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cors_plug.ex
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cors_plug.ex
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defmodule CORSPlug do
import Plug.Conn
def defaults do
[
origin: "*",
credentials: true,
max_age: 1_728_000,
headers: [
"Authorization",
"Content-Type",
"Accept",
"Origin",
"User-Agent",
"DNT",
"Cache-Control",
"X-Mx-ReqToken",
"Keep-Alive",
"X-Requested-With",
"If-Modified-Since",
"X-CSRF-Token"
],
expose: [],
methods: ["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"]
]
end
def init(options) do
options
|> prepare_cfg(Application.get_all_env(:cors_plug))
|> Keyword.update!(:expose, &Enum.join(&1, ","))
|> Keyword.update!(:methods, &Enum.join(&1, ","))
end
defp prepare_cfg(options, nil), do: Keyword.merge(defaults(), options)
defp prepare_cfg(options, env) do
defaults()
|> Keyword.merge(env)
|> Keyword.merge(options)
end
def call(conn, options) do
conn = merge_resp_headers(conn, headers(conn, options))
case conn.method do
"OPTIONS" -> conn |> send_resp(204, "") |> halt()
_method -> conn
end
end
# headers specific to OPTIONS request
defp headers(conn = %Plug.Conn{method: "OPTIONS"}, options) do
headers(%{conn | method: nil}, options) ++
[
{"access-control-max-age", "#{options[:max_age]}"},
{"access-control-allow-headers", allowed_headers(options[:headers], conn)},
{"access-control-allow-methods", options[:methods]}
]
end
# universal headers
defp headers(conn, options) do
allowed_origin = origin(options[:origin], conn)
vary_header = vary_header(allowed_origin, get_resp_header(conn, "vary"))
vary_header ++
[
{"access-control-allow-origin", allowed_origin},
{"access-control-expose-headers", options[:expose]},
{"access-control-allow-credentials", "#{options[:credentials]}"}
]
end
# Allow all requested headers
defp allowed_headers(["*"], conn) do
conn
|> get_req_header("access-control-request-headers")
|> List.first()
end
defp allowed_headers(key, _conn) do
Enum.join(key, ",")
end
# return origin if it matches regex, otherwise "null" string
defp origin(%Regex{} = regex, conn) do
req_origin = conn |> request_origin() |> to_string()
if req_origin =~ regex, do: req_origin, else: "null"
end
# get value if origin is a function
defp origin(fun, conn) when is_function(fun) do
origin(fun.(), conn)
end
# normalize non-list to list
defp origin(key, conn) when not is_list(key) do
key
|> List.wrap()
|> origin(conn)
end
# whitelist internal requests
defp origin([:self], conn) do
request_origin(conn) || "*"
end
# return "*" if origin list is ["*"]
defp origin(["*"], _conn) do
"*"
end
# return request origin if in origin list, otherwise "null" string
# see: https://www.w3.org/TR/cors/#access-control-allow-origin-response-header
defp origin(origins, conn) when is_list(origins) do
req_origin = request_origin(conn)
cond do
req_origin in origins -> req_origin
"*" in origins -> "*"
true -> "null"
end
end
defp request_origin(%Plug.Conn{req_headers: headers}) do
Enum.find_value(headers, fn {k, v} -> k =~ ~r/^origin$/i && v end)
end
# Set the Vary response header
# see: https://www.w3.org/TR/cors/#resource-implementation
defp vary_header("*", _headers), do: []
defp vary_header(_allowed_origin, []), do: [{"vary", "Origin"}]
defp vary_header(_allowed_origin, headers) do
vary = Enum.join(["Origin" | headers], ", ")
[{"vary", vary}]
end
end