In my "FTP Clients - Part 1: Web Browser Support" blog post, I mentioned creating a secured Global Listener FTP Site when you're working with FTP virtual hosts, but I didn't really explain what I meant by that or why you would want to do this. With that in mind, today's blog post is to describe how and why you might want to create a Global Listener FTP Site.
To start things off, the concept is really simple - a Global Listener FTP Site is an FTP site with no virtual host binding and anonymous access disabled. It's kind of like having a "Default FTP Site" with restricted access. Here's why this is a good idea when you're working with FTP virtual hosts - some clients default to anonymous, like web browsers, and if anonymous succeeds then the FTP client doesn't have the opportunity to enter the FTP virtual host name, so you can't get to the virtual host site.
To refresh everyone's memory, there are two different methods for binding multiple FTP host names to IP addresses in FTP7:
- FTP Virtual Host Names - This uses the "ftp.example.com|username" syntax as part of the client login in order to route FTP requests to the correct FTP site. This syntax is compatible with FTP almost every FTP clients, and should be thought of as a backwards-compatible method for binding multiple FTP host names to a single IP address.
- FTP True Host Names - This uses the FTP HOST command, which is still only an IETF draft at the moment. In the future this may be the way that FTP clients and servers automatically communicate with each other, like the "Host: www.example.com" header does for HTTP, but that may still have a few years at the very least.
Unless your FTP client allows sending custom FTP commands, you won't be able to use FTP True Host Names, so if you want to host several FTP sites on the same IP address then your only option is to use FTP Virtual Host Names. The trouble is, as I mentioned earlier, that some FTP clients (like web browsers) try to log in using anonymous first. If all of your FTP sites are bound to a virtual host name, the FTP client will get a "550-No such host is known" error from the FTP server, because the anonymous user did not specify a virtual host name as part of the USER command. On some clients you could fix that by specifying "ftp.example.com|anonymous" as your anonymous user name, but in most cases the login attempt will just fail.
If you create an FTP site that has no virtual host name, then the FTP service will have some place to send these default anonymous requests. When this FTP site does not have anonymous access enabled, the client will be prompted for their username, which will allow you to enter the "ftp.example.com|username" syntax to specify the virtual host name.
Please note that creating a Global Listener FTP site is really more of a workaround for the way that some FTP clients behave - it's certainly not required, and it only applies to situations where you are using FTP Virtual Host Names. For example, if you are using user isolation to restrict users to specific paths on a single FTP site, the Global Listener FTP site would be completely unnecessary.
Note: See my Virtual Hosts and Host Names in FTP7 blog post for more information about FTP Virtual Host Names and FTP True Host Names, and see https://datatracker.ietf.org/drafts/draft-hethmon-mcmurray-ftp-hosts/ for more information about status of the FTP HOST command.