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A simple HTTP proxy that fogs over naughty URLs

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Smokescreen is a HTTP CONNECT proxy. It proxies most traffic from Stripe to the external world (e.g., webhooks).

Smokescreen restricts which URLs it connects to: it resolves each domain name that is requested and ensures that it is a publicly routable IP and not a Stripe-internal IP. This prevents a class of attacks where, for instance, our own webhooks infrastructure is used to scan Stripe's internal network.

Smokescreen also allows us to centralize egress from Stripe, allowing us to give financial partners stable egress IP addresses and abstracting away the details of which Stripe service is making the request.

Smokescreen can be contacted over TLS. You can provide it with one or more client certificate authority certificates as well as their CRLs. Smokescreen will warn you if you load a CA certificate with no associated CRL and will abort if you try to load a CRL which cannot be used (ex.: cannot be associated with loaded CA).

Smokescreen can be provided with an ACL to determine which remote hosts a service is allowed to interact with. By default, Smokescreen will identify clients by the "common name" in the TLS certificate they present, if any. The client identification function can also be easily replaced; more on this in the usage section.


Smokescreen uses go modules to manage dependencies. The linked page contains documentation, but some useful commands are reproduced below:

  • Adding a dependency:
    go build
    go test
    go mod tidy
    will automatically fetch the latest version of any new dependencies. Running
    go mod vendor
    will vendor the dependency.
  • Updating a dependency:
    go get [email protected]
    go get [email protected]
    will bring in specific versions of a dependency. The updated dependency should be vendored using
    go mod vendor

Smokescreen uses a custom fork of goproxy to allow us to support context passing and setting granular timeouts on proxy connections.

Smokescreen is built and tested using the following Go releases. Generally, Smokescreen will only support the two most recent Go versions.

  • go1.15.x
  • go1.16.x



Here are the options you can give Smokescreen:

   --help                                      Show this help text.
   --config-file FILE                          Load configuration from FILE.  Command line options override values in the file.
   --listen-ip IP                              Listen on interface with address IP.
                                                 This argument is ignored when running under Einhorn. (default: any)
   --listen-port PORT                          Listen on port PORT.
                                                 This argument is ignored when running under Einhorn. (default: 4750)
   --timeout DURATION                          Time out after DURATION when connecting. (default: 10s)
   --proxy-protocol                            Enable PROXY protocol support.
   --deny-range RANGE                          Add RANGE(in CIDR notation) to list of blocked IP ranges.  Repeatable.
   --allow-range RANGE                         Add RANGE (in CIDR notation) to list of allowed IP ranges.  Repeatable.
   --deny-address value                        Add IP[:PORT] to list of blocked IPs.  Repeatable.
   --allow-address value                       Add IP[:PORT] to list of allowed IPs.  Repeatable.
   --egress-acl-file FILE                      Validate egress traffic against FILE
   --resolver-address ADDRESS                  Make DNS requests to ADDRESS (IP:port).  Repeatable.
   --statsd-address ADDRESS                    Send metrics to statsd at ADDRESS (IP:port). (default: "")
   --tls-server-bundle-file FILE               Authenticate to clients using key and certs from FILE
   --tls-client-ca-file FILE                   Validate client certificates using Certificate Authority from FILE
   --tls-crl-file FILE                         Verify validity of client certificates against Certificate Revocation List from FILE
   --additional-error-message-on-deny MESSAGE  Display MESSAGE in the HTTP response if proxying request is denied
   --disable-acl-policy-action POLICY ACTION   Disable usage of a POLICY ACTION such as "open" in the egress ACL
   --stats-socket-dir DIR                      Enable connection tracking. Will expose one UDS in DIR going by the name of "track-{pid}.sock".
                                                 This should be an absolute path with all symlinks, if any, resolved.
   --stats-socket-file-mode FILE_MODE          Set the filemode to FILE_MODE on the statistics socket (default: "700")
   --version, -v                               print the version


In order to override how Smokescreen identifies its clients, you must:

  • Create a new go project
  • Import Smokescreen
  • Create a Smokescreen configuration using cmd.NewConfiguration
  • Replace
    with your own
    func(request *http.Request) (string, error)
  • Call smokescreen.StartWithConfig
  • Build your new project and use the resulting executable through its CLI

Here is a fictional example that would split a client certificate's

on commas and use the first particle as the service name.
package main

import (...)

func main() { // Here is an opportunity to pass your logger conf, err := cmd.NewConfiguration(nil, nil) if err != nil { log.Fatal(err) } if conf == nil { os.Exit(1) }

conf.RoleFromRequest = func(request *http.Request) (string, error) {
    fail := func(err error) (string, error) { return "", err }

    subject := request.TLS.PeerCertificates[0].Subject
    if len(subject.OrganizationalUnit) == 0 {
        fail(fmt.Errorf("warn: Provided cert has no 'OrganizationalUnit'. Can't extract service role."))
    return strings.SplitN(subject.OrganizationalUnit[0], ".", 2)[0], nil

smokescreen.StartWithConfig(conf, nil)



An ACL can be described in a YAML formatted file. The ACL, at its top-level, contains a list of services as well as a default behavior.

Three policies are supported:

| Policy | Behavior | | ------- | -------------------------------------------------------------------------------------------------------------- | | Open | Allows all traffic for this service | | Report | Allows all traffic for this service and warns if client accesses a remote host which is not in the list | | Enforce | Only allows traffic to remote hosts provided in the list. Will warn and deny if remote host is not in the list |

A host can be specified with or without a globbing prefix

| host | valid | | ------------------- | ------- | |
| yes | |
| yes | |
| no | |
| no | |
| no | |
| hell no |

Here is a sample ACL.

Global Allow/Deny Lists

Optionally, you may specify a global allow list and a global deny list in your ACL config.

These lists override the policy, but do not override the

list for each role.

For example, specifying
in your globalallowlist will allow traffic for that domain on that role, even if that role is set to
and does not specify
in its allowed domains.

Similarly, specifying
in your globaldenylist will deny traffic for that domain on a role, even if that role is set to
. However, if the host specifies
in its
, traffic to
will be allowed on that role, regardless of policy.

If a domain matches both the

and the
, the
behavior takes priority.

Here is a sample ACL specifying these options.


  • Aditya Mukerjee
  • Andreas Fuchs
  • Andrew Dunham
  • Andrew Metcalf
  • Aniket Joshi
  • Carl Jackson
  • Craig Shannon
  • Evan Broder
  • Marc-André Tremblay
  • Ryan Koppenhaver

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