Detect domain-specific resolvers on OS X and disable DnsClient in such cases.

net/dns/dns_config_service_posix.cc

Index: net/dns/dns_config_service_posix.cc
diff --git a/net/dns/dns_config_service_posix.cc b/net/dns/dns_config_service_posix.cc
index ff2295e704a6aa3b1b6a80ac49d2d31185f9717f..fe08b54be5df54bac57e91a937e8dd0717bc2aa5 100644
--- a/net/dns/dns_config_service_posix.cc
+++ b/net/dns/dns_config_service_posix.cc
@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/files/file_path_watcher.h"
+#include "base/lazy_instance.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
 #include "base/time/time.h"
@@ -20,6 +21,61 @@
 #include "net/dns/notify_watcher_mac.h"
 #include "net/dns/serial_worker.h"
 
+#if defined(OS_MACOSX)
+#include <dlfcn.h>
+#include "third_party/apple_apsl/dnsinfo.h"
+namespace {
+
+// dnsinfo symbols are available via libSystem.dylib, but can also be present in
+// SystemConfiguration.framework. To avoid confusion, load them explicitly from
+// libSystem.dylib.
+class DnsInfoApi {
+ public:
+  typedef const char* (*dns_configuration_notify_key_t)();
+  typedef dns_config_t* (*dns_configuration_copy_t)();
+  typedef void (*dns_configuration_free_t)(dns_config_t*);
+
+  DnsInfoApi() {
+    handle_ = dlopen("/usr/lib/libSystem.dylib",
+                     RTLD_LAZY | RTLD_NOLOAD);
+    if (!handle_) return;
+    dns_configuration_notify_key =
+      reinterpret_cast<dns_configuration_notify_key_t>(
+        dlsym(handle_, "dns_configuration_notify_key"));
+    dns_configuration_copy =
+      reinterpret_cast<dns_configuration_copy_t>(
+        dlsym(handle_, "dns_configuration_copy"));
+    dns_configuration_free =
+      reinterpret_cast<dns_configuration_free_t>(
+        dlsym(handle_, "dns_configuration_free"));
+  }
+  ~DnsInfoApi() {
+    if (handle_) dlclose(handle_);
+  }
+
+  dns_configuration_notify_key_t dns_configuration_notify_key;
+  dns_configuration_copy_t dns_configuration_copy;
+  dns_configuration_free_t dns_configuration_free;
+
+ private:
+  void* handle_;
+};
+
+const DnsInfoApi& dns_info_api() {
+  base::LazyInstance<DnsInfoApi>::Leaky api = LAZY_INSTANCE_INITIALIZER;
+  return api.Get();
+}
+
+struct DnsConfigTDeleter {
+  inline void operator()(dns_config_t* ptr) const {
+    DCHECK(dns_info_api().dns_configuration_free);

[Mark Mentovai, 2013/08/06 18:05:28]

I think you’d want to be defensive and wrap the next line in an “if” instead of
or in addition to the DCHECK. The DCHECK won’t do anything in a release build. I
don’t think we actually want this to crash in release mode if
dns_configuration_notify_key and dns_configuration_copy are present but Apple
removes dns_configuration_free in some future version of the system. We’d maybe
want it to log something, but not to crash. Crashing in debug mode is fine.

[szym, 2013/08/06 22:14:19]

Done.

+    dns_info_api().dns_configuration_free(ptr);
+  }
+};
+
+}  // namespace
+#endif
+
 namespace net {
 
 #if !defined(OS_ANDROID)
@@ -31,14 +87,12 @@ const base::FilePath::CharType* kFilePathHosts =
     FILE_PATH_LITERAL("/etc/hosts");
 
 #if defined(OS_MACOSX)
-// From 10.7.3 configd-395.10/dnsinfo/dnsinfo.h
-static const char* kDnsNotifyKey =
-    "com.apple.system.SystemConfiguration.dns_configuration";
-
 class ConfigWatcher {
  public:
   bool Watch(const base::Callback<void(bool succeeded)>& callback) {
-    return watcher_.Watch(kDnsNotifyKey, callback);
+    if (!dns_info_api().dns_configuration_notify_key) return false;
+    return watcher_.Watch(dns_info_api().dns_configuration_notify_key(),
+                          callback);
   }
 
  private:
@@ -100,6 +154,31 @@ ConfigParsePosixResult ReadDnsConfig(DnsConfig* config) {
   res_nclose(&res);
 #endif
 #endif
+
+#if defined(OS_MACOSX)
+  if (!dns_info_api().dns_configuration_copy)
+    return CONFIG_PARSE_POSIX_NO_DNSINFO;
+  scoped_ptr<dns_config_t, DnsConfigTDeleter> dns_config(
+      dns_info_api().dns_configuration_copy());
+  if (!dns_config)
+    return CONFIG_PARSE_POSIX_NO_DNSINFO;
+
+  // TODO(szym): Parse dns_config_t for resolvers rather than res_state.
+  // DnsClient can't handle domain-specific unscoped resolvers.
+  unsigned num_resolvers = 0;
+  for (int i = 0; i < dns_config->n_resolver; ++i) {
+    dns_resolver_t* resolver = dns_config->resolver[i];
+    if (!resolver->n_nameserver)
+      continue;
+    if (resolver->options && !strcmp(resolver->options, "mdns"))

[Mark Mentovai, 2013/08/06 18:05:28]

Do we know anything about the “options” format? Is it possible for it to contain
multiple options?

[szym, 2013/08/06 22:14:19]

http://opensource.apple.com/source/mDNSResponder/mDNSResponder-379.37/mDNSMac...
checks:

dns_resolver_t *r = ...;
if (r->port == 5353 || r->n_nameserver == 0) {
  char *opt = r->options;
  if (opt && !strncmp(opt, "mdns", strlen(opt)))
    // assume r is multicast
}

So this is for Tiger only. On newer Mac OS it will bail on the check for
n_nameserver in line 171.

+      continue;
+    ++num_resolvers;
+  }
+  if (num_resolvers > 1) {
+    LOG(WARNING) << "dns_config has unhandled options!";
+    return CONFIG_PARSE_POSIX_UNHANDLED_OPTIONS;
+  }
+#endif
   // Override timeout value to match default setting on Windows.
   config->timeout = base::TimeDelta::FromSeconds(kDnsTimeoutSeconds);
   return result;
@@ -171,8 +250,20 @@ class DnsConfigServicePosix::ConfigReader : public SerialWorker {
 
   virtual void DoWork() OVERRIDE {
     base::TimeTicks start_time = base::TimeTicks::Now();
+    dns_config_.unhandled_options = false;
     ConfigParsePosixResult result = ReadDnsConfig(&dns_config_);
-    success_ = (result == CONFIG_PARSE_POSIX_OK);
+    switch (result) {
+      case CONFIG_PARSE_POSIX_MISSING_OPTIONS:
+      case CONFIG_PARSE_POSIX_UNHANDLED_OPTIONS:
+        dns_config_.unhandled_options = true;

[Mark Mentovai, 2013/08/06 18:05:28]

It seems like ReadDnsConfig should be responsible for setting this field.

[szym, 2013/08/06 22:14:19]

Done, although I'm not sure it's easier to follow this way.

+        // Fall through.
+      case CONFIG_PARSE_POSIX_OK:
+        success_ = true;
+        break;
+      default:
+        success_ = false;
+        break;
+    }
     UMA_HISTOGRAM_ENUMERATION("AsyncDNS.ConfigParsePosix",
                               result, CONFIG_PARSE_POSIX_MAX);
     UMA_HISTOGRAM_BOOLEAN("AsyncDNS.ConfigParseResult", success_);