synthetic/synthetic.go

package synthetic

import (
	"context"
	"encoding/hex"
	"github.com/coredns/coredns/plugin"
	"github.com/coredns/coredns/plugin/pkg/dnstest"
	clog "github.com/coredns/coredns/plugin/pkg/log"
	"github.com/coredns/coredns/plugin/test"
	"github.com/coredns/coredns/request"
	"github.com/miekg/dns"
	"net"
	"strings"
)

// Define log to be a logger with the plugin name in it. This way we can just use log.Info and
// friends to log.
var log = clog.NewWithPlugin("synthetic")

// Name implements the Handler interface.
func (s synthetic) Name() string { return "synthetic" }

// synthetic is an synthetic plugin to show how to write a plugin.
type synthetic struct {
	Next   plugin.Handler
	Config syntheticConfig
}

// ServeDNS implements the plugin.Handler interface.
//
//	This method gets called when synthetic is used in a Server
func (s synthetic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {

	// Create a new state for this request. This is used to store state and allows us to pass this
	state := request.Request{W: w, Req: r}
	log.Debug("Received request for ", state.QName(), " of type ", state.QType())

	// Continue to the next plugin in the chain, and record the response.
	rec := dnstest.NewRecorder(&test.ResponseWriter{})
	rc, err := plugin.NextOrFailure(s.Name(), s.Next, ctx, rec, r)

	// If the next plugin in the chain's recorded response is success, we go with that.
	if rc == dns.RcodeNameError {
		log.Debug("Next plugin in chain responded with acceptable response ", rc, " for ", state.QName())
		w.WriteMsg(rec.Msg)
		return rc, err
	}

	switch state.QType() {

	//
	// Handle AAAA and A requests (Forward Lookup)
	// 		Note: on the forward lookup, we're guaranteed to have a match on the domain name
	//
	case dns.TypeA, dns.TypeAAAA:

		// only continue if the first label starts with "ip-" (fall back to recorded response)
		if !strings.HasPrefix(state.Name(), "ip-") {
			w.WriteMsg(rec.Msg)
			return rc, err
		}

		// parse first dns label (now we have: "ip-<ip>")
		firstLabel := strings.Split(state.Name(), ".")[0]

		// trim "ip-" prefix (now we have: "<ip>")
		ipStr := strings.TrimPrefix(firstLabel, "ip-")

		// attempt to parse as IPv4 or IPv6
		ip := net.ParseIP(strings.ReplaceAll(ipStr, "-", "."))
		if ip == nil {
			ip = net.ParseIP(strings.ReplaceAll(ipStr, "-", ":"))
		}
		log.Debug(state.Name(), " parsed to ", ip)

		// don't continue if we couldn't parse the IP (fall back to recorded respones)
		if ip == nil {
			log.Debug("Failed to parse IP from: ", state.QName())
			w.WriteMsg(rec.Msg)
			return rc, err
		}

		// check if ip is within the synthetic network
		var found bool
		for _, n := range s.Config.net {
			if n.Contains(ip) {
				found = true
				break
			}
		}
		// don't continue if the IP is not in the synthetic network (fall back to recorded response)
		if !found {
			log.Debug("IP ", ip, " not found in synthetic network")
			w.WriteMsg(rec.Msg)
			return rc, err
		}

		// handle AAAA requests with IPv6 address
		if ip.To4() == nil && state.QType() == dns.TypeAAAA {
			log.Debug("Responding to AAAA request for ", state.QName())
			m := new(dns.Msg)
			m.SetReply(r)
			hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass()}
			m.Answer = append(m.Answer, &dns.AAAA{Hdr: hdr, AAAA: ip.To16()})
			w.WriteMsg(m)
			return dns.RcodeSuccess, nil
		}

		// handle A requests with IPv4 address
		if ip.To4() != nil && state.QType() == dns.TypeA {
			log.Debug("Responding to A request for ", state.QName())
			m := new(dns.Msg)
			m.SetReply(r)
			hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass()}
			m.Answer = append(m.Answer, &dns.A{Hdr: hdr, A: ip.To4()})
			w.WriteMsg(m)
			return dns.RcodeSuccess, nil
		}

		// handle A requests with IPv6 address (respond with empty answer and NOERROR)
		if ip.To4() == nil && state.QType() == dns.TypeA {
			log.Debug("Responding to A request for ", state.QName(), " with empty answer")
			m := new(dns.Msg)
			m.SetReply(r)
			w.WriteMsg(m)
			return dns.RcodeSuccess, nil
		}

		// handle AAAA requests with IPv4 address (respond with empty answer and NOERROR)
		if ip.To4() != nil && state.QType() == dns.TypeAAAA {
			log.Debug("Responding to AAAA request for ", state.QName(), " with empty answer")
			m := new(dns.Msg)
			m.SetReply(r)
			w.WriteMsg(m)
			return dns.RcodeSuccess, nil
		}

		// if we got here, we couldn't handle the request (fall back to recorded response)
		log.Debug("Failed to handle request for ", state.QName())
		w.WriteMsg(rec.Msg)
		return rc, err

	//
	// Handle PTR requests (Reverse Lookup)
	// 		Note: on the forward lookup, we're guaranteed to have a match on the network
	//
	case dns.TypePTR:
		ip := inArpaToIp(state.QName())
		log.Debug("Received PTR request. Parsed ", state.QName(), " to ", ip)

		// don't continue if we couldn't parse the IP (fall back to recorded response)
		if ip == nil {
			w.WriteMsg(rec.Msg)
			return rc, err
		}

		// successful reverse lookup
		forward := ipToDomainName(ip, s.Config.forward)
		log.Debug("Responding to PTR request for ", state.QName(), " with ", forward)
		m := new(dns.Msg)
		m.SetReply(r)
		hdr := dns.RR_Header{Name: state.QName(), Rrtype: state.QType(), Class: state.QClass()}
		m.Answer = append(m.Answer, &dns.PTR{Hdr: hdr, Ptr: forward})
		w.WriteMsg(m)
		return dns.RcodeSuccess, nil

	}

	// if we got here, we couldn't handle the request (fall back to recorded response)
	log.Debug("synthetic plugin not needed for ", state.QName(), " of type ", state.QType())
	w.WriteMsg(rec.Msg)
	return rc, err
}

func inArpaToIp(name string) net.IP {
	ipv4Suffix := ".in-addr.arpa."
	ipv6Suffix := ".ip6.arpa."

	if idx := strings.Index(name, ipv4Suffix); idx > 6 {
		name = name[:idx]
		parts := strings.Split(name, ".")
		if len(parts) != 4 {
			return nil
		}

		name = parts[3] + "." + parts[2] + "." + parts[1] + "." + parts[0]
		return net.ParseIP(name)
	}

	if len(name) == 73 && name[63:] == ipv6Suffix {
		// we can rely on the fact that v6 reverse hostnames have a fixed length

		// read the characters from the hostname into a buffer in reverse
		v6chars := make([]byte, 32)
		for i, j := 62, 0; i >= 0; i -= 2 {
			v6chars[j] = name[i]
			j++
		}

		// decode the characters in the buffer into 16 bytes and return it
		v6bytes := make([]byte, 16)
		if _, err := hex.Decode(v6bytes, v6chars); err != nil {
			return nil
		}

		return v6bytes
	}

	return nil
}

func ipToDomainName(ip net.IP, zone string) string {
	if ip == nil {
		return ""
	}

	if !strings.HasPrefix(zone, ".") {
		zone = "." + zone
	}

	if !strings.HasSuffix(zone, ".") {
		zone = zone + "."
	}

	sep := ":"
	if ip.To4() != nil {
		sep = "."
	}

	response := strings.Join(strings.Split(ip.String(), sep), "-")
	return "ip-" + response + zone
}