AdGuardHome/dhcpd/check_other_dhcp.go

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package dhcpd
import (
"bytes"
"crypto/rand"
"encoding/binary"
"fmt"
"math"
"net"
"os"
"time"
"github.com/AdguardTeam/golibs/log"
"github.com/krolaw/dhcp4"
"golang.org/x/net/ipv4"
)
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// CheckIfOtherDHCPServersPresent sends a DHCP request to the specified network interface,
// and waits for a response for a period defined by defaultDiscoverTime
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, 4)
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n, err := rand.Read(xID)
if n != 4 && err == nil {
err = fmt.Errorf("Generated less than 4 bytes")
}
if err != nil {
return false, wrapErrPrint(err, "Couldn't generate 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.Hour * 24 * 90).Seconds()))
binary.BigEndian.PutUint32(leaseTimeRaw, leaseTime)
options := []dhcp4.Option{
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{Code: dhcp4.OptionParameterRequestList, Value: requestList},
{Code: dhcp4.OptionMaximumDHCPMessageSize, Value: maxUDPsizeRaw},
{Code: dhcp4.OptionClientIdentifier, Value: append([]byte{0x01}, iface.HardwareAddr...)},
{Code: dhcp4.OptionIPAddressLeaseTime, Value: leaseTimeRaw},
{Code: dhcp4.OptionHostName, Value: []byte(hostname)},
}
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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 := newBroadcastPacketConn(net.IPv4(0, 0, 0, 0), 68, ifaceName)
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 on :68")
}
// send to 255.255.255.255:67
cm := ipv4.ControlMessage{}
_, err = c.WriteTo(packet, &cm, dstAddr)
// spew.Dump(n, err)
if err != nil {
return false, wrapErrPrint(err, "Couldn't send a packet to %s", dst)
}
for {
// 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")
}
// spew.Dump(n, fromAddr, err, b)
log.Tracef("Received packet (%v bytes)", n)
if n < 240 {
// packet too small for dhcp
continue
}
response := dhcp4.Packet(b[:n])
if response.OpCode() != dhcp4.BootReply ||
response.HType() != 1 /*Ethernet*/ ||
response.HLen() > 16 ||
!bytes.Equal(response.CHAddr(), iface.HardwareAddr) ||
!bytes.Equal(response.XId(), xID) {
continue
}
parsedOptions := response.ParseOptions()
if t := parsedOptions[dhcp4.OptionDHCPMessageType]; len(t) != 1 {
continue //packet without DHCP message type
}
log.Tracef("The packet is from an active DHCP server")
// that's a DHCP server there
return true, nil
}
}