# # ftp through firewall
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FTP Problem

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© April 2008 Josh

Hello - I'm having a problem with a server running SCO OpenServer 5.0.7 and ftp. ftp connections used to work fine, but now I can connect to the server via ftp, I can issue type, pwd, and cd commands successfully and receive the response-- but when I issue ls, dir, get, or put commands, I get the message: "425 Can't build data connection: Invalid argument." No file configuration changes were made. The server was moved and is behind a new firewall, but as I said, I can connect and issue some commands successfully. Port 21 is open on the firewall so I can connect.

From the server, I can ftp to other sites and all commands work.

I do suspect the firewall, but I don't know why some commands work and others dont.

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-> ftp through firewall


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Thu Apr 17 17:37:55 2008: 4049   TonyLawrence

Yes, Josh, it is the firewall, and to understand why you need to read up on how ftp actually works - what you want is to have it work in "passive" mode.

I thought I had an article here somewhere that explained this well, but can't seem to find it.. I'll look a bit more..

Thu Apr 17 19:22:37 2008: 4052   Josh

Thanks Tony.
I did try to get passive mode to work before I wrote, but the connection disconnects at the server when switching to PASV mode. Perhaps there is a config entry I'm missing?

Thu Apr 17 19:30:41 2008: 4053   TonyLawrence

Perhaps the server can't do passive mode?

Thu Apr 17 21:06:28 2008: 4054   BruceGarlock

Doesn't port 20 need to be open at the firewall as well (ftp-data).

Fri Apr 18 01:55:11 2008: 4056   Josh

I can ftp Active and Passive behind the firewall. Since neither Active nor Passive is working on the public side, it must be a firewall issue...an odd one to be sure, but a firewall issue nonetheless.
Port 20 and 21 are both open on the firewall.
I have a local Cisco engineer looking at it.
Thanks for your comments- both of you-- you helped me narrow this down very quickly.


Fri Apr 18 02:16:32 2008: 4058   TonyLawrence

Ayup - firewall is blocking you for sure.. strange symptoms though.

Tue Oct 4 17:31:23 2011: 9902   anonymous


I am having the same problem but I am not understanding what was done to make this problem go away?

Tue Oct 4 17:46:49 2011: 9903   TonyLawrence


I don't know. Unfortunately, Josh never returned to update us.

Fri Oct 7 15:52:06 2011: 9939   anonymous


Great. Now I have guilt!
I don't have my notes from that time handy, but I believe that the problem was related to Active vs. Passive FTP. I'll update again as soon as I can review my notes.

Fri Oct 7 16:01:49 2011: 9940   TonyLawrence


But at the time you said you had tried Passive, so perhaps there was more to it.

Don't feel too guilty :-)

If you can find it, great. If not, we'll survive.


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