1. Command
Netstat command parameters:
-t : Indicates that the TCP port is displayed
-u : Indicates that the UDP port is displayed
-l : Only display listening sockets (so-called sockets are programs that can read, write, and send/receive communication protocols (protocol) and data)
-p : Display process identifier and program name. Each socket/port belongs to a program.
-n : Do not perform DNS training, display IP (can speed up operation)
Summary : netstat -parameter , this parameter can be multiple letters such as tlpn or tpn, etc., choose different parameters to output different information
Two, example
1. View all current tcp ports
netstat -ntlp
2. View the usage of all 3306 ports
netstat -ntlp |grep 3306
3. The netstat -an command can see all the IPs that are connected to the local computer. It contains four parts: proto (connection method), local address (local connection address), foreign address (address for establishing connection with the local), state (Current port status). Through the detailed information of this command, you can completely monitor the connection on your computer.
The state (current port state) displayed by the netstat -an command has the following states:
LISTEN: Listen for connection requests from remote TCP ports
SYN-SENT: Waiting for a matching connection request after sending the connection request
SYN-RECEIVED: After receiving and sending a connection request, wait for the other party to confirm the connection request
ESTABLISHED: represents an open connection
FIN-WAIT-1: Waiting for the remote TCP connection interruption request, or the confirmation of the previous connection interruption request
FIN-WAIT-2: Waiting for connection interruption request from remote TCP
CLOSE-WAIT: Waiting for the connection interruption request sent from the local user
CLOSING: Waiting for the remote TCP's confirmation of the interruption of the connection
LAST-ACK: Waiting for the confirmation of the original connection interrupt request sent to the remote TCP
TIME-WAIT: Wait enough time to ensure that the remote TCP receives the confirmation of the connection interruption request
CLOSED: No connection status
View all 3306 port usage
netstat -an |grep 3306
4. Check which services and ports on a server
netstat -lamp
5. View all UDP ports
netstat -nupl
Reference documents:
https://www.cnblogs.com/ruanraun/p/port.html
https://www.cnblogs.com/xiyangbaixue/articles/3630542.html