How to Detect Open UDP Ports with hping3 and netcat (nc)

Test Environment

Server OS: Ubuntu 20.04.6; Client OS: CentOS 7; Test tools: hping3 and nc. If the server firewall is enabled, open the required ports or temporarily disable the firewall.

Server Configuration

Check which UDP ports are listening using netstat -ulnp. The server has ports 5353, 37430, and 631 open.

Client Configuration

Install hping3 and nc on the client.

Configure YUM repository from Alibaba:

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 tools:

yum install hping3 nc -y

Testing UDP Ports

Using hping3

Detecting open UDP ports

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

The client shows 100% packet loss.

Detecting closed UDP ports

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

The client reports 100% packet loss and an ICMP "Port Unreachable" message.

Summary: If a UDP port responds, there is no message; if it does not, an ICMP Port Unreachable is shown.

Using nc (netcat)

Detecting open UDP ports

nc -vuz 192.168.211.201 5353
nc -vuz 192.168.211.201 37430

When the port is open, nc prints "UDP packet sent successfully".

Detecting closed UDP ports

nc -vuz 192.168.211.201 5354
nc -vuz 192.168.211.201 37431

If the port is not open, nc reports "Connection refused".

Additional one‑liner to check status:

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

Now you understand how to detect whether a UDP port is open or closed using hping3 and nc; feel free to try these commands yourself.