How to check which ports are open in Linux

##a means all

n means not to query dns

t means tcp protocol

u means udp protocol

p means query occupied program

l means query is listening Program

netstat -nupl | grep 3306
#这个表示查找处于监听状态的，端口号为3306的进程

Linux method to check port usage status and close port

https://blog.csdn.net/wudiyi815/article/details/7473097

Premise: First of all, you must know that the port does not exist independently, it is attached to the process. When a process is opened, its corresponding port is opened; when the process is closed, the port is closed. If a process is opened again next time, the corresponding port will also be opened again. Don't purely understand it as closing a certain port, but you can disable a certain port.

1. You can use "netstat -anp" to check which ports are open.

(Note: Adding parameter '-n' will convert the application to port display, that is, an address in digital format, such as: nfs->2049, ftp->21, so it can be turned on Two terminals, one by one corresponding to the port number corresponding to the program)

2. Then you can use "lsof -i:$PORT" to view the program that uses the port ($PORT refers to the corresponding port Number). Or you can also check the file /etc/services to find out the service corresponding to the port.

(Note: Some ports cannot be found through netstat. A more reliable method is "sudo nmap -sT -O localhost")

3. To close a certain port port, you can:

1) Disable the port through the iptables tool, such as:

iptables -A INPUT -p tcp --dport $PORT -j DROP
iptables -A OUTPUT -p tcp --dport $PORT -j DROP

2) Or close the corresponding application , the port will be closed naturally, such as:

"kill -9 PID" (PID: process number)

For example: through "netstat -anp | grep ssh"

Display: tcp 0 127.0.0.1:2121 0.0.0.0:* LISTEN 7546/ssh

then: "kill -9 7546"