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