The Linux SSH client has many configuration options to choose from, and sometimes you may find yourself wishing you could automatically change certain options depending on what network you're connected to (ie: home, work, school, etc).

connection-specific-ssh-config is a simple script written in python, which can change your ssh configuration file (usually found in ~/.ssh/config) depending on what network you're currently connected to.

Written by Mike Peralta and released to the public via GPLv3 on Sunday, May 27th, 2018

Requirements

Linux (possibly others)
Python 3
- pyenv
- pipenv
NetworkManager and nmcli (tested on v1.46)
One of the following: root access, the ability to place executable files inside NetworkManager's dispatcher directory, or the ability to call this script whenever connection details change

Note: This script has only been tested with NetworkManager and nmcli 1.46, but should work with any other daemon or application that can call this script with the required parameters.

You can also try using this script with a different daemon (or your own custom script). All you need to do is call this script whenever some change in the network occurs, with the following parameters:

$ cd path/to/connection-specific-ssh-config && pipenv run python main.py --config <config_path> --adapter <adapter> --command <command>

Where config_path is the path to a yaml configuration file, explained below
Where adapter is the name of the adapter/interface that has changed states
Where command is the type of change occurring. Currently, the script only does anything when receiving the up command.

Installation (NetworkManager)

This section to help with installation.

Python Environment

Install pyenv
Install pipenv

Setting up the pipenv environment is pretty straight forward:

$ cd /path/to/connection-specific-ssh-config
$ pyenv update
$ pyenv install
$ pip install --upgrade pip pipenv
$ pipenv sync

You should then be able to get a help menu with:

$ cd /path/to/connection-specific-ssh-config
$ pipenv run python main.py --help

SSH Config Preparation

Move your old ssh configuration file (typically at ~/.ssh/config) to a safe backup, like:
- mv ~/.ssh/config config-backup
Create as many customized ssh configuration files as you need, such as config-remote or config-home. Do not name any of them config.

Configuration File

Create a configuration file somewhere safe, such as /path/to/connection-specific-ssh-config.yaml (explained in detail further below)

Network Manager Dispatch

Open a terminal and navigate to NetworkManager's dispatcher directory, often found here:
- cd /etc/NetworkManager/dispatcher.d
- There may be subdirectories underneath. Just ignore them.
Create a short bash script inside the dispatcher directory that will launch connection-specific-ssh-config. For instance, you might name this file /etc/NetworkManager/dispatcher.d/99-launch-connection-specific-ssh-config
Inside your launcher script, put the following contents:
```
 #!/bin/bash

 cd /path/to/connection-specific-ssh-config && pipenv run python main.py --config /path/to/your/config.yaml --interface "$1" --command "$2"
```
- This will take the two important variables which NetworkManager has passed along to your launcher script, and give them to connection-specific-ssh-config
Repeat steps 1-3 for each additional user who would like connection based ssh configuration files.

Example Scenarios

Why would you use this script? Suppose the following scenarios:

You have various servers you connect to when at work, which have long server names. You might put host aliases inside your laptop's SSH configuration file (at ~/.ssh/config) to speed up your ssh commands. But when you come home, you don't want those shortcuts anymore. Thus, you need one ssh configuration file for work, and another for home
One of your work locations has very slow internet, so whenever you're there, you need to change several SSH configuration options (such as ServerAliveInterval, and others), so you can work more efficiently there.
You have several servers which you can connect directly to from work. Your work has no VPN, so when at home, these servers aren't exactly online; You must first connect to a gateway server on your employer's network, then use that gateway server as a proxy to reach the other internal servers. This is generally called Transparent Multihop over SSH (example). Thus, it would really be convenient to have one ssh configuration file for home, and one for work.

Each of these scenarios can be solved by the connection-specific-ssh-config script. Simply create as many ssh configuration files as you need, and each can be dynamically symlinked as you connec to different networks.

Configuration File

Configuration files for connection-specific-ssh-config are written in YAML and are relatively simple:

You need a section called options, which holds two variables:
1. ssh-dir, which specifies your ssh configuration directory (typically ~/.ssh/)
2. default-target, which specifies the default target to use if the currently connected wifi access point does not match anything configured.
You then need one or more sections which specify targets that associate ssh configuration files with network connection names:
- The key adapter can specify one adapter to watch
- The key adapters can specify multiple adapters, as a list
- The key ssid can specify one connection name
- The key ssids can specify multiple connection names, as a list
- The key config-file-name specifies the name of the ssh configuration file that should apply if this target activates

Here's an example configuration file to help you get started:

options:

  default-target: remote
  ssh-dir: /home/your-username/.ssh

targets:

  home:
    adapters:
      - wlo1
    ssids:
      - Some Wifi AP Name
      - Some Other Wifi AP Name
    config-file-name: config-home

  remote:
    adapters:
      - wlo1
    config-file-name: config-remote

The above configuration file will instruct the script to use the ssh config file /home/your-username/.ssh/config-home while you're connected to one of the access points named in the ssids list of the home target. Otherwise, the default config file /home/your-username/.ssh/config-remote would be used. All of this would only apply to the adapter wlo1, but you could specify more adapters.

The script works by simply creating a symlink where your original ssh configuration file was (typically ~/.ssh/config), pointing to the ssh configuration determined to be the one you want active. Note that this of course means the script will fail (for safety reasons), if your original ssh configuration file still exists as a normal file.