[[!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.

For instance, it's 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 of the remote forwarding is not shown.
   Debugging the ssh server shows that this information was present
   in ssh1, but is absent in ssh2.
 * 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]]