DHCP in a LAN PARTY environment

[Tami]

DHCP in a LAN PARTY environment.
Author: Chris 'Nimby' Hendrie
Editor: Mary 'dirt' Richert


Most people who have home networks use DHCP. It works; it's easy; it's plug, play, and ignore. But what about using that same DHCP service in a larger setting? How can you handle 1,000 requests for IP addresses when the first wave of players start thier systems? And what about that dreaded half-life time on your DHCP release? Here are some ways to solve these problems.

Scaling DHCP for your event:
For most lans, all you do is hook your home router up to a port on the core switch, and have everyone grab an IP that way. That works fine for a 20-person LAN, but what about 200? The
flood of traffic would overwhelm a smaller unit like a home router. At this point, you need a dedicated DHCP server. The server can work with other services for your network, such as DNS and other Ni-provided services, and in fact, placing DHCP with DNS can be a hidden plus for your network management (We will cover this further in a future article). You'll need a minimum of a PIII 550mhz with 256MB RAM for a 150-person LAN, and I recommend a Linux or a BSD distro you're comfortable with.

The recommended DHCPD package is from the ISC . The current version, as of this writing, is 3.0.2rc3.

The dreaded renewal:
A big problem with DHCP is the half-life on the IP lease: the midway point when your user's system will start pinging back asking "Can I still keep this IP?" To limit the load on a network, and keep the DHCP server from being overwhelmed at the midway point, it's recommended that you set your DHCP lease time to TWICE the time of your event. So, if you're having a two-day event, make your DHCP lease time at least 4 days. That means that your gamers' systems won't be asking for a new IP untill they're already home and on thier local networks again.

Size matters:
The above is for small to mid-sized events, with one DHCP provider and a solid DHCPD server software solution. For larger events, though, you'll need two more things to make DHCP work properly.

First, redundancy, redundancy, redundancy. You can't have your only DHCP server die in the middle of the LAN, come back online, and then try to reassing all new IP addresses. For events of more than 500 people, having a secondary and separate DHCPD service is critical, as is sharing the dhcpd.leases file is critical as well -- having a secondary box that knows nothing about your network does you no good.

Second, you need a way to move the DHCP across the network easily. At Quakecon 2004, Linksys was kind enough to provide managed table switches that included a feature called DHCP Helper. Instead of 2,800+ users polling the primary DHCP provider, the switch polls for its
connected users and pulls down the needed IPs. Imagine the load difference on the server from 2,800+ polls to around 150. Switches with this capability can reduce the load on your core network gear and provide a more stable distribution of DHCP IP addresses.

With proper use of DHCP, you can take a major stress off the NOC staff at any sized lan. The basics in this article should give you a good introduction to proper use of DHCP.