Resolver: improved PTR response processing.

The previous code only parsed the first answer, without checking its
type, and required a compressed RR name.

The new code checks the RR type, supports responses with multiple
answers, and doesn't require the RR name to be compressed.

This has a side effect in limited support of CNAME.  If a response
includes both CNAME and PTR RRs, like when recursion is enabled on
the server, PTR RR is handled.

Full CNAME support in PTR response is not implemented in this change.

src/core/ngx_resolver.c

@@ -2401,7 +2401,7 @@ ngx_resolver_process_ptr(ngx_resolver_t *r, u_char *buf, size_t n,
     int32_t               ttl;
     ngx_int_t             octet;
     ngx_str_t             name;
-    ngx_uint_t            i, mask, qident, class;
+    ngx_uint_t            mask, type, class, qident, a, i, start;
     ngx_queue_t          *expire_queue;
     ngx_rbtree_t         *tree;
     ngx_resolver_an_t    *an;
@@ -2554,44 +2554,90 @@ valid:
 
     i += sizeof(ngx_resolver_qs_t);
 
-    if (i + 2 + sizeof(ngx_resolver_an_t) >= n) {
+    for (a = 0; a < nan; a++) {
+
+        start = i;
+
+        while (i < n) {
+
+            if (buf[i] & 0xc0) {
+                i += 2;
+                goto found;
+            }
+
+            if (buf[i] == 0) {
+                i++;
+                goto test_length;
+            }
+
+            i += 1 + buf[i];
+        }
+
         goto short_response;
+
+    test_length:
+
+        if (i - start < 2) {
+            err = "invalid name in DNS response";
+            goto invalid;
+        }
+
+    found:
+
+        if (i + sizeof(ngx_resolver_an_t) >= n) {
+            goto short_response;
+        }
+
+        an = (ngx_resolver_an_t *) &buf[i];
+
+        type = (an->type_hi << 8) + an->type_lo;
+        class = (an->class_hi << 8) + an->class_lo;
+        len = (an->len_hi << 8) + an->len_lo;
+        ttl = (an->ttl[0] << 24) + (an->ttl[1] << 16)
+            + (an->ttl[2] << 8) + (an->ttl[3]);
+
+        if (class != 1) {
+            ngx_log_error(r->log_level, r->log, 0,
+                          "unexpected RR class %ui", class);
+            goto failed;
+        }
+
+        if (ttl < 0) {
+            ttl = 0;
+        }
+
+        ngx_log_debug3(NGX_LOG_DEBUG_CORE, r->log, 0,
+                      "resolver qt:%ui cl:%ui len:%uz",
+                      type, class, len);
+
+        i += sizeof(ngx_resolver_an_t);
+
+        switch (type) {
+
+        case NGX_RESOLVE_PTR:
+
+            goto ptr;
+
+        case NGX_RESOLVE_CNAME:
+
+            break;
+
+        default:
+
+            ngx_log_error(r->log_level, r->log, 0,
+                          "unexpected RR type %ui", type);
+        }
+
+        i += len;
     }
 
-    /* compression pointer to *.arpa */
+    /* unlock addr mutex */
 
-    if (buf[i] != 0xc0 || buf[i + 1] != sizeof(ngx_resolver_hdr_t)) {
-        err = "invalid in-addr.arpa or ip6.arpa name in DNS response";
-        goto invalid;
-    }
+    ngx_log_error(r->log_level, r->log, 0,
+                  "no PTR type in DNS response");
+    return;
 
-    an = (ngx_resolver_an_t *) &buf[i + 2];
-
-    class = (an->class_hi << 8) + an->class_lo;
-    len = (an->len_hi << 8) + an->len_lo;
-    ttl = (an->ttl[0] << 24) + (an->ttl[1] << 16)
-        + (an->ttl[2] << 8) + (an->ttl[3]);
-
-    if (class != 1) {
-        ngx_log_error(r->log_level, r->log, 0,
-                      "unexpected RR class %ui", class);
-        goto failed;
-    }
-
-    if (ttl < 0) {
-        ttl = 0;
-    }
-
-    ngx_log_debug3(NGX_LOG_DEBUG_CORE, r->log, 0,
-                  "resolver qt:%ui cl:%ui len:%uz",
-                  (an->type_hi << 8) + an->type_lo,
-                  class, len);
-
-    i += 2 + sizeof(ngx_resolver_an_t);
-
-    if (i + len > n) {
-        goto short_response;
-    }
+ptr:
 
     if (ngx_resolver_copy(r, &name, buf, buf + i, buf + n) != NGX_OK) {
         goto failed;