fix: parse subnet cidr and calculate gateway

cns/imdsclient/imdsclient.go

@@ -1,17 +1,24 @@
-// Copyright 2017 Microsoft. All rights reserved.
-// MIT License
-
 package imdsclient
 
 import (
+	"bytes"
 	"encoding/json"
 	"encoding/xml"
 	"fmt"
+	"io"
 	"math"
+	"net"
 	"net/http"
-	"strings"
 
 	"github.com/Azure/azure-container-networking/cns/logger"
+	"github.com/pkg/errors"
+)
+
+var (
+	// ErrNoPrimaryInterface indicates the imds respnose does not have a primary interface indicated.
+	ErrNoPrimaryInterface = errors.New("no primary interface found")
+	// ErrInsufficientAddressSpace indicates that the CIDR space is too small to include a gateway IP; it is 1 IP.
+	ErrInsufficientAddressSpace = errors.New("insufficient address space to generate gateway IP")
 )
 
 // GetNetworkContainerInfoFromHost retrieves the programmed version of network container from Host.
@@ -45,6 +52,7 @@ func (imdsClient *ImdsClient) GetNetworkContainerInfoFromHost(networkContainerID
 }
 
 // GetPrimaryInterfaceInfoFromHost retrieves subnet and gateway of primary NIC from Host.
+// TODO(rbtr): this is not a good client contract, we should return the resp.
 func (imdsClient *ImdsClient) GetPrimaryInterfaceInfoFromHost() (*InterfaceInfo, error) {
 	logger.Printf("[Azure CNS] GetPrimaryInterfaceInfoFromHost")
 
@@ -53,64 +61,87 @@ func (imdsClient *ImdsClient) GetPrimaryInterfaceInfoFromHost() (*InterfaceInfo,
 	if err != nil {
 		return nil, err
 	}
-
 	defer resp.Body.Close()
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to read response body")
+	}
 
-	logger.Printf("[Azure CNS] Response received from NMAgent for get interface details: %v", resp.Body)
+	logger.Printf("[Azure CNS] Response received from NMAgent for get interface details: %s", string(b))
 
 	var doc xmlDocument
-	decoder := xml.NewDecoder(resp.Body)
-	err = decoder.Decode(&doc)
-	if err != nil {
-		return nil, err
+	if err := xml.NewDecoder(bytes.NewReader(b)).Decode(&doc); err != nil {
+		return nil, errors.Wrap(err, "failed to decode response body")
 	}
 
 	foundPrimaryInterface := false
 
 	// For each interface.
 	for _, i := range doc.Interface {
-		// Find primary Interface.
-		if i.IsPrimary {
-			interfaceInfo.IsPrimary = true
-
-			// Get the first subnet.
-			for _, s := range i.IPSubnet {
-				interfaceInfo.Subnet = s.Prefix
-				malformedSubnetError := fmt.Errorf("Malformed subnet received from host %s", s.Prefix)
-
-				st := strings.Split(s.Prefix, "/")
-				if len(st) != 2 {
-					return nil, malformedSubnetError
-				}
-
-				ip := strings.Split(st[0], ".")
-				if len(ip) != 4 {
-					return nil, malformedSubnetError
-				}
-
-				interfaceInfo.Gateway = fmt.Sprintf("%s.%s.%s.1", ip[0], ip[1], ip[2])
-				for _, ip := range s.IPAddress {
-					if ip.IsPrimary {
-						interfaceInfo.PrimaryIP = ip.Address
-					}
+		// skip if not primary
+		if !i.IsPrimary {
+			continue
+		}
+		interfaceInfo.IsPrimary = true
+
+		// Get the first subnet.
+		for _, s := range i.IPSubnet {
+			interfaceInfo.Subnet = s.Prefix
+			gw, err := calculateGatewayIP(s.Prefix)
+			if err != nil {
+				return nil, err
+			}
+			interfaceInfo.Gateway = gw.String()
+			for _, ip := range s.IPAddress {
+				if ip.IsPrimary {
+					interfaceInfo.PrimaryIP = ip.Address
 				}
-
-				imdsClient.primaryInterface = interfaceInfo
-				break
 			}
 
-			foundPrimaryInterface = true
+			imdsClient.primaryInterface = interfaceInfo
 			break
 		}
+
+		foundPrimaryInterface = true
+		break
 	}
 
-	var er error
-	er = nil
 	if !foundPrimaryInterface {
-		er = fmt.Errorf("Unable to find primary NIC")
+		return nil, ErrNoPrimaryInterface
 	}
 
-	return interfaceInfo, er
+	return interfaceInfo, nil
+}
+
+// calculateGatewayIP parses the passed CIDR string and returns the first IP in the range.
+func calculateGatewayIP(cidr string) (net.IP, error) {
+	_, subnet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return nil, errors.Wrap(err, "received malformed subnet from host")
+	}
+
+	// check if we have enough address space to calculate a gateway IP
+	// we need at least 2 IPs (eg the IPv4 mask cannot be greater than 31)
+	// since the zeroth is reserved and the gateway is the first.
+	mask, bits := subnet.Mask.Size()
+	if mask == bits {
+		return nil, ErrInsufficientAddressSpace
+	}
+
+	// the subnet IP is the zero base address, so we need to increment it by one to get the gateway.
+	gw := make([]byte, len(subnet.IP))
+	copy(gw, subnet.IP)
+	for idx := len(gw) - 1; idx >= 0; idx-- {
+		gw[idx]++
+		// net.IP is a binary byte array, check if we have overflowed and need to continue incrementing to the left
+		// along the arary or if we're done.
+		// it's like if we have a 9 in base 10, and add 1, it rolls over to 0 so we're not done - we need to move
+		// left and increment that digit also.
+		if gw[idx] != 0 {
+			break
+		}
+	}
+	return gw, nil
 }
 
 // GetPrimaryInterfaceInfoFromMemory retrieves subnet and gateway of primary NIC that is saved in memory.

cns/imdsclient/imdsclient_test.go

@@ -0,0 +1,56 @@
+package imdsclient
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerateGatewayIP(t *testing.T) {
+	tests := []struct {
+		name    string
+		cidr    string
+		want    net.IP
+		wantErr bool
+	}{
+		{
+			name: "base case",
+			cidr: "10.0.0.0/8",
+			want: net.IPv4(10, 0, 0, 1),
+		},
+		{
+			name: "nonzero start",
+			cidr: "10.177.233.128/27",
+			want: net.IPv4(10, 177, 233, 129),
+		},
+		{
+			name:    "invalid",
+			cidr:    "test",
+			wantErr: true,
+		},
+		{
+			name:    "no available",
+			cidr:    "255.255.255.255/32",
+			wantErr: true,
+		},
+		{
+			name: "max IPv4",
+			cidr: "255.255.255.255/31",
+			want: net.IPv4(255, 255, 255, 255),
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := calculateGatewayIP(tt.cidr)
+			if tt.wantErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+			assert.Truef(t, tt.want.Equal(got), "want %s, got %s", tt.want.String(), got.String())
+		})
+	}
+}

cns/imdsclient/testdata/interfaces.xml

@@ -0,0 +1,7 @@
+<Interfaces>
+    <Interface MacAddress="002248263DBD" IsPrimary="true">
+        <IPSubnet Prefix="10.240.0.0/16">
+            <IPAddress Address="10.240.0.4" IsPrimary="true"/>
+        </IPSubnet>
+    </Interface>
+</Interfaces>