fix(net): stop dhcp client and release all v4 addr on linkdown (#16)

This commit fixes jetkvm/kvm#12 by disabling the udhcpc client when the
link goes down, it then removes all the active IPv4 addresses from the
deivce.

Once the link comes back up, it re-activates the udhcpc client so it can
fetch a new IPv4 address for the device.

This doesn't make any changes to the IPv6 side of things yet.

network.go

@@ -6,6 +6,7 @@ import (
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
 	"net"
+	"os/exec"
 	"time"
 
 	"github.com/vishvananda/netlink"
@@ -25,6 +26,23 @@ type LocalIpInfo struct {
 	MAC  string
 }
 
+// setDhcpClientState sends signals to udhcpc to change it's current mode
+// of operation. Setting active to true will force udhcpc to renew the DHCP lease.
+// Setting active to false will put udhcpc into idle mode.
+func setDhcpClientState(active bool) {
+	var signal string;
+	if active {
+		signal = "-SIGUSR1"
+	} else {
+		signal = "-SIGUSR2"
+	}
+
+	cmd := exec.Command("/usr/bin/killall", signal, "udhcpc");
+	if err := cmd.Run(); err != nil {
+		fmt.Printf("network: setDhcpClientState: failed to change udhcpc state: %s\n", err)
+	}
+}
+
 func checkNetworkState() {
 	iface, err := netlink.LinkByName("eth0")
 	if err != nil {
@@ -47,9 +65,26 @@ func checkNetworkState() {
 		fmt.Printf("failed to get addresses for eth0: %v\n", err)
 	}
 
+	// If the link is going down, put udhcpc into idle mode.
+	// If the link is coming back up, activate udhcpc and force it to renew the lease.
+	if newState.Up != networkState.Up {
+		setDhcpClientState(newState.Up)
+	}
+
 	for _, addr := range addrs {
 		if addr.IP.To4() != nil {
-			newState.IPv4 = addr.IP.String()
+			if !newState.Up && networkState.Up {
+				// If the network is going down, remove all IPv4 addresses from the interface.
+				fmt.Printf("network: state transitioned to down, removing IPv4 address %s\n", addr.IP.String())
+				err := netlink.AddrDel(iface, &addr)
+				if err != nil {
+					fmt.Printf("network: failed to delete %s", addr.IP.String())
+				}
+
+				newState.IPv4 = "..."
+			} else {
+				newState.IPv4 = addr.IP.String()
+			}
 		} else if addr.IP.To16() != nil && newState.IPv6 == "" {
 			newState.IPv6 = addr.IP.String()
 		}