provisioner: allow tftp access from admin network only (bsc#1019111)

The tftp server is for hosts on the admin network only. But it can be
accessed from outside if the admin network is routable. This patch adds
an iptables rule to prevent access from outside the admin network.

(cherry picked from commit a699d6c)

Author: djoreilly

chef/cookbooks/provisioner/recipes/setup_base_images.rb

@@ -304,7 +304,8 @@
       owner "root"
       group "root"
       mode "0644"
-      variables(tftproot: tftproot, admin_ip: admin_ip)
+      variables(tftproot: tftproot, admin_ip: admin_ip,
+                admin_subnet: admin_net.subnet, admin_netmask: admin_net.netmask)
     end
 
     service "tftp.service" do

chef/cookbooks/provisioner/templates/default/tftp.service.erb

@@ -3,5 +3,7 @@ Description=Tftp Server
 
 [Service]
 Type=simple
+ExecStartPre=/usr/sbin/iptables -A INPUT -d <%= @admin_ip %> ! -s <%= @admin_subnet %>/<%= @admin_netmask %> -p udp -m udp --dport 69 -j DROP
 ExecStart=/usr/sbin/in.tftpd -u tftp -s <%= @tftproot %> -m /etc/tftpd.conf -L -a <%= @admin_ip %> -B 1024 -v
+ExecStopPost=/usr/sbin/iptables -D INPUT -d <%= @admin_ip %> ! -s <%= @admin_subnet %>/<%= @admin_netmask %> -p udp -m udp --dport 69 -j DROP
 Restart=on-failure