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A foundation for building Elixir Phoenix apps — SaaS, social networks, ERP systems, marketplaces, and more
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Files
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Files
lib/phoenix_kit_web/plugs/crawler_blocker.ex
defmodule PhoenixKitWeb.Plugs.CrawlerBlocker do
@moduledoc """
Best-effort application-level enforcement of the Crawlers module's bot
policy: requests whose `User-Agent` matches a **blocked** group's bots are
answered `403` instead of the page.
## What this is, honestly
robots.txt is a request; this plug is for bots that ignore it. It is
**advisory-grade**: anything can send any `User-Agent`, so a scraper that
lies passes through, and real enforcement belongs at the CDN or reverse
proxy. The settings page says the same to the operator. It is also
deliberately **default-off** (`crawlers_block_at_app` setting) — three
settings reads per request is a real cost, even ETS-cached, and silent 403s
are a support burden the operator should opt into knowingly.
Robots.txt-only tokens (`Google-Extended`, `Applebot-Extended`) have no
request UA of their own and are excluded from matching by the registry
(`Bots.ua_fragments/1`), so blocking AI training can never 403 Googlebot.
## Wiring
Core pipes its own kit routes through this plug. The host's routes are the
host's: add it to the host `:browser` pipeline for full coverage —
pipeline :browser do
# ...
plug PhoenixKitWeb.Plugs.CrawlerBlocker
end
All reads are guarded: with no database (installer context, doctor) the plug
passes everything through.
"""
@behaviour Plug
import Plug.Conn
alias PhoenixKit.Modules.Crawlers
alias PhoenixKit.Modules.Crawlers.Bots
@impl Plug
def init(opts), do: opts
@impl Plug
def call(conn, _opts) do
if blocking_active?() do
enforce(conn)
else
conn
end
end
defp enforce(conn) do
ua =
conn
|> get_req_header("user-agent")
|> List.first()
if is_binary(ua) and blocked_ua?(String.downcase(ua)) do
conn
|> put_resp_content_type("text/plain")
|> send_resp(403, "Forbidden")
|> halt()
else
conn
end
end
defp blocking_active? do
Crawlers.module_enabled?() and Crawlers.block_at_app_enabled?()
rescue
_ -> false
catch
:exit, _ -> false
end
defp blocked_ua?(downcased_ua) do
Crawlers.blocked_groups()
|> Bots.ua_fragments()
|> Enum.any?(&String.contains?(downcased_ua, &1))
rescue
_ -> false
catch
:exit, _ -> false
end
end