Here is a fun one.
I have inherited a Windows 2008 R2 Standard Server that is acting as the company's FTP server. The server has IIS installed and configured with 9 different FTP sites all configured to use host headers. As far as the FTP server goes, everything is functioning well with the exception to one little issue. One customer has said that she used to be able to use Internet Explorer and access her ftp site and browse and download documents and this has always worked well. However now when she tries to access her ftp site with Internet Explorer, she just gets page cannot be displayed error.
After working through a series of troubleshooting steps I have come to discover she cannot connect with Internet Explorer only when she is inside the network. When she is working from home, she is able to access the FTP site without issue using Internet Explorer. But when she comes into work and tries to access the site it fails. However, if she uses Windows Explorer to access the site there is no issue if she is inside or outside the network. FireFox works inside or outside the network as well as FileZilla. The issue is only occurring when using Internet Explorer when inside our network. I haven't taken the time to test other version of Internet Explorer like 8 or 7, but I figured that it could not be that simple. But I have been able to confirm that its not a configuration issue with her client because I can easily reproduce the issue on my computer which is running Windows 7 Enterprise, x64.
Using WireShark and seeing what is actually occuring during the connection phase, WireShark is showing the FTP server is sending a "530 Valid hostname is expected" response back to the client. When not inside the network and connecting across the internet, the FTP server is sending a "331 Valid hostname is expected". My assumption is when inside the network, Internet Explorer is trying to connect anonymously and fails immediately. But instead of a dialog box appearing where you can enter a username and password, the connection dies and IE displays "Internet Explorer cannot display the webpage".
So I'm at a lost cause as to why this would work externally without issue when there is a firewall in the way which could potentially cause it to fail but not work internally. Again, if using Windows Explorer there is no issue, it only happens when using Internet Explorer.
Thanks for the help in advance.