Host Name is sometimes empty - asp.net

Question

In one of my applications (ASP.NET/VB.NET), I need to read the Client Machine Name. Based on the Client Machine we trigger a Point of Sale payment device to accept the payment. On each of these systems we have a stand alone software installed which communicates to the bank using HTTP requests. I am using the following .NET code to read the Client Computer Name.
Dim name As String = String.Empty
Dim hostEntry = Dns.GetHostEntry(HttpContext.Current.Request.UserHostAddress)
If hostEntry.HostName.Contains(".") Then
name = hostEntry.HostName.Substring(0, hostEntry.HostName.IndexOf("."))
Else
name = hostEntry.HostName.Trim
End If
In the development environment, all our systems are in a domain ("xyz.com") and we don't have any issues. In the customer location they don't have a domain name setup. My above logic works well in some of the systems in the client environment and is able to make payments but in most of the systems our logic fails and is not able to read the host name. Any help will be appreciated.

Answer 1

Your question doesn't have the specifics required to answer your question. There are many questions that need to be answered about both environments to give a correct answer. Since I can't ask questions, I will make some assumptions which might apply to future readers of this post and be able to help them out.
I would ask a question but my profile was forked for some unknown reason and I don't have the required reputation to ask a question. That being said I will run through the list of issues I can identify off the bat and suggest solutions for the issue and hopefully one will lead you to a solution.
So...
1) You state you need to read the client machine name. However, if your application isn't running on an internal LAN (aka an intranet) you can't read client machine names period. So this could be your first problem.
2) Combining point 1 and given that you are reading the IP Address from UserHostAddress of the client to look up a DNS host name and when the host look up succeeds you are taking the first part of the name up until the first "." it should be safe to assume that this an intranet application running on a LAN in both your development environment and at the client environment. With that assumption and given the statement that all machines are given an domain of xyz.com it can be assumed that DNS in your development environment is being dynamically updated from presumably through Active Directory (AD). In such case, whenever a client machine on development network requests an IP address, presumably through AD, the DHCP server integrated with AD issues the new IP Address. When it does and the DHCP offer is acknowledged and accepted by the client AD updates DNS (which on a windows network is also AD integrated) by adding a host entry with the computer name of the client machine pointing to the IP Address. Additionally a DNS pointer, depending on configuration, can be added to AD's DNS which allows an IP Address lookup to resolve to the record (which in this case would be the Client's machine name). So with your development environment (presumably running on Windows Active Directory Domain) everything works. Addi tonally, by default the top level domain name (XYZ.COM) gets appended to the clients computer name in initial DNS requests from the client.
3) Your client is not running a domain which leaves further questions. Are they running windows? If they are running windows is it as a non-Ad environment, for example a work group. First assumption would be they are not AD integrated or otherwise you most likely wouldn't be having this problem although I can think of a few rare case scenarios where they might. However, odds are the relevant questions are What DNS server are they running and what DHCP sever are they running? Your application is trying to use a client IP Address on their network and the host name lookup based on their IP is failing so it tells me in their environment for one reason or another you can't get a host name from the IP Address of the client. Mind you if they could be on AD and configured entirely correctly their DNS server is just overwhelmed and not responding within 2 seconds causing the name lookup failure but that is the rate case. With more information I could help more.
3) Assuming in 2 that they are not on AD, do you have the ability manually code host names on the computer your application is running? For example, lets say yourapp.exe runs client-server-01 and clients connect to it. Then on client-server-01 you could add static DNS entries in the host file for each PC on the client network that you expect to connect. On the other hand if your application is running locally on the client PCs you could pass the machine name as a header in the web request and then read it from the Request.Headers variable on the server.
4) Again, making another assumption the clients are web based and your application in the client environment is being hosted on the server... Is the server on a DMZ outside the client environment? If so the client environment may likely be configured, per best practices, that the server host your web app is in a DMZ and DNS requests to the box are forwarded to the client's ISP and not back into their network that has the DNS server capable of resolving an internal IP to a client machine name. If this is the case you need to send the client machine name as a variable from your client or code local IPs to host names in the servers host file (assuming the internal network isn't behind NAT and exposes the real client machine's IP) or request that the DMZ'd server can access the internal DNS and configure the access accordingly.
....
The list really goes on and on but I think I highlighted the problems for 99% of the situations and provided answers to their various solutions.

Answer 2

You can try to take it from X-Forwarded-For header
The X-Forwarded-For (XFF) HTTP header field is a common method for
identifying the originating IP address of a client connecting to a web
server through an HTTP proxy or load balancer.
This is what X-Forwarded-For should return:
X-Forwarded-For: client, proxy1, proxy2
Here some example code:
string ip = Request.ServerVariables["HTTP_X_FORWARDED_FOR"] ;
if (!string.IsNullOrEmpty(ip))
{
string[] ipRange = ip.Split(',');
ip = ipRange[0];
}
else
{
ip = Request.ServerVariables["REMOTE_ADDR"];
}

Answer 3

There was an issue with Firewall setup on the client machine.Due to that our .NET code was failing. After adding an exception to all the incoming requests from xyz.com. My code is working without any issues.
Thank you guys #Alexander Higgins, #halfer for the help.