当通过 ftp 连接到 example.com 时，在提示输入较长字符串的密码之前会失败：
user@client:~> ftp example.com
Connected to example.com.
220---------- Welcome to Pure-FTPd [privsep] [TLS] ----------
220-You are user number 2 of 50 allowed.
220-Local time is now 17:56. Server port: 21.
220-This is a private system - No anonymous login
220-IPv6 connections are also welcome on this server.
220 You will be disconnected after 15 minutes of inactivity.
Name (client:username): [email protected]
421 Service not available, remote server has closed connection.
ftp: Login failed.
但如果我尝试使用较小的字符串登录：
user@client:~> ftp example.com
Connected to example.com.
220---------- Welcome to Pure-FTPd [privsep] [TLS] ----------
220-You are user number 2 of 50 allowed.
220-Local time is now 17:58. Server port: 21.
220-This is a private system - No anonymous login
220-IPv6 connections are also welcome on this server.
220 You will be disconnected after 15 minutes of inactivity.
Name (client:username): [email protected]
331 User [email protected] OK. Password required
Password:
这到底是什么问题呢？