# Simple way to set up Split DNS

Tom
Eastep
2008
Thomas M. Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License.

## What is Split DNS

Split DNS is simply a configuration in which the IP address to which a DNS name resolves is dependent on the location of the client. It is most often used in a NAT environment to insure that local clients resolve the DNS names of local servers to their RFC 1918 addresses while external clients resolve the same server names to their public counterparts.

## Why would I want to use Split DNS?

See Shorewall FAQ 2.

## Setting up Split DNS

Setting up Split DNS is extremely simple:

1. Be sure that your firewall/router can resolve external DNS names.

2. Install the dnsmasq package (http://www.thekelleys.org.uk/dnsmasq/doc.html) and arrange for it to start at boot time. There are many dnsmasq HOWTOs on the Internet.

3. Add your local hosts to `/etc/hosts` on the firewall/router using their local RFC 1918 addresses. Here's an example:

       #
       # hosts         This file describes a number of hostname-to-address
       #               mappings for the TCP/IP subsystem.  It is mostly
       #               used at boot time, when no name servers are running.
       #               On small systems, this file can be used instead of a
       #               "named" name server.
       # Syntax:
       #
       # IP-Address  Full-Qualified-Hostname  Short-Hostname
       #

       127.0.0.1       localhost

       # special IPv6 addresses
       ::1             localhost ipv6-localhost ipv6-loopback

       fe00::0         ipv6-localnet

       ff00::0         ipv6-mcastprefix
       ff02::1         ipv6-allnodes
       ff02::2         ipv6-allrouters
       ff02::3         ipv6-allhosts
       127.0.0.2       ursa.shorewall.net ursa
       172.20.1.1      linksys.shorewall.net     linksys
       192.168.0.1     opensuse.shorewall.net    opensuse
       192.168.0.2     debian.shorewall.net      debian
       192.168.0.3     ubuntu.shorewall.net      ubuntu
       192.168.0.4     fedora.shoreawll.net      fedora
       192.168.0.5     opensuse11.shorewall.net  opensuse11
       192.168.0.6     centos.shorewall.net      centos
       192.168.0.7     debian32.shorewall.net    debian32
       192.168.0.8     fedora9.shorewall.net     fedora9
       206.124.146.254 blarg.shorewall.net       blarg

4. Configure your local network hosts to use the firewall/router as their DNS server. If your local hosts are configured using DHCP, that is a simple one-line change to the DHCP configuration.

And that's it! Your local clients will resolve those names in the firewall/router's `/etc/hosts` file as defined in that file. All other names will be resolved using the firewall/router's Name Server as defined in `/etc/resolv.conf`.

Example:

From an Internet Host:

    gateway:~ # host linksys.shorewall.net
    linksys.shorewall.net has address 206.124.146.180
    gateway:~ #

From ubuntu (192.168.0.3):

    teastep@ubuntu:~$ host linksys
    linksys.shorewall.net has address 172.20.1.1
    teastep@ubuntu:~$