++dynamic port forwarding patch + blog entry

Signed-off-by: Nico Schottelius <nico@bento.schottelius.org>

blog/openssh-6.2-add-callback-functionality-using-dynamic-remote-port-forwarding.mdwn

@@ -0,0 +1,98 @@
+[[!meta title="OpenSSH 6.2: Add callback functionality (using dynamic remote port forwarding)"]]
+
+## Introduction
+
+This article describes a patch to OpenSSH 6.2 that I wrote to enable
+**ssh callback** using dynamic ports. This is rather useful to have
+for various types of software, including [[cdist|software/cdist]]
+and [[ccollect|software/ccollect]].
+
+## Background
+
+Assume you have two hosts: 
+
+ * A **target host**
+ * A **control host**
+    (backup server in case of [[ccollect|software/ccollect]],
+     configuration server in case of [[cdist|software/cdist]])
+
+Assume further that the target host can directly reach the control
+host, but the control host cannot connect to the target host directly.
+
+This is for instance the case, 
+when the target host is hidden by NAT
+or protected by a firewall.
+
+## Approaches
+
+### Create a tunnel from the target host to the control host
+
+A very simple solution is to create a static tunnel
+from the target host to the control host, which allows
+the control host to connect back:
+
+    targethost% ssh -R 42523:localhost:22 controlhost
+    controlhost% ssh -p 42523 localhost
+
+The drawback is that the remote port needs to be defined
+beforehand and both sides needs to know about it.
+
+This is especially nasty, if you have a lot of
+target hosts that need to be backed up / configured.
+
+### Use dynamic port allocation
+
+The [OpenSSH](http://openssh.org/) developers seem to have
+spotted this problem and include an option to use a random
+free port: If port 0 is chosen as the remote
+forwarding port, the port is dynamically chosen by the
+ssh server, which in our case runs on the controlhost.
+
+Even better, the port information is also displayed on stdout:
+
+    targethost% ssh -R 0:localhost:22 controlhost
+    Allocated port 59818 for remote forward to localhost:22
+
+The problem here is: The shell on the remote side does not
+know which port was chosen, as it is only printed on stdout
+by the **ssh client**.
+
+
+### Expose remote forwarding ports
+
+[[This patch|openssh-6.2p1-expose-remote-port-forwarding.diff]]
+against OpenSSH 6.2p1 creates a new environment variable
+***SSH_REMOTE_FORWARDING_PORTS***, which contains all ports
+that are used for remote forwarding:
+
+    targethost % ssh -R 1234:localhost:22 controlhost
+    controlhost % echo $SSH_REMOTE_FORWARDING_PORTS
+    1234
+
+As this works for all remotely forwarded ports, this can
+also be used for dynamic port assignments:
+
+    targethost % ssh -R 0:localhost:22 controlhost
+    controlhost % echo $SSH_REMOTE_FORWARDING_PORTS
+    54294
+
+If more than one port forwarding definition is given, they are listed
+space separated:
+
+    targethost % ssh -R 0:localhost:22 -R 1234:localhost:22 controlhost
+    controlhost % echo $SSH_REMOTE_FORWARDING_PORTS
+    59056 1234
+
+## Limitations
+
+The given patch has some known limitations:
+
+ * The destination is not shown, as it is not known for ssh2 connections
+ * The number of listed ports is limited by the buffer size of 256 characters
+
+## Future
+
+The patch will be submitted to the 
+[openssh-unix-dev mailinglist](https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev) for discussion.
+
+[[!tag net unix]]

blog/openssh-6.2-add-callback-functionality-using-dynamic-remote-port-forwarding/openssh-6.2p1-expose-remote-port-forwarding.diff

@@ -0,0 +1,82 @@
+diff -ru openssh-6.2p1/channels.c openssh-6.2p1.patched/channels.c
+--- openssh-6.2p1/channels.c	2012-12-02 23:50:55.000000000 +0100
++++ openssh-6.2p1.patched/channels.c	2013-05-15 23:26:17.119989982 +0200
+@@ -2865,6 +2865,52 @@
+ 	return success;
+ }
+ 
++/*
++ * Write list of remote forwarding ports into an existing buffer 
++ */
++void
++channel_list_rport_listener(char *buf, size_t size)
++{
++	u_int i, j, num_ports = 0;
++	int offset = 0;
++	int *ports;
++	int skip;
++
++	ports = xcalloc(channels_alloc, sizeof(int));
++
++	for (i = 0; i < channels_alloc; i++) {
++		skip = 0;
++		Channel *c = channels[i];
++		if (c == NULL || c->type != SSH_CHANNEL_RPORT_LISTENER)
++			continue;
++
++		/* Skip already added ports - IPv4 + IPv6 == same port twice */
++		for(j = 0; j < num_ports; j++) {
++			if (ports[j] == c->listening_port) {
++				skip = 1;
++				break;
++			}
++		}
++
++		if(skip) continue;
++
++		ports[num_ports] = c->listening_port;
++		num_ports++;
++
++		if(!offset) {
++			offset += snprintf(&buf[offset], size - offset, "%d", c->listening_port);
++		} else
++			offset += snprintf(&buf[offset], size - offset, " %d", c->listening_port);
++
++		if(offset >= size) {
++			error("Exceeded buffer space for remote forwarding ports listing");
++			break;
++		}
++	}
++
++	xfree(ports);
++}
++
+ int
+ channel_cancel_rport_listener(const char *host, u_short port)
+ {
+Only in openssh-6.2p1.patched/: .channels.c.swp
+diff -ru openssh-6.2p1/channels.h openssh-6.2p1.patched/channels.h
+--- openssh-6.2p1/channels.h	2012-04-22 03:21:10.000000000 +0200
++++ openssh-6.2p1.patched/channels.h	2013-05-09 23:21:37.385423623 +0200
+@@ -222,6 +222,7 @@
+ void	 channel_cancel_cleanup(int);
+ int	 channel_close_fd(int *);
+ void	 channel_send_window_changes(void);
++void	 channel_list_rport_listener(char *buf, size_t size);
+ 
+ /* protocol handler */
+ 
+diff -ru openssh-6.2p1/session.c openssh-6.2p1.patched/session.c
+--- openssh-6.2p1/session.c	2013-03-15 01:22:37.000000000 +0100
++++ openssh-6.2p1.patched/session.c	2013-05-15 23:27:12.459989713 +0200
+@@ -1235,6 +1235,9 @@
+ 	xfree(laddr);
+ 	child_set_env(&env, &envsize, "SSH_CONNECTION", buf);
+ 
++	channel_list_rport_listener(buf, sizeof buf);
++	child_set_env(&env, &envsize, "SSH_REMOTE_FORWARDING_PORTS", buf);
++
+ 	if (s->ttyfd != -1)
+ 		child_set_env(&env, &envsize, "SSH_TTY", s->tty);
+ 	if (s->term)
+Only in openssh-6.2p1.patched/: .session.c.swp