Copyright © 2008 Thomas M. Eastep
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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
# # 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 188.8.131.52 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
From an Internet Host:
gateway:~ # host linksys.shorewall.net linksys.shorewall.net has address 184.108.40.206 gateway:~ #
From ubuntu (192.168.0.3):
teastep@ubuntu:~$ host linksys linksys.shorewall.net has address 172.20.1.1 teastep@ubuntu:~$