2016隆昌洪灾:为何我设置的PASV端口范围无效？

我在SERV-U中开启被动模式，设置了PASV端口范围20021-20025 在路由器中开了这些端口的映射，并在防火墙里开放了这些端口。后用FXP登陆

WinSock 2.0 -- OpenSSL 0.9.8a 11 Oct 2005
[R] 正在连接到 ******.dns4me.com -> DNS=*******.dns4me.com IP=**.**.55.253 PORT=21
[R] 已连接到 ******.dns4me.com
[R] 220 Serv-U FTP Server v6.3 for WinSock ready...
[R] USER zsdlt
[R] 331 User name okay, need password.
[R] PASS (hidden)
[R] 230 User logged in, proceed.
[R] SYST
[R] 215 UNIX Type: L8
[R] FEAT
[R] 211-Extension supported
[R] CLNT
[R] MDTM
[R] MDTM YYYYMMDDHHMMSS[+-TZ];filename
[R] SIZE
[R] SITE PSWD;EXEC;SET;INDEX;ZONE;CHMOD;MSG
[R] REST STREAM
[R] XCRC filename;start;end
[R] MODE Z
[R] MLST Type*;Size*;Create;Modify*;Win32.ea*;
[R] 211 End
[R] CLNT FlashFXP 3.4.0.1145
[R] 200 Noted.
[R] PWD
[R] 257 "/" is current directory.
[R] TYPE A
[R] 200 Type set to A.
[R] MODE Z
[R] 200 MODE Z ok.
[R] PASV
[R] 227 Entering Passive Mode (**,**,55,253,5,232)
[R] 正在打开数据连接 IP: **.**.55.253 端口: 1512
[R] QUIT
[R] 221 Goodbye!
[R] 已注销: *****.dns4me.com
[R] 数据 Socket 错误:连接失败

数据显示FXP在要求连接1512端口，为什么我设置的端口范围无法约束FTP客户端？我换过其它FTP客户端，都是同样的毛病。请高手帮忙分析，谢谢!