Add -hostprefix in DHCP daemon to run the daemon as container

This diff adds -hostprefix option in dhcp daemon. This option
could be used to run dhcp daemon as container because container
cannot touch host's netns directly. The diff changes dhcp daemon
to touch procfs mounted to another path, like '/hostfs/proc'.

plugins/ipam/dhcp/daemon.go

@@ -39,8 +39,9 @@ const resendCount = 3
 var errNoMoreTries = errors.New("no more tries")
 
 type DHCP struct {
-	mux    sync.Mutex
-	leases map[string]*DHCPLease
+	mux             sync.Mutex
+	leases          map[string]*DHCPLease
+	hostNetnsPrefix string
 }
 
 func newDHCP() *DHCP {
@@ -58,7 +59,8 @@ func (d *DHCP) Allocate(args *skel.CmdArgs, result *current.Result) error {
 	}
 
 	clientID := args.ContainerID + "/" + conf.Name
-	l, err := AcquireLease(clientID, args.Netns, args.IfName)
+	hostNetns := d.hostNetnsPrefix + args.Netns
+	l, err := AcquireLease(clientID, hostNetns, args.IfName)
 	if err != nil {
 		return err
 	}
@@ -140,7 +142,7 @@ func getListener() (net.Listener, error) {
 	}
 }
 
-func runDaemon(pidfilePath string) error {
+func runDaemon(pidfilePath string, hostPrefix string) error {
 	// since other goroutines (on separate threads) will change namespaces,
 	// ensure the RPC server does not get scheduled onto those
 	runtime.LockOSThread()
@@ -161,6 +163,7 @@ func runDaemon(pidfilePath string) error {
 	}
 
 	dhcp := newDHCP()
+	dhcp.hostNetnsPrefix = hostPrefix
 	rpc.Register(dhcp)
 	rpc.HandleHTTP()
 	http.Serve(l, nil)