How to Detect Open UDP Ports with hping3 and netcat on Linux

Test Environment

Server OS: Ubuntu 20.04.6. Client OS: CentOS 7. Tools: hping3 and netcat ( nc). Ensure UDP ports are allowed through the firewall or temporarily disable the firewall.

Server Configuration

List listening UDP ports: netstat -ulnp Typical output shows ports 5353, 37430 and 631 open.

Client Configuration

Install hping3 and nc on the client.

Configure YUM repositories using Aliyun mirrors:

curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
curl -o /etc/yum.repos.d/epel.repo https://mirrors.aliyun.com/repo/epel-7.repo

Install the packages:

yum install -y hping3 nc

Testing UDP Ports

Compare detection of open versus closed UDP ports using both hping3 and nc.

Using hping3

Open ports (5353, 37430)

hping3 --udp 192.168.211.201 -p 5353 -c 4
hping3 --udp 192.168.211.201 -p 37430 -c 4

Result: 100 % packet loss with no ICMP error – indicates the port is reachable.

Closed ports (5354, 37431)

hping3 --udp 192.168.211.201 -p 5354 -c 4
hping3 --udp 192.168.211.201 -p 37431 -c 4

Result: 100 % packet loss plus an “ICMP Port Unreachable” message – indicates the port is not listening.

Using netcat (nc)

Open ports

nc -vuz 192.168.211.201 5353
nc -vuz 192.168.211.201 37430

Result: “UDP packet sent successfully”.

Closed ports

nc -vuz 192.168.211.201 5354
nc -vuz 192.168.211.201 37431

Result: “Connection refused”.

Scriptable online/offline check:

nc -vuz 192.168.211.201 5353 < /dev/null &> /dev/null && echo "online" || echo "offline"
nc -vuz 192.168.211.201 5354 < /dev/null &> /dev/null && echo "online" || echo "offline"

Conclusion

Both hping3 and nc can be used to verify UDP port availability. An open UDP port produces no error with hping3 (or a success message with nc), while a closed port generates an ICMP “Port Unreachable” response ( hping3) or “Connection refused” ( nc). The commands above can be reproduced on any Linux system with the same tools installed.