shim/task: relax VM network config validation

docs/vm-configuration.md

@@ -41,9 +41,10 @@ that take the following fields:
 - `mac` (reqired): MAC address
 - `dhcp` (optional, defaults to false): tells the VM to retrieve the IPv4 address
   of this interface through DHCPv4.
-- `addr` (required): IP address with subnet mask (in CIDR notation). This field
-  can be specified either zero times (when `dhcp` is specified), or at most
-  twice (once per IP family).
+- `addr` (optional): IP address with subnet mask (in CIDR notation). This field
+  can be omitted (specified zero times) either when `dhcp` is enabled or when the
+  interface is intended to be L2-only (no IP assigned), and may be specified at
+  most twice otherwise (once per IP family).
 - `features` (optional, defaults to 0): Bitwise-OR separated list of virtio-net
   features. Supported features:
   - `VIRTIO_NET_F_CSUM`: Device handles packets with partial checksum offload
@@ -61,6 +62,10 @@ that take the following fields:
 
 Note that the first network specified will be used as the default gateway.
 
+If neither `addr` nor `dhcp` are specified, the network interface will have no IP
+address assigned and will be used exclusively to switch packets between the VM
+and the host.
+
 #### gvisor-tap-vsock
 
 Here's an example using [containers/gvisor-tap-vsock](https://github.com/containers/gvisor-tap-vsock).

internal/shim/task/networking_unix.go

@@ -154,8 +154,8 @@ func parseNetwork(annotation string) (network, error) {
 		}
 	}
 
-	if n.endpoint == "" || n.mode == "" || n.mac == nil || (!n.dhcp && !n.addr4.IsValid() && !n.addr6.IsValid()) {
-		return network{}, fmt.Errorf("either 'endpoint', 'mode', 'mac', 'dhcp' or 'addr' is missing")
+	if n.endpoint == "" || n.mode == "" || n.mac == nil {
+		return network{}, fmt.Errorf("missing required field(s): endpoint, mode, mac")
 	}
 
 	return n, nil

internal/vminit/vmnetworking/vmnetworking.go

@@ -45,8 +45,8 @@ type Network struct {
 }
 
 func (nw Network) Validate() error {
-	if nw.MAC == nil || (!nw.Addr4.IsValid() && !nw.Addr6.IsValid() && !nw.DHCP) {
-		return errors.New("must specify either addr or dhcp")
+	if nw.MAC == nil {
+		return errors.New("must specify MAC address")
 	}
 	if (nw.Addr4.IsValid() || nw.Addr6.IsValid()) && nw.DHCP {
 		return errors.New("cannot specify both addr and dhcp")