Lighttpd CORS reverse proxy for transmission

lighttpd_transmission_proxy.conf

# Lighttpd as a CORS reverse proxy to Transmission XML-RPC API
server.modules = (
        "mod_rewrite",
        "mod_setenv",
        "mod_proxy"
)
server.document-root = "/var/www"

# CORS preflighted request (HTTP OPTIONS) not handled yet by Transmission : Emulate a 200 OK
$HTTP["request-method"] =~ "^OPTIONS$" {
        url.rewrite = ( "^.*$" => "/index.html") 
}
# Proxy other requests to Transmission (YMMV)
$HTTP["request-method"] =~ "^(GET|POST)$" {
        proxy.server = (
                        "/bt/rpc" => (
                                "transmission-rpc" => (
                                        "host" => "127.0.0.1",
                                        "port" => 9091
                                        )
                                )
                       )
}
# CORS Headers
setenv.add-response-header = ( 
        "Access-Control-Allow-Origin" => "*", 
        "Access-Control-Allow-Headers" => "accept, origin, x-requested-with, content-type, x-transmission-session-id",
        "Access-Control-Expose-Headers" => "X-Transmission-Session-Id",
        "Access-Control-Allow-Methods" => "GET, POST, OPTIONS"
)

The cors headers are needed only if the method is OPTIONS so you can try:

$HTTP["request-method"] =~ "^(OPTIONS)$" {
  setenv.add-response-header = ( 
          "Access-Control-Allow-Origin" => "*", 
          "Access-Control-Allow-Headers" => "accept, origin, x-requested-with, content-type, x-transmission-session-id",
          "Access-Control-Expose-Headers" => "X-Transmission-Session-Id",
          "Access-Control-Allow-Methods" => "GET, POST, OPTIONS"
  )
}