I've noticed on these boards at www.isaserver.org that a lot of questions come up regarding FTP. While there are still some unexplained mysteries regarding several of the aspects of how ISA Server handles some FTP connections, there are other areas that are able to be clarified. One of those is how Internet Explorer handles the FTP protocol.
Changing the Internet Explorer FTP Client Mode
People sometimes report to us that they get errors when connecting to an FTP server using Internet Explorer. One of the reasons for this may be that the server supports only PORT or PASV mode connections. If the IE is configured as the wrong FTP client type, then the error appears. ISA Server supports both types of connections, but you need to know how to configure Internet Explorer as each type of FTP client.
This article refers to Internet Explorer 5.01. For information on Internet Explorer 5.0, check out Stefaan Pouseele's article.
By default, Internet Explorer is configured as a PASV Mode FTP client. Some servers may not support PASV FTP clients. Therefore you might want Internet Explorer to send PORT commands to the FTP server instead. To Internet Explorer's FTP client mode, perform the following steps:
- Open Internet Explorer. Click the Tools menu, and then click the Internet Options command.
- Click the Advanced tab. You will see what appears in the figure below.
- Note the status of the Enable folder view for FTP sites checkbox. When this option is enabled, Internet Explorer sends PASV commands to the FTP server. Remove the checkmark from the checkbox, and Internet Explorer send PORT commands to the FTP Server
- The change should take place immediately. If you don't notice a change, close all instances of Internet Explorer and then open Internet Explorer up again.
Credit goes to Martin Grasdal for informing me of this handy tip!
How ISA Server Client Types Affect Internet Explorer FTP Behavior
Internet Explorer will access FTP sites differently depending on the ISA Server client type. ISA Server clients can be configured as one or more of the following client types:
- SecureNAT Client
- Firewall Client
- Web Proxy Client
It is important to note that these clients are not mutually exclusive. A single machine can be configured as all three client types and take advantage of the features provided by each client. This means that a single machine can be configured as a SecureNAT, Firewall and Web Proxy client without any adverse interaction between the client configuration settings.
If a machine is configured as a SecureNAT or Firewall Client, Internet Explorer will detect that it can send requests to the server through the Firewall service. When the request goes through the Firewall service, you can get full FTP client functionality. Note that you do not lose FTP caching. It appears that Internet Explorer is able to take advantage of both Web Proxy and Firewall service features when clients are configured to support both services.
An example of what you see when you connect to an FTP site as a SecureNAT or Firewall Client through Internet Explorer appears below.
The situation is different if the machine is configured as only a Web Proxy client. You would most often see this when you have installed ISA Server in Cache Mode. When ISA Server is installed in Cache Mode, the Firewall Service is not installed and clients can only access the Web Protocols (HTTP, HTTPS, FTP and Gopher).
The Web Proxy service only allows FTP downloads. When you connect to an FTP site via Internet Explorer as a Web Proxy client only, you will see the following, somewhat cryptic, warning dialog box:
The "read-only" status means that you cannot upload content to the FTP server. Therefore, if you require FTP uploads to a web site or a FTP server, you will have to configure as a SecureNAT or Firewall Client, in addition to making it a Web Proxy client. Note that you do not need to disable the machine from being a Web Proxy client; just add one of the other client types to the configuration.
I hope this article was helpful to you and clears up some questions you had about IE as a FTP client. If you have any questions about this article, please post a question to the message board or send an email to me at email@example.com with the title of this article in the subject line. - Tom.