dns

DNS client and server implementations using the Go project's dnsmessage package
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recursor.go (1948B)

      1 package main
      2 
      3 import (
      4 	"fmt"
      5 	"golang.org/x/net/dns/dnsmessage"
      6 	"os"
      7 
      8 	"olowe.co/dns"
      9 )
     10 
     11 // okQType returns true if t is a query type that we can resolve by
     12 // recursively querying nameservers.
     13 func okQType(t dnsmessage.Type) bool {
     14 	switch t {
     15 	case dnsmessage.TypeA, dnsmessage.TypeNS, dnsmessage.TypeCNAME, dnsmessage.TypeSOA, dnsmessage.TypePTR, dnsmessage.TypeMX, dnsmessage.TypeTXT, dnsmessage.TypeAAAA, dnsmessage.TypeSRV, dnsmessage.TypeOPT:
     16 		return true
     17 	}
     18 	return false
     19 }
     20 
     21 // rejectHandler is a safeguard to prevent queries we don't want (or support)
     22 // to be recursively resolved. It returns true if the message was rejected.
     23 func rejectHandler(w dns.ResponseWriter, qmsg *dnsmessage.Message) bool {
     24 	if !qmsg.Header.RecursionDesired {
     25 		dns.Refuse(w, qmsg)
     26 		return true
     27 	} else if qmsg.Header.OpCode != dns.OpCodeQUERY {
     28 		dns.Refuse(w, qmsg)
     29 		return true
     30 	} else if len(qmsg.Questions) != 1 {
     31 		dns.FormatError(w, qmsg)
     32 		return true
     33 	}
     34 	q := qmsg.Questions[0]
     35 	if !okQType(q.Type) {
     36 		dns.NotImplemented(w, qmsg)
     37 		return true
     38 	} else if q.Class != dnsmessage.ClassINET {
     39 		dns.NotImplemented(w, qmsg)
     40 		return true
     41 	}
     42 	return false
     43 }
     44 
     45 func handler(w dns.ResponseWriter, qmsg *dnsmessage.Message) {
     46 	if rejected := rejectHandler(w, qmsg); rejected {
     47 		return
     48 	}
     49 
     50 	var rmsg dnsmessage.Message
     51 	rmsg.Header.ID = qmsg.Header.ID
     52 	rmsg.Header.Response = true
     53 	rmsg.Header.RecursionAvailable = true
     54 	rmsg.Questions = qmsg.Questions
     55 	rmsg.RecursionDesired = true
     56 
     57 	q := qmsg.Questions[0]
     58 	resolved, err := resolveFromRoot(q)
     59 	if err != nil {
     60 		fmt.Fprintln(os.Stderr, err)
     61 		rmsg.Header.RCode = dnsmessage.RCodeServerFailure
     62 		w.WriteMsg(rmsg)
     63 		return
     64 	}
     65 	rmsg.Header.RCode = resolved.Header.RCode
     66 	rmsg.Answers = resolved.Answers
     67 	if len(rmsg.Answers) == 0 {
     68 		rmsg.Authorities = resolved.Authorities
     69 		w.WriteMsg(rmsg)
     70 		return
     71 	}
     72 	w.WriteMsg(rmsg)
     73 }
     74 
     75 func main() {
     76 	fmt.Fprintln(os.Stderr, dns.ListenAndServe("udp", "", handler))
     77 }