[Windows] Adds optional loopbackDSR argument to cni config.

Adds a bool to the cni config that will add a policy that allows for loopbackDSR on an interface. Updates relevant documentation. Allows L2Tunnel networks to be used for L2Bridge plugin.

pkg/hns/endpoint_windows.go

@@ -77,6 +77,9 @@ func GenerateHnsEndpoint(epInfo *EndpointInfo, n *NetConf) (*hcsshim.HNSEndpoint
 		}
 	}
 
+	if n.LoopbackDSR {
+		n.ApplyLoopbackDSR(&epInfo.IpAddress)
+	}
 	if hnsEndpoint == nil {
 		hnsEndpoint = &hcsshim.HNSEndpoint{
 			Name:           epInfo.EndpointName,
@@ -117,14 +120,8 @@ func GenerateHcnEndpoint(epInfo *EndpointInfo, n *NetConf) (*hcn.HostComputeEndp
 	if hcnEndpoint == nil {
 		routes := []hcn.Route{
 			{
-				NextHop: GetIpString(&epInfo.Gateway),
-				DestinationPrefix: func() string {
-					destinationPrefix := "0.0.0.0/0"
-					if ipv6 := epInfo.Gateway.To4(); ipv6 == nil {
-						destinationPrefix = "::/0"
-					}
-					return destinationPrefix
-				}(),
+				NextHop:           GetIpString(&epInfo.Gateway),
+				DestinationPrefix: GetDefaultDestinationPrefix(&epInfo.Gateway),
 			},
 		}
 
@@ -138,6 +135,9 @@ func GenerateHcnEndpoint(epInfo *EndpointInfo, n *NetConf) (*hcn.HostComputeEndp
 		}
 		ipConfigs := []hcn.IpConfig{hcnIpConfig}
 
+		if n.LoopbackDSR {
+			n.ApplyLoopbackDSR(&epInfo.IpAddress)
+		}
 		hcnEndpoint = &hcn.HostComputeEndpoint{
 			SchemaVersion:      hcn.Version{Major: 2},
 			Name:               epInfo.EndpointName,

pkg/hns/netconf_windows.go

@@ -17,6 +17,9 @@ package hns
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
+	"net"
+
 	"github.com/Microsoft/hcsshim/hcn"
 	"github.com/buger/jsonparser"
 	"github.com/containernetworking/cni/pkg/types"
@@ -26,9 +29,16 @@ import (
 // NetConf is the CNI spec
 type NetConf struct {
 	types.NetConf
+	// ApiVersion is either 1 or 2, which specifies which hns APIs to call
+	ApiVersion int `json:"ApiVersion"`
+	// V2 Api Policies
 	HcnPolicyArgs []hcn.EndpointPolicy `json:"HcnPolicyArgs,omitempty"`
-	Policies      []policy             `json:"policies,omitempty"`
-	RuntimeConfig RuntimeConfig        `json:"runtimeConfig"`
+	// V1 Api Policies
+	Policies []policy `json:"policies,omitempty"`
+	// Options to be passed in by the runtime
+	RuntimeConfig RuntimeConfig `json:"runtimeConfig"`
+	// If true, adds a policy to endpoints to support loopback direct server return
+	LoopbackDSR bool `json:"loopbackDSR"`
 }
 
 type RuntimeDNS struct {
@@ -45,6 +55,31 @@ type policy struct {
 	Value json.RawMessage `json:"value"`
 }
 
+func GetDefaultDestinationPrefix(ip *net.IP) string {
+	destinationPrefix := "0.0.0.0/0"
+	if ipv6 := ip.To4(); ipv6 == nil {
+		destinationPrefix = "::/0"
+	}
+	return destinationPrefix
+}
+
+func (n *NetConf) ApplyLoopbackDSR(ip *net.IP) {
+	value := fmt.Sprintf(`"Destinations" : ["%s"]`, ip.String())
+	if n.ApiVersion == 2 {
+		hcnLoopbackRoute := hcn.EndpointPolicy{
+			Type:     "OutBoundNAT",
+			Settings: []byte(fmt.Sprintf("{%s}", value)),
+		}
+		n.HcnPolicyArgs = append(n.HcnPolicyArgs, hcnLoopbackRoute)
+	} else {
+		hnsLoopbackRoute := policy{
+			Name:  "EndpointPolicy",
+			Value: []byte(fmt.Sprintf(`{"Type": "OutBoundNAT", %s}`, value)),
+		}
+		n.Policies = append(n.Policies, hnsLoopbackRoute)
+	}
+}
+
 // If runtime dns values are there use that else use cni conf supplied dns
 func (n *NetConf) GetDNS() types.DNS {
 	dnsResult := n.DNS