![]() Nevertheless, in any case, the firewall should be aware that it is looking for FTP connections in order to apply the correct filtering mode. But, anyway, if we must use FTP, the passive mode is usually simpler to pass through firewalls and should be used by default. One easy way to add and check hash signatures of files is just compressing them as ZIP files, or using our preferred compression scheme (as long as it has file checking capabilities, of course). transferring files to temporary folders or filenames, and renaming only a knowingly successfully transfer.sending files with trails, headers, or separated metadata files.The more commonly used methods to do this are: So, it’s up to us to check if the file transfer has been completed successfully. Aside from that, in case of connection losses, endpoints usually keep the partially transferred files. If we have too many small files, the FTP’s one data connection for each transfer design imposes some extra overhead that RSYNC would avoid. For bulk transfers, another sound option is RSYNC over SSH, VPN, or SSHFS. Or, at least, use it over a VPN or SSH tunnel. For ensuring data and credential privacy, we should try crypto-enabled counterparts, like SFTP, SCP, or HTTPS. Anyone in-between the data flow could capture not only the files but also the credential needed to initiate new data transfers. Why? First of all, FTP traffic is cleartext, which means that the user and password, and the data transfers themselves, are sent without any privacy concern. Well, if we can, we should avoid FTP whatsoever. The default on most clients (web browsers are the notable exception) is to use active mode by default: The graph below shows the two FTP operating modes. ![]() ![]() By the way, one advantage of using HTTP or HTTPS for transferring files is just that: There’s no need for multiple data connections or firewall deep inspection tricks. So, with Passive Mode, only the firewall at the server-side needs to fully inspect the traffic to open the required ports for data transfer. The Passive Mode simplifies this, not requiring that the client’s firewall opens an inbound port triggered by what is negotiated within the Control connection. It requires full packet inspection using port triggering, a firewall rule that activates one port after valid traffic is detected on another, to allow the server to actually reach the client’s callback listening port. At that time, configuring the firewall to allow active mode operation callback connections was quite tricky. The passive mode was added later to the specification – almost at the same time that Internet host administrators understood the need of using network firewalls (check our firewall intro tutorial) and proper network segmentation. In contrast, in the Passive Mode, all connections are opened from the client to the server. In Active Mode, the server actively opens the data connections (by default, it uses TCP port 20 as its source) calling back the client. So, the main difference between active and passive modes is what side will open, and what will listen, for data connections. Data Connections: each data transfer, including directory listing, opens its own Data Connection, which is closed after the stream finishes.Control Connection: the first to be open, by default, using TCP port 21, where the client sends commands to the server.The standard FTP URL has the following syntax: operates using multiple connections: Options are only available in full-fledged FTP clients. It allows even a client to command transfer between two different servers, or execute specific routines on the server. Accordingly, FTP has options to use different file encodings (binary, ASCII, and EBCDIC file), data transfer modes (stream, block, and compressed – quite limited), and operating modes (active, and passive). At that time, there were still a lot of non-compatible proprietary hardware and software architectures. For its simplicity, it has been one of the standards for non-assisted batch file transfer routines in regular datacenter operations worldwide. The FTP was designed to allow easy file transfer and remote file management to a multivendor distributed environment.
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