net: allow Resolver to use a custom dialer

In some cases it is desirable to customize the way the DNS server is
contacted, for instance to use a specific LocalAddr. While most
operating-system level resolvers do not allow this, we have the
opportunity to do so with the Go resolver. Most of the code was
already in place to allow tests to override the dialer. This exposes
that functionality, and as a side effect eliminates the need for a
testing hook.

Fixes #17404

Change-Id: I1c5e570f8edbcf630090f8ec6feb52e379e3e5c0
Reviewed-on: https://go-review.googlesource.com/37260
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/net/lookup.go b/src/net/lookup.go
index 463b374aff..818f91c3dc 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -107,6 +107,15 @@ type Resolver struct {
 	// with resolvers that process AAAA queries incorrectly.
 	StrictErrors bool
 
+	// Dial optionally specifies an alternate dialer for use by
+	// 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.
+	Dial func(ctx context.Context, network, addr string) (Conn, error)
+
 	// TODO(bradfitz): optional interface impl override hook
 	// TODO(bradfitz): Timeout time.Duration?
 }