After you run the adb forward tcp:hostport tcp:emuport command, your android running in emulator can listen on emuport and communicate with external applications running on host machine. From the external application's point of view, it's connecting to hostport rather than emuport. The adb is responsible for forwarding messages between the two.
But adb has a limitation of only accepting connections from the same machine, you can't connect from a machine that adb isn't running on. This limitation makes it a little bit inconvenient to do network programming. Especially on windows, if you want to use wireshark to capture the packages between emulator and external app. By default, wireshark isn't able to capture loopback packages on windows. So, we change the behavior of adb to accept remote connections.
(Another way to overcome this is instead of capturing network package on host side, we can use tcpdump on emulator side. For example, use "/data/tcpdump -s 1500 -vv -w /data/net.pcap udp port 5060" to capture all packages for udp port 5060 and save them in /data/net.pcap file. The -s parameter specifies the size of each packet whose default value is 68, too small to get some useful information truncated. The /data/net.pcap file can also be examined with wireshark.)
How to modify adb behavior
If we run netstat command on host side, we can find out that adb server process binds to the loopback address. So it can't be accessed via external address.
The adb register a port forwarding information through install_listener function. And install_listener calls local_name_to_fd function to bind to a local address. Inside local_name_to_fd, it starts a tcp server through socket_loopback_server. To change this, we can use socket_inaddr_any_server which binds to all ipaddresses. In this way, the adb server can forward a request from remote machine to emulator.
How to compile
In order to get the modified adb, we need to compile it. To do so, we first need to download all of anroid's source code. Then, at the root of andoird source directory, run ./build/envsetup.sh. Finally, run make adb.
howto build SDK
Modified adb v1.0.31 can be downloaded at: https://drive.google.com/#folders/0B9fylkYUFfYTTGJKN0tURzJFelE