net: allow LookupSRV on non-standard DNS names

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5313043

src/pkg/net/lookup_plan9.go

@@ -157,12 +157,21 @@ func LookupCNAME(name string) (cname string, err os.Error) {
 }
 
 // LookupSRV tries to resolve an SRV query of the given service,
-// protocol, and domain name, as specified in RFC 2782. In most cases
-// the proto argument can be the same as the corresponding
-// Addr.Network(). The returned records are sorted by priority 
-// and randomized by weight within a priority.
+// protocol, and domain name.  The proto is "tcp" or "udp".
+// The returned records are sorted by priority and randomized
+// by weight within a priority.
+//
+// LookupSRV constructs the DNS name to look up following RFC 2782.
+// That is, it looks up _service._proto.name.  To accommodate services
+// publishing SRV records under non-standard names, if both service
+// and proto are empty strings, LookupSRV looks up name directly.
 func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.Error) {
-	target := "_" + service + "._" + proto + "." + name
+	var target string
+	if service == "" && proto == "" {
+		target = name
+	} else {
+		target = "_" + service + "._" + proto + "." + name
+	}
 	lines, err := queryDNS(target, "srv")
 	if err != nil {
 		return

src/pkg/net/lookup_test.go

@@ -26,6 +26,15 @@ func TestGoogleSRV(t *testing.T) {
 	if len(addrs) == 0 {
 		t.Errorf("no results")
 	}
+
+	// Non-standard back door.
+	_, addrs, err = LookupSRV("", "", "_xmpp-server._tcp.google.com")
+	if err != nil {
+		t.Errorf("back door failed: %s", err)
+	}
+	if len(addrs) == 0 {
+		t.Errorf("back door no results")
+	}
 }
 
 func TestGmailMX(t *testing.T) {

src/pkg/net/lookup_unix.go

@@ -94,12 +94,21 @@ func LookupCNAME(name string) (cname string, err os.Error) {
 }
 
 // LookupSRV tries to resolve an SRV query of the given service,
-// protocol, and domain name, as specified in RFC 2782. In most cases
-// the proto argument can be the same as the corresponding
-// Addr.Network(). The returned records are sorted by priority
-// and randomized by weight within a priority.
+// protocol, and domain name.  The proto is "tcp" or "udp".
+// The returned records are sorted by priority and randomized
+// by weight within a priority.
+//
+// LookupSRV constructs the DNS name to look up following RFC 2782.
+// That is, it looks up _service._proto.name.  To accommodate services
+// publishing SRV records under non-standard names, if both service
+// and proto are empty strings, LookupSRV looks up name directly.
 func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.Error) {
-	target := "_" + service + "._" + proto + "." + name
+	var target string
+	if service == "" && proto == "" {
+		target = name
+	} else {
+		target = "_" + service + "._" + proto + "." + name
+	}
 	var records []dnsRR
 	cname, records, err = lookup(target, dnsTypeSRV)
 	if err != nil {

src/pkg/net/lookup_windows.go

@@ -91,9 +91,23 @@ func LookupCNAME(name string) (cname string, err os.Error) {
 	return
 }
 
+// LookupSRV tries to resolve an SRV query of the given service,
+// protocol, and domain name.  The proto is "tcp" or "udp".
+// The returned records are sorted by priority and randomized
+// by weight within a priority.
+//
+// LookupSRV constructs the DNS name to look up following RFC 2782.
+// That is, it looks up _service._proto.name.  To accommodate services
+// publishing SRV records under non-standard names, if both service
+// and proto are empty strings, LookupSRV looks up name directly.
 func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err os.Error) {
+	var target string
+	if service == "" && proto == "" {
+		target = name
+	} else {
+		target = "_" + service + "._" + proto + "." + name
+	}
 	var r *syscall.DNSRecord
-	target := "_" + service + "._" + proto + "." + name
 	e := syscall.DnsQuery(target, syscall.DNS_TYPE_SRV, 0, nil, &r, nil)
 	if int(e) != 0 {
 		return "", nil, os.NewSyscallError("LookupSRV", int(e))