Support port forwarding of literal IPv6 addresses (#85)

* Support port forwarding of literal IPv6 addresses

To disambiguate between colons as host:port separators and as IPv6 address separators, literal IPv6 addresses use square brackets around the address (https://en.wikipedia.org/wiki/IPv6_address#Literal_IPv6_addresses_in_network_resource_identifiers).  So host ::1, port 22 is written as [::1]:22, and therefore a simple concatenation of host, colon, and port doesn't work.  Fortunately net.JoinHostPort already implements this functionality, so with a bit of type gymnastics we can generate dest in an IPv6-safe way.

* Support port forwarding of literal IPv6 addresses

To disambiguate between colons as host:port separators and as IPv6 address separators, literal IPv6 addresses use square brackets around the address (https://en.wikipedia.org/wiki/IPv6_address#Literal_IPv6_addresses_in_network_resource_identifiers).  So host ::1, port 22 is written as [::1]:22, and therefore a simple concatenation of host, colon, and port doesn't work.  Fortunately net.JoinHostPort already implements this functionality, so with a bit of type gymnastics we can generate dest in an IPv6-safe way.

Author: mfielding

tcpip.go

@@ -1,9 +1,9 @@
 package ssh
 
 import (
-	"fmt"
 	"io"
 	"net"
+	"strconv"
 
 	gossh "golang.org/x/crypto/ssh"
 )
@@ -29,8 +29,8 @@ func directTcpipHandler(srv *Server, conn *gossh.ServerConn, newChan gossh.NewCh
 		return
 	}
 
-	dest := fmt.Sprintf("%s:%d", d.DestinationHost, d.DestinationPort)
-
+	dest := net.JoinHostPort(d.DestinationHost, strconv.FormatInt(int64(d.DestinationPort), 10))
+	
 	var dialer net.Dialer
 	dconn, err := dialer.DialContext(ctx, "tcp", dest)
 	if err != nil {

tcpip_test.go

@@ -2,9 +2,9 @@ package ssh
 
 import (
 	"bytes"
-	"fmt"
 	"io/ioutil"
 	"net"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -34,7 +34,7 @@ func newTestSessionWithForwarding(t *testing.T, forwardingEnabled bool) (net.Lis
 	_, client, cleanup := newTestSession(t, &Server{
 		Handler: func(s Session) {},
 		LocalPortForwardingCallback: func(ctx Context, destinationHost string, destinationPort uint32) bool {
-			addr := fmt.Sprintf("%s:%d", destinationHost, destinationPort)
+			addr := net.JoinHostPort(destinationHost, strconv.FormatInt(int64(destinationPort), 10))
 			if addr != l.Addr().String() {
 				panic("unexpected destinationHost: " + addr)
 			}