package dhcpd import ( "crypto/rand" "encoding/binary" "fmt" "math" "net" "os" "time" "github.com/hmage/golibs/log" "github.com/krolaw/dhcp4" ) func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) { iface, err := net.InterfaceByName(ifaceName) if err != nil { return false, wrapErrPrint(err, "Couldn't find interface by name %s", ifaceName) } // get ipv4 address of an interface ifaceIPNet := getIfaceIPv4(iface) if ifaceIPNet == nil { return false, fmt.Errorf("Couldn't find IPv4 address of interface %s %+v", ifaceName, iface) } srcIP := ifaceIPNet.IP src := net.JoinHostPort(srcIP.String(), "68") dst := "255.255.255.255:67" // form a DHCP request packet, try to emulate existing client as much as possible xId := make([]byte, 8) n, err := rand.Read(xId) if n != 8 && err == nil { err = fmt.Errorf("Generated less than 8 bytes") } if err != nil { return false, wrapErrPrint(err, "Couldn't generate 8 random bytes") } hostname, err := os.Hostname() if err != nil { return false, wrapErrPrint(err, "Couldn't get hostname") } requestList := []byte{ byte(dhcp4.OptionSubnetMask), byte(dhcp4.OptionClasslessRouteFormat), byte(dhcp4.OptionRouter), byte(dhcp4.OptionDomainNameServer), byte(dhcp4.OptionDomainName), byte(dhcp4.OptionDomainSearch), 252, // private/proxy autodiscovery 95, // LDAP byte(dhcp4.OptionNetBIOSOverTCPIPNameServer), byte(dhcp4.OptionNetBIOSOverTCPIPNodeType), } maxUDPsizeRaw := make([]byte, 2) binary.BigEndian.PutUint16(maxUDPsizeRaw, 1500) leaseTimeRaw := make([]byte, 4) leaseTime := uint32(math.RoundToEven(time.Duration(time.Hour * 24 * 90).Seconds())) binary.BigEndian.PutUint32(leaseTimeRaw, leaseTime) options := []dhcp4.Option{ {dhcp4.OptionParameterRequestList, requestList}, {dhcp4.OptionMaximumDHCPMessageSize, maxUDPsizeRaw}, {dhcp4.OptionClientIdentifier, append([]byte{0x01}, iface.HardwareAddr...)}, {dhcp4.OptionIPAddressLeaseTime, leaseTimeRaw}, {dhcp4.OptionHostName, []byte(hostname)}, } packet := dhcp4.RequestPacket(dhcp4.Discover, iface.HardwareAddr, nil, xId, false, options) // resolve 0.0.0.0:68 udpAddr, err := net.ResolveUDPAddr("udp4", src) if err != nil { return false, wrapErrPrint(err, "Couldn't resolve UDP address %s", src) } // spew.Dump(udpAddr, err) if !udpAddr.IP.To4().Equal(srcIP) { return false, wrapErrPrint(err, "Resolved UDP address is not %s", src) } // resolve 255.255.255.255:67 dstAddr, err := net.ResolveUDPAddr("udp4", dst) if err != nil { return false, wrapErrPrint(err, "Couldn't resolve UDP address %s", dst) } // bind to 0.0.0.0:68 log.Tracef("Listening to udp4 %+v", udpAddr) c, err := net.ListenPacket("udp4", src) if c != nil { defer c.Close() } // spew.Dump(c, err) // spew.Printf("net.ListenUDP returned %v, %v\n", c, err) if err != nil { return false, wrapErrPrint(err, "Couldn't listen to %s", src) } // send to 255.255.255.255:67 n, err = c.WriteTo(packet, dstAddr) // spew.Dump(n, err) if err != nil { return false, wrapErrPrint(err, "Couldn't send a packet to %s", dst) } // wait for answer log.Tracef("Waiting %v for an answer", defaultDiscoverTime) // TODO: replicate dhclient's behaviour of retrying several times with progressively bigger timeouts b := make([]byte, 1500) c.SetReadDeadline(time.Now().Add(defaultDiscoverTime)) n, _, err = c.ReadFrom(b) if isTimeout(err) { // timed out -- no DHCP servers return false, nil } if err != nil { return false, wrapErrPrint(err, "Couldn't receive packet") } if n > 0 { b = b[:n] } // spew.Dump(n, fromAddr, err, b) if n < 240 { // packet too small for dhcp return false, wrapErrPrint(err, "got packet that's too small for DHCP") } response := dhcp4.Packet(b[:n]) if response.HLen() > 16 { // invalid size return false, wrapErrPrint(err, "got malformed packet with HLen() > 16") } parsedOptions := response.ParseOptions() _, ok := parsedOptions[dhcp4.OptionDHCPMessageType] if !ok { return false, wrapErrPrint(err, "got malformed packet without DHCP message type") } // that's a DHCP server there return true, nil }