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I'd like to have multi FTP accounts:

  • ftp.domain1.com binding to port 21
  • ftp.domain2.com binding to port 21
  • ftp.domain3.com binding to port 21

IIS 7.5 always show "This ftp site cannot be started. Another ftp site may be using the same port"

Is it not possible to use the default port 21 for all ftp sites?

Thanks.

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  • Only one application can use a single IP address/transport protocol/port at a time. FTP uses port 21 for control, but it uses port 20 for data transfer. You need to make sure both ports are open.
    – Ron Maupin
    Commented Dec 28, 2015 at 9:22

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No, you can not. At least not universally.

The combination of IP:port identifies something unique. You can only tie one application to such a combination. That means one FTP server, or one mail server, or one webserver, ...

Note that webservers do get around the limitations of that by also sending the name of the website in their request, so they basically steer on a triplet. Most FTP client and servers do not.*

*Most as in it has been suggested for FTP and a RFC is released. But that does not mean that all clients support it. You might get it working in a small office with controlled clients, but do not yet try to use it as a general working solution on an Internet accessible FTP server with unknown clients. Instead consider one site with three directories. Or something else than ancient FTP.

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  • Almost. One application could use FTP on TCP port 21, while a second application on the same IP address could use FTP on UDP port 21. The uniqueness includes the transport protocol.
    – Ron Maupin
    Commented Dec 28, 2015 at 11:20
  • Good creative thinking. :)
    – Hennes
    Commented Dec 28, 2015 at 11:21

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