Network: Automatically figure out the gateway and netmask by inspecting

the lxc bridge interface

network.go

@@ -1,12 +1,12 @@
 package docker
 
 import (
+	"fmt"
 	"net"
 )
 
 const (
-	networkGateway   = "10.0.3.1"
-	networkPrefixLen = 24
+	networkBridgeIface = "lxcbr0"
 )
 
 type NetworkInterface struct {
@@ -19,11 +19,42 @@ func allocateIPAddress() string {
 	return "10.0.3.2"
 }
 
+func getBridgeAddr(name string) (net.Addr, error) {
+	iface, err := net.InterfaceByName(name)
+	if err != nil {
+		return nil, err
+	}
+	addrs, err := iface.Addrs()
+	if err != nil {
+		return nil, err
+	}
+	var addrs4 []net.Addr
+	for _, addr := range addrs {
+		ip := (addr.(*net.IPNet)).IP
+		if ip4 := ip.To4(); len(ip4) == net.IPv4len {
+			addrs4 = append(addrs4, addr)
+		}
+	}
+	switch {
+	case len(addrs4) == 0:
+		return nil, fmt.Errorf("Bridge %v has no IP addresses", name)
+	case len(addrs4) > 1:
+		return nil, fmt.Errorf("Bridge %v has more than 1 IPv4 address", name)
+	}
+	return addrs4[0], nil
+}
+
 func allocateNetwork() (*NetworkInterface, error) {
+	bridgeAddr, err := getBridgeAddr(networkBridgeIface)
+	if err != nil {
+		return nil, err
+	}
+	bridge := bridgeAddr.(*net.IPNet)
+	ipPrefixLen, _ := bridge.Mask.Size()
 	iface := &NetworkInterface{
 		IpAddress:   allocateIPAddress(),
-		IpPrefixLen: networkPrefixLen,
-		Gateway:     net.ParseIP(networkGateway),
+		IpPrefixLen: ipPrefixLen,
+		Gateway:     bridge.IP,
 	}
 	return iface, nil
 }