Qt5 Bind TCP Socket on Multihomed Network



  • I am having troubling using the new bind feature of the QTcpSocket class in Qt5. Any help figuring this out would be appreciated.

    I have a multihomed server that contains two NICs each with a separate IP address. I have setup routing on the servers so that sending from the source address is sent out of the appropriate NIC regardless of the target remote address. That is, sending from x.x.x.0 goes out over eth0 and x.x.x.1 goes out over eth1 regardless of who the data is being sent to. These NICs are connected via ethernet to long range Wifi links that are then connected to a switch. These wifi links act as a transparent bridge and can rather be seen as two ethernet cables (but they are limited in bandwidth). The switch is then connected to a computer. The goal is to transfer data between the server and the computer, and to use the two wifi links in parallel to increase bandwidth. Although the server is physically a server, the software has the computer running as the software server (as others connect to it). That is, the physical server (software client) opens TCP sockets and attempts to connect to the listening computer (software server).

    I use the bind feature of Qt5 to bind one TCP socket to the eth0 IP address and bind another TCP socket to the eth1 IP address. I have verified this works with other programs like PING or file transfer via SCP. See http://qt-project.org/doc/qt-5.0/qtnetwork/qabstractsocket.html#bind

    When I call bind the call succeeds and subsequent requests for the local IP address returns the correct value. E.g. socket->bind(ip) returns true and then socket->localAddress() equals ip. However, when I call connectToHost the localAddress is lost when it starts the connection attempt and after connecting it has a different localAddress that is not the one I wanted it to have.

    Can anyone shed light into what is going on? I am trying to avoid rewriting the software to reverse the roles of software client / server as these programs are quite big. Thanks for the help.


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