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ssh-forward-proxy

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SSH proxy server to allow clients access to a SSH remote by authenticating as the proxy, instead of the client.

Usage

The proxy is provided in the script in bin/ssh-forward-proxy.py. Note that this package must be installed on an intermediate/proxy machine (rather than your local machine). You can use the proxy in two ways: as a standalone SSH server; or as a netcat alternative in the ProxyCommand option (relay).

Relay

python ssh-forward-proxy.py [-i IDENTITY_FILE] [--no-host-key-check] [--server-key KEY] relay PORT HOST USER

If you use the proxy as a relay, you must also have an SSH server. A very basic (and insecure) SSH server is provided in bin/simply-ssh-server.py but it is recommended you provide your own (e.g. OpenSSH).

Using the relay is easy. Instead of doing:

ssh -p1234 user@remote.host command ...

do:

PROXY_CMD="ssh proxy.host python /path/to/ssh-forward-proxy.py relay %p %h %r"
ssh -o StrictHostKeyChecking=no -o ProxyCommand="$PROXY_CMD" -p1234 user@remote.host command ...

assuming that the proxy server is proxy.host.

Standalone server

python ssh-forward-proxy.py [-i IDENTITY_FILE] [--no-host-key-check] [--server-key KEY] server [PORT|22 [HOST]]

The standalone proxy server has no security. It allows all connections without authentication. Make sure you set up your firewall appropriately.

First, run the proxy on the intermediate/proxy server (say, on port 4000):

python /path/to/ssh-forward-proxy.py server 4000

The proxy figures out which host to forward to through the __HOST__ environment variable:

export __HOST__=user@remote.host:1234
ssh -o StrictHostKeyChecking=no -o SendEnv=__HOST__ -p4000 proxy.host command ...

Additional notes

In both methods above, the given ssh command will authenticate to user@remote.host:1234 using SSH keys or forwarded SSH agents on the intermediate/proxy server.

The StrictHostKeyChecking=no is necessary since the host key you receive will actually be provided by the proxy server and not remote.host. (An alternative is to set up a secondary known_hosts file for proxied SSH connections).

Host key

The proxy server will use a default host key provided in this python package. You can specify an alternative with the --server-key option.

Identity file

The -i option can be used to specify a public SSH key that the proxy will use to authenticate with the remote. It is not used to authenticate clients.

Using with Docker

ssh-forward-proxy was made with the sole purpose of facilitating git clone-ing of private git repositories during docker build without pre-cloning the repositories or adding SSH keys into docker images.

You can pull a pre-built docker image with docker pull lincheney/ssh-forward-proxy.

Procedure

We will assume that for a typical build process, we do:

  1. ssh from your local machine to a server (with the -A option for agent forwarding).
  2. Run docker build, including:
  3. COPY a private SSH key into the image.
  4. Install software, possibly from private git repositories.

Copying a private SSH key into the image is less than ideal since that key is now permanently embedded in the image. Even if you later rm the key, it will still exist in the lower layers. An ideal situtation would be to use the the forwarded SSH agent from step 1.

With ssh-forward-proxy, we can instead do this:

  1. ssh from your local machine to a server (with the -A option for agent forwarding).
  2. Launch the proxy:
docker run -d -v "$SSH_AUTH_SOCK:/tmp/ssh.sock" -e SSH_AUTH_SOCK=/tmp/ssh.sock --name ssh-proxy -p 127.0.0.1:4000:4000 lincheney/ssh-forward-proxy server 4000
  1. Run docker build, which now does:
  2. COPY git-ssh.sh / The git_ssh.sh file is a script containing:
```bash
#!/bin/sh
export __HOST__="$1"
ssh -o SendEnv=__HOST__ -o StrictHostKeyChecking=no -p4000 proxy.host "$2"
```
  1. ENV GIT_SSH /git_ssh.sh
  2. Install software.
  3. Remove the proxy container, docker rm -f ssh-proxy

No private SSH key will appear in the resulting image.

In this case, the standalone server was used and the -p 127.0.0.1:4000:4000 option ensures that the proxy is only accessible on the server (and not externally, since it has no security). We can use the relay instead; the process is largely the same but the git_ssh.sh will be different.

You should replace proxy.host (in the git_ssh.sh script) with an appropriate hostname/IP address. You cannot use 127.0.0.1 because that will refer to the building container rather than the proxy.