dynamic preseed file for ubuntu using sinatra

To build ubuntu physical ubuntu servers we use ubuntu preseed.

This works great but if you use a static preseed file you end up building a host that doesn’t have its hostname or static ip address set. This means that you have to manually set it afterward and we decided to automate it.

BTW it took us a while to figure out how to set a static ip in a preseed file.We blogged about it here:network-preseeding-debianubuntu-with-a-static

To do this we wrote a small sinatra app that dynamically generates the preseed file with the hostname and static ip address.

This is done by looking up the mac address of the requested host from the arp table and comparing it to a pipe delimited file that contains the mac address, what the static ip should be and its hostname.

The list is stored in a file named ip2mac.txt and was populated by a script.

The ip2mac.txt file looks like this:

172.28.0.71|a4:ba:db:35:e6:09|chi-devops11a  172.28.0.72|78:2b:cb:03:c5:44|chi-devops11b

Instead of calling a static preseed file from the pxelinux.cfg/default file we instead make a request to the sinatra app which generates it dynamically.The line in the default file we use looks like this:

append console=tty0 console=ttyS1,115200n8 initrd=ubuntu-10.04-server-amd64-  initrd.gz auto=true priority=critical preseed/url=http://172.27.0.115:4567/lucid-preseed-noraid interface=eth0 netcfg/dhcp_timeout=60 console-setup/ask_detect=false console-setup/layoutcode=us console-keymaps-at/keymap=us locale=en_US --

When the request is made the sinatra app does the following:

*  1. looks up the mac address of the request from the apr table*  2. compares the mac address to the matching line in ip2mac.txt*  3. uses the ip and hostname to populate hostname and ip variables in the preseed file*  4. returns the preseed file to the host making the request

The code:

require 'rubygems' # skip this line in Ruby 1.9  require 'sinatra'  require "erb"  require 'logger'  def log(message)    flog = Logger.new('foo.log')    flog.info(message)  end  def lookup_mac(mac)    rr = Array.new    hostfile = File.open("ip2mac.txt","r")    hostfile.readline    hostfile.each do |line|      list_ip,list_mac,name = line.split('|')      if mac.match(list_mac)    rr.push(list_ip)      end    end  return rr[0]  end  def get_mac_address()    ip =  @env['REMOTE_ADDR']    cmd = "arp -n " + ip.chomp + " | grep -v Address | awk '{print $3}'"    mac  = `#{cmd}`   return mac  end  def rev_lookup(ip)    cmd = "host " + ip + " | awk '{print $5}'"    hostname = `#{cmd}`fqdn = hostname.chop.chopreturn fqdn  end  get '/lucid-preseed-noraid' do    mac = get_mac_address()    log(mac)    ips = lookup_mac(mac)    log(ips)    fqdns = rev_lookup(ips)    @ip = ips    @fqdn = fqdns    log(fqdns)    erb :lucid_preseed_noraid  end  get '/lucid-preseed-nosrv' do    mac = get_mac_address()    log(mac)    ips = lookup_mac(mac)    log(ips)    fqdns = rev_lookup(ips)    @ip = ips    @fqdn = fqdns    log(fqdns)    erb :lucid_preseed_nosrv  end  get '/' do    "ops11"  end

To start the sinatra app just run the following:

ruby preseeder.rb
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