net: add LookupNS(domain string)

Fixes #4224.

R=golang-dev, dave, minux.ma, mikioh.mikioh, alex.brainman, rsc, herbert.fischer
CC=golang-dev
https://golang.org/cl/6675043

src/pkg/net/dnsclient.go

@@ -244,3 +244,8 @@ func (s byPref) sort() {
 	}
 	sort.Sort(s)
 }
+
+// An NS represents a single DNS NS record.
+type NS struct {
+	Host string
+}

src/pkg/net/lookup.go

@@ -47,6 +47,11 @@ func LookupMX(name string) (mx []*MX, err error) {
 	return lookupMX(name)
 }
 
+// LookupNS returns the DNS NS records for the given domain name.
+func LookupNS(name string) (ns []*NS, err error) {
+	return lookupNS(name)
+}
+
 // LookupTXT returns the DNS TXT records for the given domain name.
 func LookupTXT(name string) (txt []string, err error) {
 	return lookupTXT(name)

src/pkg/net/lookup_plan9.go

@@ -201,6 +201,21 @@ func lookupMX(name string) (mx []*MX, err error) {
 	return
 }
 
+func lookupNS(name string) (ns []*NS, err error) {
+	lines, err := queryDNS(name, "ns")
+	if err != nil {
+		return
+	}
+	for _, line := range lines {
+		f := getFields(line)
+		if len(f) < 4 {
+			continue
+		}
+		ns = append(ns, &NS{f[3]})
+	}
+	return
+}
+
 func lookupTXT(name string) (txt []string, err error) {
 	lines, err := queryDNS(name, "txt")
 	if err != nil {

src/pkg/net/lookup_test.go

@@ -52,6 +52,20 @@ func TestGmailMX(t *testing.T) {
 	}
 }
 
+func TestGmailNS(t *testing.T) {
+	if testing.Short() || !*testExternal {
+		t.Logf("skipping test to avoid external network")
+		return
+	}
+	ns, err := LookupNS("gmail.com")
+	if err != nil {
+		t.Errorf("failed: %s", err)
+	}
+	if len(ns) == 0 {
+		t.Errorf("no results")
+	}
+}
+
 func TestGmailTXT(t *testing.T) {
 	if testing.Short() || !*testExternal {
 		t.Logf("skipping test to avoid external network")

src/pkg/net/lookup_unix.go

@@ -119,6 +119,19 @@ func lookupMX(name string) (mx []*MX, err error) {
 	return
 }
 
+func lookupNS(name string) (ns []*NS, err error) {
+	_, records, err := lookup(name, dnsTypeNS)
+	if err != nil {
+		return
+	}
+	ns = make([]*NS, len(records))
+	for i, r := range records {
+		r := r.(*dnsRR_NS)
+		ns[i] = &NS{r.Ns}
+	}
+	return
+}
+
 func lookupTXT(name string) (txt []string, err error) {
 	_, records, err := lookup(name, dnsTypeTXT)
 	if err != nil {

src/pkg/net/lookup_windows.go

@@ -129,6 +129,21 @@ func lookupMX(name string) (mx []*MX, err error) {
 	return mx, nil
 }
 
+func lookupNS(name string) (ns []*NS, err error) {
+	var r *syscall.DNSRecord
+	e := syscall.DnsQuery(name, syscall.DNS_TYPE_NS, 0, nil, &r, nil)
+	if e != nil {
+		return nil, os.NewSyscallError("LookupNS", e)
+	}
+	defer syscall.DnsRecordListFree(r, 1)
+	ns = make([]*NS, 0, 10)
+	for p := r; p != nil && p.Type == syscall.DNS_TYPE_NS; p = p.Next {
+		v := (*syscall.DNSPTRData)(unsafe.Pointer(&p.Data[0]))
+		ns = append(ns, &NS{syscall.UTF16ToString((*[256]uint16)(unsafe.Pointer(v.Host))[:]) + "."})
+	}
+	return ns, nil
+}
+
 func lookupTXT(name string) (txt []string, err error) {
 	var r *syscall.DNSRecord
 	e := syscall.DnsQuery(name, syscall.DNS_TYPE_TEXT, 0, nil, &r, nil)