godns/handler.go

package main

import (
	"crypto/md5"
	"fmt"
	"github.com/miekg/dns"
	log "github.com/sirupsen/logrus"
	"meow.tf/joker/godns/cache"
	"meow.tf/joker/godns/hosts"
	"meow.tf/joker/godns/resolver"
	"meow.tf/joker/godns/utils"
	"net"
	"time"
)

type Handler struct {
	resolver        *resolver.Resolver
	middleware []MiddlewareFunc
	cache, negCache cache.Cache
	hosts           hosts.Hosts
}

type MiddlewareFunc func(w dns.ResponseWriter, r *dns.Msg, m *dns.Msg) *dns.Msg

func TsigMiddleware(secretKey string) MiddlewareFunc {
	return func(w dns.ResponseWriter, r *dns.Msg, m *dns.Msg) *dns.Msg {
		if r.IsTsig() != nil {
			if w.TsigStatus() == nil {
				m.SetTsig(r.Extra[len(r.Extra)-1].(*dns.TSIG).Hdr.Name, dns.HmacSHA256, 300, time.Now().Unix())
			}
		}

		return m
	}
}

func NewHandler(r *resolver.Resolver, resolverCache, negCache cache.Cache, h hosts.Hosts) *Handler {
	return &Handler{r, make([]MiddlewareFunc, 0), resolverCache, negCache, h}
}

// do handles a dns request.
// network will decide which network type it is (udp, tcp, https, etc)
func (h *Handler) do(network string, w dns.ResponseWriter, req *dns.Msg) {
	switch req.Opcode {
	case dns.OpcodeQuery:
		h.query(network, w, req)
	case dns.OpcodeUpdate:
		if req.IsTsig() == nil {
			m := new(dns.Msg)
			m.SetRcode(req, dns.RcodeBadSig)
			w.WriteMsg(m)
			return
		}

		packed, err := req.Pack()

		if err != nil {
			m := new(dns.Msg)
			m.SetRcode(req, dns.RcodeBadSig)
			w.WriteMsg(m)
			return
		}

		if err := dns.TsigVerify(packed, "", "", false); err != nil {
			m := new(dns.Msg)
			m.SetRcode(req, dns.RcodeBadSig)
			w.WriteMsg(m)
			return
		}

		h.update(network, w, req)
	}
}

// writeMsg writes a *dns.Msg after passing it through middleware.
func (h *Handler) writeMsg(w dns.ResponseWriter, r *dns.Msg, m *dns.Msg) {
	for _, f := range h.middleware {
		m = f(w, r, m)
	}

	w.WriteMsg(m)
}

// query handles dns queries.
func (h *Handler) query(network string, w dns.ResponseWriter, req *dns.Msg) {
	q := req.Question[0]
	question := resolver.Question{Name: utils.UnFqdn(q.Name), Type: q.Qtype, Class: dns.ClassToString[q.Qclass]}

	var remote net.IP

	switch t := w.RemoteAddr().(type) {
	case *net.TCPAddr:
		remote = t.IP
	case *net.UDPAddr:
		remote = t.IP
	default:
		return
	}

	log.WithFields(log.Fields{
		"remote": remote,
		"question": question.String(),
	}).Debug("Lookup question")

	key := KeyGen(question)

	// Check cache first
	mesg, err := h.cache.Get(key)

	if err != nil {
		if mesg, err = h.negCache.Get(key); err != nil {
			log.WithField("question", question.String()).Debug("no negative cache hit")
		} else {
			log.WithField("question", question.String()).Debug("negative cache hit")
			m := new(dns.Msg)
			m.SetRcode(req, dns.RcodeServerFailure)
			w.WriteMsg(m)
			return
		}
	} else {
		log.WithField("question", question.String()).Debug("hit cache")

		// we need this copy against concurrent modification of Id
		msg := *mesg
		msg.Id = req.Id
		h.writeMsg(w, req, &msg)

		return
	}

	// Query hosts
	if h.hosts != nil {
		if host, err := h.hosts.Get(question.Type, question.Name); err == nil {
			m := new(dns.Msg)
			m.SetReply(req)

			hdr := dns.RR_Header{
				Name: q.Name,
				Rrtype: question.Type,
				Class: dns.ClassINET,
				Ttl: uint32(host.TTL / time.Second),
			}

			switch question.Type {
			case dns.TypeA:
				for _, val := range host.Values {
					m.Answer = append(m.Answer, &dns.A{Hdr: hdr, A: net.ParseIP(val).To4()})
				}
			case dns.TypeAAAA:
				for _, val := range host.Values {
					m.Answer = append(m.Answer, &dns.AAAA{Hdr: hdr, AAAA: net.ParseIP(val).To16()})
				}
			case dns.TypeCNAME:
				for _, val := range host.Values {
					m.Answer = append(m.Answer, &dns.CNAME{Hdr: hdr, Target: val})
				}
			case dns.TypeTXT:
				m.Answer = append(m.Answer, &dns.TXT{Hdr: hdr, Txt: host.Values})
			}

			// Insert into cache before using any middleware
			err = h.cache.Set(key, m)

			if err != nil {
				log.WithError(err).Error("Unable to insert hosts entry into cache")
			}

			// Write the message
			h.writeMsg(w, req, m)

			log.WithField("question", question.Name).Debug("Found entry in hosts file")
			return
		} else {
			log.WithField("question", question.Name).Debug("No entry found in hosts file")
		}
	}

	mesg, err = h.resolver.Lookup(network, req)

	if err != nil {
		log.WithError(err).Warn("Query resolution error")

		m := new(dns.Msg)
		m.SetRcode(req, dns.RcodeServerFailure)
		w.WriteMsg(m)

		// cache the failure, too!
		if err = h.negCache.Set(key, nil); err != nil {
			log.WithError(err).Warn("Negative cache save failed")
		}
		return
	}

	// Cache if mesg.Answer length > 0
	// This is done BEFORE sending it so we don't modify the request first
	if len(mesg.Answer) > 0 {
		err = h.cache.Set(key, mesg)

		fields := log.Fields{
			"question": question.String(),
		}

		log.WithFields(fields).Debug("Insert record into cache")

		if err != nil {
			fields["error"] = err

			log.WithFields(fields).Warn("Unable to add to cache")
		}
	}

	// Write message
	h.writeMsg(w, req, mesg)
}

func (h *Handler) update(network string, w dns.ResponseWriter, req *dns.Msg) {
	for _, question := range req.Question {
		if _, ok := dns.IsDomainName(question.Name); !ok {
			continue
		}

		for _, rr := range req.Ns {
			hdr := rr.Header()

			if hdr.Class == dns.ClassANY && hdr.Rdlength == 0 {
				// Delete record
				continue
			}


		}
	}
}

func (h *Handler) Bind(net string) func(w dns.ResponseWriter, req *dns.Msg) {
	return func(w dns.ResponseWriter, req *dns.Msg) {
		h.do(net, w, req)
	}
}

func KeyGen(q resolver.Question) string {
	h := md5.New()
	h.Write([]byte(q.String()))
	x := h.Sum(nil)
	key := fmt.Sprintf("%x", x)
	return key
}