Ytdyklly

Is there an easy way to transfer files between two SSH/SFTP servers? The perfect solution would be FileZilla, but it only lets you create a connection between local and remote, but not between remote and remote.

Theoretically I could open two Nautilus windows and connect to some ssh://server1/path/to/folder and ssh://server2/path/to/folder and then just pull the files from one to the other side. My experience is that this is very unstable. Transmitting files in size sum of i.e. 10MB is no problem, but transferring i.e. 10GB often resulted in Nautilus hanging itself up and remaining there in need of ps -e | grep nautilus -> kill -9 <pid>. I also tested the same thing with Nemo and Caja. While Nemo tends to be more stable than the two others, it still is not perfect and also breaks from time to time. FileZilla is extremely stable, never really got it to break, but it is not very flexible due to the mentioned fact that it can only connect to a single SSH server.

Of course I could also mount a folder with sshfs, but this is kind of an inconvenient solution. Too much pre-work to do to get a simple transfer running.

Is there any app that can handle transfers between two SSH servers without breaking? Perfect would be something like FileZilla, that picks up the job again if the connection got interrupted.

If you are on an Ubuntu version that is still supported, then
your scp command will provide the -3 switch which enables
copying files from remote1 to remote2 via localhost:

me@local:~> scp -3 user1@remote1:/path/to/file1 user2@remote2:/path/to/file2

You can also omit the -3 switch, but then you will need the
public key (id_rsa.pub) of user1@remote1 in the
file authorized_keys of user2@remote2:

me@local:~> scp user1@remote1:/path/to/file1 user2@remote2:/path/to/file2

scp then under the hood does a ssh user1@remote1 first and from there
scp /path/to/file1 user2@remote2:/path/to/file2. That's why the credential must be
distributed different from the -3 solution.

Perhaps you could use one of several GUI front-ends to rsync:

Is there any GUI application for command rsync?

Or perhaps you could use rsync directly from the command line to connect to both remote servers:

"How to rsync files between two remotes"

I often log in to one server with ssh, then from that server's command line use rsync to push or pull files to the other remote server -- that's generally much quicker than trying to transfer the files through some 3rd computer.

The rsync is smart enough to do some work, then if anything goes wrong and interrupts the process, it can later resume right where it left off.

There's probably something I'm missing, but can't you SSH into one and connect to the other from it? Or do you want to avoid transferring one's credentials to the other? Or can they not access each other?

You need to use SCP protocol.
scp file you want to transfer login@address_of_second_server:/path_where_you_want_to_save