Question

Given a range of IP addresses entered by a user (through various means), I want to identify which of these machines have software running that I can talk to.

Here’s the basic process:

1. Ping these addresses to find available machines

2. Connect to a known socket on the available machines

3. Send a message to the successfully established sockets

4. Compare the response to the expected response

Steps 2-4 are straight forward for me. What is the best way to implement the first step in .NET?

I’m looking at the System.Net.NetworkInformation.Ping class. Should I ping multiple addresses simultaneously to speed up the process? If I ping one address at a time with a long timeout it could take forever. But with a small timeout, I may miss some machines that are available.

Sometimes pings appear to be failing even when I know that the address points to an active machine. Do I need to ping twice in the event of the request getting discarded?

To top it all off, when I scan large collections of addresses with the network cable unplugged, Ping throws a NullReferenceException in FreeUnmanagedResources(). !?

Any pointers on the best approach to scanning a range of IPs like this?

Answer 1

Since not all machines respond to pings (depending on firewall settings) I suggest skipping step 1 if possible.

If you know the machines you will be connecting to respond to pings the ping class works well enough. It only sends 1 packet so ping more than once in case it gets dropped. Also, in my experience the ping class will often throw an exception instead of returning a PingReply object if the host is unreachable.

This is my suggested implementation:

public bool
   Ping (string host, int attempts, int timeout)
   {
      System.Net.NetworkInformation.Ping  ping = 
                                       new System.Net.NetworkInformation.Ping ();

      System.Net.NetworkInformation.PingReply  pingReply;

      for (int i = 0; i < attempts; i++)
      {
         try
         {
            pingReply = ping.Send (host, timeout); 

            // If there is a successful ping then return true.
            if (pingReply != null &&
                pingReply.Status == System.Net.NetworkInformation.IPStatus.Success)
               return true;
         }
         catch
         {
            // Do nothing and let it try again until the attempts are exausted.
            // Exceptions are thrown for normal ping failurs like address lookup
            // failed.  For this reason we are supressing errors.
         }
      }

      // Return false if we can't successfully ping the server after several attempts.
      return false;
   }