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I'm trying to solve this error searching the web, without any success.

The error is:

425 Can't open data connection for transfer of "/"

My software:

  • Windows server 2013 and last filezilla server

My topology:

  • Server wired to router - Router with a dynamic IP from ISP, dynamic dns updated.

My settings:

Where I opened my ports

  • router: local and public 21, local 51000 public 51000-61000, local and public 990 (ALL tcp)
  • Windows Firewall: both directions, ports 21, 51000-61000, 990.

On my lan it works perfectly. I think there's something wrong with router port forwarding configuration. Here it is.

My port forwarding configuration on router

Is there something I'm doing wrong?

I add a log of my ftp server:

(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> PBSZ 0
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> 200 PBSZ=0
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> PROT P
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> 200 Protection level set to P
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> PWD
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> 257 "/" is current directory.
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> TYPE I
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> 200 Type set to I
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> PASV
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> 227 Entering Passive Mode (my,ip,addr,ess,205,93)
(000013)06/11/2017 21:50:57 - user (my.ip.add.ress)> MLSD
(000013)06/11/2017 21:51:07 - user (my.ip.add.ress)> 425 Can't open data connection for transfer of "/"

I add the ftp server configuration screens too just in case.

General Settings

PASV Mode settings

FTP over TLS

  • What is the error from client-side? What client are you using? Does the my,ip,addr,ess in the 227 response match the IP that the client is connecting to? – Martin Prikryl Nov 7 '17 at 12:57
  • Remote client and server have the same error. my,ip,addr,ess is the correct one. – Raikoug Nov 7 '17 at 16:56
  • Well, the router mapping seems suspicious, as you have correctly guessed. It looks like you have all public ports in range 51000-61000 mapped to a single internal port 51000. What would explain, why the client is not able to connect to port 52573. – Martin Prikryl Nov 8 '17 at 7:07
  • Well I guess I have few choices: 1) Change router (very hard one, since is ISP provided). --- 2) I have a DMZ option to bypass any port forwarding to my server. Very dangerous I think. --- 3) SFTP, I think all trafic just pass from port 22 or the custom 1 i chose.. – Raikoug Nov 8 '17 at 10:08
  • If your router is not able to map a range of internal ports, you can configure your FTP server to use only a narrow range (like 51000-51010) and configure those ports one-by-one. – Martin Prikryl Nov 8 '17 at 10:12
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The router mapping seems suspicious, as you have correctly guessed. It looks like you have all public ports in range 51000-61000 mapped to a single internal port 51000. What would explain, why the client is not able to connect to port 52573.

If your router is not able to map a range of internal ports, you can configure your FTP server to use only a narrow range (like 51000-51010) and configure those ports one-by-one.

See also How many data channel ports do I need for an FTPS server running behind NAT?

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