1.
Connected. Waiting for response.
220 Serv-U FTP Server v4.0 for WinSock ready...
USER anonymous
530 Sorry, no ANONYMOUS access allowed.
QUIT
這個就是不許匿名登錄啦

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
一般是密碼輸入錯誤時出現的信息，但對于Serv-U來說，如果沒有這個帳號（test）存在，也會產生同樣的出錯信息。

3.
Connecting to xxx.xxx.xxx.xxx, Port 21 (#1)
ERROR: Connection timed out
這個表明對方未開機，也可能是對方不在你能訪問的范圍內

4.
Connecting to xxx.xxx.xxx.xxx Port 21 (#1)
ERROR: Connection refused
這個一般表明對方已開機，但未開啟FTP服務（沒有開Serv-U）
也可能為對方不提供在這個端口上的服務

5.
Connecting to xxx.xxx.xxx.xxx, Port 21 (#1)
Connected. Waiting for response.
Disconnected from server.
Connection attempt failed. Waiting for retry...
出現這個信息實在是很倒霉，你極有可能被對方Ban了。
如果只是Ban幾分鐘or幾小時or一天還好，不然只有和站長說說好話，讓他給你解封吧。

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
這個有點麻煩，你的IP不在站長允許訪問的IP范圍內，只有和站長聯系，讓他把你的IP網段加入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，這個很常見吧，用戶太多

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
每個IP只能開一個下載，就不要用多線程啦，小心Ban了你。