/
dhcp.go
64 lines (53 loc) · 1.62 KB
/
dhcp.go
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package dhcp
import (
"math/rand"
"net"
"runtime"
"time"
"github.com/krolaw/dhcp4"
"github.com/libp2p/go-reuseport"
"github.com/pkg/errors"
)
var xid = make([]byte, 4)
var broadcastAddr, _ = net.ResolveUDPAddr("udp", "255.255.255.255:67")
func GetDNSServer() ([]string, error) {
iface, err := PickInternetInterface()
if err != nil {
return nil, errors.Wrap(err, "pick interface")
}
rand.Read(xid)
pack := dhcp4.RequestPacket(dhcp4.Discover, iface.HardwareAddr, net.IPv4(0, 0, 0, 0), xid, true, []dhcp4.Option{
{Code: dhcp4.OptionRequestedIPAddress, Value: []byte(iface.IP.To4())},
{Code: dhcp4.End},
})
var conn net.PacketConn
if runtime.GOOS == "windows" {
if conn, err = reuseport.ListenPacket("udp4", iface.IP.String()+":68"); err != nil {
return nil, errors.Wrap(err, "listen dhcp")
}
} else {
if conn, err = reuseport.ListenPacket("udp4", "0.0.0.0:68"); err != nil {
return nil, errors.Wrap(err, "listen dhcp")
}
}
defer conn.Close()
conn.SetDeadline(time.Now().Add(3 * time.Second))
if _, err := conn.WriteTo([]byte(pack), broadcastAddr); err != nil {
return nil, errors.Wrap(err, "write broadcast")
}
buf := make([]byte, 1500 /*MTU*/)
n, _, err := conn.ReadFrom(buf)
if err != nil {
return nil, errors.Wrap(err, "read dhcp offer")
}
pack = dhcp4.Packet(buf[:n])
dnsBytes := pack.ParseOptions()[dhcp4.OptionDomainNameServer]
if len(dnsBytes) < 4 || len(dnsBytes)%4 != 0 {
return nil, errors.New("invalid DNS setting in upstream network device")
}
ips := []string{}
for i := 0; i < len(dnsBytes); i += 4 {
ips = append(ips, net.IP(dnsBytes[i:i+4]).String())
}
return ips, nil
}