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ssh_force_command

Considerations

ssh is an angular stone in *nix administration.

Allowing remote access while restricting the actions permitted in the server is a very common scenario, and force command is just the ingredient needed for this purpose.

Requires Public key authentication:

  1. The file ~/.ssh/authorized_keys lists the public keys that are permitted for logging in. When the user logs in, the ssh program tells the server which key pair it would like to use for authentication. The client proves that it has access to the private key and the server checks that the corresponding public key is authorized to accept the account.

  2. ForceCommand Forces the execution of the command specified in ~/.ssh/authorized_key , ignoring any command supplied by the client and ~/.ssh/rc if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a Match block.

  3. SSH_ORIGINAL_COMMAND This variable contains the original command line if a forced command is executed. It can be used to extract the original arguments.

Purpose

This software is based in two elements, and executable and a config file written in yaml.

The binary should be set as the forced command in ~/.ssh/authorized_keys, and ~/.ssh/authorized_forced_commands.yml must store the allowed actions for this particular public key.

Goals:

  1. Improve external access safety.

The launcher is compiled and the configuration file is checked using the same security patterns as the rest of the ssh machinery.

  1. Simplify the management of allowed commands.

By standardize the configuration yaml file, way more easy and flexible than a random piped separated text file.

Installation

Obviously you need go installed in your machine.

Then just:

go get -v github.com/klashxx/ssh_force_command

And the executable will be compiled and placed in your $GOPATH/bin directory.

Configuration

A config file authorized_forced_commands.yml must be placed in the ~/.ssh directory.

Safety rules:

  • Should not be accesible by others group.
  • Owner must be ssh user.
  • Group should be ssh user group.

It's written in yaml and the format is pretty self explanatory:

tag: my_tag
commands:
  - path: command1
    description: my first desc
    env: null
  - path: /path/to/command2
    description: my second desc
    env:
      - VAR1=/var1/value
      - VAR2=value2

NOTE: ssh_force_command uses the current process's environment, if env is NOT null listed variables will be appended before execution.

Example

In the remote box

  1. Place ssh_force_command binary and this test script (named test_ssh_force_command.sh) in your HOME dir:
#!/bin/bash
echo "just a simple test"
echo "parameters: $@"
echo "VAR1: ${VAR1:-not_set}"
echo "VAR2: ${VAR2:-not_set}"
exit 0
  1. Create the configuration file ~/.ssh/authorized_forced_commands.yml with the appropriate permissions:
tag: test
commands:
    - path: ~/test_ssh_force_command.sh
      description: very dummy test
      env: null
  1. Set the forced command for the corresponding key in ~/.ssh/authorized_keys, example:
command="/home/user/ssh_force_command",no-pty ssh-rsa ZZZZB3NzaC1yc2EAAAABIwAAAQEAqxekXWvfwc74bSZxyzTxPpWaogaeMCKlXE8tgEAN/jS8+28x2h/PGzI4ij9H3aZHLayjL7PY1Uj3SETG913+NOTGONNAWORK+r9vPzyRwbJLh3dkbvYdsC0drbsqIN+3K7mGIT8U/Aw9i5oZpNZ/mpEO+dT2ymMLvLJL+sizNK7Aw10x1YWOBTEVKf6C5E/dtmWYWKyx14tpBxlh6wxiofb2hDO9i6TU/N3PKNZ/xToIDTGMpOO9mbPT6v3DRof0fIgBF3rPNaIPLUWKuwjmP4JbAiP76L93DM+Mwhc1cw7H6+oOljpTSRxmTQi20iohqVQonAhlY1w== [email protected]

In the local machine

Just execute the ssh command:

ssh user@remote_server "~/test_ssh_force_command.sh arg1 arg2"

The output should be:

just a simple test
parameters: arg1 arg2
VAR1: not_set
VAR2: not_set

Back to the remote box

Let's add some env variables to the config file, and another command:

tag: test
commands:
  - path: ~/test_ssh_force_command.sh
    description: adding env vars
    env:
      - VAR1=/var1/value
      - VAR2=foo
  - path: ls
    description: a simple ls
    env: null

Now, from the local machine, the ssh execution output must be:

just a simple test
parameters: arg1 arg2
VAR1: /var1/value
VAR2: foo

And you should be able to list the content of any remote dir where the exec user has permissions.