How to send a response with HAProxy without passing the request to web servers

Edit for HAProxy 2.2 and above: In case you need to support a whitelist of origins, Lua scripts can now generate the entire response without having to pass the request to the backend server. Sample Lua script with simple integration instructions can be found here: https://github.com/haproxytech/haproxy-lua-cors

The only way to do this is in HAProxy 1.5.14 is by manually triggering the 503 error (no servers available to handle the request) and setting the error page to the file with custom CORS headers.

backend cors_headers
    errorfile 503 /path/to/custom/file.http

The file.http should contain the desired headers and 2 empty lines at the end

HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://www.example.com
Access-Control-Max-Age: 31536000
Content-Length: 0
Cache-Control: private


<REMOVE THIS LINE COMPLETELY>

This "method" has a couple of limitations:

• there is no way to check the origin before sending the CORS headers, so you will either have to have a static list of allowed origins or you will have to allow all origins

• lack of dynamic headers: you can't do

  http-response set-header Date %[date(),http_date]

or set Expires header.

Note: if you are updating the HTTP file dynamically over time, to apply the changes to the HAProxy you will have to restart it. It can be a graceful restart or a hard restart, in either case the new file will be loaded, cached and served immediately.

Good news, HAProxy 2.2 just introduced the "Native Response Generator" feature. It works with the http-request return directive, and can be used for serving static files or text strings, including dynamic parameters. The goal is to avoid the usual hacks with errorfile.

Taking advantage of another directive introduced in version 2.2 (http-after-response), the OP goal could be achieved with the following:

backend cors_headers
    # http-response won't work here as the response is generated by HAP
    http-after-response set-header Access-Control-Allow-Origin \
        "%[req.hdr(Origin)]"
    http-after-response set-header Access-Control-Max-Age "31536000"
    http-request return status 200 content-type "text/plain" string "" if TRUE

The set-header and http-request return can be made conditional with an if clause based on the request headers or origin, depending on your needs (see the doc for examples).

With this technique the headers and response can use variables:

http-request return status 200 content-type "text/plain" \
    lf-string "Hello, you are: %[src]"