net: use DNS over TCP when use-vc is set in resolv.conf

There is a DNS resolution bug in Kubernetes (UDP response packets get dropped by conntrack, causing timeouts in DNS queries).

The recommended workaround on Linux is to configure the resolver to use TCP for DNS queries, by setting the use-vc option in resolv.conf.

With this PR, the pure Go resolver searches for "use-vc" in resolv.conf and switches to TCP when found.

Fixes #29358

Change-Id: I26b935cae2c80e5bb9955da83299a8dea84591de
GitHub-Last-Rev: 70bc00fe41f44f0b2b3cfebe67bbcc45701968cf
GitHub-Pull-Request: golang/go#29594
Reviewed-on: https://go-review.googlesource.com/c/go/+/156366
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 15 insertions, 3 deletions

diff --git a/src/net/dnsclient_unix.go b/src/net/dnsclient_unix.go
index 4e7462b66fa..7ed4ea8708c 100644
--- a/src/net/dnsclient_unix.go
+++ b/src/net/dnsclient_unix.go
@@ -26,6 +26,12 @@ import (
 	"golang.org/x/net/dns/dnsmessage"
 )
 
+const (
+	// to be used as a useTCP parameter to exchange
+	useTCPOnly  = true
+	useUDPOrTCP = false
+)
+
 var (
 	errLameReferral              = errors.New("lame referral")
 	errCannotUnmarshalDNSMessage = errors.New("cannot unmarshal DNS message")
@@ -131,13 +137,19 @@ func dnsStreamRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte)
 }
 
 // exchange sends a query on the connection and hopes for a response.
-func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration) (dnsmessage.Parser, dnsmessage.Header, error) {
+func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP bool) (dnsmessage.Parser, dnsmessage.Header, error) {
 	q.Class = dnsmessage.ClassINET
 	id, udpReq, tcpReq, err := newRequest(q)
 	if err != nil {
 		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
 	}
-	for _, network := range []string{"udp", "tcp"} {
+	var networks []string
+	if useTCP {
+		networks = []string{"tcp"}
+	} else {
+		networks = []string{"udp", "tcp"}
+	}
+	for _, network := range networks {
 		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
 		defer cancel()
 
@@ -241,7 +253,7 @@ func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string,
 		for j := uint32(0); j < sLen; j++ {
 			server := cfg.servers[(serverOffset+j)%sLen]
 
-			p, h, err := r.exchange(ctx, server, q, cfg.timeout)
+			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP)
 			if err != nil {
 				dnsErr := &DNSError{
 					Err:    err.Error(),