ssh: add support for banners

According to RFC 4252 section 5.4, the banner is sent between the
ssh-connection request and responding to user authentication.

Original support for server sending banner by joshua stein <jcs@jcs.org>

Fixes golang/go#19567

Change-Id: I729b3c8e5fd2c0068609d1590b61e92f40d87ea4
Reviewed-on: https://go-review.googlesource.com/71790
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/client.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"os"
 	"sync"
 	"time"
 )
@@ -187,6 +188,10 @@ func Dial(network, addr string, config *ClientConfig) (*Client, error) {
 // net.Conn underlying the the SSH connection.
 type HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
 
+// BannerCallback is the function type used for treat the banner sent by
+// the server. A BannerCallback receives the message sent by the remote server.
+type BannerCallback func(message string) error
+
 // A ClientConfig structure is used to configure a Client. It must not be
 // modified after having been passed to an SSH function.
 type ClientConfig struct {
@@ -209,6 +214,12 @@ type ClientConfig struct {
 	// FixedHostKey can be used for simplistic host key checks.
 	HostKeyCallback HostKeyCallback
 
+	// BannerCallback is called during the SSH dance to display a custom
+	// server's message. The client configuration can supply this callback to
+	// handle it as wished. The function BannerDisplayStderr can be used for
+	// simplistic display on Stderr.
+	BannerCallback BannerCallback
+
 	// ClientVersion contains the version identification string that will
 	// be used for the connection. If empty, a reasonable default is used.
 	ClientVersion string
@@ -255,3 +266,13 @@ func FixedHostKey(key PublicKey) HostKeyCallback {
 	hk := &fixedHostKey{key}
 	return hk.check
 }
+
+// BannerDisplayStderr returns a function that can be used for
+// ClientConfig.BannerCallback to display banners on os.Stderr.
+func BannerDisplayStderr() BannerCallback {
+	return func(banner string) error {
+		_, err := os.Stderr.WriteString(banner)
+
+		return err
+	}
+}