x/crypto/ssh: Add timeout for dialing

Fixes golang/go#14941

Change-Id: I2b3a976d451d311519fab6cdabdc98a4a4752e31
Reviewed-on: https://go-review.googlesource.com/21136
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/client.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"net"
 	"sync"
+	"time"
 )
 
 // Client implements a traditional SSH client that supports shells,
@@ -169,7 +170,7 @@ func (c *Client) handleChannelOpens(in <-chan NewChannel) {
 // to incoming channels and requests, use net.Dial with NewClientConn
 // instead.
 func Dial(network, addr string, config *ClientConfig) (*Client, error) {
-	conn, err := net.Dial(network, addr)
+	conn, err := net.DialTimeout(network, addr, config.Timeout)
 	if err != nil {
 		return nil, err
 	}
@@ -210,4 +211,9 @@ type ClientConfig struct {
 	// string returned from PublicKey.Type method may be used, or
 	// any of the CertAlgoXxxx and KeyAlgoXxxx constants.
 	HostKeyAlgorithms []string
+
+	// Timeout is the maximum amount of time for the TCP connection to establish.
+	//
+	// A Timeout of zero means no timeout.
+	Timeout time.Duration
 }