net: make go DNS use localhost if resolv.conf is missing or empty

Per resolv.conf man page, "If this file does not exist, only the name
server on the local machine will be queried."

This behavior also occurs if file is present but unreadable,
or if no nameservers are listed.

Fixes #10566

Change-Id: Id5716da0eae534d5ebfafea111bbc657f302e307
Reviewed-on: https://go-review.googlesource.com/9380
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 57 insertions, 17 deletions

diff --git a/src/net/conf_test.go b/src/net/conf_test.go
index 46e91bc0a1..43be546d07 100644
--- a/src/net/conf_test.go
+++ b/src/net/conf_test.go
@@ -19,6 +19,15 @@ type nssHostTest struct {
 
 func nssStr(s string) *nssConf { return parseNSSConf(strings.NewReader(s)) }
 
+// represents a dnsConfig returned by parsing a nonexistent resolv.conf
+var defaultResolvConf = &dnsConfig{
+	servers:  defaultNS,
+	ndots:    1,
+	timeout:  5,
+	attempts: 2,
+	err:      os.ErrNotExist,
+}
+
 func TestConfHostLookupOrder(t *testing.T) {
 	tests := []struct {
 		name      string
@@ -31,6 +40,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 			c: &conf{
 				forceCgoLookupHost: true,
 				nss:                nssStr("foo: bar"),
+				resolv:             defaultResolvConf,
 			},
 			hostTests: []nssHostTest{
 				{"foo.local", hostLookupCgo},
@@ -40,7 +50,8 @@ func TestConfHostLookupOrder(t *testing.T) {
 		{
 			name: "ubuntu_trusty_avahi",
 			c: &conf{
-				nss: nssStr("hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4"),
+				nss:    nssStr("hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4"),
+				resolv: defaultResolvConf,
 			},
 			hostTests: []nssHostTest{
 				{"foo.local", hostLookupCgo},
@@ -53,8 +64,9 @@ func TestConfHostLookupOrder(t *testing.T) {
 		{
 			name: "freebsdlinux_no_resolv_conf",
 			c: &conf{
-				goos: "freebsd",
-				nss:  nssStr("foo: bar"),
+				goos:   "freebsd",
+				nss:    nssStr("foo: bar"),
+				resolv: defaultResolvConf,
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupFilesDNS}},
 		},
@@ -62,15 +74,17 @@ func TestConfHostLookupOrder(t *testing.T) {
 		{
 			name: "openbsd_no_resolv_conf",
 			c: &conf{
-				goos: "openbsd",
+				goos:   "openbsd",
+				resolv: defaultResolvConf,
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupFiles}},
 		},
 		{
 			name: "solaris_no_nsswitch",
 			c: &conf{
-				goos: "solaris",
-				nss:  &nssConf{err: os.ErrNotExist},
+				goos:   "solaris",
+				nss:    &nssConf{err: os.ErrNotExist},
+				resolv: defaultResolvConf,
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupCgo}},
 		},
@@ -138,14 +152,18 @@ func TestConfHostLookupOrder(t *testing.T) {
 		{
 			name: "linux_no_nsswitch.conf",
 			c: &conf{
-				goos: "linux",
-				nss:  &nssConf{err: os.ErrNotExist},
+				goos:   "linux",
+				nss:    &nssConf{err: os.ErrNotExist},
+				resolv: defaultResolvConf,
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupDNSFiles}},
 		},
 		{
 			name: "files_mdns_dns",
-			c:    &conf{nss: nssStr("hosts: files mdns dns")},
+			c: &conf{
+				nss:    nssStr("hosts: files mdns dns"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupFilesDNS},
 				{"x.local", hostLookupCgo},
@@ -153,7 +171,10 @@ func TestConfHostLookupOrder(t *testing.T) {
 		},
 		{
 			name: "dns_special_hostnames",
-			c:    &conf{nss: nssStr("hosts: dns")},
+			c: &conf{
+				nss:    nssStr("hosts: dns"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupDNS},
 				{"x\\.com", hostLookupCgo},     // punt on weird glibc escape
@@ -164,6 +185,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 			name: "mdns_allow",
 			c: &conf{
 				nss:          nssStr("hosts: files mdns dns"),
+				resolv:       defaultResolvConf,
 				hasMDNSAllow: true,
 			},
 			hostTests: []nssHostTest{
@@ -173,7 +195,10 @@ func TestConfHostLookupOrder(t *testing.T) {
 		},
 		{
 			name: "files_dns",
-			c:    &conf{nss: nssStr("hosts: files dns")},
+			c: &conf{
+				nss:    nssStr("hosts: files dns"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupFilesDNS},
 				{"x", hostLookupFilesDNS},
@@ -182,7 +207,10 @@ func TestConfHostLookupOrder(t *testing.T) {
 		},
 		{
 			name: "dns_files",
-			c:    &conf{nss: nssStr("hosts: dns files")},
+			c: &conf{
+				nss:    nssStr("hosts: dns files"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupDNSFiles},
 				{"x", hostLookupDNSFiles},
@@ -191,14 +219,20 @@ func TestConfHostLookupOrder(t *testing.T) {
 		},
 		{
 			name: "something_custom",
-			c:    &conf{nss: nssStr("hosts: dns files something_custom")},
+			c: &conf{
+				nss:    nssStr("hosts: dns files something_custom"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupCgo},
 			},
 		},
 		{
 			name: "myhostname",
-			c:    &conf{nss: nssStr("hosts: files dns myhostname")},
+			c: &conf{
+				nss:    nssStr("hosts: files dns myhostname"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupFilesDNS},
 				{"somehostname", hostLookupCgo},
@@ -206,7 +240,10 @@ func TestConfHostLookupOrder(t *testing.T) {
 		},
 		{
 			name: "ubuntu14.04.02",
-			c:    &conf{nss: nssStr("hosts: files myhostname mdns4_minimal [NOTFOUND=return] dns mdns4")},
+			c: &conf{
+				nss:    nssStr("hosts: files myhostname mdns4_minimal [NOTFOUND=return] dns mdns4"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupFilesDNS},
 				{"somehostname", hostLookupCgo},
@@ -218,7 +255,10 @@ func TestConfHostLookupOrder(t *testing.T) {
 		// files.
 		{
 			name: "debian_squeeze",
-			c:    &conf{nss: nssStr("hosts: dns [success=return notfound=continue unavail=continue tryagain=continue] files [notfound=return]")},
+			c: &conf{
+				nss:    nssStr("hosts: dns [success=return notfound=continue unavail=continue tryagain=continue] files [notfound=return]"),
+				resolv: defaultResolvConf,
+			},
 			hostTests: []nssHostTest{
 				{"x.com", hostLookupDNSFiles},
 				{"somehostname", hostLookupDNSFiles},
@@ -228,7 +268,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 			name: "resolv.conf-unknown",
 			c: &conf{
 				nss:    nssStr("foo: bar"),
-				resolv: &dnsConfig{unknownOpt: true},
+				resolv: &dnsConfig{servers: defaultNS, ndots: 1, timeout: 5, attempts: 2, unknownOpt: true},
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupCgo}},
 		},