[求助]ProFtpd不支持PORT方式吗？

eax

背景描述:
小弟在Linux 下设立Proftpd遇到一点儿麻烦，请大家指点:
小弟有2台电脑，一台是台式机，安装了gentoo linux,并且运行proftpd,我称之为PC-A，此电脑有直接的公网IP
另一台电脑是便携式电脑(PC-B)，使用802.11g无线网卡通过一个802.11g无线AP上网,该AP是一个router，这个便携式电脑的ip地址是该AP分配的内部网IP

问题描述:
在PC-B上使用任何ftp客户端软件，如果是PASV方式就可以成功的连接PC-A的ftp服务器，上传下载，但是如果用PORT方式，可以成功登陆PC-A，但是一到
LIST命令就没有反应了，直到超时报错。
在PC-B上连接其他因特网的ftp站点，不管是用PORT还是PASV都是没问题的，所以我想大概问题出在PC-A的proftpd,请各位高手指点一下问题出在哪里，多谢了!

以下是PC-A的机器配置

Linux KernelNetworks 2.4.20-gentoo-r3 #2 一 5月 5 07:03:49 CST 2003
i686 AMD Athlon(tm) XP 1600+ AuthenticAMD GNU/Linux

ProFTPD 1.2.7
配置文件
# This is a basic ProFTPD configuration file (rename it to
# 'proftpd.conf' for actual use.  It establishes a single server
# and a single anonymous login.  It assumes that you have a user/group
# "nobody" and "ftp" for normal operation and anon.

ServerName          "roFTPD Installed by Eax"
ServerType          standalone
DefaultServer       on
RequireValidShell   off
AuthPAM             off
AuthPAMConfig       ftp
# Port 21 is the standard FTP port.
Port                                8721

# Umask 022 is a good standard umask to prevent new dirs and files
# from being group and world writable.
Umask                                022

# To prevent DoS attacks, set the maximum number of child processes
# to 30.  If you need to allow more than 30 concurrent connections
# at once, simply increase this value.  Note that this ONLY works
# in standalone mode, in inetd mode you should use an inetd server
# that allows you to limit maximum number of processes per service
# (such as xinetd).
MaxInstances                        30

# Set the user and group under which the server will run.
User                                proftpd
Group                                proftpd

# Normally, we want files to be overwriteable.
<Directory />
  AllowOverwrite                on
  AllowStoreRestart             on
  AllowRetrieveRestart          on
</Directory>

# A basic anonymous configuration, no upload directories.
<Anonymous ~ftp>
  User                                ftp
  Group                                ftp

  # We want clients to be able to login with "anonymous" as well as "ftp"
  UserAlias                        anonymous ftp

  # Limit the maximum number of anonymous logins
  MaxClients                        10

  # We want 'welcome.msg' displayed at login, and '.message' displayed
  # in each newly chdired directory.
  DisplayLogin                        welcome.msg
  DisplayFirstChdir                .message

  # Limit WRITE everywhere in the anonymous chroot
  <Limit WRITE>
    DenyAll
  </Limit>

</Anonymous>