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I have a Ubuntu server hosting client websites, IP: 88.208.194.208. We have Pro-FTPd set up and it runs fine when connecting through FileZilla to upload files etc. We have some clients with software that automates some file uploading so I have set up FTP accounts for each of them (actually standard user accounts in Linux, but the FTP uses them the same).

Most can connect fine but one client cannot, they get a "connection closed by host" error. They also noted that they cannot connect via Windows Explorer, which I tested and it doesn't work for me either. I get a message:

Windows cannot access this folder. Make sure you typed the file name correctly and that you have permission to access the folder.

Details: The FTP session was terminated

(I tried another server from Explorer and it connected fine so it's not WE itself.)

We have a basic iptables firewall on the server but nothing else AFAIK. iptables is not blocking anything specific.

I also checked log files and could not see the client server's IP address anywhere (ran grep in the entirety of var/logs). The client has also tried disabling firewalls etc their side with no luck.

Is this likely to be a problem on our side? Is there anything I can do to diagnose the issue?

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Can the client ping the ftp server at all? If so can you do trace route? Sounds like connectivity issue between the client and ftp server. –  Nic Young Feb 22 '12 at 16:44
    
Is your ftpd daemon using sftp or ftps?? Its possible they are using a client that is having a negotiation problem on the ssl handshake? As a quick experiment, to get more info, I would enable sftp on the SSH port 22 and have them try that instead. SSHD needs to be configured correctly though. (requires openssl v5.x or higher) ;-) –  djangofan Feb 23 '12 at 1:20
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2 Answers

Run tcpdump (saving to a file and later opening in Wireshark if you want the pretty interface) with a filter for the IP address you expect the connection to come from.

tcpdump ip host 1.2.3.4

That should confirm whether network traffic is reaching the FTP server and give you an idea of how far along things get in the process so you can direct your debugging efforts to discover whether it dies before connection, before authentication, or sometime after.

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up vote 0 down vote accepted

I contacted my host and found the problem. Both me and my client are using the same host, who completely block servers contacting each other. So we're looking for a different solution now.

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