Connect FTP server, public network CentOS through FTP command cannot connect, native cmd under the FTP command and the client can connect

Public network of Linux through the FTP command and PHP ftp_connect can not connect, tried 3 machines, can ping, but in the initial connection when the time has been waiting, not to mention the landing FTP server.
However, the local Windows machine can be connected and logged on via the FTP command under the client or CMD.
FTP server firewall is off, FTP port default 21

Reply content:

Public network of Linux through the FTP command and PHP ftp_connect can not connect, tried 3 machines, can ping, but in the initial connection when the time has been waiting, not to mention the landing FTP server.
However, the local Windows machine can be connected and logged on via the FTP command under the client or CMD.
FTP server firewall is off, FTP port default 21

Then you need to telnet to see if your interface is open to the public network.
As you say, your 21 port is inaccessible under the public network.

