This check udp port article will talk about the User Datagram Protocol, what is used for and the tool to use it.
User Datagram Protocol (UDP) is like a send and forget protocol. To check if the UDP port is open or not, you should receive a response from the port.
The UDP Socket:
With User Datagram protocol (UDP), the computer can send messages in the form of datagrams to other hosts on an Internet Protocol (IP) network. UDP uses connectionless communication which has checksums and port numbers. Checksums are for data integrity while port-numbers are used to address different functions at the source and destination of the datagram. It uses Time-sensitive applications make use of UDP. A port acts like a channel where an application on a client’s computer reaches the server. A port scan sends the client requests to server port addresses to find the active port.
How to check UDP port is open or not?
Follow below steps to check if UDP port is open or closed:
- Open a packet sniffer.
- Send a User Datagram Protocol (UDP) packet.
- After sending the UDP packet, if you receive ‘ICMP port unreachable’ message, then the UDP port is closed.
- If not, then the UDP port is open or something is blocking the ICMP.
Listening UDP ports on Windows
To listen to a specific port, follow below steps:
- Open the command prompt on your windows.
- Type below command
Netstat –an | find “UDP” | more
This will display all the listening UDP ports on your windows machine. This is useful to check if the service is listening on the server specified.
Tools to check if the UDP port is open or closed
There are various tools in the market to check if UDP port is open/available or not. These tools determine the external IP address. It also detects the open ports on the connection. These tools determine the port forwarding setup and also checks if any firewall is blocking the server application. It scans the port and network.
To check UDP port status – open or close, we can use below two steps:
- Iperf: It is a free client-server utility tool. iperf validates udp connectivity and throughput.Iperf –s –p 33001 –u is used to listen UDP traffic on port 33001. Run this to check UPD port open at the client side.
- netcat: Netcat is a utility for testing networks.nc -l –u 33001 is used to check UDP port open at the server side. Specify UDP (-u) and (-l) means listen modenc –u ip_address 33001 is used to check UDP port open at the client side. Provide the IP address and port.
To check UDP port connection between two systems, below utilities are used as follows:
Connect to the server and once the connection is established type in the window and press ENTER. The server window will display text if the connection is working. These utilities can be used for Windows, Linux, and Mac OS X environment.
UDP Port Scanner and Checker
This tool identifies the available services running on the server. It used raw IP packets to check which ports and operating system are available and running. It also checks for the firewall in case if it blocks the port. Various tools are available in the market to scan the port and check which port is open and close. These tools run on the client and server machine simultaneously. They are available for cross platforms, including Windows, Mac, and Linux.
Check UDP port open in nmap
One of the most popular ways to check UDP port open or not is nmap.
#nmap -sU -p port target
This command is used to scan the udp port. Ports to be scanned need to be specified where –sU activates UDP port scan.
For e.g. #nmap –sU –p 1-1023 192.168.1.1
In this example, the port range is from 1 to 1023 at the node 192.168.1.1. Also, instead of scanning the range of port, we can specify a specific port number. This will scan the port and check if the port is open or closed.
UDP port scan attack
UDP port scan attack can occur when some attacker sends packets on your machine. This varies on the destination port. This can let attacker determine which server application service you are running and which operating system you have. You need not worry about this attack unless you have a firewall on your system to protect from the attackers.
Another method to check UDP Open Port
Follow below alternate method to test your udp port is open or close:
- Download utilities available on Web. For e.g. Network Utilities Bundles.
- Install and launch the tool on your system.
- Click on the port check button and select the type of port.
- Click on Check Me.
These tools use local application. When the port is not reachable or giving a message displaying “Your port is not open or not reachable”, this means that the port is closed or some firewall is blocking the port.
How does UDP port scanning detection works?
After sending a UDP packet to the remote host, the following occurs:
- When UDP port is available (open). Packets are accepted but the response is not sent.
- When UDP port is closed, ICMP packet is sent in response with an error code like Destination Unreachable.
Limitations in UDP Port Scanning
Following are some of the limitations for UDP Port scanning:
- Internal packets are used for queuing incoming packets by the TCP/IP stacks of many operating systems. UDP packets that sent are not process due to limited buffer space. This can cause the slow process of port scanning. UDP port scanning is reluctant than TCP port scanning.
- The Firewall can affect the result of scanning. It can prevent the ICMP packet from reaching the scanner.
Scanning and to check UDP port can be achieved through various methods. The scanner contains a list of open UDP ports as well as the running UDP services. No response from the port anticipates that the port is open. Where an error message or an ICMP packet in the response indicates the port is closed.