From c80022204e8fc36ec487888d471de27a5ea47e17 Mon Sep 17 00:00:00 2001
From: Natanael Copa <ncopa@mirantis.com>
Date: Fri, 16 Oct 2020 16:23:54 +0000
Subject: [PATCH] net: prefer /etc/hosts over DNS when no /etc/nsswitch.conf is
 present

Do not mimic glibc behavior if /etc/nsswitch.conf is missing. This will
will likely be missing on musl libc systems and glibc systems will likely
always have it, resulting in localhost lookup being done over DNS rather
than from /etc/hosts.

Do what makes most sense rather than making any assumption about the
libc.

Fixes #35305

Change-Id: I20bd7e24131bba8eaa39a20c8950fe552364784d
GitHub-Last-Rev: 119409839d37c8c7268f5f6db19c1789d9d96074
GitHub-Pull-Request: golang/go#39685
Reviewed-on: https://go-review.googlesource.com/c/go/+/238629
Run-TryBot: Dan Peterson <dpiddy@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Peterson <dpiddy@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
---
 src/net/conf.go      |  5 -----
 src/net/conf_test.go | 13 ++++++++++---
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/net/conf.go b/src/net/conf.go
index 5340847123ce67..f1bbfedad06cb5 100644
--- a/src/net/conf.go
+++ b/src/net/conf.go
@@ -202,11 +202,6 @@ func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrde
 			// illumos defaults to "nis [NOTFOUND=return] files"
 			return fallbackOrder
 		}
-		if c.goos == "linux" {
-			// glibc says the default is "dns [!UNAVAIL=return] files"
-			// https://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html.
-			return hostLookupDNSFiles
-		}
 		return hostLookupFilesDNS
 	}
 	if nss.err != nil {
diff --git a/src/net/conf_test.go b/src/net/conf_test.go
index 4c21d56ba0ab31..1fe3cf41b1e28c 100644
--- a/src/net/conf_test.go
+++ b/src/net/conf_test.go
@@ -170,8 +170,6 @@ func TestConfHostLookupOrder(t *testing.T) {
 			},
 			hostTests: []nssHostTest{{"google.com", "myhostname", hostLookupDNSFiles}},
 		},
-		// glibc lacking an nsswitch.conf, per
-		// https://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html
 		{
 			name: "linux_no_nsswitch.conf",
 			c: &conf{
@@ -179,7 +177,16 @@ func TestConfHostLookupOrder(t *testing.T) {
 				nss:    &nssConf{err: fs.ErrNotExist},
 				resolv: defaultResolvConf,
 			},
-			hostTests: []nssHostTest{{"google.com", "myhostname", hostLookupDNSFiles}},
+			hostTests: []nssHostTest{{"google.com", "myhostname", hostLookupFilesDNS}},
+		},
+		{
+			name: "linux_empty_nsswitch.conf",
+			c: &conf{
+				goos:   "linux",
+				nss:    nssStr(""),
+				resolv: defaultResolvConf,
+			},
+			hostTests: []nssHostTest{{"google.com", "myhostname", hostLookupFilesDNS}},
 		},
 		{
 			name: "files_mdns_dns",