FTP被动模式连接被拒绝(FTP passive mode connection refused)

I’m creating an ftp client and I have a problem.

First, I create a socket where I sent USER and PASS then PASV a receive IP and PORT and then create a second data socket and try to connect, but there is a problem.

It will connect 5 times of 6, but on the 6th time I got error 111 Connection Refused and I have no idea how to fix this.

解决方案
The response text to PASV does not have a standardized format (EPSV does, though), so make sure you are parsing the IP/Port correctly, as you may receive one of many different formats used by real-world servers:

227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2
227 Entering Passive Mode. h1,h2,h3,h4,p1,p2
227 =h1,h2,h3,h4,p1,p2
Just to show a few possibilities.

If you are parsing the IP/Port correctly and still not able to connect, then either the server has too many client connections on the IP/Port and ran out of available slots for you to connect to, or else the IP/Port is being blocked by a firewall/router that the server does not know about when it reported the IP/Port to you.

Unfortunately, a connection refused error does not offer any way to differentiate between those conditions, so all you can do is attempt to connect a few times before failing the transfer, or else send an ABOR to inform the server to close the current passive IP/Port and then send a new PASV to get a new IP/Port.

机译
我正在创建一个 ftp客户端，并且遇到问题。

首先，我创建一个套接字，在此处我发送USER和PASS，然后PASV接收IP和PORT，然后创建第二个数据套接字并尝试连接，但是存在问题。

它会连接6次的5次，但在第6次出现错误 111 Connection Refused 时，我有不知道如何解决这个问题。

解决方案
对 PASV 没有标准格式（ EPSV ），因此请确保正确解析IP /端口，因为您可能会收到真实服务器使用的许多不同格式之一：

< pre> 227进入被动模式（h1，h2，h3，h4，p1，p2）。
227进入被动模式（h1，h2，h3，h4，p1，p2
227进入被动模式h1，h2，h3，h4，p1，p2
227 = h1，h2， h3，h4，p1，p2

只是为了展示一些可能性。 b
如果您正确解析IP /端口并且仍然无法连接，则服务器在IP /端口上的客户端连接太多，并且没有足够的可用插槽供您连接，否则IP /端口被防火墙/路由器阻塞，服务器不知道它何时向您报告IP /端口。

不幸的是， code>连接被拒绝错误没有提供任何区分这些条件的方法，所以你只能在失败之前尝试连接几次，否则发送一个 ABOR 通知服务器关闭当前的被动IP /端口，然后发送一个新的 PASV 来获得新的IP /端口。