sshd re-exec requires execution with an absolute path

Guri Bee

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22-10-2024

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sshd Re-exec: Why Absolute Paths Are Crucial

When configuring your SSH server (sshd), you might encounter the error "sshd re-exec requires execution with an absolute path." This error message indicates that the sshd daemon is unable to execute a command due to a missing absolute path. Let's dive into the reasons behind this behavior and how to fix it.

Understanding the Problem

The sshd daemon, responsible for handling secure shell connections, uses a mechanism called "re-exec" to run commands requested by clients. Essentially, when you execute a command through SSH, sshd spawns a new process (the re-exec process) to run the command in a secure environment.

This re-exec process needs to locate the executable file associated with your command. The problem arises when you provide a command path that is relative instead of absolute. A relative path depends on the current working directory, which may be different for the re-exec process compared to the sshd daemon itself.

This difference in working directories can cause the re-exec process to fail to find the executable file, leading to the error.

Example:

Let's say you want to run the command my_script.sh which is located in /home/user/scripts/. If you provide the command as /home/user/scripts/my_script.sh, the re-exec process can locate the script because you've provided the absolute path.

However, if you try to run the command as ./my_script.sh (relative path), the re-exec process may not find the script because its current working directory might not be /home/user/scripts/. This will trigger the "sshd re-exec requires execution with an absolute path" error.

Solutions

To overcome this issue, ensure you always provide absolute paths when configuring commands in sshd:

1. Use Absolute Paths in sshd_config:

   When specifying commands within sshd_config, such as PermitUserEnvironment, ChrootDirectory, or ForceCommand, use absolute paths to ensure they are always accessible.

   Example:

   PermitUserEnvironment yes
   ChrootDirectory /var/empty/chroot
   ForceCommand /usr/bin/bash -l
   

2. Employ Absolute Paths in Shell Scripts:

   If you are using shell scripts to handle commands, make sure to reference all executables within the script using absolute paths. This guarantees consistency regardless of the working directory.

   Example:

   #!/bin/bash
   
   # Use absolute path for 'ls' command
   /bin/ls -l /home/user/data
   

Why Absolute Paths Are Essential

Using absolute paths in SSH configuration and scripts is crucial for several reasons:

• Security: Absolute paths help prevent potential security risks by ensuring commands are executed only from specific locations. This mitigates the possibility of attackers exploiting relative paths to execute malicious code in unexpected directories.
• Reliability: Absolute paths ensure consistency across different systems and user accounts, preventing issues related to variable working directories.
• Maintainability: Absolute paths make your configurations more readable and maintainable. They provide a clear indication of the command's exact location, reducing confusion and simplifying debugging.

Conclusion

The "sshd re-exec requires execution with an absolute path" error highlights the importance of using absolute paths in SSH configurations and scripts. By adopting this practice, you ensure the reliable and secure execution of commands within your SSH environment.

Note: The examples and information in this article are based on commonly used practices. Always consult your specific SSH server documentation for the most accurate and up-to-date information on configuring commands and paths.