Multiple SSH Client Configuration

Having many hosts managed through your ${HOME}/.ssh/config file can quickly become a wildly beast of a configuration file to maintain with each host containing a few SSH client options.

Host                    prod
    HostName                mertcangokgoz.com
    User                    mertcan
    Compression             yes
    ForwardAgent            no
    IdentityFile            ~/.ssh/production
    ForwardX11              no

OpenSSH won’t allow you to split out the configuration in multiple files, such as ~/.ssh/config.dev, ~/.ssh/config.production and ~/.ssh/config.test and then include them within your main SSH configuration file ~/.ssh/config with a directive such as Include like Apache configuration allows you.

With a bash alias and a simple ssh client wrapper script, you can create a work around that will allow you to split up SSH client configuration into several files.

First, create the SSH client wrapper. Drop this file into ~/bin/ssh and ensure it is executable.

#!/bin/bash

ssh() {
    TMPDIR=~/tmp
    case "$(uname -s)" in
        Linux)
            tmp_fifo=$(mktemp -u --suffix=._ssh_fifo)
            ;;
        Darwin)
            tmp_fifo=$(mktemp -u -t ._ssh_fifo)
            ;;
        *)
            echo 'unsupported OS'
            exit
            ;;
    esac

    rm ~/tmp/._ssh_fifo* 2>/dev/null

    mkfifo "$tmp_fifo" 
    cat ~/.ssh/config ~/.ssh/config.* >"$tmp_fifo" 2>/dev/null & 
    /usr/bin/ssh -F "$tmp_fifo" "$@"
    rm "$tmp_fifo"
}

ssh $*

The SSH client wrapper creates a FIFO file consisting of all my ~/.ssh/config* files concatenated. The wrapper then calls the proper SSH client binary passing the FIFO file as the alternative configuration file.

So my shell is forced to call this wrapper, I create a simple bash alias in ~/.bash_profile

oalias ssh='~/bin/ssh'

Now everytime ssh is called, all configuration files in ~/.ssh/config* are available allowing me to split out my SSH hosts into their own separate file. Can you have more than one ~/.ssh/config file?