Allow configuration value for NDots to be zero (#735)

As per Issue #734 some people use `ndots:0` in their configuration which
is allowed by the system resolver but not by c-ares. Add support for
`ndots:0` and add a test case to validate this behavior.

Fixes Issue: #734
Fix By: Brad House (@bradh352)

RELEASE-NOTES.md

@@ -28,6 +28,7 @@ Bugfixes:
 * Fix a potential memory leak in `ares_init()`. [Issue #724](https://github.com/c-ares/c-ares/issues/724)
 * Some platforms don't have the `isascii()` function.  Implement as a macro. [PR #721](https://github.com/c-ares/c-ares/pull/721)
 * CMake: Fix Chain building if CMAKE runtime paths not set
+* NDots configuration should allow a value of zero. [PR #735](https://github.com/c-ares/c-ares/pull/735)
 
 Thanks go to these friendly people for their efforts and contributions for this release:
 

docs/ares_init_options.3

@@ -158,7 +158,7 @@ before giving up.  The default is three tries.
 The number of dots which must be present in a domain name for it to be
 queried for "as is" prior to querying for it with the default domain
 extensions appended.  The default value is 1 unless set otherwise by
-resolv.conf or the RES_OPTIONS environment variable.
+resolv.conf or the RES_OPTIONS environment variable.  Valid range is 0-15.
 .TP 18
 .B ARES_OPT_MAXTIMEOUTMS
 .B int \fImaxtimeout\fP;

src/lib/ares_init.c

@@ -152,10 +152,6 @@ static ares_status_t init_by_defaults(ares_channel_t *channel)
     channel->tries = DEFAULT_TRIES;
   }
 
-  if (channel->ndots == 0) {
-    channel->ndots = 1;
-  }
-
   if (ares__slist_len(channel->servers) == 0) {
     /* Add a default local named server to the channel unless configured not
      * to (in which case return an error).
@@ -284,6 +280,9 @@ int ares_init_options(ares_channel_t           **channelptr,
     return ARES_ENOMEM;
   }
 
+  /* One option where zero is valid, so set default value here */
+  channel->ndots = 1;
+
   status = ares__channel_threading_init(channel);
   if (status != ARES_SUCCESS) {
     goto done;

src/lib/ares_options.c

@@ -316,7 +316,7 @@ ares_status_t ares__init_by_options(ares_channel_t            *channel,
   }
 
   if (optmask & ARES_OPT_NDOTS) {
-    if (options->ndots <= 0) {
+    if (options->ndots < 0) {
       optmask &= ~(ARES_OPT_NDOTS);
     } else {
       channel->ndots = (size_t)options->ndots;

src/lib/ares_private.h

@@ -581,7 +581,7 @@ void          ares_queue_notify_empty(ares_channel_t *channel);
   } while (0)
 
 #define ARES_CONFIG_CHECK(x)                                             \
-  (x && x->lookups && ares__slist_len(x->servers) > 0 && x->ndots > 0 && \
+  (x && x->lookups && ares__slist_len(x->servers) > 0 && \
    x->timeout > 0 && x->tries > 0)
 
 ares_bool_t   ares__subnet_match(const struct ares_addr *addr,

src/lib/ares_sysconfig.c

@@ -954,7 +954,7 @@ static ares_status_t ares__init_sysconfig_libresolv(ares_sysconfig_t *sysconfig)
     }
   }
 
-  if (res.ndots > 0) {
+  if (res.ndots >= 0) {
     sysconfig->ndots = (size_t)res.ndots;
   }
   if (res.retry > 0) {

src/lib/ares_sysconfig_files.c

@@ -414,9 +414,6 @@ static ares_status_t process_option(ares_sysconfig_t *sysconfig,
   }
 
   if (strcmp(key, "ndots") == 0) {
-    if (valint == 0) {
-      return ARES_EFORMERR;
-    }
     sysconfig->ndots = valint;
   } else if (strcmp(key, "retrans") == 0 || strcmp(key, "timeout") == 0) {
     if (valint == 0) {

test/ares-test-mock-ai.cc

@@ -354,6 +354,53 @@ TEST_P(MockExtraOptsNDots5TestAI, SimpleQuery) {
   EXPECT_THAT(result.ai_, IncludesV4Address("123.45.67.8"));
 }
 
+class MockExtraOptsNDots0TestAI : public MockExtraOptsNDotsTestAI {
+ public:
+  MockExtraOptsNDots0TestAI() : MockExtraOptsNDotsTestAI(0) {}
+};
+
+TEST_P(MockExtraOptsNDots0TestAI, SimpleQuery) {
+  DNSPacket rsp_ndots0;
+  rsp_ndots0.set_response().set_aa()
+    .add_question(new DNSQuestion("ndots0", T_A))
+    .add_answer(new DNSARR("ndots0", 100, {1, 2, 3, 4}));
+  ON_CALL(server_, OnRequest("ndots0", T_A))
+    .WillByDefault(SetReply(&server_, &rsp_ndots0));
+
+  DNSPacket rsp_ndots0_first;
+  rsp_ndots0_first.set_response().set_aa()
+    .add_question(new DNSQuestion("ndots0.first.com", T_A))
+    .add_answer(new DNSARR("ndots0.first.com", 100, {99, 99, 99, 99}));
+  ON_CALL(server_, OnRequest("ndots0.first.com", T_A))
+    .WillByDefault(SetReply(&server_, &rsp_ndots0_first));
+
+  DNSPacket rsp_ndots0_second;
+  rsp_ndots0_second.set_response().set_aa()
+    .add_question(new DNSQuestion("ndots0.second.org", T_A))
+    .add_answer(new DNSARR("ndots0.second.org", 100, {98, 98, 98, 98}));
+  ON_CALL(server_, OnRequest("ndots0.second.org", T_A))
+    .WillByDefault(SetReply(&server_, &rsp_ndots0_second));
+
+  DNSPacket rsp_ndots0_third;
+  rsp_ndots0_third.set_response().set_aa()
+    .add_question(new DNSQuestion("ndots0.third.gov", T_A))
+    .add_answer(new DNSARR("ndots0.third.gov", 100, {97, 97, 97, 97}));
+  ON_CALL(server_, OnRequest("ndots0.third.gov", T_A))
+    .WillByDefault(SetReply(&server_, &rsp_ndots0_third));
+
+  AddrInfoResult result;
+  struct ares_addrinfo_hints hints = {};
+  hints.ai_family = AF_INET;
+  hints.ai_flags = ARES_AI_NOSORT;
+  ares_getaddrinfo(channel_, "ndots0", NULL, &hints, AddrInfoCallback, &result);
+  Process();
+  EXPECT_TRUE(result.done_);
+  EXPECT_EQ(ARES_SUCCESS, result.status_);
+  std::stringstream ss;
+  ss << result.ai_;
+  EXPECT_EQ("{addr=[1.2.3.4]}", ss.str());
+}
+
 class MockFlagsChannelOptsTestAI
     : public MockChannelOptsTest,
       public ::testing::WithParamInterface< std::pair<int, bool> > {
@@ -762,6 +809,9 @@ INSTANTIATE_TEST_SUITE_P(AddressFamiliesAI, MockExtraOptsTestAI,
 INSTANTIATE_TEST_SUITE_P(AddressFamiliesAI, MockExtraOptsNDots5TestAI,
       ::testing::ValuesIn(ares::test::families_modes), PrintFamilyMode);
 
+INSTANTIATE_TEST_SUITE_P(AddressFamiliesAI, MockExtraOptsNDots0TestAI,
+      ::testing::ValuesIn(ares::test::families_modes), PrintFamilyMode);
+
 INSTANTIATE_TEST_SUITE_P(AddressFamiliesAI, MockNoCheckRespChannelTestAI,
 			::testing::ValuesIn(ares::test::families_modes), PrintFamilyMode);