Default multicast interface address to "0.0.0.0" if absent

It aligns with what a lot of other apps are doing, e.g. tsplay.

Also added notes in docker to adjust network setting if wants to use
multicast in docker container.

Change-Id: I666a8979cc041c904f7ff8e3e022f800c0830d5d

docs/source/docker_instructions.md

@@ -32,6 +32,14 @@ This runs the container and maps `host_media_path` to `media` in the container:
 $ docker run -v /host_media_path/:/media -it --rm google/shaka-packager
 ```
 
+Note that the networking in the container is containerized by default, so if
+you want to access UDP multicast in the host network, you will need to configure
+the network explicitly. You may do this with `--net=host` option, i.e.
+
+```shell
+$ docker run -v /host_media_path/:/media -it --net=host --rm google/shaka-packager
+```
+
 Then in the container, run the packager command, e.g.:
 
 ```shell

docs/source/options/udp_file_options.rst

@@ -11,7 +11,7 @@ options:
 :interface=<addr>, source=<addr>:
 
     Multicast group interface address. Only the packets sent to this address is
-    received.
+    received. Default to "0.0.0.0" if not specified.
 
 :timeout=<microseconds>:
 

packager/file/udp_file.cc

@@ -207,12 +207,6 @@ bool UdpFile::Open() {
     struct ip_mreq multicast_group;
     multicast_group.imr_multiaddr = local_in_addr;
 
-    if (options->interface_address().empty()) {
-      LOG(ERROR) << "Interface address is required for multicast, which can be "
-                    "specified in udp url, e.g. "
-                    "udp://ip:port?interface=interface_ip.";
-      return false;
-    }
     if (inet_pton(AF_INET, options->interface_address().c_str(),
                   &multicast_group.imr_interface) != 1) {
       LOG(ERROR) << "Malformed IPv4 interface address "

packager/file/udp_options.h

@@ -31,12 +31,12 @@ class UdpOptions {
   UdpOptions() = default;
 
   /// IP Address.
-  std::string address_;
+  std::string address_ = "0.0.0.0";
   uint16_t port_ = 0;
   /// Allow or disallow reusing UDP sockets.
   bool reuse_ = false;
   // Address of the interface over which to receive UDP multicast streams.
-  std::string interface_address_;
+  std::string interface_address_ = "0.0.0.0";
   /// Timeout in microseconds. 0 to indicate unlimited timeout.
   unsigned timeout_us_ = 0;
 };

packager/file/udp_options_unittest.cc

@@ -25,7 +25,7 @@ TEST_F(UdpOptionsTest, AddressAndPort) {
   EXPECT_EQ(88u, options->port());
   // The below fields are not set.
   EXPECT_FALSE(options->reuse());
-  EXPECT_EQ("", options->interface_address());
+  EXPECT_EQ("0.0.0.0", options->interface_address());
   EXPECT_EQ(0u, options->timeout_us());
 }
 
@@ -62,7 +62,7 @@ TEST_F(UdpOptionsTest, Reuse) {
   EXPECT_EQ("224.1.2.30", options->address());
   EXPECT_EQ(88u, options->port());
   EXPECT_TRUE(options->reuse());
-  EXPECT_EQ("", options->interface_address());
+  EXPECT_EQ("0.0.0.0", options->interface_address());
   EXPECT_EQ(0u, options->timeout_us());
 }