net: support all PacketConn and Conn returned by Resolver.Dial

Allow the Resolver.Dial func to return instances of Conn other than
*TCPConn and *UDPConn. If the Conn is also a PacketConn, assume DNS
messages transmitted over the Conn adhere to section 4.2.1. "UDP usage".
Otherwise, follow section 4.2.2. "TCP usage".

Provides a hook mechanism so that DNS queries generated by the net
package may be answered or modified before being sent to over the
network.

Updates #19910

Change-Id: Ib089a28ad4a1848bbeaf624ae889f1e82d56655b
Reviewed-on: https://go-review.googlesource.com/45153
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 12 insertions, 12 deletions

diff --git a/src/net/dnsclient_unix.go b/src/net/dnsclient_unix.go
index 75d70d39893..acbf6c3b2ad 100644
--- a/src/net/dnsclient_unix.go
+++ b/src/net/dnsclient_unix.go
@@ -36,14 +36,14 @@ type dnsConn interface {
 	dnsRoundTrip(query *dnsMsg) (*dnsMsg, error)
 }
 
-func (c *UDPConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
-	return dnsRoundTripUDP(c, query)
+// dnsPacketConn implements the dnsConn interface for RFC 1035's
+// "UDP usage" transport mechanism. Conn is a packet-oriented connection,
+// such as a *UDPConn.
+type dnsPacketConn struct {
+	Conn
 }
 
-// dnsRoundTripUDP implements the dnsRoundTrip interface for RFC 1035's
-// "UDP usage" transport mechanism. c should be a packet-oriented connection,
-// such as a *UDPConn.
-func dnsRoundTripUDP(c io.ReadWriter, query *dnsMsg) (*dnsMsg, error) {
+func (c *dnsPacketConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
 	b, ok := query.Pack()
 	if !ok {
 		return nil, errors.New("cannot marshal DNS message")
@@ -69,14 +69,14 @@ func dnsRoundTripUDP(c io.ReadWriter, query *dnsMsg) (*dnsMsg, error) {
 	}
 }
 
-func (c *TCPConn) dnsRoundTrip(out *dnsMsg) (*dnsMsg, error) {
-	return dnsRoundTripTCP(c, out)
+// dnsStreamConn implements the dnsConn interface for RFC 1035's
+// "TCP usage" transport mechanism. Conn is a stream-oriented connection,
+// such as a *TCPConn.
+type dnsStreamConn struct {
+	Conn
 }
 
-// dnsRoundTripTCP implements the dnsRoundTrip interface for RFC 1035's
-// "TCP usage" transport mechanism. c should be a stream-oriented connection,
-// such as a *TCPConn.
-func dnsRoundTripTCP(c io.ReadWriter, query *dnsMsg) (*dnsMsg, error) {
+func (c *dnsStreamConn) dnsRoundTrip(query *dnsMsg) (*dnsMsg, error) {
 	b, ok := query.Pack()
 	if !ok {
 		return nil, errors.New("cannot marshal DNS message")