Added ability to specify net ns at link creation

Creating a link in one namespace and then moving can
be problematic since there could be a iface name conflict.
However, it is not always possible to switch into the target
namespace either -- e.g. creating a macvlan interface must
be done in the namespace of the parent interface.

link.go

@@ -10,6 +10,11 @@ type Link interface {
 	Type() string
 }
 
+type (
+	NsPid int
+	NsFd  int
+)
+
 // LinkAttrs represents data shared by most link types
 type LinkAttrs struct {
 	Index        int
@@ -20,6 +25,7 @@ type LinkAttrs struct {
 	Flags        net.Flags
 	ParentIndex  int         // index of the parent link device
 	MasterIndex  int         // must be the index of a bridge
+	Namespace    interface{} // nil | NsPid | NsFd
 }
 
 // Device links cannot be created via netlink. These links

link_linux.go

@@ -312,6 +312,20 @@ func LinkAdd(link Link) error {
 		req.AddData(mtu)
 	}
 
+	if base.Namespace != nil {
+		var attr *nl.RtAttr
+		switch base.Namespace.(type) {
+		case NsPid:
+			val := nl.Uint32Attr(uint32(base.Namespace.(NsPid)))
+			attr = nl.NewRtAttr(syscall.IFLA_NET_NS_PID, val)
+		case NsFd:
+			val := nl.Uint32Attr(uint32(base.Namespace.(NsFd)))
+			attr = nl.NewRtAttr(nl.IFLA_NET_NS_FD, val)
+		}
+
+		req.AddData(attr)
+	}
+
 	linkInfo := nl.NewRtAttr(syscall.IFLA_LINKINFO, nil)
 	nl.NewRtAttrChild(linkInfo, nl.IFLA_INFO_KIND, nl.NonZeroTerminated(link.Type()))