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

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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: 70bc00fe41
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>
This commit is contained in:
jfbus 2019-04-18 12:39:24 +00:00 • committed by Brad Fitzpatrick
parent e900964e0f
commit 825ff1e317
7 changed files with 90 additions and 6 deletions
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18
src/net/dnsclient_unix.go
View file

@ -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(),