Caddyfile Concepts

This document will help you learn about the HTTP Caddyfile in detail.

1. Structure
   • Blocks
   • Directives
   • Tokens and quotes
2. Global options
3. Addresses
4. Matchers
5. Placeholders
6. Snippets
7. Named Routes
8. Comments
9. Environment variables

Structure

The Caddyfile's structure can be described visually:

Key points:

• An optional global options block can be the very first thing in the file.

• Snippets or named routes may optionally appear next.

• Otherwise, the first line of the Caddyfile is always the address(es) of the site to serve.

• All directives and matchers must go in a site block. There is no global scope or inheritance across site blocks.

• If there is only one site block, its curly braces { } are optional.

A Caddyfile consists of at least one or more site blocks, which always starts with one or more addresses for the site. Any directives appearing before the address will be confusing to the parser.

Blocks

Opening and closing a block is done with curly braces:

... {
	...
}

• The open curly brace { must be at the end of its line and preceded by a space.

• The close curly brace } must be on its own line.

When there is only one site block, the curly braces (and indentation) are optional. This is for convenience to quickly define a single site, for example, this:

localhost

reverse_proxy /api/* localhost:9001
file_server

is equivalent to:

localhost {
	reverse_proxy /api/* localhost:9001
	file_server
}

when you have only a single site block; it's a matter of preference.

To configure multiple sites with the same Caddyfile, you must use curly braces around each one to separate their configurations:

example1.com {
	root * /www/example.com
	file_server
}

example2.com {
	reverse_proxy localhost:9000
}

If a request matches multiple site blocks, the site block with the most specific matching address is chosen. Requests don't cascade into to other site blocks.

Directives

Directives are functional keywords which customize how the site is served. They must appear within site blocks. For example, a complete file server config might look like this:

localhost {
	file_server
}

Or a reverse proxy:

localhost {
	reverse_proxy localhost:9000
}

In these examples, file_server and reverse_proxy are directives. Directives are the first word on a line in a site block.

In the second example, localhost:9000 is an argument because it appears on the same line after the directive.

Sometimes directives can open their own blocks. Subdirectives appear on the beginning of each line within directive blocks:

localhost {
	reverse_proxy localhost:9000 localhost:9001 {
		lb_policy first
	}
}

Here, lb_policy is a subdirective to reverse_proxy (it sets the load balancing policy to use between backends).

Unless otherwise documented, directives cannot be used within other directive blocks. For example, basic_auth cannot be used within file_server because the file server does not know how to do authentication; but you can use directives within route, handle, and handle_path blocks because they are specifically designed to group directives together.

Note that when the HTTP Caddyfile is adapted, HTTP handler directives are sorted according to a specific default directive order unless in a route block, so the order of appearance of the directives does not matter except in route blocks.

Tokens and quotes

The Caddyfile is lexed into tokens before being parsed. Whitespace is significant in the Caddyfile, because tokens are separated by whitespace.

Often, directives expect a certain number of arguments; if a single argument has a value with whitespace, it would be lexed as two separate tokens:

directive abc def

This could be problematic and return errors or unexpected behavior.

If abc def is supposed to be the value of a single argument, it needs to be quoted:

directive "abc def"

Quotes can be escaped if you need to use quotes in quoted tokens, too:

directive "\"abc def\""

To avoid escaping quotes, you can instead use backticks ` ` to enclose tokens; for example:

directive `{"foo": "bar"}`

Inside quoted tokens, all other characters are treated literally, including spaces, tabs, and newlines. Multi-line tokens are thus possible:

directive "first line
	second line"

Heredocs are also supported:

example.com {
	respond <<HTML
		<html>
		  <head><title>Foo</title></head>
		  <body>Foo</body>
		</html>
		HTML 200
}

The opening heredoc marker must start with <<, followed by any text (uppercase letters recommended). The closing heredoc marker must be the same text (in the above example, HTML). The opening marker can be escaped with \<< to prevent heredoc parsing, if needed.

The closing marker can be indented, which causes every line of text to have that much indentation stripped (inspired by PHP) which is nice for readability inside blocks while giving great control of the whitespace in the token text. The trailing newline is also stripped, but can be retained by adding an extra blank line before the closing marker.

Additional tokens may follow the closing marker as arguments to the directive (such as in the example above, the status code 200).

Global options

A Caddyfile may optionally start with a special block that has no keys, called a global options block:

{
	...
}

If present, it must be the very first block in the config.

It is used to set options that apply globally, or not to any one site in particular. Inside, only global options can be set; you cannot use regular site directives in them.

For example, to enable the debug global option, which is commonly used to produce verbose logs for troubleshooting:

{
	debug
}

Read the Global Options page to learn more.

Addresses

An address always appears at the top of the site block, and is usually the first thing in the Caddyfile.

These are examples of valid addresses:

From the address, Caddy can potentially infer the scheme, host and port of your site. If the address is without a port, the Caddyfile will choose the port matching the scheme if specified, or the default port of 443 will be assumed.

If you specify a hostname, only requests with a matching Host header will be honored. In other words, if the site address is localhost, then Caddy will not match requests to 127.0.0.1.

Wildcards (*) may be used, but only to represent precisely one label of the hostname. For example, *.example.com matches foo.example.com but not foo.bar.example.com, and * matches localhost but not example.com. See the wildcard certificates pattern for a practical example.

To catch all hosts, omit the host portion of the address, for example, simply https://. This is useful when using On-Demand TLS, when you don't know the domains ahead of time.

If multiple sites share the same definition, you can list all of them together, separated with spaces and commas (at least one space is necessary). The following three examples are equivalent:

# Comma separated site addresses
localhost:8080, example.com, www.example.com {
	...
}

or

# Space separated site addresses
localhost:8080 example.com www.example.com {
	...
}

or

# Comma and new-line separated site addresses
localhost:8080,
example.com,
www.example.com {
	...
}

An address must be unique; you cannot specify the same address more than once.

Placeholders cannot be used in addresses, but you may use Caddyfile-style environment variables in them:

{$DOMAIN:localhost} {
	...
}

By default, sites bind on all network interfaces. If you wish to override this, use the bind directive or the default_bind global option to do so.

Matchers

HTTP handler directives apply to all requests by default (unless otherwise documented).

Request matchers can be used to classify requests by a given criteria. With matchers, you can specify exactly which requests a certain directive applies to.

For directives that support matchers, the first argument after the directive is the matcher token. Here are some examples:

root *           /var/www  # matcher token: *
root /index.html /var/www  # matcher token: /index.html
root @post       /var/www  # matcher token: @post

Matcher tokens can be omitted entirely to match all requests; for example, * does not need to be given if the next argument does not look like a path matcher.

Read the Request Matchers page to learn more.

Placeholders

Placeholders are a simple way to inject dynamic values into your static configuration. They can be used as arguments to directives and subdirectives.

Placeholders are bounded on either side by curly braces { } and contain the identifier inside, for example: {foo.bar}. The opening placeholder brace can be escaped \{like.this} to prevent replacement. Placeholder identifiers are typically namespaced with dots to avoid collisions across modules.

Which placeholders are available depends on the context. Not all placeholders are available in all parts of the config. For example, the HTTP app sets placeholders that are only available in areas of the config related to handling HTTP requests (i.e. in HTTP handler directives and matchers, but not in tls configuration). Some directives or matchers may set their own placeholders too which can be used by anything that follows them. Some placeholders are globally available.

You can use any placeholders in the Caddyfile, but for convenience you can also use some of these equivalent shorthands which are expanded when the Caddyfile is parsed:

Not all config fields support placeholders, but most do where you would expect it. Support for placeholders needs to have been explicitly added to those fields. Plugin authors can read this article to learn how to add support for placeholders in their own modules.

Snippets

You can define special blocks called snippets by giving them a name surrounded in parentheses:

(logging) {
	log {
		output file /var/log/caddy.log
		format json
	}
}

And then you can reuse this anywhere you need, using the special import directive:

example.com {
	import logging
}

www.example.com {
	import logging
}

The import directive can also be used to include other files in its place. If the argument does not match a defined snippet, it will be tried as a file. It also supports globs to import multiple files. As a special case, it can appear anywhere within the Caddyfile (except as an argument to another directive), including outside of site blocks:

{
	email admin@example.com
}

import sites/*

You can pass arguments to an imported configuration (snippets or files) and use them like so:

(snippet) {
	respond "Yahaha! You found {args[0]}!"
}

a.example.com {
	import snippet "Example A"
}

b.example.com {
	import snippet "Example B"
}

Read the import directive page to learn more.

Named Routes

⚠️ Experimental

Named routes use syntax similar to snippets; they're a special block defined outside of site blocks, prefixed with &( and ending in ) with the name in between.

&(app-proxy) {
	reverse_proxy app-01:8080 app-02:8080 app-03:8080
}

And then you can reuse this named route within any site:

example.com {
	invoke app-proxy
}

www.example.com {
	invoke app-proxy
}

This is particularly useful to reduce memory usage if the same route is needed in many different sites, or if multiple different matcher conditions are needed to invoke the same route.

Read the invoke directive page to learn more.

Comments

Comments start with # and proceed until the end of the line:

# Comments can start a line
directive  # or go at the end

The hash character # for a comment cannot appear in the middle of a token (i.e. it must be preceded by a space or appear at the beginning of a line). This allows the use of hashes within URIs or other values without requiring quoting.

Environment variables

If your configuration relies on environment variables, you can use them in the Caddyfile:

{$ENV}

Environment variables in this form are substituted before Caddyfile parsing begins, so they can expand to empty values (i.e. ""), partial tokens, complete tokens, or even multiple tokens and lines.

For example, an environment variable UPSTREAMS="app1:8080 app2:8080 app3:8080" would expand to multiple tokens:

example.com {
	reverse_proxy {$UPSTREAMS}
}

A default value can be specified for when the environment variable is not found, by using : as the delimiter between the variable name and the default value:

{$DOMAIN:localhost} {

}

If you want to defer the substitution of an environment variable until runtime, you can use the standard {env.*} placeholders. Note that not all config parameters support these placeholders though, since module developers need to add a line of code to perform the replacement. If it doesn't seem to work, please file an issue to request support for it.

For example, if you have the caddy-dns/cloudflare plugin installed and wish to configure the DNS challenge, you can pass your CLOUDFLARE_API_TOKEN environment variable to the plugin like this:

{
	acme_dns cloudflare {env.CLOUDFLARE_API_TOKEN}
}

If you're running Caddy as a systemd service, see these instructions for setting service overrides to define your environment variables.