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/*
* SPDX-FileCopyrightText: 2021-2023 Espressif Systems (Shanghai) CO LTD
*
* SPDX-License-Identifier: Unlicense OR CC0-1.0
*/
#include <sys/param.h>
#include <inttypes.h>
#include "esp_log.h"
#include "esp_system.h"
#include "esp_check.h"
#include "esp_netif.h"
#include "lwip/err.h"
#include "lwip/sockets.h"
#include "lwip/sys.h"
#include "lwip/netdb.h"
#include "dns_server.h"
#define DNS_PORT (53)
#define DNS_MAX_LEN (256)
#define OPCODE_MASK (0x7800)
#define QR_FLAG (1 << 7)
#define QD_TYPE_A (0x0001)
#define ANS_TTL_SEC (300)
static const char *TAG = "example_dns_redirect_server";
// DNS Header Packet
typedef struct __attribute__((__packed__))
{
uint16_t id;
uint16_t flags;
uint16_t qd_count;
uint16_t an_count;
uint16_t ns_count;
uint16_t ar_count;
} dns_header_t;
// DNS Question Packet
typedef struct {
uint16_t type;
uint16_t class;
} dns_question_t;
// DNS Answer Packet
typedef struct __attribute__((__packed__))
{
uint16_t ptr_offset;
uint16_t type;
uint16_t class;
uint32_t ttl;
uint16_t addr_len;
uint32_t ip_addr;
} dns_answer_t;
// DNS server handle
struct dns_server_handle {
bool started;
TaskHandle_t task;
int num_of_entries;
dns_entry_pair_t entry[];
};
/*
Parse the name from the packet from the DNS name format to a regular .-seperated name
returns the pointer to the next part of the packet
*/
static char *parse_dns_name(char *raw_name, char *parsed_name, size_t parsed_name_max_len)
{
char *label = raw_name;
char *name_itr = parsed_name;
int name_len = 0;
do {
int sub_name_len = *label;
// (len + 1) since we are adding a '.'
name_len += (sub_name_len + 1);
if (name_len > parsed_name_max_len) {
return NULL;
}
// Copy the sub name that follows the the label
memcpy(name_itr, label + 1, sub_name_len);
name_itr[sub_name_len] = '.';
name_itr += (sub_name_len + 1);
label += sub_name_len + 1;
} while (*label != 0);
// Terminate the final string, replacing the last '.'
parsed_name[name_len - 1] = '\0';
// Return pointer to first char after the name
return label + 1;
}
// Parses the DNS request and prepares a DNS response with the IP of the softAP
static int parse_dns_request(char *req, size_t req_len, char *dns_reply, size_t dns_reply_max_len, dns_server_handle_t h)
{
if (req_len > dns_reply_max_len) {
return -1;
}
// Prepare the reply
memset(dns_reply, 0, dns_reply_max_len);
memcpy(dns_reply, req, req_len);
// Endianess of NW packet different from chip
dns_header_t *header = (dns_header_t *)dns_reply;
ESP_LOGD(TAG, "DNS query with header id: 0x%X, flags: 0x%X, qd_count: %d",
ntohs(header->id), ntohs(header->flags), ntohs(header->qd_count));
// Not a standard query
if ((header->flags & OPCODE_MASK) != 0) {
return 0;
}
// Set question response flag
header->flags |= QR_FLAG;
uint16_t qd_count = ntohs(header->qd_count);
header->an_count = htons(qd_count);
int reply_len = qd_count * sizeof(dns_answer_t) + req_len;
if (reply_len > dns_reply_max_len) {
return -1;
}
// Pointer to current answer and question
char *cur_ans_ptr = dns_reply + req_len;
char *cur_qd_ptr = dns_reply + sizeof(dns_header_t);
char name[128];
// Respond to all questions based on configured rules
for (int qd_i = 0; qd_i < qd_count; qd_i++) {
char *name_end_ptr = parse_dns_name(cur_qd_ptr, name, sizeof(name));
if (name_end_ptr == NULL) {
ESP_LOGE(TAG, "Failed to parse DNS question: %s", cur_qd_ptr);
return -1;
}
dns_question_t *question = (dns_question_t *)(name_end_ptr);
uint16_t qd_type = ntohs(question->type);
uint16_t qd_class = ntohs(question->class);
ESP_LOGD(TAG, "Received type: %d | Class: %d | Question for: %s", qd_type, qd_class, name);
if (qd_type == QD_TYPE_A) {
esp_ip4_addr_t ip = { .addr = IPADDR_ANY };
// Check the configured rules to decide whether to answer this question or not
for (int i = 0; i < h->num_of_entries; ++i) {
// check if the name either corresponds to the entry, or if we should answer to all queries ("*")
if (strcmp(h->entry[i].name, "*") == 0 || strcmp(h->entry[i].name, name) == 0) {
if (h->entry[i].if_key) {
esp_netif_ip_info_t ip_info;
esp_netif_get_ip_info(esp_netif_get_handle_from_ifkey(h->entry[i].if_key), &ip_info);
ip.addr = ip_info.ip.addr;
break;
} else if (h->entry->ip.addr != IPADDR_ANY) {
ip.addr = h->entry[i].ip.addr;
break;
}
}
}
if (ip.addr == IPADDR_ANY) { // no rule applies, continue with another question
continue;
}
dns_answer_t *answer = (dns_answer_t *)cur_ans_ptr;
answer->ptr_offset = htons(0xC000 | (cur_qd_ptr - dns_reply));
answer->type = htons(qd_type);
answer->class = htons(qd_class);
answer->ttl = htonl(ANS_TTL_SEC);
ESP_LOGD(TAG, "Answer with PTR offset: 0x%" PRIX16 " and IP 0x%" PRIX32, ntohs(answer->ptr_offset), ip.addr);
answer->addr_len = htons(sizeof(ip.addr));
answer->ip_addr = ip.addr;
}
}
return reply_len;
}
/*
Sets up a socket and listen for DNS queries,
replies to all type A queries with the IP of the softAP
*/
void dns_server_task(void *pvParameters)
{
char rx_buffer[128];
char addr_str[128];
int addr_family;
int ip_protocol;
dns_server_handle_t handle = pvParameters;
while (handle->started) {
struct sockaddr_in dest_addr;
dest_addr.sin_addr.s_addr = htonl(INADDR_ANY);
dest_addr.sin_family = AF_INET;
dest_addr.sin_port = htons(DNS_PORT);
addr_family = AF_INET;
ip_protocol = IPPROTO_IP;
inet_ntoa_r(dest_addr.sin_addr, addr_str, sizeof(addr_str) - 1);
int sock = socket(addr_family, SOCK_DGRAM, ip_protocol);
if (sock < 0) {
ESP_LOGE(TAG, "Unable to create socket: errno %d", errno);
break;
}
ESP_LOGI(TAG, "Socket created");
int err = bind(sock, (struct sockaddr *)&dest_addr, sizeof(dest_addr));
if (err < 0) {
ESP_LOGE(TAG, "Socket unable to bind: errno %d", errno);
}
ESP_LOGI(TAG, "Socket bound, port %d", DNS_PORT);
while (handle->started) {
ESP_LOGI(TAG, "Waiting for data");
struct sockaddr_in6 source_addr; // Large enough for both IPv4 or IPv6
socklen_t socklen = sizeof(source_addr);
int len = recvfrom(sock, rx_buffer, sizeof(rx_buffer) - 1, 0, (struct sockaddr *)&source_addr, &socklen);
// Error occurred during receiving
if (len < 0) {
ESP_LOGE(TAG, "recvfrom failed: errno %d", errno);
close(sock);
break;
}
// Data received
else {
// Get the sender's ip address as string
if (source_addr.sin6_family == PF_INET) {
inet_ntoa_r(((struct sockaddr_in *)&source_addr)->sin_addr.s_addr, addr_str, sizeof(addr_str) - 1);
} else if (source_addr.sin6_family == PF_INET6) {
inet6_ntoa_r(source_addr.sin6_addr, addr_str, sizeof(addr_str) - 1);
}
// Null-terminate whatever we received and treat like a string...
rx_buffer[len] = 0;
char reply[DNS_MAX_LEN];
int reply_len = parse_dns_request(rx_buffer, len, reply, DNS_MAX_LEN, handle);
ESP_LOGI(TAG, "Received %d bytes from %s | DNS reply with len: %d", len, addr_str, reply_len);
if (reply_len <= 0) {
ESP_LOGE(TAG, "Failed to prepare a DNS reply");
} else {
int err = sendto(sock, reply, reply_len, 0, (struct sockaddr *)&source_addr, sizeof(source_addr));
if (err < 0) {
ESP_LOGE(TAG, "Error occurred during sending: errno %d", errno);
break;
}
}
}
}
if (sock != -1) {
ESP_LOGE(TAG, "Shutting down socket");
shutdown(sock, 0);
close(sock);
}
}
vTaskDelete(NULL);
}
dns_server_handle_t start_dns_server(dns_server_config_t *config)
{
dns_server_handle_t handle = calloc(1, sizeof(struct dns_server_handle) + config->num_of_entries * sizeof(dns_entry_pair_t));
ESP_RETURN_ON_FALSE(handle, NULL, TAG, "Failed to allocate dns server handle");
handle->started = true;
handle->num_of_entries = config->num_of_entries;
memcpy(handle->entry, config->item, config->num_of_entries * sizeof(dns_entry_pair_t));
xTaskCreate(dns_server_task, "dns_server", 4096, handle, 5, &handle->task);
return handle;
}
void stop_dns_server(dns_server_handle_t handle)
{
if (handle) {
handle->started = false;
vTaskDelete(handle->task);
free(handle);
}
}
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