Cowboy

Getting Started

{deps, [{vpndetection_cowboy, "~> 1.0"}]}.

Requires OTP 27 or newer, and Cowboy 2.13 or newer.

You need an API key. Create one in the console; the free tier's allowance is counted per source address, and a server is a single source address, so a key is what makes this usable in production rather than optional.

Build the core once, at startup, and name this module in the chain:

{ok, Core} = vpndetection_cowboy:core(#{api_key => os:getenv("VPNDETECTION_API_KEY")}),
Dispatch = cowboy_router:compile([{'_', [{"/", my_handler, []}]}]),
{ok, _} = cowboy:start_clear(http, [{port, 8080}], #{
    env => #{dispatch => Dispatch, vpndetection => Core},
    middlewares => [cowboy_router, vpndetection_cowboy, cowboy_handler]
}).

After cowboy_router, so a skip can look at the matched route, and before cowboy_handler, so a blocked request never reaches one.

The core owns the HTTP client and the answer cache, so build it once. One per request would mean neither ever helps.

Your handler reads what it found:

init(Req, State) ->
    Body = case vpndetection_cowboy:lookup(Req) of
        #{result := #{is_vpn := true}} -> <<"Hello, VPN user">>;
        _ -> <<"Hello">>
    end,
    {ok, cowboy_req:reply(200, #{}, Body, Req), State}.

By default nothing is blocked. Every request carries a lookup and your own handlers decide what that means — which is usually what you want, because whether a VPN visitor is a problem depends entirely on what they are doing.

Blocking

Set a block_condition and a matching request is answered with 403 and never reaches your handlers.

vpndetection_cowboy:core(#{api_key => Key, block_condition => #{is_vpn => true}}).

A condition is written in the shape of a result, keyed by the same names the API uses, and only the members you name are considered. That lets it reach the evidence, not just the flags:

%% one provider
#{is_vpn => true, vpn => #{provider => <<"nordvpn">>}}

%% a numeric threshold
#{resproxy => #{hits => #{gte => 5}}}

%% any of these
#{vpn => #{confidence => [<<"high">>, <<"medium">>]}}

%% a list is OR
[#{is_tor => true}, #{is_resproxy => true}]

Values are matched by equality, strings without regard to case. A list means any-of. A map of gte/gt/lte/lt bounds a number and combines into a range; every bound you give must hold. Members set to false, null or undefined are ignored, so a condition states the signals you act on; one that constrains nothing would match every request, and is refused when the core is built rather than silently blocking all your traffic.

Atom and binary keys both work, so a condition can come straight out of json:decode/1 on your app's config file:

{ok, Raw} = file:read_file("policy.json"),
vpndetection_cowboy:core(#{api_key => Key, block_condition => json:decode(Raw)}).

Write strings as binaries. An Erlang string is a list of codepoints, so "nordvpn" and an any-of of those numbers are the same term. A bare string is accepted anyway, but binaries have no such ambiguity and are what json:decode/1 produces.

Replace the refusal with on_blocked, which must reply:

on_blocked => fun(Req, _Found) ->
    cowboy_req:reply(403, #{}, <<"VPN not allowed">>, Req)
end

Where the client address comes from

This is the setting that decides whether any of the above works, and it is the one thing only you can get right.

By default the middleware uses cowboy_req:peer/1. Cowboy has no trusted-proxy setting, so that is the socket peer and nothing else. If your app sits behind nginx, a load balancer, or a CDN, every visitor arrives wearing your proxy's address — which is a datacenter address, so a hosting rule would block all of them.

For an edge that writes the address into its own header, name the header:

ip_selector => vpndetection_selectors:header(<<"CF-Connecting-IP">>)   % or True-Client-IP

vpndetection_selectors:xff(0) reads the left-most X-Forwarded-For entry. Be aware that the left-most entry is whatever the caller sent, because proxies append to that header — it is only trustworthy when an edge you control overwrites it. If you know how many proxies sit in front, count from the right instead: xff(1) is the address your nearest proxy saw.

Anything else, pass your own fun. It receives a view of the request and returns an address:

ip_selector => fun(View) -> (maps:get(header, View))(<<"x-real-ip">>) end

If the address resolves to a private one, the middleware says so once through on_warn, which defaults to logger:warning/1. That is expected on localhost and is the signal to fix your configuration anywhere else.

When a lookup fails

The request is let through, and the reason is on the lookup's error key. Our outage should not become yours, so a network failure, an exhausted quota or a rejected key all fail open.

case vpndetection_cowboy:lookup(Req) of
    #{error := #{kind := Kind}} when Kind =/= undefined ->
        logger:warning("vpndetection unavailable: ~p", [Kind]);
    _ ->
        ok
end

Set fail_closed => true to block instead. Private addresses are answered locally and never fail, so this will not lock you out in development.

Cost and latency

Answers are cached per core for an hour, so a returning visitor costs nothing, and private addresses never leave the process. A cache miss is one request to our API, bounded at 2500 ms by default and not retried — on a request path, failing open quickly beats holding a visitor while we try again. Both are adjustable, and so is the cache, through a client you build yourself and pass as client.

Skip what you do not care about:

skip => fun(Req) -> cowboy_req:path(Req) =:= <<"/healthz">> end

If you already hold a vpndetection client, pass it as client and the middleware will share it rather than building a second cache.

Beyond a few million distinct visitors a day, stop calling the API per request: download the dataset and look addresses up locally instead.

Absent is not false

Only ip and is_vpn come back on every plan. A key your plan does not include is ABSENT from the result map, which means "not in your plan" rather than "checked, and no".

maps:get(is_hosting, Result, false)        % when you only want the flag
maps:get(is_hosting, Result, undefined)    % when the difference matters

A block_condition naming a member your plan does not serve can never match, so the middleware warns once instead of failing silently. Set on_missing_field => error to make it an error, which this middleware answers with a 500.