PORT

Checking Open Ports in Linux

1. Using `netstat` command

sudo netstat -tulnp

- `-t` - show TCP ports

- `-u` - show UDP ports

- `-l` - show listening ports

- `-n` - show numeric addresses instead of resolving hostnames

- `-p` - show the process using the port

2. Using `ss` command (modern replacement for netstat)

sudo ss -tulnp

Same options as netstat, but generally faster.

3. Using `lsof` command

sudo lsof -i -P -n | grep LISTEN

- `-i` - show network connections

- `-P` - show port numbers instead of service names

- `-n` - show numeric addresses

4. Checking specific port status

sudo lsof -i :22  # Check if port 22 is open

5. Using `nmap` (for scanning from another machine)

nmap -sT -O localhost

Or to scan a remote host:

nmap target-ip-address

6. Checking firewall status (if ports are blocked)

sudo iptables -L -n -v  # For iptables

sudo ufw status verbose  # For UFW (Ubuntu)

sudo firewall-cmd --list-all  # For firewalld (RHEL/CentOS/Fedora)

Note that you'll typically need root privileges (using sudo) to see all open ports and the processes using them.

Kill a Process Using a Specific Port in Linux

For port 3000 as an example:

sudo kill $(sudo lsof -t -i:3000)

Or more forcefully:

sudo kill -9 $(sudo lsof -t -i:3000)