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, 5 insertions, 3 deletions

diff --git a/src/net/lookup.go b/src/net/lookup.go
index 818f91c3dc..abc56de533 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -111,9 +111,11 @@ type Resolver struct {
 	// Go's built-in DNS resolver to make TCP and UDP connections
 	// to DNS services. The provided addr will always be an IP
 	// address and not a hostname.
-	// The Conn returned must be a *TCPConn or *UDPConn as
-	// requested by the network parameter. If nil, the default
-	// dialer is used.
+	// If the Conn returned is also a PacketConn, sent and received DNS
+	// messages must adhere to section 4.2.1. "UDP usage" of RFC 1035.
+	// Otherwise, DNS messages transmitted over Conn must adhere to section
+	// 4.2.2. "TCP usage".
+	// If nil, the default dialer is used.
 	Dial func(ctx context.Context, network, addr string) (Conn, error)
 
 	// TODO(bradfitz): optional interface impl override hook