A computer that is conected to a LAN or Internet usually offers some services. If we talk about a normal network that uses TCP/IP protocol, each of the services is going to use a port. For a PC to use a service, it needs to specify the number of the port.
The first step to know if a remote machine is running a service is testing connectivity. There a lot of programs you can use for this but today I am going to use PortQryUI tool. This utility can be downloaded from Microsoft:
I have selected the application because is very simple to use. If you want to use something more professional try NMAP; you can use Google to find it.
Once you download the utility it will present a license agreement, just click Yes:
Select the folder you want the program to be extracted:
Press Unzip. Once you do that you are going to get a folder with the program:
Now you can execute the program. This application will query a port and create a report. A port can have 3 different status:
- Listening. The tool has received an answer from the port.
- Not Listening. The utility receives a ICMP packet, which is no more than an error message indicating that there is no process to create an answer.
- Filtered. No response, so PortQryUI really doesn’t know if the machine is using that port.
Now that we have more information about the command, we are going to test if a remote computer is running some common services. For all the tests we are going to use a remote machine called server (admittedly, a pretty uncreative name.)
Windows has the command NSLOOKUP to test if a DNS is working but we are going to do this in other way. DNS servers use TCP port 53 and UDP port 53 so we only need to test this using the tool.
See how we have select BOTH protocols, instead TCP or UDP alone.
A normal http server resides on TCP port 80.
HTTPS is a version of HTTP with security features. This service uses the port 443. Do you want to know if www.google.com has this port enabled?
FTP uses the port 21. In this case the utility give us more information.
We can use this utility as our first test. It just checks if a response is received. I think that is a very easy way to start looking for problems. What do you think? Do you use any other network tool?