diff --git a/config/config.go b/config/config.go
index 30e30c4..15f9f79 100644
--- a/config/config.go
+++ b/config/config.go
@@ -2,6 +2,7 @@ package config
 
 import (
 	"flag"
+	"os"
 	"regexp"
 
 	"github.com/gorilla/securecookie"
@@ -30,6 +31,11 @@ var ForceTLS bool
 var SecureCookie *securecookie.SecureCookie
 var AdminToken string
 
+// Unsafe enables a number of unsafe features when set. It is principally
+// intended to be used in development. For example, it allows the caller to
+// specify the Docker networks to join.
+var Unsafe bool
+
 var PlaygroundDomain string
 
 var SegmentId string
@@ -60,6 +66,8 @@ func ParseFlags() {
 
 	flag.StringVar(&SegmentId, "segment-id", "", "Segment id to post metrics")
 
+	flag.BoolVar(&Unsafe, "unsafe", os.Getenv("PWD_UNSAFE") == "true", "Operate in unsafe mode")
+
 	flag.Parse()
 
 	SecureCookie = securecookie.New([]byte(CookieHashKey), []byte(CookieBlockKey))
diff --git a/provisioner/dind.go b/provisioner/dind.go
index a63daee..cdec3b1 100644
--- a/provisioner/dind.go
+++ b/provisioner/dind.go
@@ -11,6 +11,7 @@ import (
 	"strings"
 
 	lru "github.com/hashicorp/golang-lru"
+	"github.com/play-with-docker/play-with-docker/config"
 	"github.com/play-with-docker/play-with-docker/docker"
 	"github.com/play-with-docker/play-with-docker/id"
 	"github.com/play-with-docker/play-with-docker/pwd/types"
@@ -65,6 +66,12 @@ func (d *DinD) InstanceNew(session *types.Session, conf types.InstanceConfig) (*
 		}
 		conf.Hostname = nodeName
 	}
+
+	networks := []string{session.Id}
+	if config.Unsafe {
+		networks = append(networks, conf.Networks...)
+	}
+
 	containerName := fmt.Sprintf("%s_%s", session.Id[:8], d.generator.NewId())
 	opts := docker.CreateContainerOpts{
 		Image:          conf.ImageName,
@@ -76,7 +83,7 @@ func (d *DinD) InstanceNew(session *types.Session, conf types.InstanceConfig) (*
 		CACert:         conf.CACert,
 		HostFQDN:       conf.PlaygroundFQDN,
 		Privileged:     true,
-		Networks:       []string{session.Id},
+		Networks:       networks,
 		DindVolumeSize: conf.DindVolumeSize,
 		Envs:           conf.Envs,
 	}
diff --git a/pwd/instance_test.go b/pwd/instance_test.go
index 2513894..5bc7133 100644
--- a/pwd/instance_test.go
+++ b/pwd/instance_test.go
@@ -152,6 +152,15 @@ func TestInstanceNew_WithNotAllowedImage(t *testing.T) {
 
 	assert.Nil(t, err)
 
+	// Switch to unsafe mode in order to test custom networks below
+	//
+	// TODO: move config away from being a global in order that we don't
+	// have to hack setting the context in this way.
+	config.Unsafe = true
+	defer func() {
+		config.Unsafe = false
+	}()
+
 	expectedInstance := types.Instance{
 		Name:        fmt.Sprintf("%s_aaaabbbbcccc", session.Id[:8]),
 		Hostname:    "node1",
@@ -172,14 +181,14 @@ func TestInstanceNew_WithNotAllowedImage(t *testing.T) {
 		CACert:        nil,
 		Privileged:    true,
 		Envs:          []string{"HELLO=WORLD"},
-		Networks:      []string{session.Id},
+		Networks:      []string{session.Id, "arpanet"},
 	}
 	_d.On("ContainerCreate", expectedContainerOpts).Return(nil)
 	_d.On("ContainerIPs", expectedInstance.Name).Return(map[string]string{session.Id: "10.0.0.1"}, nil)
 	_s.On("InstancePut", mock.AnythingOfType("*types.Instance")).Return(nil)
 	_e.M.On("Emit", event.INSTANCE_NEW, "aaaabbbbcccc", []interface{}{"aaaabbbb_aaaabbbbcccc", "10.0.0.1", "node1", "ip10-0-0-1-aaaabbbbcccc"}).Return()
 
-	instance, err := p.InstanceNew(session, types.InstanceConfig{ImageName: "redis", Envs: []string{"HELLO=WORLD"}})
+	instance, err := p.InstanceNew(session, types.InstanceConfig{ImageName: "redis", Envs: []string{"HELLO=WORLD"}, Networks: []string{"arpanet"}})
 	assert.Nil(t, err)
 
 	assert.Equal(t, expectedInstance, *instance)
diff --git a/pwd/types/instance.go b/pwd/types/instance.go
index 4f84ed1..5b6c33c 100644
--- a/pwd/types/instance.go
+++ b/pwd/types/instance.go
@@ -40,4 +40,5 @@ type InstanceConfig struct {
 	Type           string
 	DindVolumeSize string
 	Envs           []string
+	Networks       []string
 }