net: return errNoSuchHost when no entry found in /etc/hosts and order is hostLookupFiles

When /etc/nsswitch.conf lists: "hosts: files" then LookupHost returns two nils when no entry inside /etc/hosts is found.

Change-Id: I96d68a079dfe009655c84cf0e697ce19a5bb6698
GitHub-Last-Rev: 894f066bbc
GitHub-Pull-Request: golang/go#56747
Reviewed-on: https://go-review.googlesource.com/c/go/+/450875
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/conf_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+//go:build unix
 
 package net
 

src/net/dnsclient_unix.go

@@ -551,9 +551,13 @@ func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hos
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		// Use entries from /etc/hosts if they match.
 		addrs, _ = lookupStaticHost(name)
-		if len(addrs) > 0 || order == hostLookupFiles {
+		if len(addrs) > 0 {
 			return
 		}
+
+		if order == hostLookupFiles {
+			return nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
+		}
 	}
 	ips, _, err := r.goLookupIPCNAMEOrder(ctx, "ip", name, order, conf)
 	if err != nil {

src/net/dnsclient_unix_test.go

@@ -12,7 +12,9 @@ import (
 	"fmt"
 	"os"
 	"path"
+	"path/filepath"
 	"reflect"
+	"runtime"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -2503,3 +2505,92 @@ func TestDNSConfigNoReload(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestLookupOrderFilesNoSuchHost(t *testing.T) {
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	if runtime.GOOS != "openbsd" {
+		defer setSystemNSS(getSystemNSS(), 0)
+		setSystemNSS(nssStr(t, "hosts: files"), time.Hour)
+	}
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+
+	resolvConf := dnsConfig{servers: defaultNS}
+	if runtime.GOOS == "openbsd" {
+		// Set error to ErrNotExist, so that the hostLookupOrder
+		// returns hostLookupFiles for openbsd.
+		resolvConf.err = os.ErrNotExist
+	}
+
+	if !conf.forceUpdateConf(&resolvConf, time.Now().Add(time.Hour)) {
+		t.Fatal("failed to update resolv config")
+	}
+
+	tmpFile := filepath.Join(t.TempDir(), "hosts")
+	if err := os.WriteFile(tmpFile, []byte{}, 0660); err != nil {
+		t.Fatal(err)
+	}
+	testHookHostsPath = tmpFile
+
+	const testName = "test.invalid"
+
+	order, _ := systemConf().hostLookupOrder(DefaultResolver, testName)
+	if order != hostLookupFiles {
+		// skip test for systems which do not return hostLookupFiles
+		t.Skipf("hostLookupOrder did not return hostLookupFiles")
+	}
+
+	var lookupTests = []struct {
+		name   string
+		lookup func(name string) error
+	}{
+		{
+			name: "Host",
+			lookup: func(name string) error {
+				_, err = DefaultResolver.LookupHost(context.Background(), name)
+				return err
+			},
+		},
+		{
+			name: "IP",
+			lookup: func(name string) error {
+				_, err = DefaultResolver.LookupIP(context.Background(), "ip", name)
+				return err
+			},
+		},
+		{
+			name: "IPAddr",
+			lookup: func(name string) error {
+				_, err = DefaultResolver.LookupIPAddr(context.Background(), name)
+				return err
+			},
+		},
+		{
+			name: "NetIP",
+			lookup: func(name string) error {
+				_, err = DefaultResolver.LookupNetIP(context.Background(), "ip", name)
+				return err
+			},
+		},
+	}
+
+	for _, v := range lookupTests {
+		err := v.lookup(testName)
+
+		if err == nil {
+			t.Errorf("Lookup%v: unexpected success", v.name)
+			continue
+		}
+
+		expectedErr := DNSError{Err: errNoSuchHost.Error(), Name: testName, IsNotFound: true}
+		var dnsErr *DNSError
+		errors.As(err, &dnsErr)
+		if dnsErr == nil || *dnsErr != expectedErr {
+			t.Errorf("Lookup%v: unexpected error: %v", v.name, err)
+		}
+	}
+}