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main.go
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main.go
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package main
import (
"errors"
"fmt"
"strings"
"time"
"strconv"
"github.com/levenlabs/go-llog"
"github.com/mediocregopher/lever"
"github.com/miekg/dns"
)
var dnsServerGroups [][]string
var client dns.Client
var allowTruncated bool
type NewReq struct {
Msg *dns.Msg
ReplyCh chan *dns.Msg
}
type RespReq struct {
OrigMsg *dns.Msg
Reply *dns.Msg
}
var newCh = make(chan NewReq)
var respCh = make(chan RespReq)
//need to use init and not main for main_test.go
func init() {
go reqSpin()
}
//Note: check len(msg.Answer) to make sure the response from this actually has an answer
func tryProxy(m *dns.Msg, addr string) *dns.Msg {
kv := llog.KV{"addr": addr, "query": m.Question[0].Name}
llog.Debug("calling exchange", kv)
aM, _, err := client.Exchange(m, addr)
if err != nil && (!allowTruncated || err != dns.ErrTruncated) {
kv["err"] = err
llog.Warn("forwarding error in tryProxy", kv)
return nil
}
kv["rcode"] = aM.Rcode
kv["answerCnt"] = len(aM.Answer)
llog.Debug("exchange response", kv)
return aM
}
func queryGroup(r *dns.Msg, servers []string) *dns.Msg {
chs := make([]chan *dns.Msg, len(servers))
doneCh := make(chan struct{})
for i := range servers {
chs[i] = make(chan *dns.Msg, 1)
go func(ch chan *dns.Msg, addr string) {
select {
case ch <- tryProxy(r, addr):
case <-doneCh:
}
}(chs[i], servers[i])
}
var m *dns.Msg
for i := range servers {
if m = <-chs[i]; m != nil && len(m.Answer) > 0 {
break
}
}
close(doneCh)
return m
}
func queryAllGroups(r *dns.Msg) {
var m *dns.Msg
for i := range dnsServerGroups {
m = queryGroup(r, dnsServerGroups[i])
if m != nil && len(m.Answer) > 0 {
break
}
}
respCh <- RespReq{r, m}
}
func sendFormatError(w dns.ResponseWriter, r *dns.Msg) {
m := new(dns.Msg)
m.SetRcode(r, dns.RcodeFormatError)
w.WriteMsg(m) //ignore write error
return
}
func validateRequest(r *dns.Msg) error {
if len(r.Question) == 0 {
return errors.New("empty question set")
}
typ, ok := dns.TypeToString[r.Question[0].Qtype]
if !ok || typ == "None" {
return fmt.Errorf("invalid question type: %q", typ)
}
// require at least 2 characters, for the period and another character
if len(r.Question) == 1 && len(r.Question[0].Name) < 2 {
return fmt.Errorf("invalid question name: %s", r.Question[0].Name)
}
return nil
}
func handleRequest(w dns.ResponseWriter, r *dns.Msg) {
kv := llog.KV{}
// Can be nil during testing
if raddr := w.RemoteAddr(); raddr != nil {
kv["srcAddr"] = raddr.String()
}
if err := validateRequest(r); err != nil {
kv["err"] = err
llog.Warn("invalid request", kv)
sendFormatError(w, r)
return
}
start := time.Now()
kv["question"] = r.Question[0].Name
kv["questionType"] = r.Question[0].Qtype
llog.Info("handling request", kv)
rr := NewReq{r, make(chan *dns.Msg)}
newCh <- rr
m := <-rr.ReplyCh
kv["ms"] = int64(time.Since(start).Nanoseconds() / 1e6)
if m == nil {
llog.Warn("error handling request", kv)
dns.HandleFailed(w, r)
return
}
//we need to make sure the sent ID matches the replied one
//it might be different if we combined in-flight messages
//copy the struct so we don't affect anything else handling this
m2 := *m
m2.Id = r.Id
m = &m2
//we always want to compress since there's no downsides afaik
m.Compress = true
kv["rcode"], _ = dns.RcodeToString[m.Rcode]
if len(m.Answer) > 0 {
kv["answer"] = m.Answer[0]
kv["answerCnt"] = len(m.Answer)
}
kv["len"] = m.Len()
llog.Info("responding to request", kv)
err := w.WriteMsg(m)
if err != nil {
kv["err"] = err
llog.Warn("error writing response", kv)
//no need to handle HandleFailed here because we cannot write
}
}
var version string
func main() {
l := lever.New("struggledns", nil)
l.Add(lever.Param{
Name: "--listen-addr",
Description: "Address to listen on for dns requests. Will bind to both tcp and udp",
Default: ":53",
})
l.Add(lever.Param{
Name: "--fwd-to",
Description: "Address (ip:port) of a dns server to attempt forward requests to. Specify multiple times to make multiple request attempts. Order specified dictates precedence should more than one server respond for a request",
})
l.Add(lever.Param{
Name: "--parallel",
Description: "If sent the query will be sent to all addresses in parallel",
Flag: true,
})
l.Add(lever.Param{
Name: "--timeout",
Description: "Timeout in milliseconds for each request",
Default: "300",
})
l.Add(lever.Param{
Name: "--log-level",
Description: "Minimum log level to show, either debug, info, warn, error, or fatal",
Default: "warn",
})
l.Add(lever.Param{
Name: "--allow-truncated",
Description: "If we should allow truncated responses to be proxied",
Flag: true,
})
if version != "" {
l.Add(lever.Param{
Name: "--version",
Aliases: []string{"-v"},
Description: "Print version info",
Flag: true,
})
}
l.Parse()
if l.ParamFlag("--version") {
fmt.Println(version)
return
}
addr, _ := l.ParamStr("--listen-addr")
dnsServers, _ := l.ParamStrs("--fwd-to")
combineGroups := l.ParamFlag("--parallel")
timeout, _ := l.ParamInt("--timeout")
logLevel, _ := l.ParamStr("--log-level")
llog.SetLevelFromString(logLevel)
allowTruncated = l.ParamFlag("--allow-truncated")
if combineGroups {
//combine all the servers sent into one group
dnsServerGroups = make([][]string, 1)
var groupServers []string
for i := range dnsServers {
groupServers = strings.Split(dnsServers[i], ",")
dnsServerGroups[0] = append(dnsServerGroups[0], groupServers...)
}
} else {
dnsServerGroups = make([][]string, len(dnsServers))
for i := range dnsServers {
dnsServerGroups[i] = strings.Split(dnsServers[i], ",")
}
}
client = dns.Client{
//since this is UDP, the Dial/Write timeouts don't mean much
//we really only care about setting the read
DialTimeout: time.Millisecond * 100,
WriteTimeout: time.Millisecond * 100,
ReadTimeout: time.Millisecond * time.Duration(timeout),
UDPSize: 4096,
}
handler := dns.HandlerFunc(handleRequest)
go func() {
llog.Info("listening on udp", llog.KV{"addr": addr})
err := dns.ListenAndServe(addr, "udp", handler)
llog.Fatal("error listening on udp", llog.KV{"err": err})
}()
go func() {
llog.Info("listening on tcp", llog.KV{"addr": addr})
err := dns.ListenAndServe(addr, "tcp", handler)
llog.Fatal("error listening on tcp", llog.KV{"err": err})
}()
select {}
}
func getQuestionKey(q dns.Question) string {
t := "nop"
if t1, ok := dns.TypeToString[q.Qtype]; ok {
t = t1
}
c := "nop"
if c1, ok := dns.ClassToString[q.Qclass]; ok {
c = c1
}
n := q.Name
return n + t + c
}
func getMsgKey(r *dns.Msg) string {
k := ""
for _, q := range r.Question {
k += getQuestionKey(q)
}
//RFC 1035 defines this as the max
//since some clients (read: go) don't support edns0, this is the default
//if they do send edns0 we'll raise it to what they sent
limit := dns.MinMsgSize
opt := r.IsEdns0()
if opt != nil {
limit = int(opt.UDPSize())
llog.Debug("received edns0 limit", llog.KV{"limit": limit})
}
k += strconv.Itoa(limit)
return k
}
func reqSpin() {
var k string
inFlight := make(map[string][]chan *dns.Msg)
for {
select {
case r := <-newCh:
k = getMsgKey(r.Msg)
_, ok := inFlight[k]
inFlight[k] = append(inFlight[k], r.ReplyCh)
if !ok {
go queryAllGroups(r.Msg)
}
case r := <-respCh:
k = getMsgKey(r.OrigMsg)
for _, ch := range inFlight[k] {
ch <- r.Reply
}
delete(inFlight, k)
}
}
}