Introduction

Working for an ISP with Linux servers it became apparent that the built in redundancy in many key services was either inadequate or non-existent. Of particular concern was redundancy in proxy servers. As bandwidth in Australia is relatively expensive mandatory proxies for HTTP are imposed by many ISPs. Manual proxies and the issuing of automatic proxy configuration files are particularly lacking in redundancy. To make this redundant a method of backing up HTTP and proxy servers was investigated. What was required was a generic method for a backup server to take over the role of a lame server.

The idea initially proposed was to update DNS records as required. This would change the IP address of the lame server to that of the backup server. This was found to be unsatisfactory on the following counts;

1.

The time to live on the zone files would need to be turned down severely to account for any users using DNS servers other than the master or secondary that can easily be reset for the zone in which the servers lie.

2.

Users may access servers using an IP address rather than a host name.

3.

Users may use non-DNS methods such as an /etc/hosts file to map server host names to IP addresses.

After some investigation it was found that a solution where the backup server would assume the IP address of the lame server would be ideal. This eliminated the difficulties related to the DNS based solution. The only remaining difficulty was to convince other boxen on the LAN of the change in circumstance and this is where ARP Spoofing came into the game [YV].

ARP spoofing is a method often employed by hackers to assume the identity of a host on a LAN. For this application ARP spoofing allows the backup server to take of the IP address of the lame server.