Allow forwarding headers from `auth_request` without flattening

[aslafy-z]

Currently, ingress-nginx flattens multiple occurrences of the same header returned by auth_request into a single header with comma-separated values. This behavior prevents proper forwarding of multiple headers with the same name, which is required for certain use cases such as Kubernetes impersonation headers.

For example, when the auth_request response includes:

Impersonate-Group: A
Impersonate-Group: B
Impersonate-Group: C

ingress-nginx transforms this into:

Impersonate-Group: A, B, C

This is problematic because upstream applications expecting separate headers receive a single combined header instead.

Proposal: Introduce a mechanism in Ingress NGINX that allows forwarding multiple headers as separate entries instead of flattening them.

Suggested Implementation

The following Lua script demonstrates how to properly forward multiple headers from auth_request using ngx_req.add_header, which ensures that repeated headers are preserved:

local auth_path = ngx.var.auth_path
local header_list = ngx.var.forward_headers

-- Split the comma-separated list into a table
local headers = {}
for header in string.gmatch(header_list, "[^,]+") do
  table.insert(headers, header:match("^%s*(.-)%s*$")) -- Trim spaces
end

local res = ngx.location.capture(auth_path)
if res.status ~= 200 then
    ngx.exit(res.status)
end

local ngx_req = require "ngx.req"

for _, header_name in ipairs(headers) do
  local values = res.header[header_name]
  if values then
    if type(values) ~= "table" then
      values = { values } -- Convert single value to table
    end

    for _, value in ipairs(values) do
      ngx_req.add_header(header_name, value)
    end
  end
end

Usage

This Lua script can be integrated into an Ingress NGINX configuration like this:

    set $auth_path "/_auth";
    set $forward_headers "Impersonate-Group,Impersonate-Extra";
    access_by_lua_file "/etc/nginx/lua/forward_header.lua";

Expected Behavior

With this change, multiple headers of the same name are forwarded separately to the upstream application instead of being merged into a single comma-separated value.

Current implementation

The current implementation responsible for handling headers in auth_request is located in: https://github.com/kubernetes/ingress-nginx/blob/main/internal/ingress/controller/template/template.go#L578-L611 and https://github.com/kubernetes/ingress-nginx/blob/main/rootfs/etc/nginx/template/nginx.tmpl#L1182-L1209.

Note that it already uses lua to evaluate these headers most of the time.

A modification to this logic should allow preserving multiple headers instead of flattening them.

Benefits

• Enables correct handling of headers required for Kubernetes impersonation (eg, kubernetes-dashboard)
• Improves compatibility with applications that expect multiple headers instead of a single comma-separated value.
• Provides greater flexibility in handling auth_request headers.

Would the Ingress NGINX maintainers be open to reviewing a PR for this feature?