net: add LookupAddr

R=adg, rsc
CC=golang-dev
https://golang.org/cl/3851041

src/pkg/net/dnsclient.go

@@ -15,6 +15,8 @@
 package net
 
 import (
+	"bytes"
+	"fmt"
 	"os"
 	"rand"
 	"sync"
@@ -357,9 +359,59 @@ func LookupMX(name string) (entries []*MX, err os.Error) {
 		return
 	}
 	entries = make([]*MX, len(records))
-	for i := 0; i < len(records); i++ {
+	for i := range records {
 		r := records[i].(*dnsRR_MX)
 		entries[i] = &MX{r.Mx, r.Pref}
 	}
 	return
 }
+
+// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
+// address addr suitable for rDNS (PTR) record lookup or an error if it fails
+// to parse the IP address.
+func reverseaddr(addr string) (arpa string, err os.Error) {
+	ip := ParseIP(addr)
+	if ip == nil {
+		return "", &DNSError{Error: "unrecognized address", Name: addr}
+	}
+	if ip.To4() != nil {
+		return fmt.Sprintf("%d.%d.%d.%d.in-addr.arpa.", ip[15], ip[14], ip[13], ip[12]), nil
+	}
+	// Must be IPv6
+	var buf bytes.Buffer
+	// Add it, in reverse, to the buffer
+	for i := len(ip) - 1; i >= 0; i-- {
+		s := fmt.Sprintf("%02x", ip[i])
+		buf.WriteByte(s[1])
+		buf.WriteByte('.')
+		buf.WriteByte(s[0])
+		buf.WriteByte('.')
+	}
+	// Append "ip6.arpa." and return (buf already has the final .)
+	return buf.String() + "ip6.arpa.", nil
+}
+
+// LookupAddr performs a reverse lookup for the given address, returning a list
+// of names mapping to that address.
+func LookupAddr(addr string) (name []string, err os.Error) {
+	name = lookupStaticAddr(addr)
+	if len(name) > 0 {
+		return
+	}
+	var arpa string
+	arpa, err = reverseaddr(addr)
+	if err != nil {
+		return
+	}
+	var records []dnsRR
+	_, records, err = lookup(arpa, dnsTypePTR)
+	if err != nil {
+		return
+	}
+	name = make([]string, len(records))
+	for i := range records {
+		r := records[i].(*dnsRR_PTR)
+		name[i] = r.Ptr
+	}
+	return
+}

src/pkg/net/hosts.go

@@ -19,7 +19,8 @@ var hostsPath = "/etc/hosts"
 // Simple cache.
 var hosts struct {
 	sync.Mutex
-	data map[string][]string
+	byName map[string][]string
+	byAddr map[string][]string
 	time   int64
 	path   string
 }
@@ -27,8 +28,9 @@ var hosts struct {
 func readHosts() {
 	now, _, _ := os.Time()
 	hp := hostsPath
-	if len(hosts.data) == 0 || hosts.time+cacheMaxAge <= now || hosts.path != hp {
+	if len(hosts.byName) == 0 || hosts.time+cacheMaxAge <= now || hosts.path != hp {
 		hs := make(map[string][]string)
+		is := make(map[string][]string)
 		var file *file
 		if file, _ = open(hp); file == nil {
 			return
@@ -45,12 +47,14 @@ func readHosts() {
 			for i := 1; i < len(f); i++ {
 				h := f[i]
 				hs[h] = append(hs[h], f[0])
+				is[f[0]] = append(is[f[0]], h)
 			}
 		}
 		// Update the data cache.
 		hosts.time, _, _ = os.Time()
 		hosts.path = hp
-		hosts.data = hs
+		hosts.byName = hs
+		hosts.byAddr = is
 		file.close()
 	}
 }
@@ -60,10 +64,23 @@ func lookupStaticHost(host string) []string {
 	hosts.Lock()
 	defer hosts.Unlock()
 	readHosts()
-	if len(hosts.data) != 0 {
-		if ips, ok := hosts.data[host]; ok {
+	if len(hosts.byName) != 0 {
+		if ips, ok := hosts.byName[host]; ok {
 			return ips
 		}
 	}
 	return nil
 }
+
+// rlookupStaticHosts looks up the hosts for the given address from /etc/hosts.
+func lookupStaticAddr(addr string) []string {
+	hosts.Lock()
+	defer hosts.Unlock()
+	readHosts()
+	if len(hosts.byAddr) != 0 {
+		if hosts, ok := hosts.byAddr[addr]; ok {
+			return hosts
+		}
+	}
+	return nil
+}

src/pkg/net/net_test.go

@@ -83,3 +83,40 @@ func TestDialError(t *testing.T) {
 		}
 	}
 }
+
+var revAddrTests = []struct {
+	Addr      string
+	Reverse   string
+	ErrPrefix string
+}{
+	{"1.2.3.4", "4.3.2.1.in-addr.arpa.", ""},
+	{"245.110.36.114", "114.36.110.245.in-addr.arpa.", ""},
+	{"::ffff:12.34.56.78", "78.56.34.12.in-addr.arpa.", ""},
+	{"::1", "1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa.", ""},
+	{"1::", "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.ip6.arpa.", ""},
+	{"1234:567::89a:bcde", "e.d.c.b.a.9.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.7.6.5.0.4.3.2.1.ip6.arpa.", ""},
+	{"1234:567:fefe:bcbc:adad:9e4a:89a:bcde", "e.d.c.b.a.9.8.0.a.4.e.9.d.a.d.a.c.b.c.b.e.f.e.f.7.6.5.0.4.3.2.1.ip6.arpa.", ""},
+	{"1.2.3", "", "unrecognized address"},
+	{"1.2.3.4.5", "", "unrecognized address"},
+	{"1234:567:bcbca::89a:bcde", "", "unrecognized address"},
+	{"1234:567::bcbc:adad::89a:bcde", "", "unrecognized address"},
+}
+
+func TestReverseAddress(t *testing.T) {
+	for i, tt := range revAddrTests {
+		a, e := reverseaddr(tt.Addr)
+		if len(tt.ErrPrefix) > 0 && e == nil {
+			t.Errorf("#%d: expected %q, got <nil> (error)", i, tt.ErrPrefix)
+			continue
+		}
+		if len(tt.ErrPrefix) == 0 && e != nil {
+			t.Errorf("#%d: expected <nil>, got %q (error)", i, e)
+		}
+		if e != nil && e.(*DNSError).Error != tt.ErrPrefix {
+			t.Errorf("#%d: expected %q, got %q (mismatched error)", i, tt.ErrPrefix, e.(*DNSError).Error)
+		}
+		if a != tt.Reverse {
+			t.Errorf("#%d: expected %q, got %q (reverse address)", i, tt.Reverse, a)
+		}
+	}
+}