DNS compression loop detection

- `dns_protocol`: Issue DNS-OARC#291: Implement proper loop detection during decompression

src/dns_protocol.c

@@ -53,9 +53,11 @@ static int rfc1035NameUnpack(const u_char* buf, size_t sz, off_t* off, char* nam
  off_t no = 0;
  unsigned char c;
  size_t len;
- static int loop_detect = 0;
- if (loop_detect > 2)
- return 4; /* compression loop */
+ /*
+ * loop_detect[] tracks which position in the DNS message it has
+ * jumped to so it can't jump to the same twice, aka loop
+ */
+ static unsigned char loop_detect[0x3FFF] = { 0 };
  if (ns <= 0)
  return 4; /* probably compression loop */
  do {
@@ -66,7 +68,7 @@ static int rfc1035NameUnpack(const u_char* buf, size_t sz, off_t* off, char* nam
  /* blasted compression */
  int rc;
  unsigned short s;
- off_t ptr;
+ off_t ptr, loop_ptr;
  s = nptohs(buf + (*off));
  (*off) += sizeof(s);
  /* Sanity check */
@@ -78,9 +80,13 @@ static int rfc1035NameUnpack(const u_char* buf, size_t sz, off_t* off, char* nam
  return 2; /* bad compression ptr */
  if (ptr < DNS_MSG_HDR_SZ)
  return 2; /* bad compression ptr */
- loop_detect++;
+ if (loop_detect[ptr])
+ return 4; /* compression loop */
+ loop_detect[(loop_ptr = ptr)] = 1;
+
  rc = rfc1035NameUnpack(buf, sz, &ptr, name + no, ns - no);
- loop_detect--;
+
+ loop_detect[loop_ptr] = 0;
  return rc;
  } else if (c > RFC1035_MAXLABELSZ) {
  /*