FTP and NAT devices

The representation of the IPs and ports in the PORT command and PASV reply poses another challenge for NAT devices in handling FTP. The NAT device must alter these values, so that they contain the IP of the NAT-ed client, and a port chosen by the NAT device for the data connection. The new IP and port will probably differ in length in their decimal representation from the original IP and port. This means that altering the values on the control connection by the NAT device must be done carefully, changing the TCP Sequence and Acknowledgment fields for all subsequent packets. For example: A client with an IP of 192.168.0.1, starting an active mode transfer on port 1025, will send the string "PORT 192,168,0,1,4,1". A NAT device masquerading this client with an IP of 192.168.15.5, with a chosen port of 2000 for the data connection, will need to replace the above string with "PORT 192,168,15,5,7,208". The new string is 23 characters long, compared to 20 characters in the original packet. The Acknowledgment field by the server to this packet will need to be decreased by 3 bytes by the NAT device for the client to correctly understand that the PORT command has arrived to the server. If the NAT device is not capable of correcting the Sequence and Acknowledgement fields, it will not be possible to use active mode FTP. Passive mode FTP will work in this case, because the information about the IP and port for the data connection is sent by the server, which doesn't need to be NATed. If NAT is performed on the server by the NAT device, then the exact opposite will happen. Active mode will work, but passive mode will fail. It should be noted that many NAT devices perform this protocol inspection and modify the PORT command without being explicitly told to do so by the user. This can lead to several problems. First of all, there is no guarantee that the used protocol really is FTP, or it might use some extension not understood by the NAT device. One example would be an SSL secured FTP connection. Due to the encryption, the NAT device will be unable to modify the address. As result, active mode transfers will fail only if encryption is used, much to the confusion of the user. The proper way to solve this is to tell the client which IP address and ports to use for active mode. Furthermore, the NAT device has to be configured to forward the selected range of ports to the client's machine. FTP over SSH FTP over SSH refers to the practice of tunneling a normal FTP session over an SSH connection. Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will protect only that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels) which will bypass the SSH connection, and thus have no confidentiality, integrity protection, etc. If the FTP client is configured to use passive mode and to connect to a SOCKS server interface that many SSH clients can present for tunneling, it is possible to run all the FTP channels over the SSH connection. Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security's software suite, and the GPL licensed FONC are two software packages that support this mode. FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol.