GoodbyeDPI/dnsredir.c

/*
 * DNS UDP Connection Tracker for GoodbyeDPI
 *
 * This is a simple connection tracker for DNS UDP data.
 * It's not a proper one. The caveats as follows:
 *    * Uses only source IP address and port as a hash key;
 *    * One-shot only. Removes conntrack record as soon as gets the reply;
 *    * Does not properly parse DNS request and response, only checks some bytes;
 *
 * But anyway, it works fine for DNS.
 */

#include <windows.h>
#include <time.h>
#include <stdio.h>
#include "dnsredir.h"
#include "uthash.h"

// IPv6 incompatible!
#define UDP_CONNRECORD_KEY_LEN 6

#define DNS_CLEANUP_INTERVAL_SEC 30

#ifndef debug
#define debug(...) do {} while (0)
#else
#define debug(...) printf(...)
#endif

/* HACK!
 * uthash uses strlen() for HASH_FIND_STR.
 * We have null bytes in our key, so we can't use strlen()
 * And since it's always UDP_CONNRECORD_KEY_LEN bytes long,
 * we don't need to use any string function to determine length.
 */
#undef uthash_strlen
#define uthash_strlen(s) UDP_CONNRECORD_KEY_LEN

typedef struct udp_connrecord {
    /* key (srcip[4] + srcport[2]) */
    char key[UDP_CONNRECORD_KEY_LEN];
    time_t time;         /* time when this record was added */
    uint32_t dstip;
    uint16_t dstport;
    UT_hash_handle hh;   /* makes this structure hashable */
} udp_connrecord_t;

static time_t last_cleanup = 0;
static udp_connrecord_t *conntrack = NULL;

void flush_dns_cache() {
    BOOL WINAPI (*DnsFlushResolverCache)();

    HMODULE dnsapi = LoadLibrary("dnsapi.dll");
    if (dnsapi == NULL)
    {
        printf("Can't load dnsapi.dll to flush DNS cache!\n");
        exit(EXIT_FAILURE);
    }

    DnsFlushResolverCache = (void*)GetProcAddress(dnsapi, "DnsFlushResolverCache");
    if (DnsFlushResolverCache == NULL || !DnsFlushResolverCache())
        printf("Can't flush DNS cache!");
    FreeLibrary(dnsapi);
}

inline static void construct_key(const uint32_t srcip, const uint16_t srcport, char *key) {
    debug("Construct key enter\n");
    if (key) {
        debug("Constructing key\n");

        *(uint32_t*)(key) = srcip;
        *(uint16_t*)(key + sizeof(srcip)) = srcport;
    }
    debug("Construct key end\n");
}

inline static void deconstruct_key(const char *key, udp_connrecord_t *connrecord,
                                   conntrack_info_t *conn_info) {
    debug("Deconstruct key enter\n");
    if (key && conn_info) {
        debug("Deconstructing key\n");

        conn_info->srcip = *(uint32_t*)(key);
        conn_info->srcport = *(uint16_t*)(key + sizeof(conn_info->srcip));
        conn_info->dstip = connrecord->dstip;
        conn_info->dstport = connrecord->dstport;
    }
    debug("Deconstruct key end\n");
}

static int check_get_udp_conntrack_key(const char *key, udp_connrecord_t **connrecord) {
    udp_connrecord_t *tmp_connrecord = NULL;
    if (!conntrack) return FALSE;

    HASH_FIND_STR(conntrack, key, tmp_connrecord);
    if (tmp_connrecord) {
        if (connrecord)
            *connrecord = tmp_connrecord;
        debug("check_get_udp_conntrack_key found key\n");
        return TRUE;
    }
    debug("check_get_udp_conntrack_key key not found\n");
    return FALSE;
}

static int add_udp_conntrack(const uint32_t srcip, const uint16_t srcport,
                             const uint32_t dstip, const uint16_t dstport) {
    if (!(srcip && srcport && dstip && dstport))
        return FALSE;

    udp_connrecord_t *tmp_connrecord = malloc(sizeof(udp_connrecord_t));
    construct_key(srcip, srcport, tmp_connrecord->key);

    if (!check_get_udp_conntrack_key(tmp_connrecord->key, NULL)) {
        tmp_connrecord->time = time(NULL);
        tmp_connrecord->dstip = dstip;
        tmp_connrecord->dstport = dstport;
        HASH_ADD_STR(conntrack, key, tmp_connrecord);
        debug("Added UDP conntrack\n");
        return TRUE;
    }
    debug("Not added UDP conntrack\n");
    free(tmp_connrecord);
    return FALSE;
}

void dns_cleanup() {
    udp_connrecord_t *tmp_connrecord, *tmp_connrecord2 = NULL;

    if (last_cleanup == 0) {
        last_cleanup = time(NULL);
        return;
    }

    if (difftime(time(NULL), last_cleanup) >= DNS_CLEANUP_INTERVAL_SEC) {
        last_cleanup = time(NULL);

        HASH_ITER(hh, conntrack, tmp_connrecord, tmp_connrecord2) {
            if (difftime(last_cleanup, tmp_connrecord->time) >= DNS_CLEANUP_INTERVAL_SEC) {
                HASH_DEL(conntrack, tmp_connrecord);
                free(tmp_connrecord);
            }
        }
    }
}

int dns_is_dns_packet(const char *packet_data, const UINT packet_dataLen, const int outgoing) {
    if (outgoing && (ntohs(*(const uint16_t*)(packet_data + 2)) & 0xFA00) == 0 &&
        (ntohs(*(const uint32_t*)(packet_data + 6))) == 0) {
        return TRUE;
    }
    else if (!outgoing &&
        (ntohs(*(const uint16_t*)(packet_data + 2)) & 0xF800) == 0x8000) {
        return TRUE;
    }
    return FALSE;
}

int dns_handle_outgoing(const uint32_t srcip, const uint16_t srcport,
                        const uint32_t dstip, const uint16_t dstport,
                        const char *packet_data, const UINT packet_dataLen) {

    if (packet_dataLen < 16)
        return FALSE;

    dns_cleanup();

    if (dns_is_dns_packet(packet_data, packet_dataLen, 1)) {
        /* Looks like DNS request */
        debug("trying to add srcport = %hu, dstport = %hu\n", ntohs(srcport), ntohs(dstport));
        return add_udp_conntrack(srcip, srcport, dstip, dstport);
    }
    debug("____dns_handle_outgoing FALSE: srcport = %hu, dstport = %hu\n", ntohs(srcport), ntohs(dstport));
    return FALSE;
}

int dns_handle_incoming(const uint32_t srcip, const uint16_t srcport,
                        const uint32_t dstip, const uint16_t dstport,
                        const char *packet_data, const UINT packet_dataLen,
                        conntrack_info_t *conn_info) {

    char key[UDP_CONNRECORD_KEY_LEN];
    udp_connrecord_t *tmp_connrecord = NULL;

    if (packet_dataLen < 16 || !conn_info)
        return FALSE;

    dns_cleanup();

    if (dns_is_dns_packet(packet_data, packet_dataLen, 0)) {
        /* Looks like DNS response */
        construct_key(srcip, srcport, key);
        if (check_get_udp_conntrack_key(key, &tmp_connrecord) && tmp_connrecord) {
            /* Connection exists in conntrack, moving on */
            deconstruct_key(key, tmp_connrecord, conn_info);
            HASH_DEL(conntrack, tmp_connrecord);
            free(tmp_connrecord);
            return TRUE;
        }
    }
    debug("____dns_handle_incoming FALSE: srcport = %hu, dstport = %hu\n", ntohs(srcport), ntohs(dstport));
    return FALSE;
}