Add support for ISC DHCP Relay

You can enable installation of DHCP relay instead of the server via
'dhcpd_mode' variable. By default, relays send the packets to their
network gateway, but that might not be what you really want; however
selection of correct DHCP server can be performed by specifying it in
'dhcpd_relay_servers' list.

defaults/main.yml

@@ -1,5 +1,28 @@
 ---
 
+# What service type to configure on this host:
+#   'server' - host is an ISC DHCP server, see dhcpd(8)
+#   'relay'  - host is an ISC DHCP relay, see dhcrelay(8)
+dhcpd_mode: 'relay'
+
+# What packages should be installed, depending on mode of operation
+dhcpd_base_packages_map:
+  'server': [ 'isc-dhcp-server' ]
+  'relay':  [ 'isc-dhcp-relay' ]
+
+
+# ---- ISC DHCP Relay configuration ----
+
+# List of DHCP servers which should receive the relayed packets
+dhcpd_relay_servers: [ '{{ ansible_default_ipv4.gateway }}' ]
+
+# List of network interfaces that dhcrelay should listen on
+dhcpd_relay_interfaces: []
+
+# Additional dhcrelay options
+dhcpd_relay_options: '-4'
+
+
 # ---- Global ISC DHCP Server configuration ----
 
 # Is this DHCP server authoritative?

handlers/main.yml

@@ -5,3 +5,8 @@
     name: 'isc-dhcp-server'
     state: 'restarted'
 
+- name: Restart isc-dhcp-relay
+  service:
+    name: 'isc-dhcp-relay'
+    state: 'restarted'
+

tasks/main.yml

@@ -1,11 +1,29 @@
 ---
 
+- name: Configure DHCP relay in debconf
+  debconf:
+    name: 'isc-dhcp-relay'
+    question: 'isc-dhcp-relay/{{ item.key }}'
+    vtype: 'string'
+    value: '{{ item.value }}'
+  with_dict:
+    servers:    '{{ dhcpd_relay_servers | join(" ") }}'
+    interfaces: '{{ dhcpd_relay_interfaces | join(" ") }}'
+    options:    '{{ dhcpd_relay_options }}'
+  register: dhcpd_register_relay_debconf
+  when: dhcpd_mode == 'relay'
+
 - name: Install DHCP packages
   apt:
     name: '{{ item }}'
     state: 'present'
     install_recommends: False
-  with_items: [ 'isc-dhcp-server' ]
+  with_items: dhcpd_base_packages_map[dhcpd_mode]
+
+- name: Reconfigure ISC DHCP relay
+  command: dpkg-reconfigure --frontend=noninteractive isc-dhcp-relay
+  notify: [ 'Restart isc-dhcp-relay' ]
+  when: dhcpd_register_relay_debconf|d() and dhcpd_register_relay_debconf.changed
 
 - name: Configure DHCP server
   template:
@@ -16,4 +34,5 @@
     mode: '0644'
   with_items: [ 'etc/default/isc-dhcp-server', 'etc/dhcp/dhcpd.conf' ]
   notify: [ 'Restart isc-dhcp-server' ]
+  when: dhcpd_mode == 'server'