build: fix host-gateway handling

build/build.go

@@ -645,7 +645,7 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 	}
 
 	// setup extrahosts
-	extraHosts, err := toBuildkitExtraHosts(opt.ExtraHosts, nodeDriver.IsMobyDriver())
+	extraHosts, err := toBuildkitExtraHosts(ctx, opt.ExtraHosts, nodeDriver)
 	if err != nil {
 		return nil, nil, err
 	}

build/utils.go

@@ -3,10 +3,12 @@ package build
 import (
 	"archive/tar"
 	"bytes"
+	"context"
 	"net"
 	"os"
 	"strings"
 
+	"github.com/docker/buildx/driver"
 	"github.com/docker/cli/opts"
 	"github.com/docker/docker/builder/remotecontext/urlutil"
 	"github.com/moby/buildkit/util/gitutil"
@@ -57,7 +59,7 @@ func isArchive(header []byte) bool {
 }
 
 // toBuildkitExtraHosts converts hosts from docker key:value format to buildkit's csv format
-func toBuildkitExtraHosts(inp []string, mobyDriver bool) (string, error) {
+func toBuildkitExtraHosts(ctx context.Context, inp []string, nodeDriver *driver.DriverHandle) (string, error) {
 	if len(inp) == 0 {
 		return "", nil
 	}
@@ -67,11 +69,16 @@ func toBuildkitExtraHosts(inp []string, mobyDriver bool) (string, error) {
 		if !ok || host == "" || ip == "" {
 			return "", errors.Errorf("invalid host %s", h)
 		}
-		// Skip IP address validation for "host-gateway" string with moby driver
-		if !mobyDriver || ip != mobyHostGatewayName {
-			if net.ParseIP(ip) == nil {
-				return "", errors.Errorf("invalid host %s", h)
+		// If the IP Address is a "host-gateway", replace this value with the
+		// IP address provided by the worker's label.
+		if ip == mobyHostGatewayName {
+			hgip, err := nodeDriver.HostGatewayIP(ctx)
+			if err != nil {
+				return "", errors.Wrap(err, "unable to derive the IP value for host-gateway")
 			}
+			ip = hgip.String()
+		} else if net.ParseIP(ip) == nil {
+			return "", errors.Errorf("invalid host %s", h)
 		}
 		hosts = append(hosts, host+"="+ip)
 	}

driver/manager.go

@@ -2,17 +2,17 @@ package driver
 
 import (
 	"context"
+	"net"
 	"os"
 	"sort"
 	"strings"
 	"sync"
 
-	"k8s.io/client-go/rest"
-
 	dockerclient "github.com/docker/docker/client"
 	"github.com/moby/buildkit/client"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
+	"k8s.io/client-go/rest"
 )
 
 type Factory interface {
@@ -154,6 +154,9 @@ type DriverHandle struct {
 	features                map[Feature]bool
 	historyAPISupportedOnce sync.Once
 	historyAPISupported     bool
+	hostGatewayIPOnce       sync.Once
+	hostGatewayIP           net.IP
+	hostGatewayIPErr        error
 }
 
 func (d *DriverHandle) Client(ctx context.Context) (*client.Client, error) {
@@ -178,3 +181,36 @@ func (d *DriverHandle) HistoryAPISupported(ctx context.Context) bool {
 	})
 	return d.historyAPISupported
 }
+
+func (d *DriverHandle) HostGatewayIP(ctx context.Context) (net.IP, error) {
+	d.hostGatewayIPOnce.Do(func() {
+		if !d.Driver.IsMobyDriver() {
+			d.hostGatewayIPErr = errors.New("host-gateway is only supported with the docker driver")
+			return
+		}
+		c, err := d.Client(ctx)
+		if err != nil {
+			d.hostGatewayIPErr = err
+			return
+		}
+		workers, err := c.ListWorkers(ctx)
+		if err != nil {
+			d.hostGatewayIPErr = errors.Wrap(err, "listing workers")
+			return
+		}
+		for _, w := range workers {
+			// should match github.com/docker/docker/builder/builder-next/worker/label.HostGatewayIP const
+			if v, ok := w.Labels["org.mobyproject.buildkit.worker.moby.host-gateway-ip"]; ok && v != "" {
+				ip := net.ParseIP(v)
+				if ip == nil {
+					d.hostGatewayIPErr = errors.Errorf("failed to parse host-gateway IP: %s", v)
+					return
+				}
+				d.hostGatewayIP = ip
+				return
+			}
+		}
+		d.hostGatewayIPErr = errors.New("host-gateway IP not found")
+	})
+	return d.hostGatewayIP, d.hostGatewayIPErr
+}