The order of files within /etc/ssh/sshd_config.d/ directly impacts how OpenSSH's sshd daemon interprets its configuration. The daemon reads files alphabetically, applying settings sequentially. This means later files can override earlier ones, leading to unexpected behavior if not carefully managed. A common example is setting PasswordAuthentication no in a later file, negating an earlier file's Match block intended to allow password logins for specific users or groups. Therefore, understanding and controlling file order in this directory is crucial for predictable and reliable SSH configuration.
The blog post "The order of files in /etc/ssh/sshd_config.d/ matters" by Chris Siebenmann details a subtle but significant characteristic of OpenSSH's configuration mechanism that can lead to unintended consequences if not understood. Specifically, it addresses how the order of files within the /etc/ssh/sshd_config.d/ directory directly impacts the final SSH server configuration.
OpenSSH constructs its runtime configuration by first reading the main configuration file, typically /etc/ssh/sshd_config. Following this, it processes each file within the /etc/ssh/sshd_config.d/ directory in lexicographical order. This means files are read alphabetically based on their filenames. The crucial detail is that later files can override settings defined in earlier files.
Siebenmann illustrates this with a hypothetical example involving two configuration files: 50-allow-some.conf and 60-deny-some.conf. If 50-allow-some.conf grants SSH access to a specific group and 60-deny-some.conf subsequently revokes access for a subgroup within that initially granted group, the final effective configuration will deny access to the subgroup. This is because the later file, 60-deny-some.conf, overrides the settings of the earlier file. Conversely, if the file order were reversed, the initial allowance would stand, and the denial would be effectively ignored, leading to a different security outcome.
The blog post emphasizes the importance of careful naming and ordering of configuration files within /etc/ssh/sshd_config.d/. Using numerical prefixes, as demonstrated in the examples (e.g., 50- and 60-), allows for predictable ordering and helps administrators understand the precedence of configuration directives. This approach provides a structured way to manage complex configurations and avoids unintended consequences arising from implicit, alphabetical ordering. Siebenmann advocates for mindful consideration of file order to ensure the desired SSH server behavior and security posture are achieved. The post implicitly highlights that relying solely on the content of individual configuration files without considering their loading order can lead to a misinterpretation of the final, effective SSH configuration. Therefore, understanding and managing this order is critical for proper SSH server administration.
Summary of Comments ( 83 )
https://news.ycombinator.com/item?id=43573507
Hacker News users discuss the implications of sshd_config.d file ordering, mostly agreeing it's a surprising but important detail. Several commenters highlight the potential for misconfigurations and debugging difficulties due to this behavior. One user shares a personal anecdote of troubleshooting an issue caused by this very problem, emphasizing the practical impact. Others point out the lack of clear documentation on this behavior in the man pages, suggesting it's a common pitfall. The discussion also touches upon alternative configuration approaches, like using a single file or employing tools like Puppet or Ansible to manage configurations more predictably. Some users express surprise that later files override earlier ones, contrary to their expectations. The overall sentiment reinforces the importance of awareness and careful management of sshd configuration files.
The Hacker News post discussing the importance of file order in
/etc/ssh/sshd_config.d/has generated several insightful comments. Many users shared their experiences and perspectives on how this seemingly minor detail can lead to significant configuration issues.One of the most compelling comments highlights the often-overlooked fact that this directory functions similarly to how
/etc/profile.d/and/etc/apache2/conf-enabled/(and similar directories) operate. It emphasizes that the order matters because later files can override settings from earlier ones. This comment served as a good reminder for users familiar with these other directory structures, connecting the concept across different system configurations.Another interesting point raised by a commenter is the importance of documentation and explicit ordering within the
sshd_config.ddirectory. They suggested using numbered prefixes, similar to systemd's approach, to ensure predictable and maintainable configuration loading. This proposition sparked further discussion about the best practices for managing configuration snippets in this directory, with some users advocating for explicitIncludedirectives within the mainsshd_configfile for maximum clarity and control.Several commenters shared anecdotal experiences where an unexpected file order in
sshd_config.dcaused problems in their SSH configurations. These stories provided practical examples of how seemingly minor ordering issues can lead to debugging headaches, reinforcing the blog post's central argument.One user mentioned the potential benefits and drawbacks of using an include directory like this. While acknowledging the potential for order-related issues, they pointed out that it allows for more modular and manageable configuration, especially when dealing with multiple contributors or automated configuration management tools.
The discussion also briefly touched upon the use of configuration management tools like Ansible, Puppet, Chef, or Salt. A commenter suggested that these tools could further complicate the ordering issue if not handled carefully, adding another layer of complexity to the configuration management process.
Finally, there was a comment acknowledging that while the blog post's information isn't entirely new, it serves as a valuable reminder of a potential pitfall that can easily be overlooked. This reinforces the importance of such discussions in raising awareness and promoting best practices within the system administration community.