How to check a port is open on the firewall

To check whether a port is open in the firewall on Linux, use the method appropriate for your firewall.

1. Check if the service is listening

First verify the On the database server is listening on the port:

$ ss -tulpn | grep 1536

$ netstat -tulpn | grep 1536

$ lsof -i :1536

If nothing is listening, the firewall is not the issue.

2. Check Firewalld (RHEL/OEL/CentOS 7/8/9)

Check firewall status:

$ systemctl status firewalld

$ firewall-cmd --list-ports

$ firewall-cmd --query-port=1536/tcp

Output:

yes/no

3. Check iptables

List rules:

$ iptables -L -n

Search for port 1536:

$ iptables -L -n | grep 1536

For more details:

$ iptables -L INPUT -n --line-numbers

4. Test from a Remote Server

From another server, test connectivity On the application server:

Using telnet

$ telnet hostname 1536

$ telnet erpuatappl.nicsi.in1536

Using nc (netcat)

$ nc -zv hostname 1536

$ nc -zv erpuatappl.nicsi.in 8015
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 10.24.248.33:8015.
Ncat: 0 bytes sent, 0 bytes received in 0.03 seconds.

5. Oracle Listener Specific Check

If port 1536 is for an Oracle listener:

$ lsnrctl status

Look for:

Listening Endpoints Summary...

(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=dbhost)(PORT=1536)))

Then test from the application server:

$ tnsping SERVICE_NAME