network: support conditionally binding active alt interface (#1834)

Description: Adds utility support for filtering for a viable WIFI or cellular interface to select as a potential alternate to the OS-supplied default, and an accessor for the platform-specific socket options to bind to it.
Risk Level: Low
Testing: Added unit test

Signed-off-by: Mike Schore <mike.schore@gmail.com>

library/common/network/configurator.cc

@@ -1,5 +1,7 @@
 #include "library/common/network/configurator.h"
 
+#include <net/if.h>
+
 #include "envoy/common/platform.h"
 
 #include "source/common/common/assert.h"
@@ -96,14 +98,18 @@ void Configurator::refreshDns(envoy_network_t network) {
 }
 
 std::vector<std::string> Configurator::enumerateV4Interfaces() {
-  return enumerateInterfaces(AF_INET);
+  return enumerateInterfaces(AF_INET, 0, 0);
 }
 
 std::vector<std::string> Configurator::enumerateV6Interfaces() {
-  return enumerateInterfaces(AF_INET6);
+  return enumerateInterfaces(AF_INET6, 0, 0);
 }
 
-Socket::OptionsSharedPtr Configurator::getUpstreamSocketOptions(envoy_network_t network, bool) {
+Socket::OptionsSharedPtr Configurator::getUpstreamSocketOptions(envoy_network_t network,
+                                                                bool override_interface) {
+  if (override_interface && network != ENVOY_NET_GENERIC) {
+    return getAlternateInterfaceSocketOptions(network);
+  }
   // Envoy uses the hash signature of overridden socket options to choose a connection pool.
   // Setting a dummy socket option is a hack that allows us to select a different
   // connection pool without materially changing the socket configuration.
@@ -116,7 +122,54 @@ Socket::OptionsSharedPtr Configurator::getUpstreamSocketOptions(envoy_network_t
   return options;
 }
 
-std::vector<std::string> Configurator::enumerateInterfaces([[maybe_unused]] unsigned short family) {
+Socket::OptionsSharedPtr Configurator::getAlternateInterfaceSocketOptions(envoy_network_t network) {
+  auto& v4_interface = getActiveAlternateInterface(network, AF_INET);
+  auto& v6_interface = getActiveAlternateInterface(network, AF_INET6);
+
+  auto options = std::make_shared<Socket::Options>();
+
+  // Android
+#ifdef SO_BINDTODEVICE
+  options->push_back(std::make_shared<AddrFamilyAwareSocketOptionImpl>(
+      envoy::config::core::v3::SocketOption::STATE_PREBIND, ENVOY_SOCKET_SO_BINDTODEVICE,
+      v4_interface, ENVOY_SOCKET_SO_BINDTODEVICE, v6_interface));
+#endif // SO_BINDTODEVICE
+
+  // iOS
+#ifdef IP_BOUND_IF
+  int v4_idx = if_nametoindex(v4_interface.c_str());
+  int v6_idx = if_nametoindex(v6_interface.c_str());
+  options->push_back(std::make_shared<AddrFamilyAwareSocketOptionImpl>(
+      envoy::config::core::v3::SocketOption::STATE_PREBIND, ENVOY_SOCKET_IP_BOUND_IF, v4_idx,
+      ENVOY_SOCKET_IPV6_BOUND_IF, v6_idx));
+#endif // IP_BOUND_IF
+
+  return options;
+}
+
+const std::string Configurator::getActiveAlternateInterface(envoy_network_t network,
+                                                            unsigned short family) {
+  // Attempt to derive an active interface that differs from the passed network parameter.
+  if (network == ENVOY_NET_WWAN) {
+    // Network is cellular, so look for a WiFi interface.
+    // WiFi should always support multicast, and will not be point-to-point.
+    auto interfaces =
+        enumerateInterfaces(family, IFF_UP | IFF_MULTICAST, IFF_LOOPBACK | IFF_POINTOPOINT);
+    return interfaces.size() > 0 ? interfaces[0] : "";
+  } else if (network == ENVOY_NET_WLAN) {
+    // Network is WiFi, so look for a cellular interface.
+    // Cellular networks should be point-to-point.
+    auto interfaces = enumerateInterfaces(family, IFF_UP | IFF_POINTOPOINT, IFF_LOOPBACK);
+    return interfaces.size() > 0 ? interfaces[0] : "";
+  } else {
+    return "";
+  }
+}
+
+std::vector<std::string>
+Configurator::enumerateInterfaces([[maybe_unused]] unsigned short family,
+                                  [[maybe_unused]] unsigned int select_flags,
+                                  [[maybe_unused]] unsigned int reject_flags) {
   std::vector<std::string> names{};
 
 #ifdef SUPPORTS_GETIFADDRS
@@ -127,9 +180,13 @@ std::vector<std::string> Configurator::enumerateInterfaces([[maybe_unused]] unsi
   RELEASE_ASSERT(!rc, "getifaddrs failed");
 
   for (ifa = interfaces; ifa != nullptr; ifa = ifa->ifa_next) {
-    if (ifa->ifa_addr && ifa->ifa_addr->sa_family == family) {
-      names.push_back(std::string{ifa->ifa_name});
+    if (!ifa->ifa_addr || ifa->ifa_addr->sa_family != family) {
+      continue;
+    }
+    if ((ifa->ifa_flags & (select_flags ^ reject_flags)) != select_flags) {
+      continue;
     }
+    names.push_back(std::string{ifa->ifa_name});
   }
 
   freeifaddrs(interfaces);

library/common/network/configurator.h

@@ -33,6 +33,15 @@ class Configurator : public Logger::Loggable<Logger::Id::upstream>, public Singl
    */
   std::vector<std::string> enumerateV6Interfaces();
 
+  /**
+   * @param family, network family of the interface.
+   * @param select_flags, flags which MUST be set for each returned interface.
+   * @param reject_flags, flags which MUST NOT be set for any returned interface.
+   * @returns a list of local network interfaces filtered by the providered flags.
+   */
+  std::vector<std::string> enumerateInterfaces(unsigned short family, unsigned int select_flags,
+                                               unsigned int reject_flags);
+
   /**
    * @returns the current OS default/preferred network class.
    */
@@ -62,7 +71,8 @@ class Configurator : public Logger::Loggable<Logger::Id::upstream>, public Singl
                                                     bool override_interface);
 
 private:
-  std::vector<std::string> enumerateInterfaces(unsigned short family);
+  Socket::OptionsSharedPtr getAlternateInterfaceSocketOptions(envoy_network_t network);
+  const std::string getActiveAlternateInterface(envoy_network_t network, unsigned short family);
   DnsCacheManagerSharedPtr dns_cache_manager_;
   static std::atomic<envoy_network_t> preferred_network_;
 };

test/common/network/configurator_test.cc

@@ -1,3 +1,5 @@
+#include <net/if.h>
+
 #include "test/extensions/common/dynamic_forward_proxy/mocks.h"
 
 #include "gtest/gtest.h"
@@ -37,5 +39,21 @@ TEST_F(ConfiguratorTest, RefreshDnsForOtherNetworkDoesntTriggerDnsRefresh) {
   configurator_->refreshDns(ENVOY_NET_WWAN);
 }
 
+TEST_F(ConfiguratorTest, EnumerateInterfacesFiltersByFlags) {
+  // Select loopback.
+  auto loopbacks = configurator_->enumerateInterfaces(AF_INET, IFF_LOOPBACK, 0);
+  EXPECT_EQ(loopbacks[0].rfind("lo", 0), 0);
+
+  // Reject loopback.
+  auto nonloopbacks = configurator_->enumerateInterfaces(AF_INET, 0, IFF_LOOPBACK);
+  for (const auto& interface : nonloopbacks) {
+    EXPECT_NE(interface.rfind("lo", 0), 0);
+  }
+
+  // Select AND reject loopback.
+  auto empty = configurator_->enumerateInterfaces(AF_INET, IFF_LOOPBACK, IFF_LOOPBACK);
+  EXPECT_EQ(empty.size(), 0);
+}
+
 } // namespace Network
 } // namespace Envoy