fake_resolver.go

package zgrab2

import (
	"context"
	"errors"
	"fmt"
	"golang.org/x/net/dns/dnsmessage"
	"net"
	"time"
)

// Fake DNS Resolver, to force a DNS lookup to return a pinned address
// Inspired by the golang net/dnsclient_unix_test.go code
//
// For a given IP, create a new Resolver that wraps a fake
// DNS server. This resolver will always return an IP that
// is represented by "ipstr", for DNS queries of the same
// IP type.  Otherwise, it will return a DNS lookup error.
func NewFakeResolver(ipstr string) (*net.Resolver, error) {
	ip := net.ParseIP(ipstr)
	if len(ip) < 4 {
		return nil, fmt.Errorf("Fake resolver can't use non-IP '%s'", ipstr)
	}
	fDNS := FakeDNSServer{
		IP: ip,
	}
	return &net.Resolver{
		PreferGo: true, // Needed to force the use of the Go internal resolver
		Dial:     fDNS.DialContext,
	}, nil
}

type FakeDNSServer struct {
	// Any domain name will resolve to this IP.  It can be either ipv4 or ipv6
	IP net.IP
}

// For a given DNS query, return the hard-coded IP that is part of
// FakeDNSServer.
//
// It will work with either ipv4 or ipv6 addresses; if a TypeA question
// is received, we will only return the IP if what we have to return is
// ipv4.  The same for TypeAAAA and ipv6.
func (f *FakeDNSServer) fakeDNS(s string, dmsg dnsmessage.Message) (r dnsmessage.Message, err error) {

	r = dnsmessage.Message{
		Header: dnsmessage.Header{
			ID:       dmsg.ID,
			Response: true,
		},
		Questions: dmsg.Questions,
	}
	ipv6 := f.IP.To16()
	ipv4 := f.IP.To4()
	switch t := dmsg.Questions[0].Type; {
	case t == dnsmessage.TypeA && ipv4 != nil:
		var ip [4]byte
		copy(ip[:], []byte(ipv4))
		r.Answers = []dnsmessage.Resource{
			{
				Header: dnsmessage.ResourceHeader{
					Name:   dmsg.Questions[0].Name,
					Type:   dnsmessage.TypeA,
					Class:  dnsmessage.ClassINET,
					Length: 4,
				},
				Body: &dnsmessage.AResource{
					A: ip,
				},
			},
		}
	case t == dnsmessage.TypeAAAA && ipv4 == nil:
		var ip [16]byte
		copy(ip[:], []byte(ipv6))
		r.Answers = []dnsmessage.Resource{
			{
				Header: dnsmessage.ResourceHeader{
					Name:   dmsg.Questions[0].Name,
					Type:   dnsmessage.TypeAAAA,
					Class:  dnsmessage.ClassINET,
					Length: 16,
				},
				Body: &dnsmessage.AAAAResource{
					AAAA: ip,
				},
			},
		}
	default:
		r.Header.RCode = dnsmessage.RCodeNameError
	}

	return r, nil
}

// This merely wraps a custom net.Conn, that is only good for DNS
// messages
func (f *FakeDNSServer) DialContext(ctx context.Context, network,
	address string) (net.Conn, error) {

	conn := &fakeDNSPacketConn{
		fakeDNSConn: fakeDNSConn{
			server:  f,
			network: network,
			address: address,
		},
	}
	return conn, nil
}

type fakeDNSConn struct {
	net.Conn
	server  *FakeDNSServer
	network string
	address string
	dmsg    dnsmessage.Message
}

func (fc *fakeDNSConn) Read(b []byte) (int, error) {
	resp, err := fc.server.fakeDNS(fc.address, fc.dmsg)
	if err != nil {
		return 0, err
	}

	bb := make([]byte, 2, 514)
	bb, err = resp.AppendPack(bb)
	if err != nil {
		return 0, fmt.Errorf("cannot marshal DNS message: %v", err)
	}

	bb = bb[2:]
	if len(b) < len(bb) {
		return 0, errors.New("read would fragment DNS message")
	}

	copy(b, bb)
	return len(bb), nil
}

func (fc *fakeDNSConn) Write(b []byte) (int, error) {
	if fc.dmsg.Unpack(b) != nil {
		return 0, fmt.Errorf("cannot unmarshal DNS message fake %s (%d)", fc.network, len(b))
	}
	return len(b), nil
}

func (fc *fakeDNSConn) SetDeadline(deadline time.Time) error {
	return nil
}

func (fc *fakeDNSConn) Close() error {
	return nil
}

type fakeDNSPacketConn struct {
	net.PacketConn
	fakeDNSConn
}

func (f *fakeDNSPacketConn) SetDeadline(deadline time.Time) error {
	return nil
}

func (f *fakeDNSPacketConn) Close() error {
	return f.fakeDNSConn.Close()
}