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dns-server.go
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/
dns-server.go
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package main
import (
"fmt"
"io"
"log"
"net"
"os"
"regexp"
)
// <domain-name> is a domain name represented as a series of labels, and
// terminated by a label with zero length.
func main() {
multiWriter := io.MultiWriter(os.Stdout)
log.SetPrefix("[info]: ")
log.SetFlags(log.Ldate | log.Ltime | log.Lmsgprefix)
log.New(multiWriter, log.Prefix(), log.Flags())
address, _ := net.ResolveUDPAddr("udp", "localhost:53")
in_conn, err := net.ListenUDP("udp", address)
if err != nil {
panic(fmt.Errorf("error %s starting udp in_conn on %s", err.Error(), address))
}
out_conn, err := net.ListenUDP("udp", nil)
if err != nil {
panic(fmt.Errorf("error %s starting udp out_conn on %s", err.Error(), address))
}
dns_resolver := net.UDPAddr{
IP: net.IPv4(0x08, 0x08, 0x08, 0x08),
Port: 53,
Zone: "udp",
}
for {
var query [512]byte
_, _, _, addr, _ := in_conn.ReadMsgUDP(query[:], nil)
out_conn.WriteMsgUDP(query[:], nil, &dns_resolver)
var response [512]byte
out_conn.ReadMsgUDP(response[:], nil)
domain := retrieve_domain(response[12:])
log.Println(domain)
toSkip, _ := regexp.MatchString(".*youtube.*", domain)
if toSkip {
continue
}
in_conn.WriteMsgUDP(response[:], nil, addr)
}
}
// The total length of a domain name (i.e., label octets and
// label length octets) is restricted to 255 octets or less.
func retrieve_domain(msg []byte) string {
var domain []byte
for {
size := msg[0]
if size == 0 {
break
}
size++
tmp := make([]byte, size)
copy(tmp, msg[1:])
tmp[len(tmp)-1] = 0x2e
domain = append(domain, tmp...)
msg = msg[size:]
}
return string(domain)
}