1.
Connected. Waiting for response.
220 Serv-U FTP Server v4.0 for WinSock ready...
USER anonymous
530 Sorry, no ANONYMOUS access allowed.
QUIT
这个就是不许匿名登录啦
2.
Connected. Waiting for response.
220 Serv-U FTP Server v4.0 for WinSock ready...
USER test
331 User name okay, need password.
PASS xxxxxx
530 Not logged in.
QUIT
一般是密码输入错误时出现的信息，但对于Serv-U来说，如果没有这个帐号（test）存在，也会产生同样的出错信息。
3.
Connecting to xxx.xxx.xxx.xxx, Port 21 (#1)
ERROR: Connection timed out
这个表明对方未开机，也可能是对方不在你能访问的范围内
4.
Connecting to xxx.xxx.xxx.xxx Port 21 (#1)
ERROR: Connection refused
这个一般表明对方已开机，但未开启FTP服务（没有开Serv-U）
也可能为对方不提供在这个端口上的服务
5.
Connecting to xxx.xxx.xxx.xxx, Port 21 (#1)
Connected. Waiting for response.
Disconnected from server.
Connection attempt failed. Waiting for retry...
出现这个信息实在是很倒霉，你极有可能被对方Ban了。
如果只是Ban几分钟or几小时or一天还好，不然只有和站长说说好话，让他给你解封吧。
6.
Connecting to xxx.xxx.xxx.xxx, Port 21 (#1)
Connected. Waiting for response.
USER XXX
331 User name okay, need password
PASS xxxxxx
530 Not logged in, unauthorized IP address.
QUIT
这个有点麻烦，你的IP不在站长允许访问的IP范围内，只有和站长联系，让他把你的IP网段加入Allow Access列表里吧
7.
Connected. Waiting for response.
220 Serv-U FTP Server v4.0 for WinSock ready...
USER user
421 Too many users - please try again later.
哈哈，这个很常见吧，用户太多
8.
Connected. Waiting for response.
220 Serv-U FTP Server v4.0 for WinSock ready...
USER test
530 Not logged in, only one session from same IP allowed at a time.
QUIT
每个IP只能开一个下载，就不要用多线程啦，小心Ban了你。
