net: respect resolv.conf rotate option

Instead of ranging over servers in the config, grab an offset
from the config that is used to determine indices.

When the rotate option is enabled, the offset increases which
rotates queries through servers. Otherwise, it is always 0
which uses servers in config order.

Fixes #17126

Change-Id: If233f6de7bfa42f88570055b9ab631be08a76b3e
Reviewed-on: https://go-review.googlesource.com/29233
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/net/dnsconfig_unix.go b/src/net/dnsconfig_unix.go
index 683ae71812..9c8108d11c 100644
--- a/src/net/dnsconfig_unix.go
+++ b/src/net/dnsconfig_unix.go
@@ -10,6 +10,7 @@ package net
 
 import (
 	"os"
+	"sync/atomic"
 	"time"
 )
 
@@ -29,6 +30,7 @@ type dnsConfig struct {
 	lookup     []string      // OpenBSD top-level database "lookup" order
 	err        error         // any error that occurs during open of resolv.conf
 	mtime      time.Time     // time of resolv.conf modification
+	soffset    uint32        // used by serverOffset
 }
 
 // See resolv.conf(5) on a Linux machine.
@@ -136,6 +138,17 @@ func dnsReadConfig(filename string) *dnsConfig {
 	return conf
 }
 
+// serverOffset returns an offset that can be used to determine
+// indices of servers in c.servers when making queries.
+// When the rotate option is enabled, this offset increases.
+// Otherwise it is always 0.
+func (c *dnsConfig) serverOffset() uint32 {
+	if c.rotate {
+		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
+	}
+	return 0
+}
+
 func dnsDefaultSearch() []string {
 	hn, err := getHostname()
 	if err != nil {