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query.go
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query.go
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package main
import (
"bytes"
"context"
"fmt"
"golang.org/x/net/dns/dnsmessage"
"net"
"strings"
"sync"
"time"
)
func handle(ctx context.Context, payload []byte) {
var parser dnsmessage.Parser
hdr, err := parser.Start(payload)
if err != nil {
logErr.Println(err)
return
}
qs, err := parser.AllQuestions()
if err != nil {
logErr.Println(err)
return
}
if *verbose {
var logBuf strings.Builder
fmt.Fprintf(&logBuf, "%d %s", hdr.ID, ctx.Value(clientAddrKey).(*net.UDPAddr))
for _, q := range qs {
fmt.Fprintf(&logBuf, " Query[%s] %s", q.Type.String()[4:], q.Name.String())
}
fmt.Fprintf(&logBuf, " len %d", len(payload))
logStd.Println(logBuf.String())
}
outConn, err := net.ListenUDP("udp", nil)
if err != nil {
logErr.Println(err)
return
}
defer outConn.Close() // duplicate close should only return error
query(ctx, payload, outConn)
}
func query(ctx context.Context, payload []byte, outConn *net.UDPConn) {
var (
clientSendTimer *time.Timer
clientSendTime time.Time
clientSendLock sync.Mutex
)
sentTime := time.Now()
for _, server := range servers {
if _, err := outConn.WriteToUDP(payload, server); err != nil {
logErr.Println(err)
continue
}
}
outConn.SetReadDeadline(sentTime.Add(*timeout))
for {
payload := make([]byte, 1500)
n, addr, err := outConn.ReadFromUDP(payload)
if err != nil {
return
}
if i, ok := lookupServer(addr); ok {
go sendBack(ctx, i+1, payload[:n], outConn, &clientSendTimer, &clientSendTime, &clientSendLock)
}
}
}
func sendBack(ctx context.Context, serverIndex int, msgIn []byte, outConn *net.UDPConn, clientSendTimer **time.Timer, clientSendTime *time.Time, clientSendLock *sync.Mutex) {
delay := determine(serverIndex, msgIn)
if delay < 0 {
return
}
newClientSendTime := time.Now().Add(delay)
// Lock to prevent race when answers come in simultaneously. Context is not handy for this
clientSendLock.Lock()
// if planned send time doesn't exist, go ahead.
// or if calculated send time is prior to the previously planned one, go ahead.
if clientSendTime.IsZero() || newClientSendTime.Before(*clientSendTime) {
// if there's no previous timer or stop is successful, set new planned time
if *clientSendTimer == nil || (*clientSendTimer).Stop() {
*clientSendTimer = time.AfterFunc(delay, func() {
outConn.Close()
listenerConn.WriteToUDP(msgIn, ctx.Value(clientAddrKey).(*net.UDPAddr))
})
*clientSendTime = newClientSendTime
} // If stop fails, let the previous timer fire
}
clientSendLock.Unlock()
}
func determine(serverIndex int, msgIn []byte) (delay time.Duration) {
delay = -1 // Assume DROP if parse fails
var logBuf strings.Builder
var parser dnsmessage.Parser
hdr, err := parser.Start(msgIn)
if err != nil {
logErr.Println(err)
return
}
parser.SkipAllQuestions()
answers, err := parser.AllAnswers() // parse answers in advance since there are several rules
if err != nil {
logErr.Println(err)
return
}
if *verbose {
fmt.Fprintf(&logBuf, "%d %s Answer len %d", hdr.ID, servers[serverIndex-1], len(msgIn))
for _, ans := range answers {
fmt.Fprintf(&logBuf, " %s %s TTL %d %v", ans.Header.Name, ans.Header.Type.String()[4:], ans.Header.TTL, ans.Body)
}
}
for _, rule := range rules { // rule by rule. continue if match failed
match := rule.match
if match.server != 0 && match.server != uint(serverIndex) {
continue
}
for _, ans := range answers {
if match.name != "" {
name := bytes.Trim(ans.Header.Name.Data[:ans.Header.Name.Length], ".")
switch ml, l := len(match.name), len(name); {
case ml > l:
continue
case ml < l:
if dotMName := append([]byte("."), match.name...); !bytes.EqualFold(name[len(name)-len(dotMName):], dotMName) {
continue
}
default: // ml = l
if !bytes.EqualFold(name, []byte(match.name)) {
continue
}
}
}
if match.answerType != 0 && match.answerType != ans.Header.Type {
continue
}
if match.ipset != 0 {
var ip net.IP
switch ans.Header.Type {
case dnsmessage.TypeA:
res := ans.Body.(*dnsmessage.AResource)
ip = res.A[:]
case dnsmessage.TypeAAAA:
res := ans.Body.(*dnsmessage.AAAAResource)
ip = res.AAAA[:]
default: // neither A nor AAAA, not match
continue
}
if !ipsets[match.ipset-1].containsIP(ip) {
continue
}
}
if *verbose {
switch d := rule.delay; {
case d < 0:
logBuf.WriteString(" [DROP]")
case d == 0:
logBuf.WriteString(" [ACCEPT]")
default:
fmt.Fprintf(&logBuf, " [DELAY %v]", d)
}
logStd.Println(&logBuf)
}
return rule.delay // if everything goes smoothly
}
}
if *verbose {
logBuf.WriteString(" [DROP]")
logStd.Println(&logBuf)
}
return
}