//go:build ignore
// +build ignore
// minidns is a trivial DNS server used for testing.
//
// It takes an "answers" file which contains lines with the following format:
//
// <domain> <type> <value>
//
// For example:
//
// blah A 1.2.3.4
// blah MX mx1
//
// Supported types: A, AAAA, MX, TXT.
//
// It's only meant to be used for testing, so it's not robust, performant, or
// standards compliant.
package main
import (
"bufio"
"encoding/binary"
"flag"
"fmt"
"net"
"os"
"regexp"
"strings"
"sync"
"blitiri.com.ar/go/log"
"golang.org/x/net/dns/dnsmessage"
)
var (
addr = flag.String("addr", ":53", "address to listen to (UDP)")
zonesPath = flag.String("zones", "", "file with the zones")
)
func main() {
flag.Parse()
srv := &miniDNS{
answers: map[string][]dnsmessage.Resource{},
}
if *zonesPath == "" {
log.Fatalf("-zones must be given")
}
var zonesFile *os.File
if *zonesPath == "-" {
zonesFile = os.Stdin
} else {
var err error
zonesFile, err = os.Open(*zonesPath)
if err != nil {
log.Fatalf("error opening %v: %v", *zonesPath, err)
}
}
srv.loadZones(zonesFile)
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
srv.listenAndServeUDP(*addr)
}()
go func() {
defer wg.Done()
srv.listenAndServeTCP(*addr)
}()
wg.Wait()
}
type miniDNS struct {
// Domain -> Answers.
// We always respond the same regardless of the query.
// Not great, but does the trick.
answers map[string][]dnsmessage.Resource
}
func (m *miniDNS) listenAndServeUDP(addr string) {
conn, err := net.ListenPacket("udp", addr)
if err != nil {
log.Fatalf("error listening UDP %q: %v", addr, err)
}
log.Infof("listening on %v", conn.LocalAddr())
buf := make([]byte, 64*1024)
for {
n, addr, err := conn.ReadFrom(buf)
if err != nil {
log.Infof("error reading from udp: %v", err)
continue
}
msg := &dnsmessage.Message{}
err = msg.Unpack(buf[:n])
if err != nil {
log.Infof("%v error unpacking message: %v", addr, err)
}
if lq := len(msg.Questions); lq != 1 {
log.Infof("%v/%-5d dropping packet with %d questions",
addr, msg.ID, lq)
continue
}
q := msg.Questions[0]
log.Infof("[UDP] %v/%-5d Q: %s %s %s",
addr, msg.ID, q.Name, q.Type, q.Class)
reply := m.handle(msg)
rbuf, err := reply.Pack()
if err != nil {
log.Fatalf("error packing reply: %v", err)
}
if len(rbuf) > 512 {
log.Infof("truncating UDP response of %v bytes", len(rbuf))
reply.Truncated = true
reply.Answers = nil
rbuf, err = reply.Pack()
}
conn.WriteTo(rbuf, addr)
}
}
func (m *miniDNS) listenAndServeTCP(addr string) {
ls, err := net.Listen("tcp", addr)
if err != nil {
log.Fatalf("error listening TCP %q: %v", addr, err)
}
log.Infof("listening on %v", addr)
for {
conn, err := ls.Accept()
if err != nil {
log.Infof("error accepting: %v", err)
continue
}
msg, err := readTCPMessage(conn)
if err != nil {
log.Infof("%v error reading message: %v", addr, err)
conn.Close()
continue
}
if lq := len(msg.Questions); lq != 1 {
log.Infof("%v/%-5d dropping packet with %d questions",
addr, msg.ID, lq)
conn.Close()
continue
}
q := msg.Questions[0]
log.Infof("[TCP] %v/%-5d Q: %s %s %s",
addr, msg.ID, q.Name, q.Type, q.Class)
reply := m.handle(msg)
err = writeTCPMessage(conn, reply)
if err != nil {
log.Infof("error writing reply: %v", err)
}
conn.Close()
}
}
func readTCPMessage(conn net.Conn) (*dnsmessage.Message, error) {
// Read the 2-byte length first, then the message.
lenHdr := struct{ Len uint16 }{}
err := binary.Read(conn, binary.BigEndian, &lenHdr)
if err != nil {
return nil, err
}
data := make([]byte, lenHdr.Len)
err = binary.Read(conn, binary.BigEndian, &data)
if err != nil {
return nil, err
}
msg := &dnsmessage.Message{}
err = msg.Unpack(data)
if err != nil {
return nil, fmt.Errorf("%v error unpacking message: %v", addr, err)
}
return msg, nil
}
func writeTCPMessage(conn net.Conn, msg *dnsmessage.Message) error {
rbuf, err := msg.Pack()
if err != nil {
return fmt.Errorf("error packing reply: %v", err)
}
lenHdr := struct{ Len uint16 }{Len: uint16(len(rbuf))}
err = binary.Write(conn, binary.BigEndian, lenHdr)
if err != nil {
return err
}
_, err = conn.Write(rbuf)
return err
}
func (m *miniDNS) handle(msg *dnsmessage.Message) *dnsmessage.Message {
reply := &dnsmessage.Message{
Header: dnsmessage.Header{
ID: msg.ID,
Response: true,
RCode: dnsmessage.RCodeSuccess,
},
Questions: msg.Questions,
}
q := msg.Questions[0]
if answers, ok := m.answers[q.Name.String()]; ok {
for _, ans := range answers {
if q.Type == ans.Header.Type {
log.Infof("-> %s %v", q.Type, ans.Body)
reply.Answers = append(reply.Answers, ans)
}
}
} else {
log.Infof("-> NXERROR")
reply.Header.RCode = dnsmessage.RCodeNameError
}
return reply
}
func (m *miniDNS) loadZones(f *os.File) {
scanner := bufio.NewScanner(f)
lineno := 0
for scanner.Scan() {
lineno++
line := strings.TrimSpace(scanner.Text())
if strings.HasPrefix(line, "#") || line == "" {
continue
}
vs := regexp.MustCompile("\\s+").Split(line, 3)
if len(vs) != 3 {
log.Fatalf("line %d: invalid format", lineno)
}
domain, t, value := vs[0], vs[1], vs[2]
if !strings.HasSuffix(domain, ".") {
domain += "."
}
var body dnsmessage.ResourceBody
var qType dnsmessage.Type
switch strings.ToLower(t) {
case "a":
qType = dnsmessage.TypeA
ip := net.ParseIP(value).To4()
if ip == nil {
log.Fatalf("line %d: invalid IP %q", lineno, value)
}
a := &dnsmessage.AResource{}
copy(a.A[:], ip[:4])
body = a
case "aaaa":
qType = dnsmessage.TypeAAAA
ip := net.ParseIP(value).To16()
if ip == nil {
log.Fatalf("line %d: invalid IP %q", lineno, value)
}
aaaa := &dnsmessage.AAAAResource{}
copy(aaaa.AAAA[:], ip[:16])
body = aaaa
case "mx":
qType = dnsmessage.TypeMX
if !strings.HasPrefix(value, ".") {
value += "."
}
body = &dnsmessage.MXResource{
Pref: 10,
MX: dnsmessage.MustNewName(value),
}
case "ns":
qType = dnsmessage.TypeNS
body = &dnsmessage.NSResource{
NS: dnsmessage.MustNewName(value),
}
case "txt":
qType = dnsmessage.TypeTXT
body = &dnsmessage.TXTResource{
TXT: []string{value},
}
default:
log.Fatalf("line %d: unknown type %q", lineno, t)
}
answer := dnsmessage.Resource{
Header: dnsmessage.ResourceHeader{
Name: dnsmessage.MustNewName(domain),
Type: qType,
Class: dnsmessage.ClassINET,
},
Body: body,
}
m.answers[domain] = append(m.answers[domain], answer)
}
if err := scanner.Err(); err != nil {
log.Fatalf("error reading zones: %v", err)
}
}