add DHCP IPAM plugin

The plugin binary actually functions in two modes. The first mode
is a regular CNI plugin. The second mode (when stared with "daemon" arg)
runs a DHCP client daemon. When executed as a CNI plugin, it issues
an RPC request to the daemon for actual processing. The daemon is
required since a DHCP lease needs to be maintained by periodically
renewing it. One instance of the daemon can server arbitrary number
of containers/leases.

plugins/ipam/dhcp/main.go

@@ -0,0 +1,64 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"net/rpc"
+	"os"
+
+	"github.com/appc/cni/pkg/plugin"
+	"github.com/appc/cni/pkg/skel"
+)
+
+const socketPath = "/run/cni/dhcp.sock"
+
+func main() {
+	if len(os.Args) > 1 && os.Args[1] == "daemon" {
+		runDaemon()
+	} else {
+		skel.PluginMain(cmdAdd, cmdDel)
+	}
+}
+
+func cmdAdd(args *skel.CmdArgs) error {
+	client, err := rpc.DialHTTP("unix", socketPath)
+	if err != nil {
+		return fmt.Errorf("error dialing DHCP daemon: %v", err)
+	}
+
+	result := &plugin.Result{}
+	err = client.Call("DHCP.Allocate", args, result)
+	if err != nil {
+		return fmt.Errorf("error calling DHCP.Add: %v", err)
+	}
+
+	return plugin.PrintResult(result)
+}
+
+func cmdDel(args *skel.CmdArgs) error {
+	client, err := rpc.DialHTTP("unix", socketPath)
+	if err != nil {
+		return fmt.Errorf("error dialing DHCP daemon: %v", err)
+	}
+
+	dummy := struct{}{}
+	err = client.Call("DHCP.Release", args, &dummy)
+	if err != nil {
+		return fmt.Errorf("error calling DHCP.Del: %v", err)
+	}
+
+	return nil
+}