SFTP via ProxyJump

[misterakko]

Hi.
I have a specific need and I swear that I will personally buy a license (and possibly make my company buy more if my boss okays it) if Cyberduck can do this as promised — but either I found a bug or I'm a dummy.
There’s three servers in my organization I can only contact via ProxyJump. This is what I do in the macOS Terminal (first time I have to type in the key passphrase, from the second+ time there’s no need) to get in via SSH.

alavakko@lanhost315 ~ % cat .ssh/config
Host elearn docebo
   HostName 10.71.5.153
   ProxyJump [email protected]
   HostKeyAlgorithms +ssh-rsa
   PubkeyAcceptedAlgorithms +ssh-rsa
   User root
Host chamilo
	Hostname 10.71.5.160
	ProxyJump [email protected]
	User root
Host elmigrate
	hostname 10.71.5.95
	proxyjump [email protected]
	User root
Host glpi
    HostName 10.71.5.114
    ProxyJump [email protected]
    User root
Host gitlab.com
    IdentityFile ~/.ssh/ctoalvinnova
Host *
  AddKeysToAgent yes
  UseKeychain yes
  IdentityFile ~/.ssh/id_ed25519
  IdentityFile ~/.ssh/id_rsa


alavakko@lanhost315 ~ % ssh elmigrate
Linux elmigrate 6.1.0-32-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.129-1 (2025-03-06) x86_64
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Fri Aug  1 08:36:05 2025 from 10.71.5.1
root@elmigrate:~#

I’ve always used Transmit, but it does not support ProxyJump, so I normally use CLI sftp which is a pain in the ass. Cyberduck's website says that this app can handle ProxyJumps, but when I try with the Cyberduck unregistered version I get a failure and this error message:
Exhausted available authentication methods. OpenSSH Private Key integer comparison failed. Per ricevere assistenza contatta il servizio di hosting.

[dkocher]

The error OpenSSH Private Key integer comparison failed hints at an issue with the private key used to authenticate with. Have you set the bookmark to use a openssh-key-v1 key for authentication?

Please include the log file after enabling debugoutput. You can reach the logging output in Preferences → Connection.

[misterakko]

Yes, it does work. I shall leave some suggestions for the next person to have this issue and pass through here.
First, double check that you remember the passphrase for your key. Open the Terminal and use
ssh-keygen -y
The system will ask you which key you want to use. If you only have one it will be preselected. Then you shall be asked for the passphrase. Type it, it's normal if nothing is shown onscreen, end with the ENTER key. If you typed the wrong one, you shall get "incorrect passphrase supplied to decrypt private key". If you are right, you will get a single line starting with ssh- continuing with the key (looks like a long password) and ending in your nick (looks like an email address).

Now switch to Cyberduck. It's important that in the "server" field you only type the ProxyJump shortcut that you find in your config file. In my case, I used "elmigrate" as shown in the screenshot.

You will be asked to accept the server's signature, type the key passphrase and accept the connection, then the files will appear

[dkocher]

Thank you for sharing. The requirement to use the alias from your OpenSSH configuration is docuemnted in OpenSSH Configuration Interoperability.