How do I check if a port is in use on Linux?
常常有時候要查 Linux 使用 port 的狀況，來確定 service 是否正常，就還要用 man 去查參數，寫篇文紀錄一下
檢查 Linux 目前使用的 port
- 使用以下 command
sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo netstat -tulpn | grep :443
sudo ss -tulpn | grep LISTEN
sudo ss -tulpn | grep ':22'
sudo fuser -v -n tcp 443
- 從 /etc/services 搜尋，把 PORT_NUMBER_HERE 換成要搜尋的 port
grep -E -w '[PORT_NUMBER_HERE]/(tcp|udp)' /etc/services
搜尋哪一個 process 正在 listening port 443
sudo netstat -tulpn | grep :443
sudo ss -tulpn | grep ':443'
取得所有 open port 的清單
sudo lsof -i -P -n | grep LISTEN
sudo ss -tulpn
sudo netstat -tulpn
Sample outputs:
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN 500/redis-server 12
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 438/nginx -g daemon
tcp 0 0 127.0.0.1:8080 0.0.0.0:* LISTEN 407/lighttpd
tcp 0 0 0.0.0.0:443 0.0.0.0:* LISTEN 438/nginx -g daemon
tcp6 0 0 :::80 :::* LISTEN 438/nginx -g daemon
udp 0 0 0.0.0.0:68 0.0.0.0:* 277/dhclient
Use fuser command to check if a TCP or UDP port is in use on Linux
sudo fuser -v -n <protocol> <port>
Ex: 檢查 SSHD (port 22) is in use:
sudo fuser -v -n tcp 22
Outputs:
USER PID ACCESS COMMAND
22/tcp: root 663 F.... sshd
root 9877 F.... sshd
ubuntu 9992 F.... sshd
Ex: 檢查 DNS Server (port 53) UDP port is in use:
sudo fuser -nv udp 53
Outputs:
53/udp: 392 1189
Use nc command 來檢查 TCP or UDP port is used
Try the nc command as follows to scan the open ports (TCP/22 {ssh port}) of a specified host (10.8.0.1):
nc -v -z <ip_address> <port>
nc -v -z 10.8.0.1 22
Outputs:
Connection to 10.8.0.1 22 port [tcp/ssh] succeeded!
不過如果你在防火牆(block port 22)背後，則 Output 會變成以下這樣
nc: connect to 10.8.0.1 port 23 (tcp) failed: Connection refused
