net: add support for /etc/hosts aliases using go resolver

It adds support for /etc/hosts aliases and fixes the difference between the glibc cgo and the go DNS resolver.
Examples: https://pastebin.com/Fv6UcAVr

Fixes #44741

Change-Id: I98c484fced900731fbad800278b296028a45f044
GitHub-Last-Rev: 3d47e44f11
GitHub-Pull-Request: golang/go#51004
Reviewed-on: https://go-review.googlesource.com/c/go/+/382996
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/net/dnsclient_unix.go

@@ -559,7 +559,7 @@ func (r *Resolver) goLookupHost(ctx context.Context, name string) (addrs []strin
 func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []string, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		// Use entries from /etc/hosts if they match.
-		addrs = lookupStaticHost(name)
+		addrs, _ = lookupStaticHost(name)
 		if len(addrs) > 0 || order == hostLookupFiles {
 			return
 		}
@@ -576,8 +576,9 @@ func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hos
 }
 
 // lookup entries from /etc/hosts
-func goLookupIPFiles(name string) (addrs []IPAddr) {
-	for _, haddr := range lookupStaticHost(name) {
+func goLookupIPFiles(name string) (addrs []IPAddr, canonical string) {
+	addr, canonical := lookupStaticHost(name)
+	for _, haddr := range addr {
 		haddr, zone := splitHostZone(haddr)
 		if ip := ParseIP(haddr); ip != nil {
 			addr := IPAddr{IP: ip, Zone: zone}
@@ -585,7 +586,7 @@ func goLookupIPFiles(name string) (addrs []IPAddr) {
 		}
 	}
 	sortByRFC6724(addrs)
-	return
+	return addrs, canonical
 }
 
 // goLookupIP is the native Go implementation of LookupIP.
@@ -598,11 +599,23 @@ func (r *Resolver) goLookupIP(ctx context.Context, network, host string) (addrs
 
 func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name string, order hostLookupOrder) (addrs []IPAddr, cname dnsmessage.Name, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
-		addrs = goLookupIPFiles(name)
-		if len(addrs) > 0 || order == hostLookupFiles {
-			return addrs, dnsmessage.Name{}, nil
+		var canonical string
+		addrs, canonical = goLookupIPFiles(name)
+
+		if len(addrs) > 0 {
+			var err error
+			cname, err = dnsmessage.NewName(canonical)
+			if err != nil {
+				return nil, dnsmessage.Name{}, err
+			}
+			return addrs, cname, nil
+		}
+
+		if order == hostLookupFiles {
+			return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
 		}
 	}
+
 	if !isDomainName(name) {
 		// See comment in func lookup above about use of errNoSuchHost.
 		return nil, dnsmessage.Name{}, &DNSError{Err: errNoSuchHost.Error(), Name: name, IsNotFound: true}
@@ -776,9 +789,18 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name strin
 	sortByRFC6724(addrs)
 	if len(addrs) == 0 && !(network == "CNAME" && cname.Length > 0) {
 		if order == hostLookupDNSFiles {
-			addrs = goLookupIPFiles(name)
+			var canonical string
+			addrs, canonical = goLookupIPFiles(name)
+			if len(addrs) > 0 {
+				var err error
+				cname, err = dnsmessage.NewName(canonical)
+				if err != nil {
+					return nil, dnsmessage.Name{}, err
+				}
+				return addrs, cname, nil
+			}
 		}
-		if len(addrs) == 0 && lastErr != nil {
+		if lastErr != nil {
 			return nil, dnsmessage.Name{}, lastErr
 		}
 	}

src/net/dnsclient_unix_test.go

@@ -2170,6 +2170,56 @@ func TestRootNS(t *testing.T) {
 	}
 }
 
+func TestGoLookupIPCNAMEOrderHostsAliasesFilesOnlyMode(t *testing.T) {
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	testHookHostsPath = "testdata/aliases"
+	mode := hostLookupFiles
+
+	for _, v := range lookupStaticHostAliasesTest {
+		testGoLookupIPCNAMEOrderHostsAliases(t, mode, v.lookup, absDomainName(v.res))
+	}
+}
+
+func TestGoLookupIPCNAMEOrderHostsAliasesFilesDNSMode(t *testing.T) {
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	testHookHostsPath = "testdata/aliases"
+	mode := hostLookupFilesDNS
+
+	for _, v := range lookupStaticHostAliasesTest {
+		testGoLookupIPCNAMEOrderHostsAliases(t, mode, v.lookup, absDomainName(v.res))
+	}
+}
+
+var goLookupIPCNAMEOrderDNSFilesModeTests = []struct {
+	lookup, res string
+}{
+	// 127.0.1.1
+	{"invalid.invalid", "invalid.test"},
+}
+
+func TestGoLookupIPCNAMEOrderHostsAliasesDNSFilesMode(t *testing.T) {
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+	testHookHostsPath = "testdata/aliases"
+	mode := hostLookupDNSFiles
+
+	for _, v := range goLookupIPCNAMEOrderDNSFilesModeTests {
+		testGoLookupIPCNAMEOrderHostsAliases(t, mode, v.lookup, absDomainName(v.res))
+	}
+}
+
+func testGoLookupIPCNAMEOrderHostsAliases(t *testing.T, mode hostLookupOrder, lookup, lookupRes string) {
+	ins := []string{lookup, absDomainName(lookup), strings.ToLower(lookup), strings.ToUpper(lookup)}
+	for _, in := range ins {
+		_, res, err := goResolver.goLookupIPCNAMEOrder(context.Background(), "ip", in, mode)
+		if err != nil {
+			t.Errorf("expected err == nil, but got error: %v", err)
+		}
+		if res.String() != lookupRes {
+			t.Errorf("goLookupIPCNAMEOrder(%v): got %v, want %v", in, res, lookupRes)
+		}
+	}
+}
+
 // Test that we advertise support for a larger DNS packet size.
 // This isn't a great test as it just tests the dnsmessage package
 // against itself.

src/net/hosts.go

@@ -28,6 +28,11 @@ func parseLiteralIP(addr string) string {
 	return ip.String() + "%" + zone
 }
 
+type byName struct {
+	addrs         []string
+	canonicalName string
+}
+
 // hosts contains known host entries.
 var hosts struct {
 	sync.Mutex
@@ -36,7 +41,7 @@ var hosts struct {
 	// name. It would be part of DNS labels, a FQDN or an absolute
 	// FQDN.
 	// For now the key is converted to lower case for convenience.
-	byName map[string][]string
+	byName map[string]byName
 
 	// Key for the list of host names must be a literal IP address
 	// including IPv6 address with zone identifier.
@@ -62,8 +67,9 @@ func readHosts() {
 		return
 	}
 
-	hs := make(map[string][]string)
+	hs := make(map[string]byName)
 	is := make(map[string][]string)
+
 	var file *file
 	if file, _ = open(hp); file == nil {
 		return
@@ -81,13 +87,32 @@ func readHosts() {
 		if addr == "" {
 			continue
 		}
+
+		var canonical string
 		for i := 1; i < len(f); i++ {
 			name := absDomainName(f[i])
 			h := []byte(f[i])
 			lowerASCIIBytes(h)
 			key := absDomainName(string(h))
-			hs[key] = append(hs[key], addr)
+
+			if i == 1 {
+				canonical = key
+			}
+
 			is[addr] = append(is[addr], name)
+
+			if v,ok := hs[key]; ok {
+				hs[key] = byName{
+					addrs:         append(v.addrs, addr),
+					canonicalName: v.canonicalName,
+				}
+				continue
+			}
+
+			hs[key] = byName{
+				addrs:         []string{addr},
+				canonicalName: canonical,
+			}
 		}
 	}
 	// Update the data cache.
@@ -100,8 +125,8 @@ func readHosts() {
 	file.close()
 }
 
-// lookupStaticHost looks up the addresses for the given host from /etc/hosts.
-func lookupStaticHost(host string) []string {
+// lookupStaticHost looks up the addresses and the cannonical name for the given host from /etc/hosts.
+func lookupStaticHost(host string) ([]string, string) {
 	hosts.Lock()
 	defer hosts.Unlock()
 	readHosts()
@@ -111,13 +136,13 @@ func lookupStaticHost(host string) []string {
 			lowerASCIIBytes(lowerHost)
 			host = string(lowerHost)
 		}
-		if ips, ok := hosts.byName[absDomainName(host)]; ok {
-			ipsCp := make([]string, len(ips))
-			copy(ipsCp, ips)
-			return ipsCp
+		if byName, ok := hosts.byName[absDomainName(host)]; ok {
+			ipsCp := make([]string, len(byName.addrs))
+			copy(ipsCp, byName.addrs)
+			return ipsCp, byName.canonicalName
 		}
 	}
-	return nil
+	return nil, ""
 }
 
 // lookupStaticAddr looks up the hosts for the given address from /etc/hosts.

src/net/hosts_test.go

@@ -72,7 +72,7 @@ func TestLookupStaticHost(t *testing.T) {
 func testStaticHost(t *testing.T, hostsPath string, ent staticHostEntry) {
 	ins := []string{ent.in, absDomainName(ent.in), strings.ToLower(ent.in), strings.ToUpper(ent.in)}
 	for _, in := range ins {
-		addrs := lookupStaticHost(in)
+		addrs, _ := lookupStaticHost(in)
 		if !reflect.DeepEqual(addrs, ent.out) {
 			t.Errorf("%s, lookupStaticHost(%s) = %v; want %v", hostsPath, in, addrs, ent.out)
 		}
@@ -157,7 +157,7 @@ func TestHostCacheModification(t *testing.T) {
 	ent := staticHostEntry{"localhost", []string{"127.0.0.1", "127.0.0.2", "127.0.0.3"}}
 	testStaticHost(t, testHookHostsPath, ent)
 	// Modify the addresses return by lookupStaticHost.
-	addrs := lookupStaticHost(ent.in)
+	addrs, _ := lookupStaticHost(ent.in)
 	for i := range addrs {
 		addrs[i] += "junk"
 	}
@@ -173,3 +173,42 @@ func TestHostCacheModification(t *testing.T) {
 	}
 	testStaticAddr(t, testHookHostsPath, ent)
 }
+
+var lookupStaticHostAliasesTest = []struct {
+	lookup, res string
+}{
+	// 127.0.0.1
+	{"test", "test"},
+	// 127.0.0.2
+	{"test2.example.com", "test2.example.com"},
+	{"2.test", "test2.example.com"},
+	// 127.0.0.3
+	{"test3.example.com", "3.test"},
+	{"3.test", "3.test"},
+	// 127.0.0.4
+	{"example.com", "example.com"},
+	// 127.0.0.5
+	{"test5.example.com", "test4.example.com"},
+	{"5.test", "test4.example.com"},
+	{"4.test", "test4.example.com"},
+	{"test4.example.com", "test4.example.com"},
+}
+
+func TestLookupStaticHostAliases(t *testing.T) {
+	defer func(orig string) { testHookHostsPath = orig }(testHookHostsPath)
+
+	testHookHostsPath = "testdata/aliases"
+	for _, ent := range lookupStaticHostAliasesTest {
+		testLookupStaticHostAliases(t, ent.lookup, absDomainName(ent.res))
+	}
+}
+
+func testLookupStaticHostAliases(t *testing.T, lookup, lookupRes string) {
+	ins := []string{lookup, absDomainName(lookup), strings.ToLower(lookup), strings.ToUpper(lookup)}
+	for _, in := range ins {
+		_, res := lookupStaticHost(in)
+		if res != lookupRes {
+			t.Errorf("lookupStaticHost(%v): got %v, want %v", in, res, lookupRes)
+		}
+	}
+}

src/net/testdata/aliases

@@ -0,0 +1,8 @@
+127.0.0.1 test
+127.0.0.2 test2.example.com 2.test
+127.0.0.3 3.test test3.example.com
+127.0.0.4 example.com
+127.0.0.5 test4.example.com 4.test 5.test test5.example.com
+
+# must be a non resolvable domain on the internet
+127.0.1.1 invalid.test invalid.invalid