Question

How do I get all servers IP which are listening on specific port (Ex:9090) in LAN
I have already read some basics information about multicast,anycast ,unicast and broadcast and it seems like broadcast is the one meant for my application.
I am thinking of two ideas ..

Use TCP protocol to Connect to all IP Addresses (192.168.1.1-254) in parallel on port 9090 and set a little time period to the connection timeout. just to check if there’s any responce. (but it doesn’t seem like a good idea)

Use UDP protocol and broadcast a message like “hello” and check for response then get all IP Addresses which respond.

So which idea should I pick, are there better ideas to do it?
I would also like to know if my broadcast-ed message is received to the server and how to get the IP address of it.

Answer 1

I’ve done it with TCP to search all server that is listening on a specific port.
Its not an idle solution, but it worked for me.

I’ve created a userControl with a public function Initialize to pass the port that i want to scan for servers.

public ServerDiscovery()
    {
        InitializeComponent();
    //    statusLabel.Image = imageState.Images[2];
        timer.Elapsed += timer_tick;
    }
    int port = 0;
    bool busy = false;  // to tell that the function is busy with scanning.
    public void Initialize(int port)
    {
        this.port = port;
    }

    System.Timers.Timer timer = new System.Timers.Timer(5000);
    List<SocketAsyncEventArgs> list = new List<SocketAsyncEventArgs>();
    // this list to hold all sockets that created to connect to IP .. to DISPOSE it later
    HashSet<string> usedIP = new HashSet<string>();
    //usedIP will be explained later. 

    public IPEndPoint getAddress()
    {   //this function to get the IPEndPoint of the selected server from listview.
        if (listServer.SelectedItems.Count > 0)
        {
            if (listServer.SelectedItems[0].ImageIndex == 0)
            {
                ListViewItem item = listServer.SelectedItems[0];
                IPEndPoint ep = new IPEndPoint(IPAddress.Parse(item.SubItems[1].Text), port);
                return ep;
            }
        }
        return null;
    }

    public void Refresh()   //to scan for servers
    {
        if (!busy)
        {
            usedIP.Clear();
            listServer.Items.Clear();
    //       statusLabel.Text = "Scanning for servers.";
    //        statusLabel.Image = Image.FromFile("loading.gif");
    //        btnRefresh.Enabled = false;
            busy = true;
            timer.Start();
            IPAddress[] IpA = Dns.GetHostByName(Dns.GetHostName()).AddressList;
            for (int j = 0; j < IpA.Length ; j++)
            {
                if (IpA[j].AddressFamily == AddressFamily.InterNetwork)  // to make sure it's an IPV4
                {
                    string scanIP = IpA[j].ToString().Substring(0, IpA[j].ToString().LastIndexOf(".")) + ".";
                    if (!usedIP.Contains(scanIP))
             //usedIP is a hashset that holds the first 3 parts on an ip (ex:"192.168.1." from "192.168.1.30") i used this to avoid scanning the same ip addresses more than once .. like if i had a wireless network ip ("192.168.1.5") and an Ethernet Network ip ("192.168.1.5"). so with that hashset it will scan once.
                    {
                        usedIP.Add(scanIP);
                        Parallel.For(1, 255, i =>
                        {
                            Scan(scanIP + i);
                        });
                    }
                }
            }
        }
    }

    private void Scan(string ipAdd)
    {
        Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        SocketAsyncEventArgs e = new SocketAsyncEventArgs();
        e.RemoteEndPoint = new IPEndPoint(IPAddress.Parse(ipAdd), port);
        e.UserToken = s;
        e.Completed += new EventHandler<SocketAsyncEventArgs>(e_Completed);
        list.Add(e);   // add the created socket to a list to dispose when time is up.
        s.ConnectAsync(e);
    }

    private void e_Completed(object sender, SocketAsyncEventArgs e)
    {
        if (e.ConnectSocket != null) //if there's a responce from the server [e.ConnectSocket] will not be equal null.
        {
            StreamReader sr = new StreamReader(new NetworkStream(e.ConnectSocket));
            ListViewItem item = new ListViewItem();
            string[] cmd = sr.ReadLine().Split('<');  in my server constructor this line will receive a string like "PC_NAME<Available"  ..
            item.Text = cmd[0];
            item.SubItems.Add(((IPEndPoint)e.RemoteEndPoint).Address.ToString());
            item.SubItems.Add(cmd[1]);
            if (cmd[1] == "Busy")
                item.ImageIndex = 1;
            else
                item.ImageIndex = 0;
            AddServer(item);
            list.Remove(e);  //active server should be remove from the list that holds the sockets and disposed.. because there's no need to keep connection after showing that this server is active. 
            ((Socket)e.UserToken).Dispose();
        }
    }

    delegate void AddItem(ListViewItem item);
    private void AddServer(ListViewItem item)
    {
        if (InvokeRequired)
        {
            Invoke(new AddItem(AddServer), item);  //just to add an item from a background thread.
            return;
        }
        listServer.Items.Add(item);
    }

    private void timer_tick(object sender, EventArgs e)
    {
        busy = false;     //when time's up .. set busy to false so we can scan again
        timer.Stop();
        foreach (var s in list) //dispose all sockets that's trying to connect and waiting for a response
        {
            try
            {
                ((Socket)s.UserToken).Dispose();
            }
            catch { }
        }
        //this.Invoke((MethodInvoker)delegate        // for design
       // {
       //     btnRefresh.Enabled = true;
       //     btnRefresh.BorderStyle = Border3DStyle.Raised;
       //     statusLabel.Text = "Ready.";
       //     statusLabel.Image = imageState.Images[2];
       // });
    }

    private void btnRefresh_Click(object sender, EventArgs e)
    {
       // btnRefresh.BorderStyle = Border3DStyle.Sunken;
        Refresh();
    }

   // private void listServer_KeyUp(object sender, KeyEventArgs e)
   // {
    //    if (e.KeyCode == Keys.F5)
    //    {
     //       btnRefresh_Click(sender, (EventArgs)e);
     //   }
  //  }


}

Again this’s not an ideal solution that works on any case but it worked fine with me.