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- /control/dhcp/find_active_dhcp: fix DHCP server detection

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Before this patch we couldn't receive incoming DHCP packets.
Now we bind() to 0.0.0.0 and set the required network interface
 using SO_BINDTODEVICE option.

As an improvement, we now wait until a reply to our request is
 received and skip all unknown packets.
This commit is contained in:
Simon Zolin
2019-04-16 17:36:34 +03:00
parent 69c5f175e8
commit c5ed6da5bd
3 changed files with 106 additions and 40 deletions
Download Patch File Download Diff File Expand all files Collapse all files

87
dhcpd/check_other_dhcp.go
View File

@@ -1,6 +1,7 @@
package dhcpd
import (
"bytes"
"crypto/rand"
"encoding/binary"
"fmt"
@@ -11,6 +12,7 @@ import (
"github.com/AdguardTeam/golibs/log"
"github.com/krolaw/dhcp4"
"golang.org/x/net/ipv4"
)
// CheckIfOtherDHCPServersPresent sends a DHCP request to the specified network interface,
@@ -32,13 +34,13 @@ func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) {
dst := "255.255.255.255:67"
// form a DHCP request packet, try to emulate existing client as much as possible
xID := make([]byte, 8)
xID := make([]byte, 4)
n, err := rand.Read(xID)
if n != 8 && err == nil {
err = fmt.Errorf("Generated less than 8 bytes")
if n != 4 && err == nil {
err = fmt.Errorf("Generated less than 4 bytes")
}
if err != nil {
return false, wrapErrPrint(err, "Couldn't generate 8 random bytes")
return false, wrapErrPrint(err, "Couldn't generate random bytes")
}
hostname, err := os.Hostname()
if err != nil {
@@ -89,58 +91,63 @@ func CheckIfOtherDHCPServersPresent(ifaceName string) (bool, error) {
// bind to 0.0.0.0:68
log.Tracef("Listening to udp4 %+v", udpAddr)
c, err := net.ListenPacket("udp4", src)
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 to %s", src)
return false, wrapErrPrint(err, "Couldn't listen on :68")
}
// send to 255.255.255.255:67
_, err = c.WriteTo(packet, dstAddr)
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)
}
// 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)
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)
if n < 240 {
// packet too small for dhcp
return false, wrapErrPrint(err, "got packet that's too small for DHCP")
}
log.Tracef("Received packet (%v bytes)", n)
response := dhcp4.Packet(b[:n])
if response.HLen() > 16 {
// invalid size
return false, wrapErrPrint(err, "got malformed packet with HLen() > 16")
}
if n < 240 {
// packet too small for dhcp
continue
}
parsedOptions := response.ParseOptions()
_, ok := parsedOptions[dhcp4.OptionDHCPMessageType]
if !ok {
return false, wrapErrPrint(err, "got malformed packet without DHCP message type")
}
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
}
// that's a DHCP server there
return true, nil
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
}
}