View Source Plug.Cowboy (Plug.Cowboy v2.6.1)

Adapter interface to the Cowboy2 webserver.



  • :net - If using :inet (IPv4 only - the default) or :inet6 (IPv6)

  • :ip - the ip to bind the server to. Must be either a tuple in the format {a, b, c, d} with each value in 0..255 for IPv4, or a tuple in the format {a, b, c, d, e, f, g, h} with each value in 0..65535 for IPv6, or a tuple in the format {:local, path} for a unix socket at the given path. If you set an IPv6, the :net option will be automatically set to :inet6. If both :net and :ip options are given, make sure they are compatible (i.e. give a IPv4 for :inet and IPv6 for :inet6). Also, see "Loopback vs Public IP Addresses".

  • :port - the port to run the server. Defaults to 4000 (http) and 4040 (https). Must be 0 when :ip is a {:local, path} tuple.

  • :dispatch - manually configure Cowboy's dispatch. If this option is used, the given plug won't be initialized nor dispatched to (and doing so becomes the user's responsibility).

  • :ref - the reference name to be used. Defaults to plug.HTTP (http) and plug.HTTPS (https). Note, the default reference name does not contain the port so in order to serve the same plug on multiple ports you need to set the :ref accordingly, e.g.: ref: MyPlug_HTTP_4000, ref: MyPlug_HTTP_4001, etc. This is the value that needs to be given on shutdown.

  • :compress - Cowboy will attempt to compress the response body. Defaults to false.

  • :stream_handlers - List of Cowboy stream_handlers, see Cowboy docs.

  • :protocol_options - Specifies remaining protocol options, see Cowboy docs.

  • :transport_options - A keyword list specifying transport options, see Ranch docs. By default :num_acceptors will be set to 100 and :max_connections to 16_384.

All other options given at the top level must configure the underlying socket. For HTTP connections, those options are listed under ranch_tcp. For example, you can set :ipv6_v6only to true if you want to bind only on IPv6 addresses.

For HTTPS (SSL) connections, those options are described in ranch_ssl. See https/3 for an example and read Plug.SSL.configure/1 to understand about our SSL defaults.

When using a Unix socket, OTP 21+ is required for Plug.Static and Plug.Conn.send_file/3 to behave correctly.


Safety limits

Cowboy sets different limits on URL size, header length, number of headers and so on to protect your application from attacks. For example, the request line length defaults to 10k, which means Cowboy will return 414 if a larger URL is given. You can change this under :protocol_options:

protocol_options: [max_request_line_length: 50_000]

Keep in mind though increasing those limits can pose a security risk. Other times, browsers and proxies along the way may have equally strict limits, which means the request will still fail or the URL will be pruned. You can consult all limits here.


Loopback vs Public IP Addresses

Should your application bind to a loopback address, such as ::1 (IPv6) or (IPv4), or a public one, such as ::0 (IPv6) or (IPv4)? It depends on how (and whether) you want it to be reachable from other machines.

Loopback addresses are only reachable from the same host (localhost is usually configured to resolve to a loopback address). You may wish to use one if:

  • Your app is running in a development environment (such as your laptop) and you don't want others on the same network to access it.
  • Your app is running in production, but behind a reverse proxy. For example, you might have Nginx bound to a public address and serving HTTPS, but forwarding the traffic to your application running on the same host. In that case, having your app bind to the loopback address means that Nginx can reach it, but outside traffic can only reach it via Nginx.

Public addresses are reachable from other hosts. You may wish to use one if:

  • Your app is running in a container. In this case, its loopback address is reachable only from within the container; to be accessible from outside the container, it needs to bind to a public IP address.
  • Your app is running in production without a reverse proxy, using Cowboy's SSL support.



You can configure which exceptions are logged via :log_exceptions_with_status_code application environment variable. If the status code returned by Plug.Exception.status/1 for the exception falls into any of the configured ranges, the exception is logged. By default it's set to [500..599].

config :plug_cowboy,
  log_exceptions_with_status_code: [400..599]

By default, Plug.Cowboy includes the entire conn to the log metadata for exceptions. However, this metadata may contain sensitive information such as security headers or cookies, which may be logged in plain text by certain logging backends. To prevent this, you can configure the :conn_in_exception_metadata option to not include the conn in the metadata.

config :plug_cowboy,
  conn_in_exception_metadata: false



Plug.Cowboy uses the :telemetry library for instrumentation. The following span events are published during each request:

  • [:cowboy, :request, :start] - dispatched at the beginning of the request
  • [:cowboy, :request, :stop] - dispatched at the end of the request
  • [:cowboy, :request, :exception] - dispatched at the end of a request that exits

A single event is published when the request ends with an early error:

  • [:cowboy, :request, :early_error] - dispatched for requests terminated early by Cowboy

See cowboy_telemetry for more details on the events.

To opt-out of this default instrumentation, you can manually configure cowboy with the option stream_handlers: [:cowboy_stream_h].


WebSocket support

Plug.Cowboy supports upgrading HTTP requests to WebSocket connections via the use of the Plug.Conn.upgrade_adapter/3 function, called with :websocket as the second argument. Applications should validate that the connection represents a valid WebSocket request before calling this function (Cowboy will validate the connection as part of the upgrade process, but does not provide any capacity for an application to be notified if the upgrade is not successful). If an application wishes to negotiate WebSocket subprotocols or otherwise set any response headers, it should do so before calling Plug.Conn.upgrade_adapter/3.

The third argument to Plug.Conn.upgrade_adapter/3 defines the details of how Plug.Cowboy should handle the WebSocket connection, and must take the form {handler, handler_opts, connection_opts}, where values are as follows:

  • handler is a module which implements the :cowboy_websocket behaviour. Note that this module will NOT have its c:cowboy_websocket.init/2 callback called; only the 'later' parts of the :cowboy_websocket lifecycle are supported
  • handler_opts is an arbitrary term which will be passed as the argument to c:cowboy_websocket.websocket_init/1
  • connection_opts is a map with any of Cowboy's websockets options

Link to this section Summary


A function for starting a Cowboy2 server under Elixir v1.5+ supervisors.

Runs cowboy under http.

Runs cowboy under https.

Shutdowns the given reference.

Link to this section Functions

A function for starting a Cowboy2 server under Elixir v1.5+ supervisors.

It supports all options as specified in the module documentation plus it requires the following two options:

  • :scheme - either :http or :https
  • :plug - such as MyPlug or {MyPlug, plug_opts}



Assuming your Plug module is named MyApp you can add it to your supervision tree by using this function:

children = [
  {Plug.Cowboy, scheme: :http, plug: MyApp, options: [port: 4040]}

Supervisor.start_link(children, strategy: :one_for_one)
Link to this function

http(plug, opts, cowboy_options \\ [])

View Source
@spec http(module(), Keyword.t(), Keyword.t()) ::
  {:ok, pid()} | {:error, :eaddrinuse} | {:error, term()}

Runs cowboy under http.



# Starts a new interface
Plug.Cowboy.http MyPlug, [], port: 80

# The interface above can be shutdown with
Plug.Cowboy.shutdown MyPlug.HTTP
Link to this function

https(plug, opts, cowboy_options \\ [])

View Source
@spec https(module(), Keyword.t(), Keyword.t()) ::
  {:ok, pid()} | {:error, :eaddrinuse} | {:error, term()}

Runs cowboy under https.

Besides the options described in the module documentation, this function sets defaults and accepts all options defined in Plug.SSL.configure/1.



# Starts a new interface
Plug.Cowboy.https MyPlug, [],
  port: 443,
  password: "SECRET",
  otp_app: :my_app,
  keyfile: "priv/ssl/key.pem",
  certfile: "priv/ssl/cert.pem",
  dhfile: "priv/ssl/dhparam.pem"

# The interface above can be shutdown with
Plug.Cowboy.shutdown MyPlug.HTTPS

Shutdowns the given reference.