DNS Conditional Forwarding – dnsmasq

December 19th, 2010, 12:00 pm

Why would I want to use Conditional Forwarding?

In my case, my local dns server has entries for local hostnames such as m2n.ion.lan, mongo.ion.lan, and tux.ion.lan. If I am using the vpn dns, then these address lookups would fail. By using Conditional Forwarding I can do all lookups locally, except for ones that match the remote top level domain (example.local). Anything that matches example.local would be forwarded to the remote dns server.

Problem:

  1. Connect to remote vpn server and use local DNS server
  2. Ping server.remote.local (remote FQDN) – fail
  3. Ping server.ion.lan (local FQDN) – success

Of course the remote ping fails because the local DNS server knows nothing about the remote domain. If I was to configure my machine to use the remote DNS server the opposite would happen. I would be able to ping server.remote.local, but a ping to server.ion.lan would fail.

Solution: Use dnsmasq with conditional forwarding to forward *.work.local requests to the remote dns server.

1. Install dnsmasq using your local package manager

2. Edit /etc/dnsmasq.conf

# Tells dnsmasq to forward anything with the domain of remote.local to dns server 10.25.11.2
server=/remote.local/10.25.11.2

# Listen to requests only coming from the local machine
listen-address=127.0.0.1

# Do not cache anything
# A decent dns server will already cache for your local network
cache-size=0

3. Edit /etc/resolv.conf

# Local LAN Domain
domain ion.lan

# local dnsmasq server
nameserver 127.0.0.1

# Your main dns server (dnsmasq will forward all requests to this server)
nameserver 10.20.1.1

4. Start dnsmasq

5. Test – ping a local server and remote server using the FQDN

All dns requests will be forwarded to 10.20.1.1 except any matching *.remote.local. server.remote.local will be forwarded to 10.25.11.2

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