Resolver: explicit check for compression pointers in question. Since nginx always uses exactly one entry in the question section of a DNS query, and never uses compression pointers in this entry, parsing of a DNS response in ngx_resolver_process_response() does not expect compression pointers to appear in the question section of the DNS response. Indeed, compression pointers in the first name of a DNS response hardly make sense, do not seem to be allowed by RFC 1035 (which says "a pointer to a prior occurance of the same name", note "prior"), and were never observed in practice. Added an explicit check to ngx_resolver_process_response()'s parsing of the question section to properly report an error if compression pointers nevertheless appear in the question section.

--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -1798,6 +1798,12 @@ ngx_resolver_process_response(ngx_resolv
     i = sizeof(ngx_resolver_hdr_t);
 
     while (i < (ngx_uint_t) n) {
+
+        if (buf[i] & 0xc0) {
+            err = "unexpected compression pointer in DNS response";
+            goto done;
+        }
+
         if (buf[i] == '\0') {
             goto found;
         }