pixiecore: checkpoint early bits of a full DHCP server.

I started hacking on this, but I need to take care of other bits
of the port before I embark on such a large feature. The code is
currently dead and not used by pixiecore, but I didn't want to
lose the small bits of address fiddling I've done so far.

pixiecore/dhcp_server.go

@@ -0,0 +1,100 @@
+// Copyright 2016 Google 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 pixiecore
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"time"
+
+	"go.universe.tf/netboot/dhcp4"
+	"golang.org/x/net/icmp"
+)
+
+type dhcpServer struct {
+	dhcpConn *dhcp4.Conn // Used for sending only
+	icmpConn icmp.PacketConn
+	leases   []lease
+}
+
+const (
+	leaseFree = iota
+	leaseReserved
+	leaseActive
+)
+
+type lease struct {
+	state   int
+	mac     net.HardwareAddr
+	expires time.Time
+}
+
+func newDHCPServer(address string, dhcpConn *dhcp4.Conn) (*dhcpServer, error) {
+	var ret dhcpServer
+
+	managedNet, err := findIPNet(address)
+	if err != nil {
+		return nil, err
+	}
+
+	for ip := managedNet.IP.Mask(managedNet.Mask); managedNet.Contains(ip); ip = nextIP(ip) {
+		ret.leases = append(ret.leases, lease{})
+		if ip.Equal(managedNet.IP) {
+			ret.leases[len(ret.leases)-1].state = leaseActive // Active with no expiry == eternal
+		}
+	}
+	return nil, errors.New("TODO")
+}
+
+func nextIP(ip net.IP) net.IP {
+	ip = ip.To4()
+	if ip == nil {
+		return nil
+	}
+	ret := net.IPv4(ip[0], ip[1], ip[2], ip[3])
+	for i := 3; i >= 0; i-- {
+		ret[i]++
+		if ret[i] != 0 {
+			break
+		}
+	}
+	return ret
+}
+
+func findIPNet(address string) (*net.IPNet, error) {
+	wanted := net.ParseIP(address)
+	if wanted == nil || wanted.To4() == nil {
+		return nil, fmt.Errorf("Bad IPv4 address %q", address)
+	}
+
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, addr := range addrs {
+		ipaddr, ok := addr.(*net.IPNet)
+		if !ok {
+			continue
+		}
+		if !ipaddr.IP.Equal(wanted) {
+			continue
+		}
+		return ipaddr, nil
+	}
+
+	return nil, fmt.Errorf("IP address %q not found on any network interface", address)
+}