How Is Server Software Configured? Examining the Structure of Configuration Files

Contemporary software on servers is usually configured through editing configuration files. Surprisingly, to the extent of our knowledge, the diversity of configuration file formats remains unstudied. Our goal in this work is to determine what data structures are used in configuration files and what formats are being used to encode them, in order to have a foundation for semantic analysis of their contents in the future. We examine 14133 files in 3409 packages comprising the whole set of software available in Debian stable repositories that has configuration files in the /etc directory. After eliminating files that are not configuration (such as init scripts), we assign them to categories using various criteria, some of them being the data structure they express or whether the order of statements matter. In this examination we find that even custom configuration formats can usually be expressed using one of several commonly used data structures. Some software packages, however, are configured in a Turing-complete programming language, usually the same one that the configured program was written in, and are therefore unsuitable for static analysis. Regardless, we provide a taxonomy of configuration formats, and highlight common themes in custom formats used by various packages. Ultimately, we describe a data structure that is able to hold information about all of these configuration files for further analysis.