on device wifi

main/dns_server.c

@@ -0,0 +1,163 @@
+/*
+ * MTG RFID Companion - Captive Portal DNS Server
+ *
+ * Lightweight UDP DNS server on port 53. Resolves every domain query
+ * to 192.168.4.1 to trigger automatic captive portal detection on smartphones.
+ */
+
+#include <string.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_log.h"
+#include "lwip/sockets.h"
+#include "lwip/netdb.h"
+#include "dns_server.h"
+
+static const char *TAG = "dns";
+
+#define DNS_PORT 53
+#define DNS_MAX_LEN 512
+
+static TaskHandle_t s_dns_task = NULL;
+static int s_sock = -1;
+static volatile bool s_running = false;
+
+typedef struct __attribute__((packed)) {
+    uint16_t id;
+    uint16_t flags;
+    uint16_t qdcount;
+    uint16_t ancount;
+    uint16_t nscount;
+    uint16_t arcount;
+} dns_header_t;
+
+static void dns_server_task(void *pvParameters)
+{
+    uint8_t rx_buffer[DNS_MAX_LEN];
+    uint8_t tx_buffer[DNS_MAX_LEN];
+    struct sockaddr_in client_addr;
+    socklen_t addr_len = sizeof(client_addr);
+
+    struct sockaddr_in server_addr = {
+        .sin_family = AF_INET,
+        .sin_addr.s_addr = htonl(INADDR_ANY),
+        .sin_port = htons(DNS_PORT),
+    };
+
+    s_sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);
+    if (s_sock < 0) {
+        ESP_LOGE(TAG, "Unable to create DNS socket: errno %d", errno);
+        s_running = false;
+        vTaskDelete(NULL);
+        return;
+    }
+
+    struct timeval timeout = {
+        .tv_sec = 1,
+        .tv_usec = 0,
+    };
+    setsockopt(s_sock, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
+
+    if (bind(s_sock, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
+        ESP_LOGE(TAG, "DNS socket bind failed: errno %d", errno);
+        close(s_sock);
+        s_sock = -1;
+        s_running = false;
+        vTaskDelete(NULL);
+        return;
+    }
+
+    ESP_LOGI(TAG, "Captive portal DNS server listening on UDP port 53");
+
+    while (s_running) {
+        int len = recvfrom(s_sock, rx_buffer, sizeof(rx_buffer), 0,
+                           (struct sockaddr *)&client_addr, &addr_len);
+
+        if (len < (int)sizeof(dns_header_t)) {
+            continue;
+        }
+
+        dns_header_t *req_hdr = (dns_header_t *)rx_buffer;
+        int qdcount = ntohs(req_hdr->qdcount);
+        if (qdcount < 1) {
+            continue;
+        }
+
+        /* Parse Question section to find end of question */
+        int idx = sizeof(dns_header_t);
+        while (idx < len && rx_buffer[idx] != 0) {
+            idx += 1 + rx_buffer[idx];
+        }
+        idx += 1; /* null terminator */
+        idx += 4; /* QTYPE + QCLASS */
+
+        if (idx > len || idx + 16 > DNS_MAX_LEN) {
+            continue;
+        }
+
+        /* Build Response Packet */
+        memcpy(tx_buffer, rx_buffer, idx);
+        dns_header_t *resp_hdr = (dns_header_t *)tx_buffer;
+        resp_hdr->flags = htons(0x8180); /* Standard response, recursion available, no error */
+        resp_hdr->ancount = htons(1);
+        resp_hdr->nscount = 0;
+        resp_hdr->arcount = 0;
+
+        /* Answer Section */
+        uint8_t *ans = tx_buffer + idx;
+        /* Name pointer to question at offset 12 (0xC00C) */
+        *ans++ = 0xC0;
+        *ans++ = 0x0C;
+        /* Type: A (IPv4) */
+        *ans++ = 0x00;
+        *ans++ = 0x01;
+        /* Class: IN */
+        *ans++ = 0x00;
+        *ans++ = 0x01;
+        /* TTL: 60 seconds */
+        *ans++ = 0x00;
+        *ans++ = 0x00;
+        *ans++ = 0x00;
+        *ans++ = 0x3C;
+        /* Data length: 4 bytes */
+        *ans++ = 0x00;
+        *ans++ = 0x04;
+        /* IP Address: 192.168.4.1 */
+        *ans++ = 192;
+        *ans++ = 168;
+        *ans++ = 4;
+        *ans++ = 1;
+
+        int tx_len = ans - tx_buffer;
+        sendto(s_sock, tx_buffer, tx_len, 0,
+               (struct sockaddr *)&client_addr, addr_len);
+    }
+
+    if (s_sock >= 0) {
+        close(s_sock);
+        s_sock = -1;
+    }
+    ESP_LOGI(TAG, "DNS server stopped");
+    s_dns_task = NULL;
+    vTaskDelete(NULL);
+}
+
+esp_err_t dns_server_start(void)
+{
+    if (s_running) {
+        return ESP_OK;
+    }
+    s_running = true;
+    BaseType_t ret = xTaskCreatePinnedToCore(
+        dns_server_task, "dns_server", 3072, NULL, 4, &s_dns_task, 0);
+    if (ret != pdPASS) {
+        s_running = false;
+        return ESP_ERR_NO_MEM;
+    }
+    return ESP_OK;
+}
+
+void dns_server_stop(void)
+{
+    s_running = false;
+}