net: support single-request resolv.conf option in pure Go resolver

There is a DNS resolution issue in Kubernetes (UDP response packets get dropped due to a race in conntrack between the parallel A and AAAA queries, causing timeouts in DNS queries).

A workaround is to enable single-request / single-request-reopen in resolv.conf in order to use sequential A and AAAA queries instead of parallel queries.

With this PR, the pure Go resolver searches for "single-request" and "single-request-reopen" in resolv.conf and send A and AAAA queries sequentially when found.

Fixes #29644

Change-Id: I906b3484008c1b9adf2e3e9241ea23767e29df59
GitHub-Last-Rev: d481acfb4c49d82fd474078b31a1a4697b57dadf
GitHub-Pull-Request: golang/go#29661
Reviewed-on: https://go-review.googlesource.com/c/go/+/157377
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -569,34 +569,52 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 	resolvConf.mu.RLock()
 	conf := resolvConf.dnsConfig
 	resolvConf.mu.RUnlock()
-	type racer struct {
+	type result struct {
 		p      dnsmessage.Parser
 		server string
 		error
 	}
-	lane := make(chan racer, 1)
+	lane := make(chan result, 1)
 	qtypes := [...]dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
-	var lastErr error
-	for _, fqdn := range conf.nameList(name) {
-		for _, qtype := range qtypes {
+	var queryFn func(fqdn string, qtype dnsmessage.Type)
+	var responseFn func(fqdn string, qtype dnsmessage.Type) result
+	if conf.singleRequest {
+		queryFn = func(fqdn string, qtype dnsmessage.Type) {}
+		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
+			dnsWaitGroup.Add(1)
+			defer dnsWaitGroup.Done()
+			p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
+			return result{p, server, err}
+		}
+	} else {
+		queryFn = func(fqdn string, qtype dnsmessage.Type) {
 			dnsWaitGroup.Add(1)
 			go func(qtype dnsmessage.Type) {
 				p, server, err := r.tryOneName(ctx, conf, fqdn, qtype)
-				lane <- racer{p, server, err}
+				lane <- result{p, server, err}
 				dnsWaitGroup.Done()
 			}(qtype)
 		}
+		responseFn = func(fqdn string, qtype dnsmessage.Type) result {
+			return <-lane
+		}
+	}
+	var lastErr error
+	for _, fqdn := range conf.nameList(name) {
+		for _, qtype := range qtypes {
+			queryFn(fqdn, qtype)
+		}
 		hitStrictError := false
-		for range qtypes {
-			racer := <-lane
-			if racer.error != nil {
-				if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
+		for _, qtype := range qtypes {
+			result := responseFn(fqdn, qtype)
+			if result.error != nil {
+				if nerr, ok := result.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
 					// This error will abort the nameList loop.
 					hitStrictError = true
-					lastErr = racer.error
+					lastErr = result.error
 				} else if lastErr == nil || fqdn == name+"." {
 					// Prefer error for original name.
-					lastErr = racer.error
+					lastErr = result.error
 				}
 				continue
 			}
@@ -618,12 +636,12 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 
 		loop:
 			for {
-				h, err := racer.p.AnswerHeader()
+				h, err := result.p.AnswerHeader()
 				if err != nil && err != dnsmessage.ErrSectionDone {
 					lastErr = &DNSError{
 						Err:    "cannot marshal DNS message",
 						Name:   name,
-						Server: racer.server,
+						Server: result.server,
 					}
 				}
 				if err != nil {
@@ -631,35 +649,35 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 				}
 				switch h.Type {
 				case dnsmessage.TypeA:
-					a, err := racer.p.AResource()
+					a, err := result.p.AResource()
 					if err != nil {
 						lastErr = &DNSError{
 							Err:    "cannot marshal DNS message",
 							Name:   name,
-							Server: racer.server,
+							Server: result.server,
 						}
 						break loop
 					}
 					addrs = append(addrs, IPAddr{IP: IP(a.A[:])})
 
 				case dnsmessage.TypeAAAA:
-					aaaa, err := racer.p.AAAAResource()
+					aaaa, err := result.p.AAAAResource()
 					if err != nil {
 						lastErr = &DNSError{
 							Err:    "cannot marshal DNS message",
 							Name:   name,
-							Server: racer.server,
+							Server: result.server,
 						}
 						break loop
 					}
 					addrs = append(addrs, IPAddr{IP: IP(aaaa.AAAA[:])})
 
 				default:
-					if err := racer.p.SkipAnswer(); err != nil {
+					if err := result.p.SkipAnswer(); err != nil {
 						lastErr = &DNSError{
 							Err:    "cannot marshal DNS message",
 							Name:   name,
-							Server: racer.server,
+							Server: result.server,
 						}
 						break loop
 					}

src/net/dnsclient_unix_test.go

@@ -17,6 +17,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -1621,3 +1622,76 @@ func TestTXTRecordTwoStrings(t *testing.T) {
 		t.Errorf("txt[1], got %q, want %q", txt[1], want)
 	}
 }
+
+// Issue 29644: support single-request resolv.conf option in pure Go resolver.
+// The A and AAAA queries will be sent sequentially, not in parallel.
+func TestSingleRequestLookup(t *testing.T) {
+	defer dnsWaitGroup.Wait()
+	var (
+		firstcalled int32
+		ipv4        int32 = 1
+		ipv6        int32 = 2
+	)
+	fake := fakeDNSServer{rh: func(n, s string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+		r := dnsmessage.Message{
+			Header: dnsmessage.Header{
+				ID:       q.ID,
+				Response: true,
+			},
+			Questions: q.Questions,
+		}
+		for _, question := range q.Questions {
+			switch question.Type {
+			case dnsmessage.TypeA:
+				if question.Name.String() == "slowipv4.example.net." {
+					time.Sleep(10 * time.Millisecond)
+				}
+				if !atomic.CompareAndSwapInt32(&firstcalled, 0, ipv4) {
+					t.Errorf("the A query was received after the AAAA query !")
+				}
+				r.Answers = append(r.Answers, dnsmessage.Resource{
+					Header: dnsmessage.ResourceHeader{
+						Name:   q.Questions[0].Name,
+						Type:   dnsmessage.TypeA,
+						Class:  dnsmessage.ClassINET,
+						Length: 4,
+					},
+					Body: &dnsmessage.AResource{
+						A: TestAddr,
+					},
+				})
+			case dnsmessage.TypeAAAA:
+				atomic.CompareAndSwapInt32(&firstcalled, 0, ipv6)
+				r.Answers = append(r.Answers, dnsmessage.Resource{
+					Header: dnsmessage.ResourceHeader{
+						Name:   q.Questions[0].Name,
+						Type:   dnsmessage.TypeAAAA,
+						Class:  dnsmessage.ClassINET,
+						Length: 16,
+					},
+					Body: &dnsmessage.AAAAResource{
+						AAAA: TestAddr6,
+					},
+				})
+			}
+		}
+		return r, nil
+	}}
+	r := Resolver{PreferGo: true, Dial: fake.DialContext}
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+	if err := conf.writeAndUpdate([]string{"options single-request"}); err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range []string{"hostname.example.net", "slowipv4.example.net"} {
+		firstcalled = 0
+		_, err := r.LookupIPAddr(context.Background(), name)
+		if err != nil {
+			t.Error(err)
+		}
+	}
+}

src/net/dnsconfig_unix.go

@@ -32,6 +32,7 @@ type dnsConfig struct {
 	err           error         // any error that occurs during open of resolv.conf
 	mtime         time.Time     // time of resolv.conf modification
 	soffset       uint32        // used by serverOffset
+	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
 }
 
 // See resolv.conf(5) on a Linux machine.
@@ -115,6 +116,13 @@ func dnsReadConfig(filename string) *dnsConfig {
 					conf.attempts = n
 				case s == "rotate":
 					conf.rotate = true
+				case s == "single-request" || s == "single-request-reopen":
+					// Linux option:
+					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
+					// "By default, glibc performs IPv4 and IPv6 lookups in parallel [...]
+					//  This option disables the behavior and makes glibc
+					//  perform the IPv6 and IPv4 requests sequentially."
+					conf.singleRequest = true
 				default:
 					conf.unknownOpt = true
 				}

src/net/dnsconfig_unix_test.go

@@ -102,6 +102,28 @@ var dnsReadConfigTests = []struct {
 			search:   []string{"c.symbolic-datum-552.internal."},
 		},
 	},
+	{
+		name: "testdata/single-request-resolv.conf",
+		want: &dnsConfig{
+			servers:       defaultNS,
+			ndots:         1,
+			singleRequest: true,
+			timeout:       5 * time.Second,
+			attempts:      2,
+			search:        []string{"domain.local."},
+		},
+	},
+	{
+		name: "testdata/single-request-reopen-resolv.conf",
+		want: &dnsConfig{
+			servers:       defaultNS,
+			ndots:         1,
+			singleRequest: true,
+			timeout:       5 * time.Second,
+			attempts:      2,
+			search:        []string{"domain.local."},
+		},
+	},
 }
 
 func TestDNSReadConfig(t *testing.T) {

src/net/testdata/single-request-reopen-resolv.conf

+options single-request-reopen
\ No newline at end of file

src/net/testdata/single-request-resolv.conf

+options single-request
\ No newline at end of file