Service Cannot resolve DNS

[JimS]

Hi guys,

I'm writing a simple application which pings a range of IP addresses from a database, and simply records the result back into a table.

I initially wrote it as a console app and everything worked fine. Now that I've converted the code to run as a service it throws a socket error:

System.Net.NetworkInformation.PingException: An exception occurred during a Ping request. ---> System.Net.Sockets.SocketException: No such host is known
   at System.Net.Dns.GetAddrInfo(String name)
   at System.Net.Dns.InternalGetHostByName(String hostName, Boolean includeIPv6)
   at System.Net.Dns.GetHostAddresses(String hostNameOrAddress)
   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)
   --- End of inner exception stack trace ---
   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)

In the above example the input to the ping function was an IP address.

I'm confused, because it looks like it's failing a DNS lookup, but I'm passing it an IP address, not a host name. Is this some security setting that's different because the app runs as a service? Its running as my network account which has admin rights.

Here is the sub which performs the ping...

Private Sub ping(ByVal Address As String)
        Dim _ping As New System.Net.NetworkInformation.Ping()

        Dim _options As New PingOptions()
        Dim _reply As PingReply

        myCore.log("Trying: " & Address)

        Dim time As Long = 0
        Dim hits As Integer = 0
        AvgTime = -1
        PacketLoss = 100


        Dim data As String = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" '32 Bytes
        Dim buffer As Byte()
        buffer = System.Text.Encoding.ASCII.GetBytes(data)

        Try

            _reply = _ping.Send(Address, 500, buffer, _options)

            myCore.log(_reply.Status)
            myCore.log(_reply.Address.ToString)

            time += _reply.RoundtripTime
            hits += 1


            AvgTime = time / hits
            PacketLoss = ((5 - hits) / hits) * 100

        Catch ex As Exception
            myCore.log("Error: " & Environment.NewLine & ex.ToString)

        End Try
    End Sub

Any help would be appreciated as this is starting to get frustrating!

Thanks,

Jim.

 

[JohnH]

I'm confused, because it looks like it's failing a DNS lookup, but I'm passing it an IP address, not a host name.

When you give it an IP string it uses DNS services to lookup a IP address object. You can get around this by using the Parse method of IPAddress class. Net.IPAddress.Parse("ip string") returns a IP address object without using DNS services.
But I don't think this is the problem..

Is this some security setting that's different because the app runs as a service? Its running as my network account which has admin rights.

I have tested Ping from a Windows Service (including DNS lookup) and didn't get any problems, did it on XP SP2 and tested all three service accounts plus a user account, no problems with any of them. I think you may have one of two problems, either a firewall blocking, or stricter security in newer OS (like Vista).

 

[JimS]

Thanks for the reply John. I'm using XP sp2, and the Windows firewall is disabled as I am behind a corporate firewall.

I can ping the host from a command line, and also from the same code if it runs as a console application....

I will try and manually parse the IP address as you mentioned and see if this makes any difference. I'm stumped!

 

[JimS]

This gets more interesting...

Using Net.IPAddress.Parse I get the following:

System.FormatException: An invalid IP address was specified.
   at System.Net.IPAddress.InternalParse(String ipString, Boolean tryParse)
   at System.Net.IPAddress.Parse(String ipString)

But the string being parsed is definately "172.24.6.100", one of our machines...

Really confused now.

Jim.

 

[JimS]

Ok for some reason it seems to be a probem with the string, as when I hard code the string as "172.24.6.100" it works.

 

[JimS]

Turned out to just be whitespace at the end of the IP address as read from the DB.

I feel dumb.