np422 / add-host-rb

Small utility to add hosts from dns to local /etc/hosts file

add-host

Add-host is a small utility to fetch ip information from a remote DNS server and add it to /etc/hosts and the ssh host-keys to ~/.ssh/known_hosts if the host responds to ssh.

Sometimes I connect with vpn to more than one remote site at the same time. I find it to be less error prone to use the hosts file to resolve names compared to configure multiple dns-servers with different opinions if a dns-query should give an ip-address or NXDOMAIN as response to the query.

And it doesn't take very long time before the manual task of looking up ip-addresses and adding them to /etc/hosts gets boring.

So I automated the process of finding the ip-address and adding an entry to the hosts file, and as an added bonus the script it will get the ssh host-keys for the host added and update ~/.ssh/known_hosts so already the first connectioin with ssh can be done without any interactive yes responses.

USAGE

  add-host [-v|--verbose] [-a|--alias=ALIAS] -d|--domain=DOMAIN -s|--dns-server=DNS hostname_to_add

The add-host command will resolve the ip-address of hostname_to_add by querying the dns-server specified with the option DNS.

Any existing entry in /etc/hosts for the hostname_to_add will be removed and a new entry will be written to /etc/hosts for hostname_to_add

A copy of the original /etc/hosts will be saved as /tmp/hosts-timestamp before any changes are made to the /etc/hosts file.

If the hostname_to_add is connectable over ssh, any previously used ssh host keys for the hostname/ip will be removed from ~/.ssh/known_hosts and ssh host keys for host_to_add added to ~/.ssh/known_hosts.

The environment variables AHDOMAIN, AHDNS will be used if --dns-server and --domain options aren't given. If you frequently use add-host you probably want to set the environment variables in your .bash_profile or similar.

hostname_to_add should be short-form without a dommainname. The user running add-host needs sudo privlileges or have write-permissions on /etc/hosts.

You can also add an ALIAS for hostname_to_add with the -a/--alias switch.

EXAMPLES

  $ add-host -d some.domain.com -s 10.20.20.11 prd-bapp-v021

  $ export AHDOMAIN=other.domain.org
  $ export ANDNS=172.16.14.0
  $ add-host -a ora1 oradb-vl001-ops-a
  $ add-host -a ora2 oradb-vl002-ops-b

OPTIONS

    -h, --help                       Display usage information
    -v, --verbose                    Print more output
    -d, --domain=DOMAIN              Use DOMAIN to resolve
    -s, --dns-server=DNS             Resolv host at DNS dns-server
    -a, --alias=ALIAS                Add an ALIAS for hostname_to_add

Extending add-host with custom ruby classes

This part of add-host was implemented mostly because you can, and because I thought it was fun. I doubt that any sane person actually will attempt to make serious use this functionality.

The files in the directory ~/.addhost_extensions.d/ that ends with .rb will be read using load and an include AddHostExtension is attempted.

After /etc/hosts and known_hosts have been updated, a new extension object is created with the following constructor call, and hence is what should be implemented by an extension.

  extobj = AddHostExtension.extension_class.new(ip:        ip,
                                                hostname:  hostname,
                                                hostalias: hostalias,
                                                domain:    domain,
                                                ssh:       ssh)

In the argument-list hostname/hostalias will be in short form, the AHDOMAIN is provided in domain parameter and ssh is set to true if the host added was reachable over ssh, ssh-keys were found and added to known_hosts

Then a call is made to the method extobj.do_whatever, where an extension can do pretty much whatever it wants to do with the information about the newly added host it received previously toghether with .new( .. )

I've included the extension that I actually use in the repo as an example. I didn't think the stuff i put in zsh_ssh_function_adder.rb was generic enough to include in the add-host command, as it's very specific to my personal use-case.

A minimal addhost extension my_cool_extension_class.rb is also included and can be used as starting point if you should wish to try to write an extension yourself. That file is also included inline below:

module AddHostExtension
  attr_reader :extension_class


  class MyCoolExtensionClass
    def initialize(ip:, hostname:, hostalias:, domain:, ssh:)
      @ip        = ip
      @hostname  = hostname
      @hostalias = hostalias
      @domain    = domain
      @ssh       = ssh
    end

    def do_whatever
      debug_obj_vars
    end

    private
    def debug_obj_vars
      puts @ip
      puts @hostname
      puts @hostalias
      puts @domain
      puts @ssh
    end
  end
  @extension_class = MyCoolExtensionClass
end