noteId	tags
4675876023f211efb378f1bbbb549ae0

ssh-dynconf

Functions for dynamically generating / modifying ssh config files and availability checks ssh hosts

Core functions

Check ssh hosts' availability

ssh_host_available

ssh_host_available   
   Check whether a host is available via ssh

   Arguments:      
      --host     <str> 
         The ssh host to check
         Default: localhost
      --port     <num> 
         The ssh port to check
         Default: 12345
      --timeout  <num> 
         The timout in seconds
         Default: 2

   Usage:      
      ssh_host_available \
         --host     "localhost" \
         --port     "12345" \
         --timeout  "2"

ssh_available_hosts

ssh_available_hosts   
   Check whether a host is available via ssh

   Arguments:      
      --hosts    <str> 
         The ssh host to check
         Default: localhost
      --port     <num> 
         The ssh port to check
         Default: 12345
      --timeout  <num> 
         The timout in seconds
         Default: 2

   Usage:      
      ssh_available_hosts \
         --hosts    "localhost" \
         --port     "12345" \
         --timeout  "2"

Modify ssh config file(s)

ssh_config_update

ssh_config_update   
   Add / replace / delete a ssh config keyword.
   See `man ssh_config` for possible keywords.

   Arguments:      
      --action   <str> 
         The action to perform.
         Either add|replace|delete a ssh config keyword.
         Default: replace
      --file     <str> 
         The ssh config file
         Default: ~/.ssh/example
      --keyword  <str> 
         The ssh config keyword to replace.
         Default: Host
      --value    <str> 
         The value to set for `keyword`
         Default: localhost

   Usage:      
      ssh_config_update \
         --action   "replace" \
         --file     "~/.ssh/example" \
         --keyword  "Host" \
         --value    "localhost"

Examples

The following basic examples how to use ssh-dynconf are located in the Examples/ folder:

Connecting to a host referred to as examplehost on port 50212:

ssh -F ssh_config examplehost -p 50212 

using a dynamic configuration can be achieved using a ssh_config that contains

Match originalhost examplehost exec "./make-examplehost-config.sh %p"
	Include examplehost-config

If the entered host matches 'examplehost', a script make-examplehost-config.sh is executed, e.g. containing

#!/usr/bin/env bash

source ssh-dynconf-init

# Get port from the ssh command
ssh_port="${1}"

example_config_file="examplehost-config"

# List of addresses to check
addresses=(
    "127.0.0.1"
    "some-host-name"
    "some-other-host-name"
)

# Determine the hosts where a ssh server is available
available_hosts=(
    $(
        ssh_available_hosts \
            --hosts ${addresses[@]} \
            --port "${ssh_port}" \
            --timeout 2
    )
)

if [ "${#available_hosts[@]}" -eq 0 ]; then
    echo "Host not found!">&2
    exit 1
fi

# Use the first available host
hostname="$(
    ssh_host_name \
        --host "${available_hosts[0]}"
)"
# Username for the ssh connection
username="root"

# Set hostname in example_config_file
ssh_config_update \
    --action add \
    --keyword hostname \
    --value "${hostname}" \
    --file "${example_config_file}"

# Set port in example_config_file
ssh_config_update \
    --action add \
    --keyword port \
    --value "${ssh_port}" \
    --file "${example_config_file}"

# Set username in example_config_file
ssh_config_update \
    --action add \
    --keyword user \
    --value "${username}" \
    --file "${example_config_file}"

# Set identityfile in example_config_file
ssh_config_update \
    --action add \
    --keyword IdentityFile \
    --value "\${PWD}/example_id_rsa" \
    --file "${example_config_file}"

# Copy example_config_file to ~/.ssh
# (OpenSSH does not allow variable expansion or paths relative to ./ in include statements)
# see https://man7.org/linux/man-pages/man5/ssh_config.5.html
cp "${example_config_file}" "${HOME}/.ssh/"

exit 0

to create/modify a configuration file examplehost-config with a conditional hostname for examplehost.
This examplehost-config is then included in the as configuration in the original ssh_config.