Using placeholders/variables in handler

Help
1

1. The problem I’m having:

I am using caddy, built using xcaddy, with the “non-standard” module replace-response to manipulate the content of the response. My goal is to find the phrase “localhost” and replace it with the value of the placeholder {http.request.host}, in my case an IPv4 address. Therefore I created this configuration

...
"handle":[
   {
      "handler":"replace_response",
      "replacements":[
         {
            "search":"localhost",
            "replace":"{http.request.host}"
         }
      ]
   }
]
...

As I can see in the response, this does not work. The replacement is done correctly, but instead of the expected value from the original request content "Hi, I replaced localhost" to e.g. "Hi, I replaced 192.168.172.36", the placeholder itself is placed: "Hi, I replaced {http.request.host}".

The same goes for a normal static_response. So I am not suspecting the module to be the problem.

2. Error messages and/or full log output:

2023/07/12 15:48:22.686 INFO    using provided configuration    {"config_file": "caddy-replace.json", "config_adapter": ""}
2023/07/12 15:48:22.687 INFO    admin   admin endpoint started  {"address": "localhost:2019", "enforce_origin": false, "origins": ["//localhost:2019", "//[::1]:2019", "//127.0.0.1:2019"]}
2023/07/12 15:48:22.687 INFO    tls.cache.maintenance   started background certificate maintenance      {"cache": "0xc0000fdf10"}
2023/07/12 15:48:22.687 INFO    tls     cleaning storage unit   {"description": "FileStorage:/home/user/.local/share/caddy"}
2023/07/12 15:48:22.687 INFO    http.log        server running  {"name": "example", "protocols": ["h1", "h2", "h3"]}
2023/07/12 15:48:22.687 INFO    tls     finished cleaning storage units
2023/07/12 15:48:22.687 INFO    autosaved config (load with --resume flag)      {"file": "/home/user/.config/caddy/autosave.json"}
2023/07/12 15:48:22.688 INFO    serving initial configuration

3. Caddy version:

user@ubuntu:~/Documents/caddy/xcaddy$ ./caddy version
v2.6.4 h1:2hwYqiRwk1tf3VruhMpLcYTg+11fCdr8S3jhNAdnPy8=

4. How I installed and ran Caddy:

Static self built binary via xcaddy
./caddy run --config caddy-replace.json

a. System environment:

Ubuntu

d. My complete Caddy config:

{
  "apps": {
    "http": {
      "servers": {
        "example": {
          "listen": [
            ":2015"
          ],
          "routes": [
            {
              "handle": [
                {
                  "handler": "replace_response",
                  "replacements": [
                    {
                      "search": "localhost",
                      "replace": "{http.request.host}"
                    }
                  ]
                },
                {
                  "handler": "static_response",
                  "body": "Hi, I replaced localhost"
                }
              ]
            }
          ]
        }
      }
    }
  }
}

5. Links to relevant resources:

2

Ok, after I just wrote all of this I had the idea of changing the sequence… Thank you for your indirect help, keep up the great work :smiley:

{
  "apps": {
    "http": {
      "servers": {
        "example": {
          "listen": [
            ":2015"
          ],
          "routes": [
            {
              "handle": [
                {
                  "handler": "static_response",
                  "body": "Hi {http.request.host}, I replaced localhost"
                },
                {
                  "handler": "replace_response",
                  "replacements": [
                    {
                      "search": "localhost",
                      "replace": "{http.request.host}"
                    }
                  ]
                }
              ]
            }
          ]
        }
      }
    }
  }
}
3

Hmm. My guess is that second one “works” only because you happen to be testing using http://localhost:2015.

The static_response handler being first would write the response and not invoke any further handlers.

The first one doesn’t work because placeholders are not evaluated after the response has been written; i.e. placeholders (dynamic content) are not expanded when performing replacements. This could be changed I guess, but my gut tells me it should be an opt-in to avoid surprises.

4

Hi, I have a similiar problem. Do you know if there is any other way to manipulate a response by using placeholders?

5

What are you trying to do exactly?

6

Hi @matt,
thank you for your answer! Sadly it is how you said. For example the following example still does not work:

{
  "apps": {
    "http": {
      "servers": {
        "example": {
          "listen": [
            ":2015"
          ],
          "routes": [
            {
              "handle": [
                {
                  "handler": "static_response",
                  "body": "Hi, I replaced should-be-replaced"
                },
                {
                  "handler": "replace_response",
                  "replacements": [
                    {
                      "search": "should-be-replaced",
                      "replace": "{http.request.host}"
                    }
                  ]
                }              
              ]
            }
          ]
        }
      }
    }
  }
}

Depending on the order of the two handler the response is one of the following:

  1. Hi, I replaced {http.request.host}
  2. Hi, I replaced should-be-replaced

My usecase for what I need this feature in the end is, that I want to manipulate responses of an reverse proxied application. So in the example the static_response would be a reverse_proxy . It does not support different hostnames, so links in the UI are wrong. It would be awesome to have caddy to be able to help here!

7

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