = true;
+            return net;
+        }
+    }
+    return net;
+}
+
+static std::mutex out_mutex;
+
+static void probe(uint32_t ip_host_order) {
+    uint32_t target = htonl(ip_host_order);
+    uint8_t mac[6];
+    ULONG mac_len = 6;
+
+    DWORD res = SendARP(target, 0, mac, &mac_len);
+    if (res != NO_ERROR) return;
+
+    in_addr addr{};
+    addr.s_addr = target;
+    char ipstr[INET_ADDRSTRLEN];
+    inet_ntop(AF_INET, &addr, ipstr, sizeof(ipstr));
+
+    char macstr[32];
+    snprintf(macstr, sizeof(macstr), "%02X:%02X:%02X:%02X:%02X:%02X",
+        mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+
+    std::lock_guard<std::mutex> lock(out_mutex);
+    std::cout << ipstr << "\t" << macstr << std::endl;
+}
+
+int main() {
+    WSADATA wsa;
+    if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0) {
+        std::cerr << "WSAStartup failed" << std::endl;
+        return 1;
+    }
+
+    LocalNet net = find_local_net();
+    if (!net.found) {
+        std::cerr << "no local ipv4 network found" << std::endl;
+        WSACleanup();
+        return 1;
+    }
+
+    uint32_t network = net.ip & net.mask;
+    uint32_t broadcast = network | ~net.mask;
+    uint32_t host_count = broadcast - network;
+
+    if (host_count == 0 || host_count > 65534) {
+        std::cerr << "subnet too big or too small to scan (" << host_count << " hosts)" << std::endl;
+        WSACleanup();
+        return 1;
+    }
+
+    in_addr netaddr{};
+    netaddr.s_addr = htonl(network);
+    char netstr[INET_ADDRSTRLEN];
+    inet_ntop(AF_INET, &netaddr, netstr, sizeof(netstr));
+    std::cout << "scanning " << netstr << "/" << host_count << " hosts..." << std::endl;
+
+    std::vector<std::thread> threads;
+    for (uint32_t ip = network + 1; ip < broadcast; ++ip) {
+        threads.emplace_back(probe, ip);
+        if (threads.size() >= 64) {
+            for (auto& t : threads) t.join();
+            threads.clear();
+        }
+    }
+    for (auto& t : threads) t.join();
+
+    WSACleanup();
+    return 0;
+}