Troubleshooting

How to debug your SSH connection

There are – unfortunately – a number of things that can go wrong when trying to connect to a remote system. But no need to fret: Most of these issues are quickly fixed and this guide will show you how to do so.

Common Pitfalls#

A lot of users that run into trouble when setting up SSH, come across one of the following problems. Although these points have been mentioned in conjunction with the SSH setup before, it doesn’t hurt to double check them before you dive deeper into debugging:

1. Has at least an hour passed since you uploaded your public key to our website? The propagation of your key through our systems does not happen instantaneously.
2. Does the SSH key you uploaded to your account website have the HPCGATE and HPCLOGIN attribute? You can check the uploaded key again on the JGU account website → here.
3. Is your SSH agent running and aware of your SSH key’s location? How to check this depends on which SSH client you use and is explained in more detail below.

SSH Agents#

On Linux and MacOS an SSH agent is usually running automatically once you open the terminal. You can also start the SSH agent manually using the command eval $(ssh-agent -s). Afterwards, run the command

ssh-add <path-to-private-key>

in order to introduce your SSH key to your agent. This way, you won’t have to retype your passphrase again for the rest of the session.

In case of PowerShell on Windows the equivalent command is Get-Service ssh-agent.

If you use Putty or MobaXterm, make sure Pageant is running. Search for the program and after opening it look out for its symbol on your Windows system tray. Right-click and select View keys. Use the GUI to add your SSH key.

Investigating Log Files#

On Linux and MacOS you can simply increase the verbosity level of your SSH command with the flag -vv to gain more insight into your connection attempt. For example:

ssh -vv -J <username>@hpcgate.zdv.uni-mainz.de <username>@mogon

Logging for PuTTY#

The Logging configuration panel allows you to save log files of your PuTTY sessions, for debugging, analysis or future reference.

Prev

1 2 3 4 5

Next

1. To access the PuTTY logging configuration panel click on Logging under the Session-section

2. The main option is a radio-button set that specifies whether PuTTY will log anything at all. The default option is None.

3. From the radio-button set we selected SSH packets and raw data. In this mode (which is only used by SSH connections), the SSH message packets sent over the encrypted connection are written to the log file (as well as Event Log entries). You might need this to debub a network-level problem. Be warned that if you log in using a password, the password can appear in the log file.

4. In the Log file name edit box you enter the name of the file you want to log the session to. The Browse button will let you choose a place to put the file. Alternatively you can type a pathname into the edit box.

5. The Log file name edit box has a few special features. If you use the &-character in the file name box, PuTTY will insert details of the current session in the name of the file it actually opens.

   • &Y will be replaced by the current year, as four digits.

   • &M will be replaced by the current month, as two digits.

   • &D will be replaced by the current day of the month, as two digits.

   • &T will be replaced by the current time, as six digits (HHMMSS) with no punctuation.

   • &H will be replaced by the host name you are connecting to (or the serial line, for serial connection).

   • &P will be replaced by the port number you are connecting to on the target host.

     Example:

     putty_&H_&Y&M&D_&T.log

Firewall settings#

In rare cases connections might fail because of firewall settings. If you believe that to be your case, you can confirm if other SSH connections, for example to Gitlab or Github, also fail. Please look up instructions on managing the firewall for your respective operating system.

Contacting Us#

If you have followed the steps outlined above and still can’t find the issue, we will gladly help you out! Please remember to attach the log file to your e-mail.