Pi-Hole DHCP-Server: Offer static ip-routes

Background

My WIFI is bound to my Internet-Router and it is seperated from my home-office by an routed stateful-firewall. Some WIFI devices (for example my printer) need access to this home-office ip-range.

Pi-Hole DHCP Service

The DHCP-Service of my pi-hole server usually offers just dhcp-option #3 – the default-gateway.

Disable DHCP-Option 3 (Default Gatway)

The pi-hole Web-GUI doesn’t allow to specify additional dhcp-options, so first disable offering the default-gateway-option:

pi-hole disable dhcp-server option 3 „Router (gateway) IP address“

which is not allowed (my opinion: a bug since this is no feature)

Stop here and use another dhcp-service?

Use the „Router-Option“ as fallback for „old“ clients

Fortunately RFC3442 („The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4“) https://datatracker.ietf.org/doc/html/rfc3442 states:

If the DHCP server returns both a Classless Static Routes option and
   a Router option, the DHCP client MUST ignore the Router option.

So this can stay enabled, it’ll be ignored – and, just for the case an old DHCP-Client doesn’t implement the Classless Static-Routes Option it’ll ignore it and can use the Default-Gateway-option.

Implementation

  1. leave the Web-GUI as it is
  2. go the the SSH-bash-shell
  3. add additional dhcp-options

Leave the auto-generated config-files as they are

adminname@pi-hole-server:/etc/dnsmasq.d $ ls -l
total 16
-rw-r--r-- 1 root root 1524 May 22 13:16 01-pihole.conf
-rw-r--r-- 1 root root  490 May 22 13:16 02-pihole-dhcp.conf
-rw-r--r-- 1 root root  143 Dec  8 14:04 04-pihole-static-dhcp.conf

adminname@pi-hole-server:cat 01-pihole.conf
# Pi-hole: A black hole for Internet advertisements
# (c) 2017 Pi-hole, LLC (https://pi-hole.net)
# Network-wide ad blocking via your own hardware.
#
# Dnsmasq config for Pi-hole's FTLDNS
#
# This file is copyright under the latest version of the EUPL.
# Please see LICENSE file for your rights under this license.

###############################################################################
#      FILE AUTOMATICALLY POPULATED BY PI-HOLE INSTALL/UPDATE PROCEDURE.      #
# ANY CHANGES MADE TO THIS FILE AFTER INSTALL WILL BE LOST ON THE NEXT UPDATE #
#                                                                             #
#        IF YOU WISH TO CHANGE THE UPSTREAM SERVERS, CHANGE THEM IN:          #
#                      /etc/pihole/setupVars.conf                             #
#                                                                             #
#        ANY OTHER CHANGES SHOULD BE MADE IN A SEPARATE CONFIG FILE           #
#                    WITHIN /etc/dnsmasq.d/yourname.conf                      #
###############################################################################

addn-hosts=/etc/pihole/local.list
...

add your config in a seperated config-file

So lets add „/etc/dnsmasq.d/yourname.conf“:

I’d like to create

  1. a Default-Route (0.0.0.0/0) via the WIFI-Router 172.16.1.1
  2. static routes for all private (RFC1918) address-ranges via the Firewall 172.16.1.2
sudo echo "
dhcp-option=option:classless-static-route,0.0.0.0/0,172.16.1.1,10.0.0.0/8,172.16.1.2,172.16.0.0/12,172.16.1.2,192.168.0.0/16,172.16.1.2" > 99-user-settings.conf

restart the DHCP-Service

sudo service pihole-FTL reload

refresh the DHCP-Client

Horrible – Windows 10 states a „default-gateway“ to be set ūüôĀ

C:\Users\user>ipconfig /renew "WLAN"

Windows-IP-Konfiguration

Drahtlos-LAN-Adapter WLAN:
   Verbindungsspezifisches DNS-Suffix: prod.local
   IPv4-Adresse  . . . . . . . . . . : 172.16.1.14
   Subnetzmaske  . . . . . . . . . . : 255.255.255.0
   Standardgateway . . . . . . . . . : 172.16.1.1

Check results

But the ip routing-table is fine: even Windows 10 implements RFC3442:

C:\Users\user>route print
===========================================================================
IPv4-Routentabelle
===========================================================================
Aktive Routen:
     Netzwerkziel    Netzwerkmaske          Gateway    Schnittstelle Metrik
          0.0.0.0          0.0.0.0       172.16.1.1     172.16.1.14     35
         10.0.0.0        255.0.0.0       172.16.1.2     172.16.1.14     36
        127.0.0.0        255.0.0.0   Auf Verbindung       127.0.0.1    331
        127.0.0.1  255.255.255.255   Auf Verbindung       127.0.0.1    331
       172.16.0.0      255.240.0.0       172.16.1.2     172.16.1.14     36
      192.168.0.0      255.255.0.0       172.16.1.2     172.16.1.14     36
       172.16.1.0    255.255.255.0   Auf Verbindung     172.16.1.14    291
  255.255.255.255  255.255.255.255   Auf Verbindung     172.16.1.14    291
===========================================================================

Android?

Forget it, Google seems to not priotize fixing basic low-level IP-Stack issues, it’s embarrassing: https://issuetracker.google.com/issues/36920068