Add support for service configs to c-ares resolver.

9 files changed, 353 insertions, 28 deletions

diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
index 04a7852323..f1480bb1ae 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
@@ -19,7 +19,10 @@
 #include <grpc/support/port_platform.h>
 #if GRPC_ARES == 1 && !defined(GRPC_UV)
 
+#include <limits.h>
+#include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 #include <grpc/support/alloc.h>
 #include <grpc/support/host_port.h>
@@ -31,11 +34,14 @@
 #include "src/core/ext/filters/client_channel/resolver_registry.h"
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/iomgr/combiner.h"
+#include "src/core/lib/iomgr/gethostname.h"
 #include "src/core/lib/iomgr/resolve_address.h"
 #include "src/core/lib/iomgr/timer.h"
+#include "src/core/lib/json/json.h"
 #include "src/core/lib/support/backoff.h"
 #include "src/core/lib/support/env.h"
 #include "src/core/lib/support/string.h"
+#include "src/core/lib/transport/service_config.h"
 
 #define GRPC_DNS_MIN_CONNECT_TIMEOUT_SECONDS 1
 #define GRPC_DNS_INITIAL_CONNECT_BACKOFF_SECONDS 1
@@ -54,6 +60,8 @@ typedef struct {
   char *default_port;
   /** channel args. */
   grpc_channel_args *channel_args;
+  /** whether to request the service config */
+  bool request_service_config;
   /** pollset_set to drive the name resolution process */
   grpc_pollset_set *interested_parties;
 
@@ -85,6 +93,8 @@ typedef struct {
 
   /** currently resolving addresses */
   grpc_lb_addresses *lb_addresses;
+  /** currently resolving service config */
+  char *service_config_json;
 } ares_dns_resolver;
 
 static void dns_ares_destroy(grpc_exec_ctx *exec_ctx, grpc_resolver *r);
@@ -144,6 +154,77 @@ static void dns_ares_on_retry_timer_locked(grpc_exec_ctx *exec_ctx, void *arg,
   GRPC_RESOLVER_UNREF(exec_ctx, &r->base, "retry-timer");
 }
 
+static bool value_in_json_array(grpc_json *array, const char *value) {
+  for (grpc_json *entry = array->child; entry != NULL; entry = entry->next) {
+    if (entry->type == GRPC_JSON_STRING && strcmp(entry->value, value) == 0) {
+      return true;
+    }
+  }
+  return false;
+}
+
+static char *choose_service_config(char *service_config_choice_json) {
+  grpc_json *choices_json = grpc_json_parse_string(service_config_choice_json);
+  if (choices_json == NULL || choices_json->type != GRPC_JSON_ARRAY) {
+    gpr_log(GPR_ERROR, "cannot parse service config JSON string");
+    return NULL;
+  }
+  char *service_config = NULL;
+  for (grpc_json *choice = choices_json->child; choice != NULL;
+       choice = choice->next) {
+    if (choice->type != GRPC_JSON_OBJECT) {
+      gpr_log(GPR_ERROR, "cannot parse service config JSON string");
+      break;
+    }
+    grpc_json *service_config_json = NULL;
+    for (grpc_json *field = choice->child; field != NULL; field = field->next) {
+      // Check client language, if specified.
+      if (strcmp(field->key, "clientLanguage") == 0) {
+        if (field->type != GRPC_JSON_ARRAY ||
+            !value_in_json_array(field, "c++")) {
+          service_config_json = NULL;
+          break;
+        }
+      }
+      // Check client hostname, if specified.
+      if (strcmp(field->key, "clientHostname") == 0) {
+        char *hostname = grpc_gethostname();
+        if (hostname == NULL || field->type != GRPC_JSON_ARRAY ||
+            !value_in_json_array(field, hostname)) {
+          service_config_json = NULL;
+          break;
+        }
+      }
+      // Check percentage, if specified.
+      if (strcmp(field->key, "percentage") == 0) {
+        if (field->type != GRPC_JSON_NUMBER) {
+          service_config_json = NULL;
+          break;
+        }
+        int random_pct = rand() % 100;
+        int percentage;
+        if (sscanf(field->value, "%d", &percentage) != 1 ||
+            random_pct > percentage) {
+          service_config_json = NULL;
+          break;
+        }
+      }
+      // Save service config.
+      if (strcmp(field->key, "serviceConfig") == 0) {
+        if (field->type == GRPC_JSON_OBJECT) {
+          service_config_json = field;
+        }
+      }
+    }
+    if (service_config_json != NULL) {
+      service_config = grpc_json_dump_to_string(service_config_json, 0);
+      break;
+    }
+  }
+  grpc_json_destroy(choices_json);
+  return service_config;
+}
+
 static void dns_ares_on_resolved_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                         grpc_error *error) {
   ares_dns_resolver *r = arg;
@@ -152,8 +233,40 @@ static void dns_ares_on_resolved_locked(grpc_exec_ctx *exec_ctx, void *arg,
   r->resolving = false;
   r->pending_request = NULL;
   if (r->lb_addresses != NULL) {
-    grpc_arg new_arg = grpc_lb_addresses_create_channel_arg(r->lb_addresses);
-    result = grpc_channel_args_copy_and_add(r->channel_args, &new_arg, 1);
+    static const char *args_to_remove[2];
+    size_t num_args_to_remove = 0;
+    grpc_arg new_args[3];
+    size_t num_args_to_add = 0;
+    new_args[num_args_to_add++] =
+        grpc_lb_addresses_create_channel_arg(r->lb_addresses);
+    grpc_service_config *service_config = NULL;
+    char *service_config_string = NULL;
+    if (r->service_config_json != NULL) {
+      service_config_string = choose_service_config(r->service_config_json);
+      gpr_free(r->service_config_json);
+      if (service_config_string != NULL) {
+        gpr_log(GPR_INFO, "selected service config choice: %s",
+                service_config_string);
+        args_to_remove[num_args_to_remove++] = GRPC_ARG_SERVICE_CONFIG;
+        new_args[num_args_to_add++] = grpc_channel_arg_string_create(
+            GRPC_ARG_SERVICE_CONFIG, service_config_string);
+        service_config = grpc_service_config_create(service_config_string);
+        if (service_config != NULL) {
+          const char *lb_policy_name =
+              grpc_service_config_get_lb_policy_name(service_config);
+          if (lb_policy_name != NULL) {
+            args_to_remove[num_args_to_remove++] = GRPC_ARG_LB_POLICY_NAME;
+            new_args[num_args_to_add++] = grpc_channel_arg_string_create(
+                GRPC_ARG_LB_POLICY_NAME, (char *)lb_policy_name);
+          }
+        }
+      }
+    }
+    result = grpc_channel_args_copy_and_add_and_remove(
+        r->channel_args, args_to_remove, num_args_to_remove, new_args,
+        num_args_to_add);
+    if (service_config != NULL) grpc_service_config_destroy(service_config);
+    gpr_free(service_config_string);
     grpc_lb_addresses_destroy(exec_ctx, r->lb_addresses);
   } else {
     const char *msg = grpc_error_string(error);
@@ -207,10 +320,12 @@ static void dns_ares_start_resolving_locked(grpc_exec_ctx *exec_ctx,
   GPR_ASSERT(!r->resolving);
   r->resolving = true;
   r->lb_addresses = NULL;
+  r->service_config_json = NULL;
   r->pending_request = grpc_dns_lookup_ares(
       exec_ctx, r->dns_server, r->name_to_resolve, r->default_port,
       r->interested_parties, &r->dns_ares_on_resolved_locked, &r->lb_addresses,
-      true /* check_grpclb */);
+      true /* check_grpclb */,
+      r->request_service_config ? &r->service_config_json : NULL);
 }
 
 static void dns_ares_maybe_finish_next_locked(grpc_exec_ctx *exec_ctx,
@@ -256,6 +371,10 @@ static grpc_resolver *dns_ares_create(grpc_exec_ctx *exec_ctx,
   r->name_to_resolve = gpr_strdup(path);
   r->default_port = gpr_strdup(default_port);
   r->channel_args = grpc_channel_args_copy(args->args);
+  const grpc_arg *arg = grpc_channel_args_find(
+      r->channel_args, GRPC_ARG_SERVICE_CONFIG_DISABLE_RESOLUTION);
+  r->request_service_config = !grpc_channel_arg_get_integer(
+      arg, (grpc_integer_options){false, false, true});
   r->interested_parties = grpc_pollset_set_create();
   if (args->pollset_set != NULL) {
     grpc_pollset_set_add_pollset_set(exec_ctx, r->interested_parties,
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
index 6ec3790a5f..e65723a63b 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
@@ -54,6 +54,8 @@ struct grpc_ares_request {
   grpc_closure *on_done;
   /** the pointer to receive the resolved addresses */
   grpc_lb_addresses **lb_addrs_out;
+  /** the pointer to receive the service config in JSON */
+  char **service_config_json_out;
   /** the evernt driver used by this request */
   grpc_ares_ev_driver *ev_driver;
   /** number of ongoing queries */
@@ -266,10 +268,68 @@ static void on_srv_query_done_cb(void *arg, int status, int timeouts,
   grpc_exec_ctx_finish(&exec_ctx);
 }
 
+static const char g_service_config_attribute_prefix[] = "grpc_config=";
+
+static void on_txt_done_cb(void *arg, int status, int timeouts,
+                           unsigned char *buf, int len) {
+  gpr_log(GPR_DEBUG, "on_txt_done_cb");
+  char *error_msg;
+  grpc_ares_request *r = (grpc_ares_request *)arg;
+  gpr_mu_lock(&r->mu);
+  if (status != ARES_SUCCESS) goto fail;
+  struct ares_txt_ext *reply = NULL;
+  status = ares_parse_txt_reply_ext(buf, len, &reply);
+  if (status != ARES_SUCCESS) goto fail;
+  // Find service config in TXT record.
+  const size_t prefix_len = sizeof(g_service_config_attribute_prefix) - 1;
+  struct ares_txt_ext *result;
+  for (result = reply; result != NULL; result = result->next) {
+    if (result->record_start &&
+        memcmp(result->txt, g_service_config_attribute_prefix, prefix_len) ==
+            0) {
+      break;
+    }
+  }
+  // Found a service config record.
+  if (result != NULL) {
+    size_t service_config_len = result->length - prefix_len;
+    *r->service_config_json_out = gpr_malloc(service_config_len + 1);
+    memcpy(*r->service_config_json_out, result->txt + prefix_len,
+           service_config_len);
+    for (result = result->next; result != NULL && !result->record_start;
+         result = result->next) {
+      *r->service_config_json_out = gpr_realloc(
+          *r->service_config_json_out, service_config_len + result->length + 1);
+      memcpy(*r->service_config_json_out + service_config_len, result->txt,
+             result->length);
+      service_config_len += result->length;
+    }
+    (*r->service_config_json_out)[service_config_len] = '\0';
+    gpr_log(GPR_INFO, "found service config: %s", *r->service_config_json_out);
+  }
+  // Clean up.
+  ares_free_data(reply);
+  goto done;
+fail:
+  gpr_asprintf(&error_msg, "C-ares TXT lookup status is not ARES_SUCCESS: %s",
+               ares_strerror(status));
+  grpc_error *error = GRPC_ERROR_CREATE_FROM_COPIED_STRING(error_msg);
+  gpr_free(error_msg);
+  if (r->error == GRPC_ERROR_NONE) {
+    r->error = error;
+  } else {
+    r->error = grpc_error_add_child(error, r->error);
+  }
+done:
+  gpr_mu_unlock(&r->mu);
+  grpc_ares_request_unref(NULL, r);
+}
+
 static grpc_ares_request *grpc_dns_lookup_ares_impl(
     grpc_exec_ctx *exec_ctx, const char *dns_server, const char *name,
     const char *default_port, grpc_pollset_set *interested_parties,
-    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb) {
+    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb,
+    char **service_config_json) {
   grpc_error *error = GRPC_ERROR_NONE;
   /* TODO(zyc): Enable tracing after #9603 is checked in */
   /* if (grpc_dns_trace) {
@@ -300,11 +360,12 @@ static grpc_ares_request *grpc_dns_lookup_ares_impl(
   error = grpc_ares_ev_driver_create(&ev_driver, interested_parties);
   if (error != GRPC_ERROR_NONE) goto error_cleanup;
 
-  grpc_ares_request *r = gpr_malloc(sizeof(grpc_ares_request));
+  grpc_ares_request *r = gpr_zalloc(sizeof(grpc_ares_request));
   gpr_mu_init(&r->mu);
   r->ev_driver = ev_driver;
   r->on_done = on_done;
   r->lb_addrs_out = addrs;
+  r->service_config_json_out = service_config_json;
   r->success = false;
   r->error = GRPC_ERROR_NONE;
   ares_channel *channel = grpc_ares_ev_driver_get_channel(r->ev_driver);
@@ -315,13 +376,17 @@ static grpc_ares_request *grpc_dns_lookup_ares_impl(
     grpc_resolved_address addr;
     if (grpc_parse_ipv4_hostport(dns_server, &addr, false /* log_errors */)) {
       r->dns_server_addr.family = AF_INET;
-      memcpy(&r->dns_server_addr.addr.addr4, addr.addr, addr.len);
+      struct sockaddr_in *in = (struct sockaddr_in *)addr.addr;
+      memcpy(&r->dns_server_addr.addr.addr4, &in->sin_addr,
+             sizeof(struct in_addr));
       r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
       r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
     } else if (grpc_parse_ipv6_hostport(dns_server, &addr,
                                         false /* log_errors */)) {
       r->dns_server_addr.family = AF_INET6;
-      memcpy(&r->dns_server_addr.addr.addr6, addr.addr, addr.len);
+      struct sockaddr_in6 *in6 = (struct sockaddr_in6 *)addr.addr;
+      memcpy(&r->dns_server_addr.addr.addr6, &in6->sin6_addr,
+             sizeof(struct in6_addr));
       r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
       r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
     } else {
@@ -342,8 +407,6 @@ static grpc_ares_request *grpc_dns_lookup_ares_impl(
       goto error_cleanup;
     }
   }
-  // An extra reference is put here to avoid destroying the request in
-  // on_done_cb before calling grpc_ares_ev_driver_start.
   gpr_ref_init(&r->pending_queries, 1);
   if (grpc_ipv6_loopback_available()) {
     grpc_ares_hostbyname_request *hr = create_hostbyname_request(
@@ -362,6 +425,10 @@ static grpc_ares_request *grpc_dns_lookup_ares_impl(
                r);
     gpr_free(service_name);
   }
+  if (service_config_json != NULL) {
+    grpc_ares_request_ref(r);
+    ares_search(*channel, hr->host, ns_c_in, ns_t_txt, on_txt_done_cb, r);
+  }
   /* TODO(zyc): Handle CNAME records here. */
   grpc_ares_ev_driver_start(exec_ctx, r->ev_driver);
   grpc_ares_request_unref(exec_ctx, r);
@@ -379,8 +446,8 @@ error_cleanup:
 grpc_ares_request *(*grpc_dns_lookup_ares)(
     grpc_exec_ctx *exec_ctx, const char *dns_server, const char *name,
     const char *default_port, grpc_pollset_set *interested_parties,
-    grpc_closure *on_done, grpc_lb_addresses **addrs,
-    bool check_grpclb) = grpc_dns_lookup_ares_impl;
+    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb,
+    char **service_config_json) = grpc_dns_lookup_ares_impl;
 
 void grpc_cancel_ares_request(grpc_exec_ctx *exec_ctx, grpc_ares_request *r) {
   if (grpc_dns_lookup_ares == grpc_dns_lookup_ares_impl) {
@@ -465,7 +532,8 @@ static void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx,
                     grpc_schedule_on_exec_ctx);
   grpc_dns_lookup_ares(exec_ctx, NULL /* dns_server */, name, default_port,
                        interested_parties, &r->on_dns_lookup_done, &r->lb_addrs,
-                       false /* check_grpclb */);
+                       false /* check_grpclb */,
+                       NULL /* service_config_json */);
 }
 
 void (*grpc_resolve_address_ares)(
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
index 5d2d6c993b..108333047d 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
@@ -27,29 +27,30 @@
 
 typedef struct grpc_ares_request grpc_ares_request;
 
-/* Asynchronously resolve addr. Use \a default_port if a port isn't designated
-   in addr, otherwise use the port in addr. grpc_ares_init() must be called at
-   least once before this function. \a on_done may be called directly in this
-   function without being scheduled with \a exec_ctx, it must not try to acquire
-   locks that are being held by the caller. */
+/* Asynchronously resolve \a name. Use \a default_port if a port isn't
+   designated in \a name, otherwise use the port in \a name. grpc_ares_init()
+   must be called at least once before this function. \a on_done may be
+   called directly in this function without being scheduled with \a exec_ctx,
+   so it must not try to acquire locks that are being held by the caller. */
 extern void (*grpc_resolve_address_ares)(grpc_exec_ctx *exec_ctx,
-                                         const char *addr,
+                                         const char *name,
                                          const char *default_port,
                                          grpc_pollset_set *interested_parties,
                                          grpc_closure *on_done,
                                          grpc_resolved_addresses **addresses);
 
-/* Asynchronously resolve addr. It will try to resolve grpclb SRV records in
+/* Asynchronously resolve \a name. It will try to resolve grpclb SRV records in
   addition to the normal address records. For normal address records, it uses
-  \a default_port if a port isn't designated in \a addr, otherwise it uses the
-  port in \a addr. grpc_ares_init() must be called at least once before this
+  \a default_port if a port isn't designated in \a name, otherwise it uses the
+  port in \a name. grpc_ares_init() must be called at least once before this
   function. \a on_done may be called directly in this function without being
-  scheduled with \a exec_ctx, it must not try to acquire locks that are being
-  held by the caller. */
+  scheduled with \a exec_ctx, so it must not try to acquire locks that are
+  being held by the caller. */
 extern grpc_ares_request *(*grpc_dns_lookup_ares)(
-    grpc_exec_ctx *exec_ctx, const char *dns_server, const char *addr,
+    grpc_exec_ctx *exec_ctx, const char *dns_server, const char *name,
     const char *default_port, grpc_pollset_set *interested_parties,
-    grpc_closure *on_done, grpc_lb_addresses **addresses, bool check_grpclb);
+    grpc_closure *on_done, grpc_lb_addresses **addresses, bool check_grpclb,
+    char **service_config_json);
 
 /* Cancel the pending grpc_ares_request \a request */
 void grpc_cancel_ares_request(grpc_exec_ctx *exec_ctx,
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper_fallback.c b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper_fallback.c
index b67636a3e4..f2587c4520 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper_fallback.c
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper_fallback.c
@@ -28,15 +28,16 @@ struct grpc_ares_request {
 static grpc_ares_request *grpc_dns_lookup_ares_impl(
     grpc_exec_ctx *exec_ctx, const char *dns_server, const char *name,
     const char *default_port, grpc_pollset_set *interested_parties,
-    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb) {
+    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb,
+    char **service_config_json) {
   return NULL;
 }
 
 grpc_ares_request *(*grpc_dns_lookup_ares)(
     grpc_exec_ctx *exec_ctx, const char *dns_server, const char *name,
     const char *default_port, grpc_pollset_set *interested_parties,
-    grpc_closure *on_done, grpc_lb_addresses **addrs,
-    bool check_grpclb) = grpc_dns_lookup_ares_impl;
+    grpc_closure *on_done, grpc_lb_addresses **addrs, bool check_grpclb,
+    char **service_config_json) = grpc_dns_lookup_ares_impl;
 
 void grpc_cancel_ares_request(grpc_exec_ctx *exec_ctx, grpc_ares_request *r) {}
 
diff --git a/src/core/lib/iomgr/gethostname.h b/src/core/lib/iomgr/gethostname.h
new file mode 100644
index 0000000000..9c6b9d8d42
--- /dev/null
+++ b/src/core/lib/iomgr/gethostname.h
@@ -0,0 +1,26 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#ifndef GRPC_CORE_LIB_IOMGR_GETHOSTNAME_H
+#define GRPC_CORE_LIB_IOMGR_GETHOSTNAME_H
+
+// Returns the hostname of the local machine.
+// Caller takes ownership of result.
+char *grpc_gethostname();
+
+#endif /* GRPC_CORE_LIB_IOMGR_GETHOSTNAME_H */
diff --git a/src/core/lib/iomgr/gethostname_fallback.c b/src/core/lib/iomgr/gethostname_fallback.c
new file mode 100644
index 0000000000..6229461568
--- /dev/null
+++ b/src/core/lib/iomgr/gethostname_fallback.c
@@ -0,0 +1,27 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/lib/iomgr/port.h"
+
+#ifdef GRPC_GETHOSTNAME_FALLBACK
+
+#include <stddef.h>
+
+char *grpc_gethostname() { return NULL; }
+
+#endif  // GRPC_GETHOSTNAME_FALLBACK
diff --git a/src/core/lib/iomgr/gethostname_host_name_max.c b/src/core/lib/iomgr/gethostname_host_name_max.c
new file mode 100644
index 0000000000..4d0511412e
--- /dev/null
+++ b/src/core/lib/iomgr/gethostname_host_name_max.c
@@ -0,0 +1,37 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/lib/iomgr/port.h"
+
+#ifdef GRPC_POSIX_HOST_NAME_MAX
+
+#include <limits.h>
+#include <unistd.h>
+
+#include <grpc/support/alloc.h>
+
+char *grpc_gethostname() {
+  char *hostname = (char *)gpr_malloc(HOST_NAME_MAX);
+  if (gethostname(hostname, HOST_NAME_MAX) != 0) {
+    gpr_free(hostname);
+    return NULL;
+  }
+  return hostname;
+}
+
+#endif  // GRPC_POSIX_HOST_NAME_MAX
diff --git a/src/core/lib/iomgr/gethostname_sysconf.c b/src/core/lib/iomgr/gethostname_sysconf.c
new file mode 100644
index 0000000000..51bac5d69d
--- /dev/null
+++ b/src/core/lib/iomgr/gethostname_sysconf.c
@@ -0,0 +1,37 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+#include "src/core/lib/iomgr/port.h"
+
+#ifdef GRPC_POSIX_SYSCONF
+
+#include <unistd.h>
+
+#include <grpc/support/alloc.h>
+
+char *grpc_gethostname() {
+  size_t host_name_max = (size_t)sysconf(_SC_HOST_NAME_MAX);
+  char *hostname = (char *)gpr_malloc(host_name_max);
+  if (gethostname(hostname, host_name_max) != 0) {
+    gpr_free(hostname);
+    return NULL;
+  }
+  return hostname;
+}
+
+#endif  // GRPC_POSIX_SYSCONF
diff --git a/src/core/lib/iomgr/port.h b/src/core/lib/iomgr/port.h
index c12058f890..42033d0ba4 100644
--- a/src/core/lib/iomgr/port.h
+++ b/src/core/lib/iomgr/port.h
@@ -59,6 +59,7 @@
 #define GRPC_HAVE_MSG_NOSIGNAL 1
 #define GRPC_HAVE_UNIX_SOCKET 1
 #define GRPC_LINUX_MULTIPOLL_WITH_EPOLL 1
+#define GRPC_POSIX_HOST_NAME_MAX 1
 #define GRPC_POSIX_SOCKET 1
 #define GRPC_POSIX_SOCKETADDR 1
 #define GRPC_POSIX_WAKEUP_FD 1
@@ -93,6 +94,7 @@
 #define GRPC_POSIX_SOCKET 1
 #define GRPC_POSIX_SOCKETADDR 1
 #define GRPC_POSIX_SOCKETUTILS 1
+#define GRPC_POSIX_SYSCONF 1
 #define GRPC_POSIX_WAKEUP_FD 1
 #define GRPC_TIMER_USE_GENERIC 1
 #elif defined(GPR_FREEBSD)
@@ -125,4 +127,11 @@
 #error Must define exactly one of GRPC_POSIX_SOCKET, GRPC_WINSOCK_SOCKET, GPR_CUSTOM_SOCKET
 #endif
 
+#if defined(GRPC_POSIX_HOST_NAME_MAX) && defined(GRPC_POSIX_SYSCONF)
+#error "Cannot define both GRPC_POSIX_HOST_NAME_MAX and GRPC_POSIX_SYSCONF"
+#endif
+#if !defined(GRPC_POSIX_HOST_NAME_MAX) && !defined(GRPC_POSIX_SYSCONF)
+#define GRPC_GETHOSTNAME_FALLBACK 1
+#endif
+
 #endif /* GRPC_CORE_LIB_IOMGR_PORT_H */