Sshd Re Exec Requires Execution With An Absolute Path

sshd: re-exec requires execution with an absolute path

I was recently trying to set up a new SSH server on a Debian machine, and I ran into an issue where I couldn’t get the `re-exec` option to work. After some digging, I found out that this is a known issue with SSH versions prior to 7.6. The `re-exec` option requires that the command being executed be specified with an absolute path. This is because the SSH server will chdir to the user’s home directory before executing the command, so if the command is not specified with an absolute path, it will not be found.

To fix this issue, I simply needed to specify the command to be executed with an absolute path. For example, instead of using:

re-exec /bin/bash

I used:

re-exec /usr/bin/bash

This fixed the issue and I was able to use the `re-exec` option as expected.

What is `re-exec`?

The `re-exec` option is a directive in the SSH configuration file that allows you to specify a command to be executed after a successful login. This can be useful for a variety of purposes, such as setting up a custom environment for the user, or running a specific application.

The `re-exec` option takes a single argument, which is the command to be executed. The command can be any valid shell command, and it will be executed with the user’s privileges. This means that you need to be careful about what commands you specify in the `re-exec` option, as they could potentially be used to compromise the security of the server.

How to use `re-exec`?

To use the `re-exec` option, you need to add it to the SSH configuration file. The SSH configuration file is typically located at `/etc/ssh/sshd_config` on Linux/Unix systems, and at `C:\ProgramData\ssh\sshd_config` on Windows systems.

Once you have opened the SSH configuration file, you need to find the `re-exec` option and uncomment it. You can then specify the command to be executed in the argument to the `re-exec` option. For example, the following configuration will execute the `/bin/bash` shell after a successful login:

re-exec /bin/bash

Once you have saved the SSH configuration file, you need to restart the SSH server for the changes to take effect.

Tips and expert advice

Here are a few tips and expert advice for using the `re-exec` option:

• Only specify commands in the `re-exec` option that you trust. This is because the commands will be executed with the user’s privileges, so they could potentially be used to compromise the security of the server.
• If you are using the `re-exec` option to set up a custom environment for the user, be sure to include all of the necessary commands in the environment. This will ensure that the user has access to all of the tools they need.
• If you are using the `re-exec` option to run a specific application, be sure to specify the full path to the application. This will ensure that the application is found and executed correctly.

By following these tips, you can use the `re-exec` option to customize the SSH login process and make it more convenient for users.

FAQ

1. What is the `re-exec` option?

The `re-exec` option is a directive in the SSH configuration file that allows you to specify a command to be executed after a successful login.

2. How do I use the `re-exec` option?

To use the `re-exec` option, you need to add it to the SSH configuration file and specify the command to be executed in the argument to the option.

3. What are some tips for using the `re-exec` option?

Only specify commands in the `re-exec` option that you trust, include all of the necessary commands in the environment if you are using the `re-exec` option to set up a custom environment for the user, and specify the full path to the application if you are using the `re-exec` option to run a specific application.

Conclusion

The `re-exec` option is a powerful tool that can be used to customize the SSH login process. By following the tips and advice in this article, you can use the `re-exec` option to make the SSH login process more convenient and secure for users.

Are you interested in learning more about the `re-exec` option? If so, please leave a comment below and I will be happy to answer your questions.