How to Solve the SSH Key Authentication Hanging Problem on Debian Server: A Step-by-Step Guide

Question:

I have a headless Debian server (rented VPS) with two accounts that use RSA key authentication. When I try to log in with Putty, the process hangs at the “Authenticating” message indefinitely. This happens for both accounts, even though I have root ssh enabled in the config. I have rebooted the server, removed and re-added the authorized_keys file, and verified that the key works for another account. However, none of these steps solved the problem. I need to fix this issue as soon as possible to grant access to a third-party. What could be causing the ssh key authentication to hang and how can I resolve it?

Answer:

How to Fix SSH Key Authentication Hanging Issue on Debian Server

If you have a headless Debian server (rented VPS) with two accounts that use RSA key authentication, you may encounter a problem when you try to log in with Putty. The process may hang at the “Authenticating” message indefinitely, and you may not be able to access your server. This can be frustrating, especially if you need to grant access to a third-party. In this article, we will explain what could be causing the ssh key authentication to hang and how to resolve it.

There are several possible causes for the ssh key authentication hanging issue. Some of them are:

  • A problem with your ~/.bashrc or /etc/profile files. These files are executed when you log in to your server, and they may contain commands or scripts that are blocking or slowing down the login process. For example, if you have a command that tries to access a network resource that is unavailable, it may cause the ssh session to hang.
  • A problem with your authorized_keys file. This file contains the public keys that are allowed to log in to your server. It may be corrupted, misconfigured, or have incorrect permissions. For example, if you have a command=”command” directive attached to your key, it may prevent you from logging in normally. Or, if you have a wrong key or a duplicate key in the file, it may cause a conflict.
  • A problem with your network or firewall settings. There may be something in your network or firewall that is blocking or interfering with the ssh connection. For example, if you have a DNS issue that prevents you from resolving the hostname of your server, or if you have a port that is closed or filtered, it may prevent you from logging in.
  • Possible Solutions

    Depending on the cause of the problem, there are different solutions that you can try. Some of them are:

  • Run ssh with debug options. You can use the -vvv option to run ssh with verbose output, which can help you identify where the problem is. For example, you can run ssh -vvvvv user@hostname and see what messages are displayed. If you see a message like “PTY allocation request failed on channel 0”, it may indicate a problem with your ~/.bashrc or /etc/profile files. If you see a message like “Server denied our key”, it may indicate a problem with your authorized_keys file. If you see a message like “Connection timed out”, it may indicate a problem with your network or firewall settings.
  • Run sshd with debug options. You can also run the ssh daemon on your server with debug options, which can help you see what is happening on the server side. For example, you can run /usr/sbin/sshd -d -D -p 222 on your server, which will run sshd in debug mode on port 222. Then, you can run ssh -v -C -A -X -p 222 user@hostname on your client, which will connect to the server on port 222. You can then see the output of sshd on the server and see if there is any error or warning.
  • Check and modify your ~/.bashrc or /etc/profile files. If you suspect that there is something in your ~/.bashrc or /etc/profile files that is causing the problem, you can try to modify them or bypass them. For example, you can run ssh -vvvvv user@hostname “/bin/bash –noprofile –norc -i” on your client, which will run bash without executing the ~/.bashrc or /etc/profile files. If this lets you log in, then you can edit the files and remove or comment out the problematic commands or scripts.
  • Check and modify your authorized_keys file. If you suspect that there is something in your authorized_keys file that is causing the problem, you can try to modify it or replace it. For example, you can log in to your server with another account that works, and then check the permissions, ownership, and content of the authorized_keys file. You can use chmod, chown, and nano commands to change them. You can also try to remove and re-add the public key of the account that does not work, or use a different key pair.
  • Check and modify your network or firewall settings. If you suspect that there is something in your network or firewall that is causing the problem, you can try to modify them or bypass them. For example, you can use nslookup, ping, and telnet commands to test the connectivity and resolution of your server. You can also use Network Utility or other tools to scan for open ports and see if port 22 is open. You can also check your firewall settings and see if there is any rule that is blocking or filtering the ssh traffic.
  • Conclusion

    In

this article, we have explained what could be causing the ssh key authentication to hang and how to resolve it. We hope that this article has helped you solve your problem and access your server. If you have any questions or feedback, please feel free to leave a comment below. Thank you for reading.

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