Logging a Response Header Field in Caddy Reverse Proxy without Returning to Client

[isaced]

1. The problem I’m having:

How can I read a specific field from the Response Header in a reverse proxy with Caddy and write it to a log, without returning it to the client?

For example, I want to log the UserId in my logs. I return the X-User-Id field in the reverse_proxy directive, hoping to write it to the log. Then, I use the header -X-User-Id directive to remove this field so that it is not returned to the client. However, once I remove this header, it is also absent in the logs.

2. Error messages and/or full log output:

/

3. Caddy version:

2.7

4. How I installed and ran Caddy:

a. System environment:

Docker

b. Command:

sudo docker compose up -d

c. Service/unit/compose file:

...

d. My complete Caddy config:

example.com {
	reverse_proxy myservice:8080

	log {
		output file /var/log/myservice/access.log
	}

	# Remove header
	header {
		-X-User-Id
	}
}

5. Links to relevant resources:

[francislavoie]

It’s not possible yet, but we’re working on it:

logging: Implement `extra_log` handler

This isn’t what you’re asking for, but you could enable the debug global option to get the reverse_proxy logs, which do contain the proxy’s response headers before being written to the client. You could configure a logger in global options to only include those logs from http.handlers.reverse_proxy in one file.

[francislavoie]

I had another idea which might be a little nicer, i.e. not an extra log field, but just reusing the existing user_id one in the access logs:

vars: Allow overriding `http.auth.user.id` in replacer as a special case

[isaced]

It’s cool, can it be designed to be more flexible and dynamic, not only for scenarios that require logging UserId, but also for others, such as X-Project-Id/X-Team-Id?"

[francislavoie]

extra_log (or whatever it ends up being called) will allow you to do that, yes.

[isaced]

Great, I’ll look forward to extra_log (or whatever it ends up being called)…

Thank you very much for your reply.