FTP Clients - Part 2: Explicit FTPS versus Implicit FTPS

Posted: Nov 10, 2008  1 comments  

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In part 2 of my series on FTP clients, I thought it would be best to have a discussion about the differences between Implicit FTPS and Explicit FTPS. In my recent "FTP Clients - Part 1: Web Browser Support" blog post, I referenced Implicit and Explicit FTPS with a link to my Using FTP Over SSL walkthrough. But it occurred to me that some people may not understand the difference between the two, and my upcoming blog posts are going to build upon that knowledge, so I thought that a quick discussion of these two technologies would be prudent.

FTP over SSL (FTPS)

One of the many limitations of the File Transfer Protocol (FTP) is a general lack of security; e.g. user names and passwords are transmitted in clear text, data is transferred with no encryption, etc. In order to address this situation, FTP over SSL (FTPS) was introduced in Requests for Comments (RFC) article 2228 - FTP Security Extensions, and expanded in RFC 4217 - Securing FTP with TLS to address Transport Layer Security (TLS).

Following up on these RFC articles, the FTP service for Windows Server 2008 added support for FTPS, and the FTP SSL Settings Feature in the IIS Manager allows you to configure your FTPS settings to allow or require SSL, enforce 128-bit SSL, or customize your control/data channel SSL settings.

FTP SSL Settings

Explicit FTPS

Explicit FTPS is really what RFCs 2228 and 4217 envisioned; basically the way this works is an FTP client connects over the control/command channel (usually on port 21), and then the client can negotiate SSL for either the command/control channel or the data channel using new FTP commands like AUTH, PROT, CCC, etc.

The FTP service for Windows Server 2008 allows customized settings for both the command/control channel and the data channel through the Advanced SSL Policy dialog:

Advanced SSL Policy

There are several ways that Explicit FTPS might be implemented depending on your business needs:

Control Channel Data Channel Notes
Allow Allow This configuration allows the client to decide whether any part of the FTP session should be encrypted.
Require only
for credentials
Allow This configuration protects your FTP client credentials from electronic eavesdropping, and allows the client to decide whether data transfers should be encrypted.
Require only
for credentials
Require This configuration requires that the client's credentials must be secure, and then allows the client to decide whether FTP commands should be encrypted. However, all data transfers must be encrypted.
Require Require This configuration is the most secure - the client must negotiate SSL using the FTPS-related commands before other FTP commands are allowed, and all data transfers must be encrypted.

Implicit FTPS

Implicit FTPS takes SSL one step further than simply requiring that SSL-related commands must be sent first like you can with Explicit SSL; with Implicit FTPS, an SSL handshake must be negotiated before any FTP commands can be sent by the client. In addition, even though Explicit FTPS allows the client to arbitrarily decide whether to use SSL, Implicit FTPS requires that the entire FTP session must be encrypted. Basically the way that Implicit FTPS works is that an FTP client connects to the command/control channel, in this case using port 990, and immediately performs an SSL handshake; after SSL has been negotiated, additional FTP commands for the session can be sent by the FTP client.

Using FTPS in FTP service for Windows Server 2008 follows the Internet Assigned Numbers Authority (IANA) specification that the Implicit FTPS command/control channel is on port 990 and the Implicit FTPS data channel is on port 989.

Using FTPS in Windows Server 2008

Here's the way that you specify which type of FTP over SSL (FTPS) that you are using in Windows Server 2008:

  • If you enable FTPS and you assign the FTP site to the default port of 21, you are using Explicit SSL.
  • If you enable FTPS and you assign the FTP site to port 990, you are using Implicit SSL.
  • In point of fact, if you enable FTPS and you assign the FTP site to any port other than port 990, you are always using Explicit SSL.

Note: If you are using FTP on any ports other than the defaults of 21/20 and 990/989, you must make sure that those ports are not already assigned by IANA to another protocol. For more information, see the list of assigned port numbers on IANA's web site.

Parting Thoughts

Choosing whether to use Explicit FTPS over Implicit FTPS is a personal choice, and generally this choice may depend on your business needs or your FTP client. In several FTP clients that I've tested, the FTP client chooses one form of FTPS over another as the default method, and the FTP client may require some manual configuration to use the other.

Shortly after shipping the FTP service for Windows Server 2008 we discovered an issue where the FTP service was not cleaning up Implicit SSL connections properly, and we issued a hotfix rollup package for the FTP service that is discussed in Microsoft Knowledge Base article 955136.

I hope this helps to clear things up a bit. ;-]

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