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: 1194098
GitHub-Pull-Request: #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

@@ -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 {

src/net/conf_test.go

@@ -170,16 +170,23 @@ 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{
 				goos:   "linux",
 				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",