A Strange - 421 Service not available, remote server has closed connection - problem for Linux ftp client.

Here is my machine configuration:

I have 3 window server 2003. Each one hosts a VMWare server, which in
turn contains both window based and linux based virtual machines.

All the window based virtual machines have no problem connecting to
any of the outside world FTP server. But all the linux based (i386-
redhat-linux-gnu version 2.4.20-8) virtual machines returns the "421
Service not available, remote server has closed connection" error
right after the username is entered. For instance:

ftp> open wirefreethought.com
Connected to wirefreethought.com (66.199.224.58).
220 (vsFTPd 2.0.1)
Name (wirefreethought.com:root): vince
421 Service not available, remote server has closed connection
Login failed.
No control connection for command: Transport endpoint is not connected

The strange things is, if I try "ping hostname" first, then
immediately follow by "ftp hostname", everything works fine. Any
other time, the above error occurs. Everything worked fine before,
until yesterday the problem suddenly occurs.

Any insight will be greatly appreciated!

Best Regards,
Vince