Ports are used by services running on a server to communicate over the network. For example, web servers usually listen on ports 80 and 443, mail services use ports like 25, 110, 143, 993, and 995, and control panels such as cPanel\/WHM use ports like 2082, 2083, 2086, and 2087.<\/p>\n\n\n\n
To check whether ports are open, closed, or filtered on a server, we can use the Before checking open or closed ports, ensure the following:<\/p>\n\n\n\n For AlmaLinux, CentOS, or RHEL-based servers:<\/p>\n\n\n\n For Ubuntu or Debian-based servers:<\/p>\n\n\n\n If you want to scan a remote server, note down the server IP address.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Replace Use the following command to scan all TCP ports and detect service versions:<\/p>\n\n\n\n Example:<\/p>\n\n\n\n Example output:<\/p>\n\n\n\n If you are logged in to the server and want to check which services are listening locally, use:<\/p>\n\n\n\n Alternative command:<\/p>\n\n\n\nnmap<\/code> command. nmap<\/code> is a powerful network scanning tool used to identify active services, open ports, and service versions running on a server.<\/p>\n\n\n\nPrerequisites<\/h2>\n\n\n\n
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You should have root or sudo privileges on the system from where you are running the scan.<\/li>\n\n\n\n
The nmap<\/code> tool must be installed on the server or local machine.<\/li>\n\n\n\n
You should know the IP address of the server that needs to be scanned.<\/li>\n\n\n\n
Ensure the scanning machine can reach the target server.<\/li>\n\n\n\n
Make sure the firewall or security group allows scanning, otherwise some ports may show as filtered.<\/li>\n\n\n\n
Scan only servers that you own or have permission to test.<\/li>\n<\/ol>\n\n\n\nSteps<\/h2>\n\n\n\n
Step 1: Install nmap<\/h3>\n\n\n\n
yum install nmap -y\n<\/pre>\n\n\n\n
apt install nmap -y\n<\/pre>\n\n\n\n
Step 2: Check the server IP address<\/h3>\n\n\n\n
SERVER_IP_ADDRESS\n<\/pre>\n\n\n\n
SERVER_IP_ADDRESS<\/code> with the actual IP address of the server.<\/p>\n\n\n\nStep 3: Run the port scan command<\/h3>\n\n\n\n
nmap -Pn -T4 -sV -p- SERVER_IP_ADDRESS\n<\/pre>\n\n\n\n
nmap -Pn -T4 -sV -p- 192.168.1.10\n<\/pre>\n\n\n\n
Command Explanation<\/h3>\n\n\n\n
-Pn : Treat the host as online and skip host discovery\n-T4 : Use faster scan timing\n-sV : Detect service\/version information\n-p- : Scan all ports from 1 to 65535\n<\/pre>\n\n\n\n
Step 4: Review the scan output<\/h3>\n\n\n\n
PORT STATE SERVICE VERSION\n53\/tcp open domain\n110\/tcp open pop3 Dovecot pop3d\n143\/tcp open imap Dovecot imapd\n993\/tcp open ssl\/imap Dovecot imapd\n995\/tcp open ssl\/pop3 Dovecot pop3d\n2082\/tcp open unknown\n2083\/tcp open ssl\/unknown\n2086\/tcp open unknown\n2087\/tcp open ssl\/unknown\n8080\/tcp open http Apache Tomcat\n<\/pre>\n\n\n\n
Step 5: Understand the port states<\/h3>\n\n\n\n
open : The port is open and a service is listening on it.\nclosed : The port is reachable but no service is listening.\nfiltered : The port is blocked by firewall or security rules.\n<\/pre>\n\n\n\n
Step 6: Check listening ports locally on the server<\/h3>\n\n\n\n
ss -tulnp\n<\/pre>\n\n\n\n
netstat -tulnp\n<\/pre>\n\n\n\n
Conclusion<\/h2>\n\n\n\n