How to specify a specific port with RSYNC?

`rsync` is a very powerful tool used by system administrators for backing up and synchronizing data. You can use it to copy files from one device to another device locally or remotely. It provides fast incremental file transfer by transferring only the differences between the source and the destination.

To specify a specific port with `rsync`, you should use the `-e` option followed by `ssh -p [port_number]`. Here is a basic format:

```
rsync -avz -e ‘ssh -p ’ /source/directory/ user@destination:/destination/directory/
```

For example, if you want to synchronize files to a remote server that listens on port 2222, you would specify the port like this:

```
rsync -avz -e ‘ssh -p 2222’ /home/user/Documents/ user@192.168.1.104:/home/user/Backup/
```

In this example, `-a` (archive mode) equals to `-rlptgoD`, it is a quick way of saying you want recursion and want to preserve almost everything.
`-v` (verbose) increases the amount of information you are given during the transfer. By using `-v` option, you can see what is being transferred.
`-z` (compress file data) decrease the data size.

You should replace `/home/user/Documents/` with the directory you want to synchronize, `user@192.168.1.104` with the user and IP of your remote server, and `/home/user/Backup/` with the directory where you want to store the backup on the remote server.

The `-e ‘ssh -p 2222’` part tells rsync to use SSH as the remote shell with port `2222`.

In regard to resources for this information, references include the official Rsync documentation available on the official site (https://download.samba.org/pub/rsync/rsync.html), and a variety of Unix/Linux forums and tutorials such as LinuxCommand (http://linuxcommand.org/lc3_man_pages/rsync1.html) and TecMint (https://www.tecmint.com/rsync-local-remote-file-synchronization-commands/). These sources provide an extensive and thorough overview of the rsync command, including its options, use cases, and examples.

In case your rsync connection is unsuccessful due to firewall restrictions, you should check your system’s firewall settings. If you don’t have privileges to alter these settings, you may need to consult with your network administrator.

Just remember this, while `rsync` is a very powerful tool, with great power comes great responsibility. Always double-check your commands before you press the Enter key. Using the wrong options, you could end up deleting files instead of backing them up.