"ns:" network mode to use existing network namespace

cmd/nerdctl/container/container_run.go

@@ -112,11 +112,11 @@ func setCreateFlags(cmd *cobra.Command) {
 
 	// #region network flags
 	// network (net) is defined as StringSlice, not StringArray, to allow specifying "--network=cni1,cni2"
-	cmd.Flags().StringSlice("network", []string{netutil.DefaultNetworkName}, `Connect a container to a network ("bridge"|"host"|"none"|"container:<container>"|<CNI>)`)
+	cmd.Flags().StringSlice("network", []string{netutil.DefaultNetworkName}, `Connect a container to a network ("bridge"|"host"|"none"|"container:<container>"|"ns:<path>"|<CNI>)`)
 	cmd.RegisterFlagCompletionFunc("network", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
 		return completion.NetworkNames(cmd, []string{})
 	})
-	cmd.Flags().StringSlice("net", []string{netutil.DefaultNetworkName}, `Connect a container to a network ("bridge"|"host"|"none"|<CNI>)`)
+	cmd.Flags().StringSlice("net", []string{netutil.DefaultNetworkName}, `Connect a container to a network ("bridge"|"host"|"none"|"container:<container>"|"ns:<path>"|<CNI>)`)
 	cmd.RegisterFlagCompletionFunc("net", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
 		return completion.NetworkNames(cmd, []string{})
 	})

cmd/nerdctl/container/container_run_network_linux_test.go

@@ -21,14 +21,19 @@ import (
 	"io"
 	"net"
 	"os"
+	"os/exec"
 	"regexp"
 	"runtime"
 	"strings"
 	"testing"
+	"time"
 
+	"github.com/containernetworking/plugins/pkg/ns"
+	"github.com/vishvananda/netlink"
 	"gotest.tools/v3/assert"
 	"gotest.tools/v3/icmd"
 
+	"github.com/containerd/containerd/v2/pkg/netns"
 	"github.com/containerd/errdefs"
 
 	"github.com/containerd/nerdctl/v2/cmd/nerdctl/helpers"
@@ -499,6 +504,41 @@ func TestSharedNetworkStack(t *testing.T) {
 		AssertOutContains(testutil.NginxAlpineIndexHTMLSnippet)
 }
 
+func TestRunContainerInExistingNetNS(t *testing.T) {
+	if rootlessutil.IsRootless() {
+		t.Skip("Can't create new netns in rootless mode")
+	}
+	testutil.DockerIncompatible(t)
+	base := testutil.NewBase(t)
+
+	netNS, err := netns.NewNetNS(t.TempDir() + "/netns")
+	assert.NilError(t, err)
+	err = netNS.Do(func(netns ns.NetNS) error {
+		loopback, err := netlink.LinkByName("lo")
+		assert.NilError(t, err)
+		err = netlink.LinkSetUp(loopback)
+		assert.NilError(t, err)
+		return nil
+	})
+	assert.NilError(t, err)
+	defer netNS.Remove()
+
+	containerName := testutil.Identifier(t)
+	defer base.Cmd("rm", "-f", containerName).AssertOK()
+	base.Cmd("run", "-d", "--name", containerName,
+		"--network=ns:"+netNS.GetPath(), testutil.NginxAlpineImage).AssertOK()
+	base.EnsureContainerStarted(containerName)
+	time.Sleep(3 * time.Second)
+
+	err = netNS.Do(func(netns ns.NetNS) error {
+		stdout, err := exec.Command("curl", "-s", "http://127.0.0.1:80").Output()
+		assert.NilError(t, err)
+		assert.Assert(t, strings.Contains(string(stdout), testutil.NginxAlpineIndexHTMLSnippet))
+		return nil
+	})
+	assert.NilError(t, err)
+}
+
 func TestRunContainerWithMACAddress(t *testing.T) {
 	base := testutil.NewBase(t)
 	tID := testutil.Identifier(t)

docs/command-reference.md

@@ -175,9 +175,10 @@ Isolation flags:
 
 Network flags:
 
-- :whale: `--net, --network=(bridge|host|none|container:<container>|<CNI>)`: Connect a container to a network.
+- :whale: `--net, --network=(bridge|host|none|container:<container>|ns:<path>|<CNI>)`: Connect a container to a network.
   - Default: "bridge"
-  - 'container:<name|id>': reuse another container's network stack, container has to be precreated.
+  - `container:<name|id>`: reuse another container's network stack, container has to be precreated.
+  - :nerd_face: `ns:<path>`: run inside an existing network namespace
   - :nerd_face: Unlike Docker, this flag can be specified multiple times (`--net foo --net bar`)
 - :whale: `-p, --publish`: Publish a container's port(s) to the host
 - :whale: `--dns`: Set custom DNS servers

pkg/cmd/container/kill.go

@@ -151,7 +151,7 @@ func cleanupNetwork(ctx context.Context, container containerd.Container, globalO
 		}
 
 		switch netType {
-		case nettype.Host, nettype.None, nettype.Container:
+		case nettype.Host, nettype.None, nettype.Container, nettype.Namespace:
 			// NOP
 		case nettype.CNI:
 			e, err := netutil.NewCNIEnv(globalOpts.CNIPath, globalOpts.CNINetConfPath, netutil.WithNamespace(globalOpts.Namespace), netutil.WithDefaultNetwork())

pkg/composer/serviceparser/serviceparser.go

@@ -385,7 +385,7 @@ func getNetworks(project *types.Project, svc types.ServiceConfig) ([]networkName
 			return nil, errors.New("net and network_mode must not be set together")
 		}
 		if strings.Contains(svc.NetworkMode, ":") {
-			if !strings.HasPrefix(svc.NetworkMode, "container:") {
+			if !strings.HasPrefix(svc.NetworkMode, "container:") && !strings.HasPrefix(svc.NetworkMode, "ns:") {
 				return nil, fmt.Errorf("unsupported network_mode: %q", svc.NetworkMode)
 			}
 		}

pkg/containerutil/container_network_manager.go

@@ -132,6 +132,10 @@ func NewNetworkingOptionsManager(globalOptions types.GlobalCommandOptions, netOp
 		manager = &containerNetworkManager{globalOptions, netOpts, client}
 	case nettype.CNI:
 		manager = &cniNetworkManager{globalOptions, netOpts, client, cniNetworkManagerPlatform{}}
+	case nettype.Namespace:
+		// We'll handle Namespace networking identically to Host-mode networking, but
+		// put the container in the specified network namespace instead of the root.
+		manager = &hostNetworkManager{globalOptions, netOpts, client}
 	default:
 		return nil, fmt.Errorf("unexpected container networking type: %q", netType)
 	}
@@ -491,6 +495,26 @@ func copyFileContent(src string, dst string) error {
 	return nil
 }
 
+// getHostNetworkingNamespace Returns an oci.SpecOpts representing the network namespace to
+// be used by the hostNetworkManager. When running with `--network=host` this would be the host's
+// root namespace, but `--network=ns:<path>` can be used to run a container in an existing netns.
+func getHostNetworkingNamespace(netModeArg string) (oci.SpecOpts, error) {
+	if !strings.Contains(netModeArg, ":") {
+		// Use the host root namespace by default
+		return oci.WithHostNamespace(specs.NetworkNamespace), nil
+	}
+
+	netItems := strings.Split(netModeArg, ":")
+	if len(netItems) < 2 {
+		return nil, fmt.Errorf("namespace networking argument format must be 'ns:<path>', got: %q", netModeArg)
+	}
+	netnsPath := netItems[1]
+	return oci.WithLinuxNamespace(specs.LinuxNamespace{
+		Type: specs.NetworkNamespace,
+		Path: netnsPath,
+	}), nil
+}
+
 // ContainerNetworkingOpts Returns a slice of `oci.SpecOpts` and `containerd.NewContainerOpts` which represent
 // the network specs which need to be applied to the container with the given ID.
 func (m *hostNetworkManager) ContainerNetworkingOpts(_ context.Context, containerID string) ([]oci.SpecOpts, []containerd.NewContainerOpts, error) {
@@ -525,8 +549,13 @@ func (m *hostNetworkManager) ContainerNetworkingOpts(_ context.Context, containe
 		return nil, nil, err
 	}
 
+	netModeArg := m.netOpts.NetworkSlice[0]
+	netNamespace, err := getHostNetworkingNamespace(netModeArg)
+	if err != nil {
+		return nil, nil, err
+	}
 	specs := []oci.SpecOpts{
-		oci.WithHostNamespace(specs.NetworkNamespace),
+		netNamespace,
 		withDedupMounts("/etc/hosts", withCustomHosts(etcHostsPath)),
 		withDedupMounts("/etc/resolv.conf", withCustomResolvConf(resolvConfPath)),
 	}

pkg/netutil/nettype/nettype.go

@@ -29,6 +29,7 @@ const (
 	Host
 	CNI
 	Container
+	Namespace
 )
 
 var netTypeToName = map[interface{}]string{
@@ -37,6 +38,7 @@ var netTypeToName = map[interface{}]string{
 	Host:      "host",
 	CNI:       "cni",
 	Container: "container",
+	Namespace: "ns",
 }
 
 func Detect(names []string) (Type, error) {
@@ -54,6 +56,8 @@ func Detect(names []string) (Type, error) {
 			tmp = Host
 		case "container":
 			tmp = Container
+		case "ns":
+			tmp = Namespace
 		default:
 			tmp = CNI
 		}

pkg/ocihook/ocihook.go

@@ -170,7 +170,7 @@ func newHandlerOpts(state *specs.State, dataStore, cniPath, cniNetconfPath strin
 	}
 
 	switch netType {
-	case nettype.Host, nettype.None, nettype.Container:
+	case nettype.Host, nettype.None, nettype.Container, nettype.Namespace:
 		// NOP
 	case nettype.CNI:
 		e, err := netutil.NewCNIEnv(cniPath, cniNetconfPath, netutil.WithNamespace(namespace), netutil.WithDefaultNetwork())