Implement proper local broadcast behavior

The behavior for sending and receiving local broadcast (255.255.255.255)
traffic is as follows:

Outgoing
--------
* A broadcast packet sent on a socket that is bound to an interface goes out
  that interface
* A broadcast packet sent on an unbound socket follows the route table to
  select the outgoing interface
  + if an explicit route entry exists for 255.255.255.255/32, use that one
  + else use the default route
* Broadcast packets are looped back and delivered following the rules for
  incoming packets (see next). This is the same behavior as for multicast
  packets, except that it cannot be disabled via sockopt.

Incoming
--------
* Sockets wishing to receive broadcast packets must bind to either INADDR_ANY
  (0.0.0.0) or INADDR_BROADCAST (255.255.255.255). No other socket receives
  broadcast packets.
* Broadcast packets are multiplexed to all sockets matching it. This is the
  same behavior as for multicast packets.
* A socket can bind to 255.255.255.255:<port> and then receive its own
  broadcast packets sent to 255.255.255.255:<port>

In addition, this change implicitly fixes an issue with multicast reception. If
two sockets want to receive a given multicast stream and one is bound to ANY
while the other is bound to the multicast address, only one of them will
receive the traffic.

PiperOrigin-RevId: 272792377

1 files changed, 342 insertions, 66 deletions

diff --git a/test/syscalls/linux/socket_ipv4_udp_unbound_external_networking.cc b/test/syscalls/linux/socket_ipv4_udp_unbound_external_networking.cc
index c85ae30dc..8b8993d3d 100644
--- a/test/syscalls/linux/socket_ipv4_udp_unbound_external_networking.cc
+++ b/test/syscalls/linux/socket_ipv4_udp_unbound_external_networking.cc
@@ -42,6 +42,26 @@ TestAddress V4EmptyAddress() {
   return t;
 }
 
+constexpr char kMulticastAddress[] = "224.0.2.1";
+
+TestAddress V4Multicast() {
+  TestAddress t("V4Multicast");
+  t.addr.ss_family = AF_INET;
+  t.addr_len = sizeof(sockaddr_in);
+  reinterpret_cast<sockaddr_in*>(&t.addr)->sin_addr.s_addr =
+      inet_addr(kMulticastAddress);
+  return t;
+}
+
+TestAddress V4Broadcast() {
+  TestAddress t("V4Broadcast");
+  t.addr.ss_family = AF_INET;
+  t.addr_len = sizeof(sockaddr_in);
+  reinterpret_cast<sockaddr_in*>(&t.addr)->sin_addr.s_addr =
+      htonl(INADDR_BROADCAST);
+  return t;
+}
+
 void IPv4UDPUnboundExternalNetworkingSocketTest::SetUp() {
   got_if_infos_ = false;
 
@@ -116,7 +136,7 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, SetUDPBroadcast) {
 // Verifies that a broadcast UDP packet will arrive at all UDP sockets with
 // the destination port number.
 TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
-       UDPBroadcastReceivedOnAllExpectedEndpoints) {
+       UDPBroadcastReceivedOnExpectedPort) {
   auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
   auto rcvr1 = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
   auto rcvr2 = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
@@ -136,51 +156,134 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
                          sizeof(kSockOptOn)),
               SyscallSucceedsWithValue(0));
 
-  sockaddr_in rcv_addr = {};
-  socklen_t rcv_addr_sz = sizeof(rcv_addr);
-  rcv_addr.sin_family = AF_INET;
-  rcv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
-  ASSERT_THAT(bind(rcvr1->get(), reinterpret_cast<struct sockaddr*>(&rcv_addr),
-                   rcv_addr_sz),
+  // Bind the first socket to the ANY address and let the system assign a port.
+  auto rcv1_addr = V4Any();
+  ASSERT_THAT(bind(rcvr1->get(), reinterpret_cast<sockaddr*>(&rcv1_addr.addr),
+                   rcv1_addr.addr_len),
               SyscallSucceedsWithValue(0));
   // Retrieve port number from first socket so that it can be bound to the
   // second socket.
-  rcv_addr = {};
+  socklen_t rcv_addr_sz = rcv1_addr.addr_len;
   ASSERT_THAT(
-      getsockname(rcvr1->get(), reinterpret_cast<struct sockaddr*>(&rcv_addr),
+      getsockname(rcvr1->get(), reinterpret_cast<sockaddr*>(&rcv1_addr.addr),
                   &rcv_addr_sz),
       SyscallSucceedsWithValue(0));
-  ASSERT_THAT(bind(rcvr2->get(), reinterpret_cast<struct sockaddr*>(&rcv_addr),
+  EXPECT_EQ(rcv_addr_sz, rcv1_addr.addr_len);
+  auto port = reinterpret_cast<sockaddr_in*>(&rcv1_addr.addr)->sin_port;
+
+  // Bind the second socket to the same address:port as the first.
+  ASSERT_THAT(bind(rcvr2->get(), reinterpret_cast<sockaddr*>(&rcv1_addr.addr),
                    rcv_addr_sz),
               SyscallSucceedsWithValue(0));
 
   // Bind the non-receiving socket to an ephemeral port.
-  sockaddr_in norcv_addr = {};
-  norcv_addr.sin_family = AF_INET;
-  norcv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
+  auto norecv_addr = V4Any();
+  ASSERT_THAT(bind(norcv->get(), reinterpret_cast<sockaddr*>(&norecv_addr.addr),
+                   norecv_addr.addr_len),
+              SyscallSucceedsWithValue(0));
+
+  // Broadcast a test message.
+  auto dst_addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&dst_addr.addr)->sin_port = port;
+  constexpr char kTestMsg[] = "hello, world";
+  EXPECT_THAT(
+      sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
+             reinterpret_cast<sockaddr*>(&dst_addr.addr), dst_addr.addr_len),
+      SyscallSucceedsWithValue(sizeof(kTestMsg)));
+
+  // Verify that the receiving sockets received the test message.
+  char buf[sizeof(kTestMsg)] = {};
+  EXPECT_THAT(recv(rcvr1->get(), buf, sizeof(buf), 0),
+              SyscallSucceedsWithValue(sizeof(kTestMsg)));
+  EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
+  memset(buf, 0, sizeof(buf));
+  EXPECT_THAT(recv(rcvr2->get(), buf, sizeof(buf), 0),
+              SyscallSucceedsWithValue(sizeof(kTestMsg)));
+  EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
+
+  // Verify that the non-receiving socket did not receive the test message.
+  memset(buf, 0, sizeof(buf));
+  EXPECT_THAT(RetryEINTR(recv)(norcv->get(), buf, sizeof(buf), MSG_DONTWAIT),
+              SyscallFailsWithErrno(EAGAIN));
+}
+
+// Verifies that a broadcast UDP packet will arrive at all UDP sockets bound to
+// the destination port number and either INADDR_ANY or INADDR_BROADCAST, but
+// not a unicast address.
+TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
+       UDPBroadcastReceivedOnExpectedAddresses) {
+  // FIXME(b/137899561): Linux instance for syscall tests sometimes misses its
+  // IPv4 address on eth0.
+  SKIP_IF(!got_if_infos_);
+
+  auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+  auto rcvr1 = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+  auto rcvr2 = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+  auto norcv = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+
+  // Enable SO_BROADCAST on the sending socket.
+  ASSERT_THAT(setsockopt(sender->get(), SOL_SOCKET, SO_BROADCAST, &kSockOptOn,
+                         sizeof(kSockOptOn)),
+              SyscallSucceedsWithValue(0));
+
+  // Enable SO_REUSEPORT on all sockets so that they may all be bound to the
+  // broadcast messages destination port.
+  ASSERT_THAT(setsockopt(rcvr1->get(), SOL_SOCKET, SO_REUSEPORT, &kSockOptOn,
+                         sizeof(kSockOptOn)),
+              SyscallSucceedsWithValue(0));
+  ASSERT_THAT(setsockopt(rcvr2->get(), SOL_SOCKET, SO_REUSEPORT, &kSockOptOn,
+                         sizeof(kSockOptOn)),
+              SyscallSucceedsWithValue(0));
+  ASSERT_THAT(setsockopt(norcv->get(), SOL_SOCKET, SO_REUSEPORT, &kSockOptOn,
+                         sizeof(kSockOptOn)),
+              SyscallSucceedsWithValue(0));
+
+  // Bind the first socket the ANY address and let the system assign a port.
+  auto rcv1_addr = V4Any();
+  ASSERT_THAT(bind(rcvr1->get(), reinterpret_cast<sockaddr*>(&rcv1_addr.addr),
+                   rcv1_addr.addr_len),
+              SyscallSucceedsWithValue(0));
+  // Retrieve port number from first socket so that it can be bound to the
+  // second socket.
+  socklen_t rcv_addr_sz = rcv1_addr.addr_len;
   ASSERT_THAT(
-      bind(norcv->get(), reinterpret_cast<struct sockaddr*>(&norcv_addr),
-           sizeof(norcv_addr)),
+      getsockname(rcvr1->get(), reinterpret_cast<sockaddr*>(&rcv1_addr.addr),
+                  &rcv_addr_sz),
       SyscallSucceedsWithValue(0));
+  EXPECT_EQ(rcv_addr_sz, rcv1_addr.addr_len);
+  auto port = reinterpret_cast<sockaddr_in*>(&rcv1_addr.addr)->sin_port;
+
+  // Bind the second socket to the broadcast address.
+  auto rcv2_addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&rcv2_addr.addr)->sin_port = port;
+  ASSERT_THAT(bind(rcvr2->get(), reinterpret_cast<sockaddr*>(&rcv2_addr.addr),
+                   rcv2_addr.addr_len),
+              SyscallSucceedsWithValue(0));
+
+  // Bind the non-receiving socket to the unicast ethernet address.
+  auto norecv_addr = rcv1_addr;
+  reinterpret_cast<sockaddr_in*>(&norecv_addr.addr)->sin_addr =
+      eth_if_sin_addr_;
+  ASSERT_THAT(bind(norcv->get(), reinterpret_cast<sockaddr*>(&norecv_addr.addr),
+                   norecv_addr.addr_len),
+              SyscallSucceedsWithValue(0));
 
   // Broadcast a test message.
-  sockaddr_in dst_addr = {};
-  dst_addr.sin_family = AF_INET;
-  dst_addr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
-  dst_addr.sin_port = rcv_addr.sin_port;
+  auto dst_addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&dst_addr.addr)->sin_port = port;
   constexpr char kTestMsg[] = "hello, world";
   EXPECT_THAT(
       sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
-             reinterpret_cast<struct sockaddr*>(&dst_addr), sizeof(dst_addr)),
+             reinterpret_cast<sockaddr*>(&dst_addr.addr), dst_addr.addr_len),
       SyscallSucceedsWithValue(sizeof(kTestMsg)));
 
   // Verify that the receiving sockets received the test message.
   char buf[sizeof(kTestMsg)] = {};
-  EXPECT_THAT(read(rcvr1->get(), buf, sizeof(buf)),
+  EXPECT_THAT(recv(rcvr1->get(), buf, sizeof(buf), 0),
               SyscallSucceedsWithValue(sizeof(kTestMsg)));
   EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
   memset(buf, 0, sizeof(buf));
-  EXPECT_THAT(read(rcvr2->get(), buf, sizeof(buf)),
+  EXPECT_THAT(recv(rcvr2->get(), buf, sizeof(buf), 0),
               SyscallSucceedsWithValue(sizeof(kTestMsg)));
   EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
 
@@ -190,10 +293,12 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
               SyscallFailsWithErrno(EAGAIN));
 }
 
-// Verifies that a UDP broadcast sent via the loopback interface is not received
-// by the sender.
+// Verifies that a UDP broadcast can be sent and then received back on the same
+// socket that is bound to the broadcast address (255.255.255.255).
+// FIXME(b/141938460): This can be combined with the next test
+//                     (UDPBroadcastSendRecvOnSocketBoundToAny).
 TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
-       UDPBroadcastViaLoopbackFails) {
+       UDPBroadcastSendRecvOnSocketBoundToBroadcast) {
   auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
 
   // Enable SO_BROADCAST.
@@ -201,33 +306,73 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
                          sizeof(kSockOptOn)),
               SyscallSucceedsWithValue(0));
 
-  // Bind the sender to the loopback interface.
-  sockaddr_in src = {};
-  socklen_t src_sz = sizeof(src);
-  src.sin_family = AF_INET;
-  src.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
-  ASSERT_THAT(
-      bind(sender->get(), reinterpret_cast<struct sockaddr*>(&src), src_sz),
-      SyscallSucceedsWithValue(0));
+  // Bind the sender to the broadcast address.
+  auto src_addr = V4Broadcast();
+  ASSERT_THAT(bind(sender->get(), reinterpret_cast<sockaddr*>(&src_addr.addr),
+                   src_addr.addr_len),
+              SyscallSucceedsWithValue(0));
+  socklen_t src_sz = src_addr.addr_len;
   ASSERT_THAT(getsockname(sender->get(),
-                          reinterpret_cast<struct sockaddr*>(&src), &src_sz),
+                          reinterpret_cast<sockaddr*>(&src_addr.addr), &src_sz),
               SyscallSucceedsWithValue(0));
-  ASSERT_EQ(src.sin_addr.s_addr, htonl(INADDR_LOOPBACK));
+  EXPECT_EQ(src_sz, src_addr.addr_len);
 
   // Send the message.
-  sockaddr_in dst = {};
-  dst.sin_family = AF_INET;
-  dst.sin_addr.s_addr = htonl(INADDR_BROADCAST);
-  dst.sin_port = src.sin_port;
+  auto dst_addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&dst_addr.addr)->sin_port =
+      reinterpret_cast<sockaddr_in*>(&src_addr.addr)->sin_port;
   constexpr char kTestMsg[] = "hello, world";
-  EXPECT_THAT(sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
-                     reinterpret_cast<struct sockaddr*>(&dst), sizeof(dst)),
+  EXPECT_THAT(
+      sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
+             reinterpret_cast<sockaddr*>(&dst_addr.addr), dst_addr.addr_len),
+      SyscallSucceedsWithValue(sizeof(kTestMsg)));
+
+  // Verify that the message was received.
+  char buf[sizeof(kTestMsg)] = {};
+  EXPECT_THAT(RetryEINTR(recv)(sender->get(), buf, sizeof(buf), 0),
               SyscallSucceedsWithValue(sizeof(kTestMsg)));
+  EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
+}
+
+// Verifies that a UDP broadcast can be sent and then received back on the same
+// socket that is bound to the ANY address (0.0.0.0).
+// FIXME(b/141938460): This can be combined with the previous test
+//                     (UDPBroadcastSendRecvOnSocketBoundToBroadcast).
+TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
+       UDPBroadcastSendRecvOnSocketBoundToAny) {
+  auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
 
-  // Verify that the message was not received by the sender (loopback).
+  // Enable SO_BROADCAST.
+  ASSERT_THAT(setsockopt(sender->get(), SOL_SOCKET, SO_BROADCAST, &kSockOptOn,
+                         sizeof(kSockOptOn)),
+              SyscallSucceedsWithValue(0));
+
+  // Bind the sender to the ANY address.
+  auto src_addr = V4Any();
+  ASSERT_THAT(bind(sender->get(), reinterpret_cast<sockaddr*>(&src_addr.addr),
+                   src_addr.addr_len),
+              SyscallSucceedsWithValue(0));
+  socklen_t src_sz = src_addr.addr_len;
+  ASSERT_THAT(getsockname(sender->get(),
+                          reinterpret_cast<sockaddr*>(&src_addr.addr), &src_sz),
+              SyscallSucceedsWithValue(0));
+  EXPECT_EQ(src_sz, src_addr.addr_len);
+
+  // Send the message.
+  auto dst_addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&dst_addr.addr)->sin_port =
+      reinterpret_cast<sockaddr_in*>(&src_addr.addr)->sin_port;
+  constexpr char kTestMsg[] = "hello, world";
+  EXPECT_THAT(
+      sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
+             reinterpret_cast<sockaddr*>(&dst_addr.addr), dst_addr.addr_len),
+      SyscallSucceedsWithValue(sizeof(kTestMsg)));
+
+  // Verify that the message was received.
   char buf[sizeof(kTestMsg)] = {};
-  EXPECT_THAT(RetryEINTR(recv)(sender->get(), buf, sizeof(buf), MSG_DONTWAIT),
-              SyscallFailsWithErrno(EAGAIN));
+  EXPECT_THAT(RetryEINTR(recv)(sender->get(), buf, sizeof(buf), 0),
+              SyscallSucceedsWithValue(sizeof(kTestMsg)));
+  EXPECT_EQ(0, memcmp(buf, kTestMsg, sizeof(kTestMsg)));
 }
 
 // Verifies that a UDP broadcast fails to send on a socket with SO_BROADCAST
@@ -237,15 +382,12 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendBroadcast) {
 
   // Broadcast a test message without having enabled SO_BROADCAST on the sending
   // socket.
-  sockaddr_in addr = {};
-  socklen_t addr_sz = sizeof(addr);
-  addr.sin_family = AF_INET;
-  addr.sin_port = htons(12345);
-  addr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
+  auto addr = V4Broadcast();
+  reinterpret_cast<sockaddr_in*>(&addr.addr)->sin_port = htons(12345);
   constexpr char kTestMsg[] = "hello, world";
 
   EXPECT_THAT(sendto(sender->get(), kTestMsg, sizeof(kTestMsg), 0,
-                     reinterpret_cast<struct sockaddr*>(&addr), addr_sz),
+                     reinterpret_cast<sockaddr*>(&addr.addr), addr.addr_len),
               SyscallFailsWithErrno(EACCES));
 }
 
@@ -274,21 +416,10 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendUnicastOnUnbound) {
                      reinterpret_cast<struct sockaddr*>(&addr), addr_sz),
               SyscallSucceedsWithValue(sizeof(kTestMsg)));
   char buf[sizeof(kTestMsg)] = {};
-  ASSERT_THAT(read(rcvr->get(), buf, sizeof(buf)),
+  ASSERT_THAT(recv(rcvr->get(), buf, sizeof(buf), 0),
               SyscallSucceedsWithValue(sizeof(kTestMsg)));
 }
 
-constexpr char kMulticastAddress[] = "224.0.2.1";
-
-TestAddress V4Multicast() {
-  TestAddress t("V4Multicast");
-  t.addr.ss_family = AF_INET;
-  t.addr_len = sizeof(sockaddr_in);
-  reinterpret_cast<sockaddr_in*>(&t.addr)->sin_addr.s_addr =
-      inet_addr(kMulticastAddress);
-  return t;
-}
-
 // Check that multicast packets won't be delivered to the sending socket with no
 // set interface or group membership.
 TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
@@ -609,8 +740,9 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
 }
 
 // Check that two sockets can join the same multicast group at the same time,
-// and both will receive data on it.
-TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendMulticastToTwo) {
+// and both will receive data on it when bound to the ANY address.
+TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
+       TestSendMulticastToTwoBoundToAny) {
   auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
   std::unique_ptr<FileDescriptor> receivers[2] = {
       ASSERT_NO_ERRNO_AND_VALUE(NewSocket()),
@@ -624,8 +756,72 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendMulticastToTwo) {
     ASSERT_THAT(setsockopt(receiver->get(), SOL_SOCKET, SO_REUSEPORT,
                            &kSockOptOn, sizeof(kSockOptOn)),
                 SyscallSucceeds());
-    // Bind the receiver to the v4 any address to ensure that we can receive the
-    // multicast packet.
+    // Bind to ANY to receive multicast packets.
+    ASSERT_THAT(
+        bind(receiver->get(), reinterpret_cast<sockaddr*>(&receiver_addr.addr),
+             receiver_addr.addr_len),
+        SyscallSucceeds());
+    socklen_t receiver_addr_len = receiver_addr.addr_len;
+    ASSERT_THAT(getsockname(receiver->get(),
+                            reinterpret_cast<sockaddr*>(&receiver_addr.addr),
+                            &receiver_addr_len),
+                SyscallSucceeds());
+    EXPECT_EQ(receiver_addr_len, receiver_addr.addr_len);
+    EXPECT_EQ(
+        htonl(INADDR_ANY),
+        reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_addr.s_addr);
+    // On the first iteration, save the port we are bound to. On the second
+    // iteration, verify the port is the same as the one from the first
+    // iteration. In other words, both sockets listen on the same port.
+    if (bound_port == 0) {
+      bound_port =
+          reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port;
+    } else {
+      EXPECT_EQ(bound_port,
+                reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port);
+    }
+
+    // Register to receive multicast packets.
+    ASSERT_THAT(setsockopt(receiver->get(), IPPROTO_IP, IP_ADD_MEMBERSHIP,
+                           &group, sizeof(group)),
+                SyscallSucceeds());
+  }
+
+  // Send a multicast packet to the group and verify both receivers get it.
+  auto send_addr = V4Multicast();
+  reinterpret_cast<sockaddr_in*>(&send_addr.addr)->sin_port = bound_port;
+  char send_buf[200];
+  RandomizeBuffer(send_buf, sizeof(send_buf));
+  ASSERT_THAT(RetryEINTR(sendto)(sender->get(), send_buf, sizeof(send_buf), 0,
+                                 reinterpret_cast<sockaddr*>(&send_addr.addr),
+                                 send_addr.addr_len),
+              SyscallSucceedsWithValue(sizeof(send_buf)));
+  for (auto& receiver : receivers) {
+    char recv_buf[sizeof(send_buf)] = {};
+    ASSERT_THAT(
+        RetryEINTR(recv)(receiver->get(), recv_buf, sizeof(recv_buf), 0),
+        SyscallSucceedsWithValue(sizeof(recv_buf)));
+    EXPECT_EQ(0, memcmp(send_buf, recv_buf, sizeof(send_buf)));
+  }
+}
+
+// Check that two sockets can join the same multicast group at the same time,
+// and both will receive data on it when bound to the multicast address.
+TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
+       TestSendMulticastToTwoBoundToMulticastAddress) {
+  auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+  std::unique_ptr<FileDescriptor> receivers[2] = {
+      ASSERT_NO_ERRNO_AND_VALUE(NewSocket()),
+      ASSERT_NO_ERRNO_AND_VALUE(NewSocket())};
+
+  ip_mreq group = {};
+  group.imr_multiaddr.s_addr = inet_addr(kMulticastAddress);
+  auto receiver_addr = V4Multicast();
+  int bound_port = 0;
+  for (auto& receiver : receivers) {
+    ASSERT_THAT(setsockopt(receiver->get(), SOL_SOCKET, SO_REUSEPORT,
+                           &kSockOptOn, sizeof(kSockOptOn)),
+                SyscallSucceeds());
     ASSERT_THAT(
         bind(receiver->get(), reinterpret_cast<sockaddr*>(&receiver_addr.addr),
              receiver_addr.addr_len),
@@ -636,6 +832,9 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendMulticastToTwo) {
                             &receiver_addr_len),
                 SyscallSucceeds());
     EXPECT_EQ(receiver_addr_len, receiver_addr.addr_len);
+    EXPECT_EQ(
+        inet_addr(kMulticastAddress),
+        reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_addr.s_addr);
     // On the first iteration, save the port we are bound to. On the second
     // iteration, verify the port is the same as the one from the first
     // iteration. In other words, both sockets listen on the same port.
@@ -643,6 +842,83 @@ TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest, TestSendMulticastToTwo) {
       bound_port =
           reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port;
     } else {
+      EXPECT_EQ(
+          inet_addr(kMulticastAddress),
+          reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_addr.s_addr);
+      EXPECT_EQ(bound_port,
+                reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port);
+    }
+
+    // Register to receive multicast packets.
+    ASSERT_THAT(setsockopt(receiver->get(), IPPROTO_IP, IP_ADD_MEMBERSHIP,
+                           &group, sizeof(group)),
+                SyscallSucceeds());
+  }
+
+  // Send a multicast packet to the group and verify both receivers get it.
+  auto send_addr = V4Multicast();
+  reinterpret_cast<sockaddr_in*>(&send_addr.addr)->sin_port = bound_port;
+  char send_buf[200];
+  RandomizeBuffer(send_buf, sizeof(send_buf));
+  ASSERT_THAT(RetryEINTR(sendto)(sender->get(), send_buf, sizeof(send_buf), 0,
+                                 reinterpret_cast<sockaddr*>(&send_addr.addr),
+                                 send_addr.addr_len),
+              SyscallSucceedsWithValue(sizeof(send_buf)));
+  for (auto& receiver : receivers) {
+    char recv_buf[sizeof(send_buf)] = {};
+    ASSERT_THAT(
+        RetryEINTR(recv)(receiver->get(), recv_buf, sizeof(recv_buf), 0),
+        SyscallSucceedsWithValue(sizeof(recv_buf)));
+    EXPECT_EQ(0, memcmp(send_buf, recv_buf, sizeof(send_buf)));
+  }
+}
+
+// Check that two sockets can join the same multicast group at the same time,
+// and with one bound to the wildcard address and the other bound to the
+// multicast address, both will receive data.
+TEST_P(IPv4UDPUnboundExternalNetworkingSocketTest,
+       TestSendMulticastToTwoBoundToAnyAndMulticastAddress) {
+  auto sender = ASSERT_NO_ERRNO_AND_VALUE(NewSocket());
+  std::unique_ptr<FileDescriptor> receivers[2] = {
+      ASSERT_NO_ERRNO_AND_VALUE(NewSocket()),
+      ASSERT_NO_ERRNO_AND_VALUE(NewSocket())};
+
+  ip_mreq group = {};
+  group.imr_multiaddr.s_addr = inet_addr(kMulticastAddress);
+  // The first receiver binds to the wildcard address.
+  auto receiver_addr = V4Any();
+  int bound_port = 0;
+  for (auto& receiver : receivers) {
+    ASSERT_THAT(setsockopt(receiver->get(), SOL_SOCKET, SO_REUSEPORT,
+                           &kSockOptOn, sizeof(kSockOptOn)),
+                SyscallSucceeds());
+    ASSERT_THAT(
+        bind(receiver->get(), reinterpret_cast<sockaddr*>(&receiver_addr.addr),
+             receiver_addr.addr_len),
+        SyscallSucceeds());
+    socklen_t receiver_addr_len = receiver_addr.addr_len;
+    ASSERT_THAT(getsockname(receiver->get(),
+                            reinterpret_cast<sockaddr*>(&receiver_addr.addr),
+                            &receiver_addr_len),
+                SyscallSucceeds());
+    EXPECT_EQ(receiver_addr_len, receiver_addr.addr_len);
+    // On the first iteration, save the port we are bound to and change the
+    // receiver address from V4Any to V4Multicast so the second receiver binds
+    // to that. On the second iteration, verify the port is the same as the one
+    // from the first iteration but the address is different.
+    if (bound_port == 0) {
+      EXPECT_EQ(
+          htonl(INADDR_ANY),
+          reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_addr.s_addr);
+      bound_port =
+          reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port;
+      receiver_addr = V4Multicast();
+      reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port =
+          bound_port;
+    } else {
+      EXPECT_EQ(
+          inet_addr(kMulticastAddress),
+          reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_addr.s_addr);
       EXPECT_EQ(bound_port,
                 reinterpret_cast<sockaddr_in*>(&receiver_addr.addr)->sin_port);
     }