Problem with site-to-site encryption between Serv-U servers

Hello. I have 2 servers both running Serv-U 4.1 in Implicit SSL mode (Allow only SSL/TLS sessions). I can upload/download from my workstation to/from any of the two servers without any problem and I can see the icon with yellow padlock in Serv-U's Activity window during transfer. I can also send file from ftp.smartftp.com, connected in Explicit SSL mode, to any of the two servers and the data channel is encrypted with the yellow padlock shown. The problem occurs when I send file between the two servers. It is not encrypted. Prior to transfer, I can see that SmartFTP issues PROT C command (it doesn't in previous case). That's probably why the data transfer is not encrypted. But I don't know why SmartFTP has to issue that command. Any ideas?

SmartFTP: 1.0.981.36
AUTH Mode: TLS
Data Connection Mode: Private Data Connection

Serv-u doesn't support SSCN. Thus it doesnt support encrypted FXP transfer. Thats the reason SmartFTP falls back to "clear" data connection mode (PROT C).

-Mat

Serv-u doesn't support SSCN. Thus it doesnt support encrypted FXP transfer. Thats the reason SmartFTP falls back to "clear" data connection mode (PROT C).

-Mat
But I can FXP from ftp.smartftp.com (seems to be BlackMoon) to the Serv-U server with data encrypted. Do you mean that the sending server has to support SSCN while the receiving server doesn't have to? Cheers.

Have just done more experiment. Apart from previous post:

- FXP from ftp.smartftp.com to my Serv-U 4.1 site, the data is encrypted.

I have more results testing with RaidenFTPD,

- FXP from RaidenFTPD 2.4 to Serv-U 4.1, the data is encrypted.
- FXP from Serv-U 4.1 to RaidenFTPD 2.4, the data is encrypted.

But the problem occurs when:
- FXP from Serv-U 4.1 to Serv-U 4.1, SmartFTP simply issues PROT C to disable data encryption.

Seems to me that Serv-U 4.1 does support data encryption when FXP but SmartFTP doesn't know how to handle the control sequence when FXP between two Serv-U servers?

RaidenFTPd, Blackmoon FTP (smartftp.com) supports SSCN but ServU doesnt. Only one of the two servers your FXPing requres SSCN suport. That explains everything. SmartFTP is doing the right thing.
-Mat

Ok. I understand now. Although Serv-U supports secure FXP, it doesn't support any of the two commands SSCN or CPSV sent by FTP client to initiate secure FXP. Since CPSV isn't supported by SmartFTP so another server that supports SSCN is needed in order to initiate the secure data connection for Serv-U. Thanks.