How do I check if a port is in use on Linux?

2024年2月27日 · 閱讀時間約 1 分鐘

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