Ftp – Why can’t I connect to some ftp servers from behind the win2008 NAT

I want to connect to an FTP server from my local network, which is behind a NAT.

The gateway is a Win 2008 server, configured with routing and remote services / Nat enabled. The problem exists from XP, Vista sp1, and Vista sp2 computers.

Here's what I get (using the default DOS ftp client, but I get the same thing using FileZilla)

ftp> open ftp.foo.com
Connected to ftp.foo.com.
220-

[here I wait for 30s before getting the following line]

Connection closed by remote host.
ftp>

As you can see I'm disconnected before the login process, so I don't have the opportunity to use PASSV.

Here's what works :

• everything else (ie, every non-ftp related connection)
• connection to this specific server from my NAT gateway
• connection on other servers from my behind-nat computer
• direct internet connection to this specific ftp server from my computer (ie, without using the NAT server)

The company providing the ftp says the problem is not on their end, since I can connect from my NAT server. The same reasoning leads to the problem not being on my side neither (I can connect on other ftp servers)

What should I check ?

EDIT : here's what I get when I sniff the connection on the gateway while attempting to connect from my behind-the-nat computer :

On the Internet facing interface :

No.     Time        Source                Destination           Protocol Info
   1312 320.576218  213.186.xx.xxx        192.168.1.1           FTP      Response: 220-
   1313 320.576602  213.186.xx.xxx        192.168.1.1           FTP      Response: 220-Bienvenue, 
   1315 320.610911  213.186.xx.xxx        192.168.1.1           FTP      Response: 220-

And on my internal LAN interface :

    No.     Time        Source                Destination           Protocol Info
       9288 118.698920  213.186.xx.xx        192.168.0.100         FTP      Response: 220-

       9293 118.890406  213.186.xx.xx        192.168.0.100         FTP      Response: 220-Bienvenue, 
    Frame 9293 (166 bytes on wire, 166 bytes captured)
    Ethernet II, Src: Tp-LinkT_c6:e2:3f (00:21:27:c6:e2:3f), Dst: Giga-Byt_22:b0:99 (00:1f:d0:22:b0:99)
    Internet Protocol, Src: 213.186.xx.xx(213.186.xx.xx), Dst: 192.168.0.100 (192.168.0.100)
    Transmission Control Protocol, Src Port: ftp (21), Dst Port: jini-discovery (4160), Seq: 7, Ack: 1, Len: 112
        Source port: ftp (21)
        Destination port: jini-discovery (4160)
        Sequence number: 7    (relative sequence number)
        [Next sequence number: 119    (relative sequence number)]
        Acknowledgement number: 1    (relative ack number)
        Header length: 20 bytes
        Flags: 0x18 (PSH, ACK)
        Window size: 65536 (scaled)
        Checksum: 0xa5bc [incorrect, should be 0x96cb (maybe caused by "TCP checksum offload"?)]
        [SEQ/ACK analysis]
    File Transfer Protocol (FTP)

   9306 119.187327  213.186.xx.xx        192.168.0.100         FTP      [TCP Retransmission] Response: 220-Bienvenue, 
   9326 119.787350  213.186.xx.xx        192.168.0.100         FTP      [TCP Retransmission] Response: 220-Bienvenue, 
No.     Time        Source                Destination           Protocol Info
   9373 120.987450  213.186.xx.xx        192.168.0.100         FTP      [TCP Retransmission] Response: 220-Bienvenue, 

Any idea of why this CRC problem might be happening ?

btw, here are my mtu settings (on the gateway)and they are identical on my computer.

C:\Windows\system32>netsh interface ipv4 show interfaces
Idx  Met   MTU   State        Name
---  ---  -----  -----------  -------------------
  1   50 4294967295  connected    Loopback Pseudo-Interface 1
 10   30   1500  connected    Local Area Connection
 13   20   1500  connected    Local Area Connection 2

And here's some MTU analysis on my computer :

Pinging 192.168.0.1 with 1472 bytes of data:
Reply from 192.168.0.1: bytes=1472 time<1ms TTL=128

Pinging 192.168.0.1 with 1473 bytes of data:
Packet needs to be fragmented but DF set.

Here's what I get if I try to connect using telnet :

220-

Connection to host lost.

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