resolv: Implement strict-error stub resolver option (bug 27929)

For now, do not enable this mode by default due to the potential
impact on compatibility with existing deployments.

Reviewed-by: DJ Delorie <dj@redhat.com>

NEWS

@@ -15,6 +15,16 @@ Major new features:
   the RES_OPTIONS=-no-aaaa environment variable performs AAAA DNS
   queries when the glibc DNS stub resolver is used.
 
+* The DNS stub resolver now supports the strict-error option.  If
+  activated, getaddrinfo for the AF_UNSPEC address family (with dual
+  A/AAAA DNS lookups) attemps to obtain an A/AAAA response pair from
+  another DNS server if one of the responses indicates failure.  Without
+  the strict-error option, getaddrinfo returns the A record data it has
+  obtained even if the AAAA query failed.  The new strict error mode is
+  incompatible with some DNS environments which do not follow the RFCs,
+  which is why this mode is not enabled by default.  A future version
+  of the library may turn it on by default, however.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
   [Add deprecations, removals and changes affecting compatibility here]

resolv/res_init.c

@@ -695,6 +695,7 @@ res_setoptions (struct resolv_conf_parser *parser, const char *options)
             { STRnLEN ("use-vc"),  RES_USEVC },
             { STRnLEN ("trust-ad"), RES_TRUSTAD },
             { STRnLEN ("no-aaaa"), RES_NOAAAA },
+            { STRnLEN ("strict-error"), RES_STRICTERR },
           };
 #define noptions (sizeof (options) / sizeof (options[0]))
           bool negate_option = *cp == '-';

resolv/res_send.c

@@ -1234,15 +1234,31 @@ send_dg(res_state statp,
 
 		if (thisansp_error) {
 		next_ns:
-			if (recvresp1 || (buf2 != NULL && recvresp2)) {
+		        /* Outside of strict-error mode, use the first
+			   response even if the second response is an
+			   error.  This allows parallel resolution to
+			   succeed even if the recursive resolver
+			   always answers with SERVFAIL for AAAA
+			   queries (which still happens in practice
+			   unfortunately).
+
+			   In strict-error mode, always switch to the
+			   next server and try to get a response from
+			   there.  */
+			if ((statp->options & RES_STRICTERR) == 0)
+			  {
+			    if (recvresp1 || (buf2 != NULL && recvresp2))
+			      {
 				*resplen2 = 0;
 				return resplen;
 			      }
+
 			    if (buf2 != NULL && !single_request)
 			      {
 				/* No data from the first reply.  */
 				resplen = 0;
-			    /* We are waiting for a possible second reply.  */
+				/* We are waiting for a possible
+				   second reply.  */
 				if (matching_query == 1)
 				  recvresp1 = 1;
 				else
@@ -1250,6 +1266,7 @@ send_dg(res_state statp,
 
 				goto wait;
 			      }
+			  }
 
 			/* don't retry if called from dig */
 			if (!statp->pfcode)

resolv/resolv.h

@@ -133,6 +133,7 @@ struct res_sym {
 #define RES_NORELOAD    0x02000000 /* No automatic configuration reload.  */
 #define RES_TRUSTAD     0x04000000 /* Request AD bit, keep it in responses.  */
 #define RES_NOAAAA      0x08000000 /* Suppress AAAA queries.  */
+#define RES_STRICTERR   0x10000000 /* Report more DNS errors as errors.  */
 
 #define RES_DEFAULT	(RES_RECURSE|RES_DEFNAMES|RES_DNSRCH)
 

resolv/tst-resolv-res_init-skeleton.c

@@ -129,6 +129,7 @@ print_resp (FILE *fp, res_state resp)
         print_option_flag (fp, &options, RES_NORELOAD, "no-reload");
         print_option_flag (fp, &options, RES_TRUSTAD, "trust-ad");
         print_option_flag (fp, &options, RES_NOAAAA, "no-aaaa");
+        print_option_flag (fp, &options, RES_STRICTERR, "strict-error");
         fputc ('\n', fp);
         if (options != 0)
           fprintf (fp, "; error: unresolved option bits: 0x%x\n", options);
@@ -741,6 +742,15 @@ struct test_case test_cases[] =
      "nameserver 192.0.2.1\n"
      "; nameserver[0]: [192.0.2.1]:53\n"
     },
+    {.name = "strict-error flag",
+     .conf = "options strict-error\n"
+     "nameserver 192.0.2.1\n",
+     .expected = "options strict-error\n"
+     "search example.com\n"
+     "; search[0]: example.com\n"
+     "nameserver 192.0.2.1\n"
+     "; nameserver[0]: [192.0.2.1]:53\n"
+    },
     { NULL }
   };
 

resolv/tst-resolv-semi-failure.c

@@ -67,6 +67,9 @@ response (const struct resolv_response_context *ctx,
   resolv_response_close_record (b);
 }
 
+/* Set to 1 if strict error checking is enabled.  */
+static int do_strict_error;
+
 static void
 check_one (void)
 {
@@ -83,7 +86,10 @@ check_one (void)
       struct addrinfo *ai;
       int ret = getaddrinfo ("www.example", "80", &hints, &ai);
       const char *expected;
-      if (ret == 0 && ai->ai_next != NULL)
+      /* In strict-error mode, a switch to the second name server
+         happens, and both responses are received, so a single
+         response is a bug.  */
+      if (do_strict_error || (ret == 0 && ai->ai_next != NULL))
         expected = ("address: STREAM/TCP 192.0.2.17 80\n"
                     "address: STREAM/TCP 2001:db8::1 80\n");
       else
@@ -99,6 +105,7 @@ check_one (void)
 static int
 do_test (void)
 {
+  for (do_strict_error = 0; do_strict_error < 2; ++do_strict_error)
     for (int do_single_lookup = 0; do_single_lookup < 2; ++do_single_lookup)
       {
         struct resolv_test *aux = resolv_test_start
@@ -107,6 +114,8 @@ do_test (void)
              .response_callback = response,
            });
 
+        if (do_strict_error)
+          _res.options |= RES_STRICTERR;
         if (do_single_lookup)
           _res.options |= RES_SNGLKUP;