Reverse Proxy not sending all Link headers

Help
1

Hi All,

I’m running into a strange issue with caddy and reverse_proxy. I have a caddyfile setup to serve site[dot]com and anything that hits site[dot]com/api/ gets forwarded to a reverse proxy at 127.0.0.1:8081.

The 127.0.0.1:8081 site enables vulcain. Vulcain should return Link headers when it detects the header Preload: /api/resource/* if it can find it.

For whatever reason site.com is not receiving all the Link headers back from the proxy.

If i run the following command and hit site.com from outside docker the response only returns 1 Link header.

//Only returns one link header for some reason.
curl -i -H  'Preload: "/departments/*"' https://site.com/api/people/1
//Other headers
Link: <https://site.com/api/docs.jsonld>; rel="http://www.w3.org/ns/hydra/core#apiDocumentation"

If I ssh into the php server and run the following command it returns all the expected link headers.

//When ssh into the php server. It returns the link headers correctly
//curl -i -H  'Preload: "/departments/*"' http://localhost:8081/people/1

HTTP/1.1 200 OK
//Other headers
Link: <http://localhost:8081/docs.jsonld>; rel="http://www.w3.org/ns/hydra/core#apiDocumentation"
Link: </departments/1>; rel=preload; as=fetch
Link: </departments/3>; rel=preload; as=fetch

I’ve tried adding

header_down Link {http.request.header.Link} to my caddy file but it returns Link:;
I’ve also tried header_down +Link {http.request.header.Link} but it returns

Link: <http://localhost:8081/docs.jsonld>; rel="http://www.w3.org/ns/hydra/core#apiDocumentation"
Link:

I’m not exactly sure whats going on. The proxy is obviously receiving the first Link header, but for whatever reason its not including the others. Can anyone point me into the right direction?

Below is my current caddy file.

site.com {
    tls /certs/certificate.crt /certs/private.key
    root * /app/scale-angular/dist/scale/browser
    # Now proxy all /api/* requests to the backend
    handle_path /api/* {
        reverse_proxy http://127.0.0.1:8081 {
            header_up Host {http.request.host}
            header_up X-Real-IP {http.request.remote}
            header_up X-Forwarded-For {http.request.remote}
            header_up X-Forwarded-Proto {http.request.scheme}
            header_up X-Forwarded-Prefix /api
            header_up Prefer {http.request.header.Prefer}  # vulcain
            header_up Preload {http.request.header.Preload}  # vulcain
        }
    }

    handle {
       @notFileOrDir {
          not {
             file {
                try_files {path}
             }
          }
       }
       rewrite @notFileOrDir /index.html
       file_server
    }
}

:8081 {
    handle {
        root * /app/api/public
        vulcain
        php_server        
    }
}
2

I’m trying to replicate your situation, but I can’t get your result.

For example:

{
	http_port 8080
}

## My Caddy instance pretending to be your site.com
:8080 {
	reverse_proxy http://127.0.0.1:8081
}

## My Caddy instance pretending to be your Vulcain instance
:8081 {
	  header {
			+Link value1
			+Link value2
			+Link value3
		}
    respond "Vulcain"
}

## My Caddy instance pretending to be your Vulcain instance
$ curl -I http://localhost:8081
HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8
Link: value1
Link: value2
Link: value3
Server: Caddy
Date: Wed, 26 Mar 2025 00:08:36 GMT
Content-Length: 7

## My Caddy instance pretending to be your site.com
curl -I http://localhost:8080
HTTP/1.1 200 OK
Content-Length: 7
Content-Type: text/plain; charset=utf-8
Date: Wed, 26 Mar 2025 00:08:41 GMT
Link: value1
Link: value2
Link: value3
Server: Caddy
Server: Caddy

The proxy instance on port 8080 doesn’t touch the proxied headers at all.

I’m on Caddy v2.9.1

2 Likes
3

It looks like you’re right. I modified my caddyfile to reflect what you have and I do see the link headers I defined in the request now.

:8081 {
    header {
        +Link value1
        +Link value2
        +Link value3
    }

    vulcain
    root * /app/api/public
    php_server
}

Is it possible to dump the response headers before they get sent back to the proxy? Maybe symfony isn’t actually returning all the headers properly.

4

You can try enabling debug in the global options:

{
    debug
}

Or, if you only want to focus on your Vulcain instance, you can log its access log - which includes request and response headers - by adding the log directive:

:8081 {
    header {
        +Link value1
        +Link value2
        +Link value3
    }

    vulcain
    root * /app/api/public
    php_server
    log
}

Hope it helps!

1 Like
5

I finally managed to figure out the issue. Vulcain needed to go before the reverse_proxy. The preload='/api/departments/*' had a conflict with header_up X-Forwarded-Prefix /api. It looks like api-platform gets confused with the forward prefx and preload. Changing preload to preload='/departments/*' fixes the issues.

6

Please, share the working config, if you can. In case someone else runs into the same problem.

2 Likes
7

The below CaddyFile works for me now. When sending the Preload headers, I need to omit anything /api. Ie: Preload: /api/department/* is invalid. must be Preload: /department/*

site.com {
    tls /certs/certificate.crt /certs/private.key
    root * /app/scale-angular/dist/scale/browser

    # Proxy /api/* requests to Symfony API (running with vulcain at :8081)
    handle_path /api/* {
         vulcain
         reverse_proxy http://127.0.0.1:8081 {
            header_up X-Forwarded-For {http.request.remote}
            header_up X-Forwarded-Proto {http.request.scheme}
            header_up X-Forwarded-Prefix /api
            header_up Prefer {http.request.header.Prefer} # Required for Vulcain support
            header_up Preload {http.request.header.Preload} # Required for Vulcain support
        }
    }
    # Fallback for SPA routes
    handle {
        @notFileOrDir {
            not {
                file {
                    try_files {path}
                }
            }
        }
        rewrite @notFileOrDir /index.html
        file_server
    }
}

:8081 {
    root * /app/api/public
    php_server
}
2 Likes