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resolver.go
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resolver.go
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package dns
import (
"bufio"
"crypto/rand"
"fmt"
"math/big"
"net"
"strings"
"golang.org/x/net/dns/dnsmessage"
)
const ROOT_SERVERS = "198.41.0.4,170.247.170.2,192.33.4.12,199.7.91.13,192.203.230.10,192.5.5.241,192.112.36.4,198.97.190.53,192.36.148.17,192.58.128.30,193.0.14.129,199.7.83.42,202.12.27.33"
func HandlePacket(pc net.PacketConn, addr net.Addr, buffer []byte) {
if err := handlePacket(pc, addr, buffer); err != nil {
fmt.Printf("handlePacket error [%s]: %s\n", addr.String(), err)
}
}
func handlePacket(pc net.PacketConn, addr net.Addr, buffer []byte) error {
p := dnsmessage.Parser{}
header, err := p.Start(buffer)
if err != nil {
return err
}
question, err := p.Question()
if err != nil {
return err
}
res, err := dnsQuery(getRootServers(), question)
if err != nil {
return err
}
res.Header.ID = header.ID
resBuffer, err := res.Pack()
if err != nil {
return err
}
_, err = pc.WriteTo(resBuffer, addr)
if err != nil {
return err
}
return nil
}
func dnsQuery(servers []net.IP, question dnsmessage.Question) (*dnsmessage.Message, error) {
fmt.Printf("Question: %v\n", question)
for i := 0; i < 3; i++ {
dnsAnswer, header, err := outgoingDnsQuery(servers, question)
if err != nil {
return nil, err
}
parsedAnswers, err := dnsAnswer.AllAnswers()
if err != nil {
return nil, err
}
if header.Authoritative {
return &dnsmessage.Message{
Header: dnsmessage.Header{Response: true},
Answers: parsedAnswers,
}, nil
}
authorities, err := dnsAnswer.AllAuthorities()
if err != nil {
return nil, err
}
if len(authorities) == 0 {
return &dnsmessage.Message{
Header: dnsmessage.Header{
RCode: dnsmessage.RCodeNameError,
},
}, nil
}
nameservers := make([]string, len(authorities))
for k, auth := range authorities {
if auth.Header.Type == dnsmessage.TypeNS {
nameservers[k] = auth.Body.(*dnsmessage.NSResource).NS.String()
}
}
fmt.Printf("servers: %+v\n", nameservers)
additionals, err := dnsAnswer.AllAdditionals()
if err != nil {
return nil, err
}
newResolverServersFound := false
servers = []net.IP{}
for _, additional := range additionals {
if additional.Header.Type == dnsmessage.TypeA {
for _, ns := range nameservers {
if additional.Header.Name.String() == ns {
newResolverServersFound = true
servers = append(servers, additional.Body.(*dnsmessage.AResource).A[:])
}
}
}
}
if !newResolverServersFound {
for _, ns := range nameservers {
if !newResolverServersFound {
q := dnsmessage.Question{
Name: dnsmessage.MustNewName(ns),
Type: dnsmessage.TypeA,
Class: dnsmessage.ClassINET,
}
res, err := dnsQuery(getRootServers(), q)
if err != nil {
fmt.Printf("Warning: lookup of nameserver %s failed: %s\n", ns, err)
} else {
newResolverServersFound = true
for _, ans := range res.Answers {
if ans.Header.Type == dnsmessage.TypeA {
servers = append(servers, ans.Body.(*dnsmessage.AResource).A[:])
}
}
}
}
}
}
}
return &dnsmessage.Message{
Header: dnsmessage.Header{
RCode: dnsmessage.RCodeServerFailure,
},
}, nil
}
func outgoingDnsQuery(servers []net.IP, question dnsmessage.Question) (*dnsmessage.Parser, *dnsmessage.Header, error) {
fmt.Printf("New outgoing DNS query for %s, servers: %+v\n", question.Name.String(), servers)
max := ^uint16(0)
rn, err := rand.Int(rand.Reader, big.NewInt(int64(max)))
if err != nil {
return nil, nil, err
}
msg := dnsmessage.Message{
Header: dnsmessage.Header{
ID: uint16(rn.Int64()),
Response: false,
OpCode: dnsmessage.OpCode(0),
},
Questions: []dnsmessage.Question{question},
}
buff, err := msg.Pack()
if err != nil {
return nil, nil, err
}
var conn net.Conn
for _, server := range servers {
conn, err = net.Dial("udp", server.String()+":53")
if err == nil {
break
}
}
if conn == nil {
return nil, nil, fmt.Errorf("Failed to stablish connection with the servers: %s", err)
}
_, err = conn.Write(buff)
if err != nil {
return nil, nil, err
}
answer := make([]byte, 512)
n, err := bufio.NewReader(conn).Read(answer)
if err != nil {
return nil, nil, err
}
err = conn.Close()
if err != nil {
return nil, nil, err
}
p := dnsmessage.Parser{}
header, err := p.Start(answer[:n])
if conn == nil {
return nil, nil, fmt.Errorf("Parser start error: %s", err)
}
questions, err := p.AllQuestions()
if err != nil {
return nil, nil, err
}
if len(questions) != len(msg.Questions) {
return nil, nil, fmt.Errorf("Answer packet does not have the same amouunt of questions")
}
err = p.SkipAllQuestions()
if err != nil {
return nil, nil, err
}
return &p, &header, nil
}
func getRootServers() []net.IP {
rootServers := []net.IP{}
for _, rs := range strings.Split(ROOT_SERVERS, ",") {
ip := net.ParseIP(rs)
if ip != nil {
rootServers = append(rootServers, ip)
} else {
fmt.Printf("Error parsing IP address of: %s server\n", rs)
}
}
return rootServers
}