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HPC @ Uni.lu

High Performance Computing in Luxembourg

FAQ: Why Can't I Access the Cluster Anymore?

There are several possibilities and usually the error message can give you some hints.

Your account has expired

Please send us an email to hpc-sysadmins@uni.lu with the current end date of your contract and we will extend your account accordingly.

Permission denied (publickey)

Basically, you are NOT able to connect to the access servers until your SSH public key is configured. There can be several reason that explain the permission denied message:

  • Ensure you have run your SSH agent
  • If you have a new computer or for some other reason you have generated a new ssh key, please update your ssh keys on the IPA user portal.
  • You are using (deprecated) DSA keys. As per the OpenSSH website: “OpenSSH 7.0 and greater similarly disable the ssh-dss (DSA) public key algorithm. It too is weak and we recommend against its use”. Solution: generate a new RSA keypair (3092 bit or more) and send it to hpc-sysadmins@uni.lu. For more information on keys, see this website. Please note that Ed25519 keys do not currently work with the Debian 7 (Wheezy) servers.
  • Your public key is corrupted, please resend it to hpc-sysadmins@uni.lu
  • We have taken the cluster down for maintenance and we forgot to activate the banner message mentioning this. Please check the calendar, the latest Twitter messages (box on the right of this page) and the messages sent on the hpc-users mailing list.

Be careful with permission changes to your $HOME

If you change your home directory to be writeable by the group, ssh will not let you connect anymore. It requires drwxr-xr-x or 755 (or less) on your $HOME and ~/.ssh, and -rw-r–r– or 644 (or less) on ~/.ssh/authorized_keys.

File and folder permissions can be verified at any time using stat $path, e.g.:

$> stat $HOME
$> stat $HOME/.ssh
$> stat $HOME/.ssh/authorized_keys

Check out the description of the notation of file permissions in both symbolic and numeric mode.

Open a ticket

If you cannot solve your problem, do not hesitate to open a ticket on hpc-tracker.