Define Linux Network Devices

The proposed "netdevices" field provides a declarative way to
specify which host network devices should be moved into a container's
network namespace.

This approach is similar than the existing "devices" field used for block
devices but uses a dictionary keyed by the interface name instead.

The proposed scheme is based on the existing representation of network
device by the `struct net_device`
https://docs.kernel.org/networking/netdevices.html.

This proposal focuses solely on moving existing network devices into
the container namespace. It does not cover the complexities of
network configuration or network interface creation, emphasizing the
separation of device management and network configuration.

Signed-off-by: Antonio Ojea <[email protected]>

Author: aojea

Diff for: config-linux.md

@@ -189,6 +189,112 @@ In addition to any devices configured with this setting, the runtime MUST also s
 * [`/dev/ptmx`][pts.4].
   A [bind-mount or symlink of the container's `/dev/pts/ptmx`][devpts].
 
+## <a name="configLinuxNetworkDevices" />Network Devices
+
+Linux network devices are entities that send and receive data packets. They are
+not represented as files in the `/dev` directory. Instead, they are represented
+by the [`net_device`][net_device] data structure in the Linux kernel. Network
+devices can belong to only one network namespace and use a set of operations
+distinct from regular file operations. Network devices can be categorized as
+**physical** or **virtual**:
+
+* **Physical network devices** correspond to hardware interfaces, such as
+    Ethernet cards (e.g., `eth0`, `enp0s3`). They are directly associated with
+    physical network hardware.
+* **Virtual network devices** are software-defined interfaces, such as loopback
+    devices (`lo`), virtual Ethernet pairs (`veth`), bridges (`br0`), VLANs, and
+    MACVLANs. They are created and managed by the kernel and do not correspond
+    to physical hardware.
+
+This schema focuses solely on moving existing network devices identified by name
+from the host network namespace into the container network namespace. It does
+not cover the complexities of network device creation or network configuration,
+such as IP address assignment, routing, and DNS setup.
+
+**`netDevices`** (object, OPTIONAL) - A set of network devices that MUST be made
+available in the container. The runtime is responsible for moving these devices;
+the underlying mechanism is implementation-defined.
+
+The name of the network device is the entry key. Entry values are objects with
+the following properties:
+
+* **`name`** *(string, OPTIONAL)* - the name of the network device inside the
+    container namespace. If not specified, the host name is used.
+
+The runtime MUST check if moving the network interface to the container
+namespace is possible. If a network device with the specified name already
+exists in the container namespace, the runtime MUST [generate an error](runtime.md#errors),
+unless the user has provided a template by appending
+`%d` to the new name. In that case, the runtime MUST allow the move, and the
+kernel will generate a unique name for the interface within the container's
+network namespace.
+
+The runtime MUST preserve the existing network interface attributes, as defined
+by the kernel, including IP addresses, enabling users to preconfigure the
+interfaces.
+
+The runtime MUST set the network device state to "up" after moving it to the
+network namespace to allow the container to send and receive network traffic
+through that device.
+
+### Namespace Lifecycle and Container Termination
+
+The runtime MUST NOT actively manage the interface's lifecycle and configuration
+*within* the container's network namespace. This is because network interfaces
+are inherently tied to the network namespace itself, and their lifecycle is
+therefore managed by the owner of the network namespace. Typically, this
+ownership and management are handled by higher-level container runtime
+orchestrators, rather than the processes running directly within the container.
+
+The runtime **MUST NOT** attempt to move the interface out of the namespace
+before deletion. This design decision is based on the following:
+
+* **Namespace Ownership:** Network interfaces are tied to the network namespace,
+    which may not always be directly managed by the runtime.
+* **Abrupt Termination:** Even when the runtime manages the namespace, it cannot
+    reliably participate in its deletion if the container's processes terminate
+    abruptly (e.g., due to a crash).
+
+During the network namespace deletion the kernel's built-in namespace cleanup
+mechanisms take over, as described in [network_namespaces(7)][net_namespaces.7]:
+"When a network namespace is freed (i.e., when the last process in the namespace
+terminates), its physical network devices are moved back to the initial network
+namespace." All the network namespace migratable physical network devices are
+moved to the default network namespace, while virtual devices (veth, macvlan,
+...) are destroyed.
+
+If users require custom handling of interface lifecycle during namespace
+deletion, they can utilize existing features within the namespace orchestrator
+or employ post-stop hooks.
+
+**Physical Interface Renaming and Systemd**
+
+When a physical interface is renamed within a container and the container's
+network namespace is later deleted, the kernel will move the interface back to
+the root namespace with its renamed name. To ensure predictable interface names
+in the root namespace, users can utilize systemd's `udevd` and `networkd` rules.
+Refer to [systemd Predictable Network Interface Names][predictable-network-interfaces-names] for more information on configuring
+predictable names.
+
+When a physical interface is renamed within a container and the container's
+network namespace is later deleted, the kernel will move the interface back to
+the root namespace with its renamed name. In case of a name conflict in the root
+namespace, the kernel will rename it to `dev%d`. To ensure predictable interface
+names in the root namespace, users can utilize systemd's `udevd` and `networkd`
+rules. Refer to [systemd Predictable Network Interface Names][predictable-network-interfaces-names]
+for more information on configuring predictable names.
+
+### Example
+
+#### Moving a device with a renamed interface inside the container:
+
+```json
+"netDevices": {
+    "eth0" : {
+        "name": "container_eth0"
+    }
+}
+
 ## <a name="configLinuxControlGroups" />Control groups
 
 Also known as cgroups, they are used to restrict resource usage for a container and handle device access.
@@ -975,6 +1081,9 @@ subset of the available options.
 [mknod.1]: https://man7.org/linux/man-pages/man1/mknod.1.html
 [mknod.2]: https://man7.org/linux/man-pages/man2/mknod.2.html
 [namespaces.7_2]: https://man7.org/linux/man-pages/man7/namespaces.7.html
+[net_device]: https://docs.kernel.org/networking/netdevices.html
+[net_namespaces.7]: https://man7.org/linux/man-pages/man7/network_namespaces.7.html
+[predictable-network-interfaces-names]: https://systemd.io/PREDICTABLE_INTERFACE_NAMES
 [null.4]: https://man7.org/linux/man-pages/man4/null.4.html
 [personality.2]: https://man7.org/linux/man-pages/man2/personality.2.html
 [pts.4]: https://man7.org/linux/man-pages/man4/pts.4.html

Diff for: features-linux.md

@@ -228,3 +228,17 @@ Irrelevant to the availability of Intel RDT on the host operating system.
   }
 }
 ```
+
+## <a name="linuxFeaturesNetDevices" />NetDevices
+
+**`netDevices`** (object, OPTIONAL) represents the runtime's implementation status of Linux network devices.
+
+* **`enabled`** (bool, OPTIONAL) represents whether the runtime supports the capability to move Linux network devices into the container's network namespace.
+
+### Example
+
+```json
+"netDevices": {
+  "enabled": true
+}
+```

Diff for: schema/config-linux.json

@@ -9,6 +9,12 @@
                     "$ref": "defs-linux.json#/definitions/Device"
                 }
             },
+            "netDevices": {
+                "type": "object",
+                "additionalProperties": {
+                    "$ref": "defs-linux.json#/definitions/NetDevice"
+                }
+            },
             "uidMappings": {
                 "type": "array",
                 "items": {

Diff for: schema/defs-linux.json

@@ -189,6 +189,14 @@
                 }
             }
         },
+        "NetDevice": {
+            "type": "object",
+            "properties": {
+                "name": {
+                    "type": "string"
+                }
+            }
+        },
         "weight": {
             "$ref": "defs.json#/definitions/uint16"
         },

Diff for: schema/features-linux.json

@@ -110,6 +110,14 @@
                         }
                     }
                 }
+            },
+            "netDevices": {
+                "type": "object",
+                "properties": {
+                    "enabled": {
+                        "type": "boolean"
+                    }
+                }
             }
         }
     }

Diff for: schema/test/config/bad/linux-netdevice.json

@@ -0,0 +1,13 @@
+{
+    "ociVersion": "1.0.0",
+    "root": {
+        "path": "rootfs"
+    },
+    "linux": {
+        "netDevices": {
+            "eth0": {
+                "name": 23
+            }
+        }
+    }
+}

Diff for: schema/test/config/good/linux-netdevice.json

@@ -0,0 +1,15 @@
+{
+    "ociVersion": "1.0.0",
+    "root": {
+        "path": "rootfs"
+    },
+    "linux": {
+        "netDevices": {
+            "eth0": {
+                "name": "container_eth0"
+            },
+            "ens4": {},
+            "ens5": {}
+        }
+    }
+}

Diff for: schema/test/features/good/runc.json

@@ -182,6 +182,9 @@
         },
         "selinux": {
             "enabled": true
+        },
+        "netDevices": {
+            "enabled": true
         }
     },
     "annotations": {

Diff for: specs-go/config.go

@@ -236,6 +236,8 @@ type Linux struct {
 	Namespaces []LinuxNamespace `json:"namespaces,omitempty"`
 	// Devices are a list of device nodes that are created for the container
 	Devices []LinuxDevice `json:"devices,omitempty"`
+	// NetDevices are key-value pairs, keyed by network device name on the host, moved to the container's network namespace.
+	NetDevices map[string]LinuxNetDevice `json:"netDevices,omitempty"`
 	// Seccomp specifies the seccomp security settings for the container.
 	Seccomp *LinuxSeccomp `json:"seccomp,omitempty"`
 	// RootfsPropagation is the rootfs mount propagation mode for the container.
@@ -491,6 +493,12 @@ type LinuxDevice struct {
 	GID *uint32 `json:"gid,omitempty"`
 }
 
+// LinuxNetDevice represents a single network device to be added to the container's network namespace
+type LinuxNetDevice struct {
+	// Name of the device in the container namespace
+	Name string `json:"name,omitempty"`
+}
+
 // LinuxDeviceCgroup represents a device rule for the devices specified to
 // the device controller
 type LinuxDeviceCgroup struct {

Diff for: specs-go/features/features.go

@@ -48,6 +48,7 @@ type Linux struct {
 	Selinux         *Selinux         `json:"selinux,omitempty"`
 	IntelRdt        *IntelRdt        `json:"intelRdt,omitempty"`
 	MountExtensions *MountExtensions `json:"mountExtensions,omitempty"`
+	NetDevices      *NetDevices      `json:"netDevices,omitempty"`
 }
 
 // Cgroup represents the "cgroup" field.
@@ -143,3 +144,10 @@ type IDMap struct {
 	// Nil value means "unknown", not "false".
 	Enabled *bool `json:"enabled,omitempty"`
 }
+
+// NetDevices represents the "netDevices" field.
+type NetDevices struct {
+	// Enabled is true if network devices support is compiled in.
+	// Nil value means "unknown", not "false".
+	Enabled *bool `json:"enabled,omitempty"`
+}