Use a keyed hash function instead of a CRC to lookup questions sharin…

…g the same query id

dnscrypt.h

@@ -127,6 +127,7 @@ struct context {
     uint8_t crypt_publickey[crypto_box_PUBLICKEYBYTES];
     uint8_t crypt_secretkey[crypto_box_SECRETKEYBYTES];
     uint64_t nonce_ts_last;
+    unsigned char hash_key[crypto_shorthash_KEYBYTES];
 };
 
 int dnscrypt_cmp_client_nonce(const uint8_t

main.c

@@ -407,6 +407,8 @@ main(int argc, const char **argv)
         exit(1);
     }
 
+    randombytes_buf(c.hash_key, sizeof c.hash_key);
+
     if ((c.event_loop = event_base_new()) == NULL) {
         logger(LOG_ERR, "Unable to initialize the event loop.");
         exit(1);

rfc1035.c

@@ -170,52 +170,30 @@ extract_name(struct dns_header *header, size_t plen, unsigned char **pp, char
     }
 }
 
-/* CRC the question section. This is used to safely detect query 
+/* Hash the question section. This is used to safely detect query
    retransmision and to detect answers to questions we didn't ask, which 
    might be poisoning attacks. Note that we decode the name rather 
-   than CRC the raw bytes, since replies might be compressed differently. 
+   than hash the raw bytes, since replies might be compressed differently. 
    We ignore case in the names for the same reason. Return all-ones
    if there is not question section. */
-unsigned int 
-questions_crc(struct dns_header *header, size_t plen, char *name)
+uint64_t
+questions_hash(struct dns_header *header, size_t plen, char *name, const unsigned char key[crypto_shorthash_KEYBYTES])
 {
-  int q;
-  unsigned int crc = 0xffffffff;
-  unsigned char *p1, *p = (unsigned char *)(header+1);
-
-  for (q = ntohs(header->qdcount); q != 0; q--) 
-    {
-      if (!extract_name(header, plen, &p, name, 1, 4))
-	return crc; /* bad packet */
-
-      for (p1 = (unsigned char *)name; *p1; p1++)
-	{
-	  int i = 8;
-	  char c = *p1;
-
-	  if (c >= 'A' && c <= 'Z')
-	    c += 'a' - 'A';
-
-	  crc ^= c << 24;
-	  while (i--)
-	    crc = crc & 0x80000000 ? (crc << 1) ^ 0x04c11db7 : crc << 1;
-	}
-
-      /* CRC the class and type as well */
-      for (p1 = p; p1 < p+4; p1++)
-	{
-	  int i = 8;
-	  crc ^= *p1 << 24;
-	  while (i--)
-	    crc = crc & 0x80000000 ? (crc << 1) ^ 0x04c11db7 : crc << 1;
-	}
-
-      p += 4;
-      if (!CHECK_LEN(header, p, plen, 0))
-	return crc; /* bad packet */
-    }
-
-  return crc;
+  unsigned char qb[MAXDNAME + 4];
+  uint64_t hash = 0xffffffffffffffffULL;
+  unsigned char *p = (unsigned char *)(header+1);
+  size_t name_len;
+
+  if (ntohs(header->qdcount) != 1 ||
+      !extract_name(header, plen, &p, name, 1, 4) ||
+      (name_len = strlen(name)) > (sizeof qb - 4)) {
+      return hash;
+  }
+  memcpy(qb, name, name_len);
+  memcpy(qb + name_len, p, 4);
+  crypto_shorthash((unsigned char *) &hash, qb, name_len + 4ULL, key);
+
+  return hash;
 }
 
 static unsigned char *skip_name(unsigned char *ansp, struct dns_header *header, size_t plen, int extrabytes)

rfc1035.h

@@ -3,8 +3,9 @@
 
 #include "compat.h"
 #include "dns-protocol.h"
+#include <sodium.h>
 
-unsigned int questions_crc(struct dns_header *header, size_t plen, char *buff);
+uint64_t questions_hash(struct dns_header *header, size_t plen, char *buff, const unsigned char key[crypto_shorthash_KEYBYTES]);
 int extract_name(struct dns_header *header, size_t plen, unsigned char **pp, char
         *name, int isExtract, int extrabytes);
 int add_resource_record(struct dns_header *header, unsigned int nameoffset, unsigned char **pp, 

udp_request.c

@@ -1,3 +1,4 @@
+
 #include "dnscrypt.h"
 
 typedef struct SendtoWithRetryCtx_ {
@@ -349,7 +350,7 @@ client_to_proxy_cb(evutil_socket_t client_proxy_handle, short ev_flags,
     }
 
     udp_request->id = ntohs(header->id);
-    udp_request->crc = questions_crc(header, dns_query_len, c->namebuff);
+    udp_request->hash = questions_hash(header, dns_query_len, c->namebuff, c->hash_key);
 
     udp_request->timeout_timer =
         evtimer_new(udp_request->context->event_loop, timeout_timer_cb,
@@ -372,18 +373,17 @@ client_to_proxy_cb(evutil_socket_t client_proxy_handle, short ev_flags,
 }
 
 /*
- * Find corresponding request by DNS id and crc of questions.
- * Don't check crc if not know (0xffffffff).
+ * Find corresponding request by DNS id and hash of questions.
+ * Don't check hash if not know (0xffffffffffffffff).
  */
 static UDPRequest *
-lookup_request(struct context *c, uint16_t id, unsigned int crc)
+lookup_request(struct context *c, uint16_t id, uint64_t hash)
 {
     UDPRequest *scanned_udp_request;
     TAILQ_FOREACH(scanned_udp_request, &c->udp_request_queue, queue) {
         if (id == scanned_udp_request->id
-            && (scanned_udp_request->crc == crc || crc == 0xffffffff)) {
+            && (scanned_udp_request->hash == hash || hash == 0xffffffffffffffffULL)) {
             return scanned_udp_request;
-            break;
         }
     }
     return NULL;
@@ -425,8 +425,8 @@ resolver_to_proxy_cb(evutil_socket_t proxy_resolver_handle, short ev_flags,
 
     struct dns_header *header = (struct dns_header *)dns_reply;
     uint16_t id = ntohs(header->id);
-    unsigned int crc = questions_crc(header, dns_reply_len, c->namebuff);
-    udp_request = lookup_request(c, id, crc);
+    uint64_t hash = questions_hash(header, dns_reply_len, c->namebuff, c->hash_key);
+    udp_request = lookup_request(c, id, hash);
     if (udp_request == NULL) {
         logger(LOG_ERR, "Received a reply that doesn't match any active query");
         return;

udp_request.h

@@ -12,7 +12,7 @@ typedef struct UDPRequestStatus_ {
 
 typedef struct UDPRequest_ {
     uint16_t id;
-    unsigned int crc;
+    uint64_t hash;
     TAILQ_ENTRY(UDPRequest_) queue;
     uint8_t client_nonce[crypto_box_HALF_NONCEBYTES];
     uint8_t nmkey[crypto_box_BEFORENMBYTES];