CONF File Documentation

Summary

A .conf file is a Configuration Information File: plain-text settings that tell a program or system service how to behave. It is the Unix and Linux cousin of a .cfg or .ini file, and its MIME type is text/plain. The syntax inside depends on which program owns it (Apache directives, nginx blocks, Key Value lines, INI-style key=value). Open and edit it with a code editor (Notepad++ or VS Code, nano or vim on Linux), never a word processor. Always copy the file first: a wrong change can stop a service from starting.

Technical details

FeatureValue
Full nameConfiguration Information File
File extension.conf
MIME typetext/plain
Format typeGeneric plain-text configuration file (syntax is application-defined)
DeveloperNone — a Unix/Linux convention used by many applications
CategorySettings file
Typical location/etc and its subfolders on Linux/Unix systems
EncodingPlain text (usually UTF-8 / ASCII)
Magic numberNone (identified by the owning program, not a signature)
Comment syntaxLines beginning with # (sometimes ;)
Common syntaxesApache directives, nginx brace blocks, Key Value, INI key=value
Read whenAt program / service startup
Open standardNo (no single specification)
Edit withNotepad++, VS Code, nano, vim, gedit — never a word processor
Line endingsUse Unix (LF) for Linux server configs
Related extensions.cfg, .ini, .config, .yaml, .toml, .json

What is a CONF file?

CONF is short for “configuration”, and a .conf file stores settings that control how a program, or very often a system service, behaves. It is essentially the same idea as a .cfg or .ini file, but the .conf name is a Unix and Linux convention: it is what fills the /etc directory on a Linux box (httpd.conf, nginx.conf, sshd_config, smb.conf, resolv.conf and hundreds more). There is no single CONF standard. It is a generic extension that thousands of unrelated programs reuse, so the exact syntax inside depends entirely on which software owns the file.

Nearly all .conf files are plain text, and that is the whole point: administrators hand-edit them to change how a service runs. There is no magic number to identify one and no fixed binary layout, because there is no format, only a naming convention. You identify a .conf by its location and name (for example /etc/ssh/sshd_config) or by reading its directives, not by a signature.

Why the syntax differs per application

The internal format differs by program, and the same .conf extension can hold several quite different grammars. A few that you will meet constantly:

# Apache httpd: directive blocks
<VirtualHost *:80>
    ServerName example.com
    DocumentRoot /var/www/example
</VirtualHost>

# nginx: brace-delimited blocks
server {
    listen 80;
    server_name example.com;
    root /var/www/example;
}

# OpenSSH sshd_config: Key Value lines
Port 22
PermitRootLogin no
PasswordAuthentication yes

# INI-style: [sections] and key=value
[database]
host=127.0.0.1
port=5432

Apache uses directive blocks delimited by tags such as <VirtualHost>; nginx uses C-like brace blocks ending in semicolons; OpenSSH uses bare Key Value lines with no equals sign; and many applications use INI-style key=value grouped under [section] headers. None of these is “the” .conf format; each program defines and parses its own.

Common building blocks across CONF files

Despite the differing grammars, most .conf files share the same kinds of building block. There are directives or key-value lines, which are the actual settings: an option name and its value. There are blocks or sections that group related settings, whether that is an Apache container tag, an nginx brace block, or an INI [section]. There are comment lines, conventionally prefixed with # (some formats use ;), which the program ignores and which are used both for documentation and for temporarily disabling a setting. Many .conf files also use include directives that pull in other files, such as Apache’s Include or the widespread conf.d/*.conf pattern, so one main config can assemble many smaller fragments. And throughout, values are things the program reads at startup: file paths, network ports, and on/off flags.

Read at startup: why a typo takes a service down

The defining behavioural fact about a .conf is when it is read: at program or service startup. The service parses the whole file, and if it hits a syntax error (a missing brace, an unclosed tag, an unrecognised directive) or an invalid value, it commonly refuses to start at all rather than starting with a broken configuration. That is why a single careless edit to nginx.conf or sshd_config can take a web server offline or, worse, lock you out of a remote machine over SSH.

Two habits follow directly from this. First, back up the file before you touch it: cp file file.bak gives you a one-command restore. Second, use the program’s own configuration test where one exists, before restarting the service. nginx -t checks nginx syntax and reports the exact line of any error; apachectl configtest does the same for Apache; sshd -t validates an SSH config. Running the test first means you find the mistake while the running service is still up.

Editing a CONF file without corrupting it

Because a .conf is plain text, the tool has to preserve it as plain text. Use a code-aware editor: Notepad++ or Visual Studio Code on Windows, TextEdit in plain-text mode (Format > Make Plain Text) on macOS, or nano, vim, gedit or Emacs on Linux. Do not use a word processor. WordPad and Microsoft Word can silently insert formatting or rich-text structure that is invisible on screen but corrupts the file for the program that reads it.

Line endings are the other common trap. Linux services expect Unix (LF) line endings, and a file saved from a Windows editor with CRLF endings can confuse some parsers. In Notepad++, set Edit > EOL Conversion > Unix (LF) before saving a Linux config. Beyond the mechanics, change one setting at a time so that if the service fails to start you know exactly which line to blame, and treat configuration copied from an untrusted tutorial with caution, since a config can point a service at unexpected paths or relax a security setting.

CONF, CFG and INI: the same job, different names

It is worth being explicit that .conf, .cfg, .ini and .config are, in practice, interchangeable names for plain-text configuration files. They do the same job; the difference is convention. .conf is the Unix and Linux tradition, seen throughout /etc. .cfg and .ini are more common on Windows and in desktop applications. Renaming one to another does not change the syntax inside, and it only makes sense if a specific program insists on a particular extension. The program that reads a live .conf still expects its original filename, so do not rename the file a running service depends on.

Frequently asked questions

How do I open a CONF file?

.conf files are plain text. Open them with Notepad++ or VS Code on Windows, TextEdit in plain-text mode on macOS, or nano, vim or gedit on Linux, where they mostly live in /etc. Avoid word processors, which can corrupt the file with hidden formatting.

What is the difference between CONF, CFG and INI?

Practically none. All three are conventional names for plain-text configuration files. .conf is the Unix and Linux convention (think /etc), while .cfg and .ini are more common on Windows. The internal syntax depends on the program, not the extension.

My service will not start after I edited its CONF — what do I do?

Restore the backup you made and restart the service. If you did not back it up, re-check your change for typos, missing braces or semicolons, and wrong line endings, and run the program’s config-test command (such as nginx -t) to find the bad line. Many programs log the exact line number of the error.

References