No Placeholders in Active Health Checking Headers

[PoneyClairDeLune]

Version

Caddy: 2.7.2

Bug description

When defining a header in the active health checker, placeholders as header values do not get interpreted altogether.

Expected behaviour

Placeholders set in active health checker headers should be interpreted normally as everywhere else in the Caddyfile.

Steps of bug reproduction

1. Set a new web server for request catching. This can either be a Caddy server, or any new web server spun up with any language.
   Example Caddyfile snippet and WingBlade (JavaScript) web server snippet below.

http://127.0.0.1:8081 {
	log {
		level INFO
	}
	handle /api/example {
		respond "OK"
	}
}

"use strict";

WingBlade.web.serve(async function (request) {
	console.info("New request received. Header info below.");
	request.headers.forEach((value, key) => {
		console.info(`${key}: ${value}`);
	});
	return new Response("OK", {
		status: 200
	});
});

2. Set a header in a new reverse proxied target with an active health checker. Example Caddyfile snippet:

...
health_uri "/api/example"
health_headers {
	X-Hostname "{system.hostname}"
}
...

3. Reload the Caddyfile.
4. Check the sent headers in the target. Prettified Caddy log entry and WingBlade service log outputs are below.

{
	"level": "info",
	"ts": 1691110717.1085844,
	"logger": "http.log.access",
	"msg": "handled request",
	"request": {
		"remote_ip": "127.0.0.1",
		"remote_port": "41368",
		"client_ip": "127.0.0.1",
		"proto": "HTTP/1.1",
		"method": "GET",
		"host": "127.0.0.1:8081",
		"uri": "/api/example",
		"headers": {
			"User-Agent": ["Go-http-client/1.1"],
			"X-Hostname": ["{system.hostname}"],
			"Accept-Encoding": ["gzip"]
		}
	},
	"bytes_read": 0,
	"user_id": "",
	"duration": 0.003900064,
	"size": 2,
	"status": 200,
	"resp_headers": {
		"Server": ["Caddy"],
		"Content-Type": ["text/plain; charset=utf-8"]
	}
}

New request received. Header info below.
User-Agent: Go-http-client/1.1
X-Hostname: {system.hostname}
Accept-Encoding: gzip

[mholt]

I guess we didn't enable placeholders there because most placeholders are only available in the context of an HTTP request, which active health checks aren't. But the system and env var placeholders are always available, so that'd make sense.

After 2.7.3 I can add support for this. But not before 😅

[francislavoie]

Implemented in #5861, if you'd like to try it out!

[PoneyClairDeLune]

@francislavoie tysm! It's now working correctly.

New request received. Header info below.
accept-encoding: gzip
host: 127.0.0.1:8000
user-agent: Go-http-client/1.1
x-hostname: testenv