Lua library for enabling CORS in HAProxy.

Cross-origin Request Sharing allows you to permit client-side code running within a different domain to call your services. This module extends HAProxy so that it can:

• set an Access-Control-Allow-Methods header in response to a preflight request
• set an Access-Control-Allow-Headers header in response to a preflight request
• set an Access-Control-Max-Age header in response to a preflight request
• set an Access-Control-Allow-Origin header to whitelist a domain. Note that this header should only ever return either a single domain or an asterisk (*). Otherwise, it would have been possible to hardcode all permitted domains without the need for Lua scripting.

This library checks the incoming Origin header, which contains the calling code's domain, and tries to match it with the list of permitted domains. If there is a match, that domain is sent back in the Access-Control-Allow-Origin header.

It also sets the Vary header to Accept-Encoding,Origin so that caches do not reuse cached CORS responses for different origins.

• HAProxy must be compiled with Lua support.

Copy the cors.lua file to the server where you run HAProxy.

Load the cors.lua file via the lua-load directive in the global section of your HAProxy configuration:

global
    lua-load /path/to/cors.lua

In your frontend or listen section, capture the client's Origin request header by adding http-request lua.cors Its parameters are:

• The first parameter is a comma-delimited list of HTTP methods that can be used. This is used to set the Access-Control-Allow-Methods header.
• The second parameter is comma-delimited list of origins that are permitted to call your service. This is used to set the Access-Control-Allow-Origin header.
• The third parameter is a comma-delimited list of custom headers that can be used. This is used to set the Access-Control-Allow-Headers header.

Each of these parameters can be set to an asterisk (*) to allow all values.

Within the same frontend or listen section, add the http-response lua.cors action to attach CORS headers to responses from backend servers.

Example 1: Allow specific methods, origins and headers

http-request lua.cors "GET,PUT,POST" "example.com,localhost,localhost:8080", "X-Custom-Header1,X-Custom-Header2"

http-response lua.cors 

Example 2: Allow all methods, origins, and headers

http-request lua.cors "*" "*", "*"

http-response lua.cors 

This module handles preflight OPTIONS requests, but it does it differently depending on if you are using HAProxy 2.2 and above. For 2.2, the module intercepts the preflight request and returns it immediately without contacting the backend server.

For versions prior to 2.2, the module must forward the request to the backend server and then attach the CORS headers to the response as it passes back through the load balancer.

This module returns the following CORS headers for a preflight request:

• Access-Control-Allow-Methods - set to the HTTP methods you set with http-request lua cors in the haproxy.cfg file
• Access-Conrol-Allow-Headers - set to the HTTP headers you set with http-request lua cors in the haproxy.cfg file
• Access-Control-Max-Age - set to 600

Check the example directory for a working demo. It uses Docker Compose to run HAProxy and a web server in containers. Go to http://localhost to test it. It demonstrates a preflight request by clicking the "PUT data" button.