Ancient topic, but I ran into similar problems recently and figured my $.02 might help somebody.
In my case, we're running IIS 7.5 behind a slightly older version of ASA, which we're in the process of replacing. We have an existing FTP site and my plan was to simply add FTPS support with the certificate & maybe getting our network admins to open up a few ports. IIS has a similar masquerade setting for each FTP site named "External IP Address of Firewall", which is, itself, misleading.
the TL/DR version: If your FTP server allows you to specify a masquerade IP & a range of ports used for PASV connections, you SHOULD be able to fix this by opening up those ports & disabling ftp inspection.
Due to some some other constraints, I wasn't able to get inspection disabled on our ASA, so I had to make some compromises. Here's what I observed/learned:
- The ASA can only inspect non-encrypted traffic. duh?
- The default behavior of the ASA is to inspect a number of protocols, including FTP.
- the client authenticates on server port 21 and determines the feature set supported by the server.
- The client, if so configured, will send a PASV request to the server
- The server will respond with "227 entering passive mode (a,b,c,d,e,f)" where a.b.c.d is the server address and e*256 + f = port number
- The a.b.c.d address will be the internal IP unless the masquerade address is configured
- the FTP inspection will rewrite the a.b.c.d address to the external IP and open up the specified port for this client.
- If the a.b.c.d address IS the external address, the response packet is discarded. *This might be due to the strict option, which I could not verify.
- CuteFTP will recognize a non-routable IP in the PASV response and attempt to use the server's external address instead.
- the ASA can't read the SSL-encrypted FTPS traffic, so it bypasses the inspection and works.
So in our case, when I set the masquerade IP, I was able to connect just fine via FTPS, but regular FTP would fail. When I removed the masquerade IP, I was still able to connect to both FTP and FTPS using CuteFTP, but our primary client wasn't able to connect to FTPS. (their system wasn't "smart" enough to translate the non-routable IP...)
So my lame workaround was using two separate sites: one that used a masquerade IP and required SSL, the other site that didn't.
TMI, but maybe it helps somebody work through this.