Open TCP Ports and Verify

TCP ports in AlmaLinux/Red Hat 8 can be blocked either using the IPTABLES service or using the firewall service. The following is an example of how to use the IPTABLES service to open a TCP port and then test it using the netcat utility.

1. Check if a port is blocked. In this example, we can check if port 80 is accepting traffic by entering the iptables command with grep: $ iptables -nL | grep 80

If the iptables command returns no data, the port needs to be opened.

2. To open TCP Port 80, enter the iptables command as follows: $ iptables -I INPUT -p tcp --dport 80 -j ACCEPT
3. Re-enter the iptables -nL command to see if the port is now open. $ iptables -nL | grep 80 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80
4. Install the netcat utility. $ yum install nc
5. Run netcat as a server listing on port 80. $ nc -l -p 80
6. From another server, install the netcat utility. $ yum install nc
7. Run netcat to the server you are running iperf3 to verify that port 80 is accepting commands.

$ nc -z -v 192.168.16.7 80

Ncat: Version 7.50 ( https://nmap.org/ncat )

Ncat: Connected to 192.168.16.7:80.

Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.

nc

Open TCP Port Example

Open TCP Port 80:

# nc -z -v 192.168.40.41 80

Ncat: Version 7.50 ( https://nmap.org/ncat )

Ncat: Connected to 192.168.40.41:80.

Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.

Closed TCP Port Example

Closed TCP Port 80:

# nc -z -v 192.168.40.31 80

Ncat: Version 7.50 ( https://nmap.org/ncat )

Ncat: Connection refused.