Copyright © 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”.
2008/07/14
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.
See Shorewall FAQ 2.
Setting up Split DNS is extremely simple:
Be sure that your firewall/router can resolve external DNS names.
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.
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-localneta
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
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:~$