- Active-mode FTP
- Passive-mode FTP
Active-mode FTP ConnectionsActive-mode FTP is sometimes referred to as "client-managed" because the client sends a PORT command to the server (over the control connection) that requests the server to establish a data connection from TCP Port 20 on the server, to the client, using the TCP port that is specified by the PORT command.
The FTP client sends the PORT command to the FTP server in the following format:
NOTE: In Windows NT 4.0 and Windows 2000, the valid default ephemeral port range is 1024-5000. Increasing the ephemeral range in Windows NT 4.0 and Windows 2000 requires adding a value to the system registry.
For additional information on increasing the default ephemeral range, click the article number below to view the article in the Microsoft Knowledge Base:
|Instruction||Sent From||Sent To|
|250 CWD command successful.||10.0.0.10:21||192.168.4.29:8190|
|200 PORT command successful.||10.0.0.10:21||192.168.4.29:8190|
|<file listing is transferred>||10.0.0.10:20||192.168.4.29:8191|
|226 Transfer complete.||10.0.0.10:21||192.168.4.29:8190|
Passive-mode FTP ConnectionsPassive-mode FTP is sometimes referred to as "server-managed", because after the client issues a PASV command, the server responds to that PASV instruction with one of its ephemeral ports that will be used as the server-side port of the data connection. After a data connection command is issued by the client, the server connects to the client using the port immediately above the client-side port of the control connection. The following is a typical sequence for a passive-mode FTP connection:
|Instruction||Sent From||Sent To|
|250 CWD command successful.||10.0.0.10:21||192.168.4.29:7971|
|227 Entering Passive Mode (192,168,4,29,9,227).||10.0.0.10:21||192.168.4.29:7971|
|<file listing is transferred>||10.0.0.10:2531||192.168.4.29:7972|
|226 Transfer complete.||10.0.0.10:21||192.168.4.29:7971|
Common Problems Encountered With FTPThe most common problem encountered when you use FTP over the Internet results when you attempt transfers through a Network Boundary Securing Device (NBSD) such as a proxy, firewall, or Network Address Translation (NAT) device. In most cases the NBSD allows the control connection to be established over TCP 21 (that is, the user can successfully log on to the FTP server), but when the user attempts a data transfer such as DIR, LS, GET, or PUT, the FTP client appears to stop responding because the NBSD is blocking the data connection port that is specified by the client. If the NBSD supports logging, you can verify port blocking by reviewing the deny/reject logs on the NBSD.
In addition to causing problems for NBSD administrators, FTP is commonly misunderstood as a secure means for transferring data, because the FTP server can be configured to require a valid username and password combination prior to granting access. Users should be aware that neither the credentials specified at logon nor the data itself is encrypted or encoded in any way. All FTP data can be easily intercepted and analyzed by any station on any network between the FTP client and FTP server.
FTP Clients Provided by MicrosoftThe following table lists the FTP clients provided by Microsoft, and the connection mode that each client supports:
|FTP Client||Transfer Mode|
|Internet Explorer 5.1 and earlier||Passive|
|Internet Explorer 5.5 and later||Both|
|FrontPage ver.1.1 to Windows XP||Active|
File Transfer AlternativesBecause of the NBSD configuration issues and security concerns with FTP, several alternatives to standard FTP are used. One common alternative to FTP is the use of HTTP as a file transfer method, because most firewalls allow HTTP connections over TCP 80 and HTTPS connections over TCP 443. Although Microsoft has supported HTTP-based file transfers for several years in products such as the FrontPage Server Extensions and the Posting Acceptor, the recognized standard for HTTP file transfers is WebDAV, the HTTP extensions for distributed authoring and versioning. Defined by RFC 2518, WebDAV is built into IIS 5.0, and allows the user to use WebDAV shares (that is, folders that are published on a WebDAV-enabled Web server) in much the same way that network shares are used, provided that the connection is made by a client that is capable of communicating with WebDAV (such as Internet Explorer 5.0 and later).
NOTE: For more information on RFC 2518, see the following Web site:
The following resources contain more in-depth information about the File Transfer Protocol service:
Running Microsoft® Internet Information Server
Authors: Leonid Braginski and Matthew Powell
Publisher: Microsoft Press, July 1998
WebDAV, WebFolder, and MSIPP Resources
The following resources contain more in-depth information about WebDAV, WebFolders, and the Microsoft Internet Publishing Provider:
WebDAV in 2 Minutes
The following resources contain more in-depth information on how to secure an IP network, which can provide additional security to protect information that is transmitted by using FTP:
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