DHCP lease issues with broadband connections

For many organizations the broadband connection to their ISP is controlled by a single network node or NIC that must get a DHCP lease in order to connect to the Internet properly. DHCP leases for broadband connections are set to variable lengths of time after which the lease is released and must be renewed. A typical lease length is from one to seven days, and when assignments are made or changed these changes are propagated to the DNS server. A lease is assigned to a NIC based on the MAC address.

Some ISPs set a pool of leases for a longer time period, and another pool for a shorter time period. If you have a connected firewall and the end of your longer lease is reached you may find that the firewall may not "grab" one of the longer leases on its first renew, or even that the renew command doesn't renew successfully at all. The DHCP address is "stuck" in the firewall and requires a full system restart to properly renew.

It may take several attempts in a situation like this to successfully grab a new lease. When dealing with Microsoft ISA Server in this situation you may also find that IPCONFIG /RELEASE followed by IPCONFIG /RENEW also doesn't work effectively. You are then forced to enable and disable the NIC in ISA Server in the Network Connections folder in order to get another lease, or to restart the server.

This situation arises in many cable modem setups. I suspect it is because the original network design for some broadbands assumed

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that the computer connected to the network would be turned off every night. With 24X7 connections people routed their connection to a device or server that could serve multiple systems. There are good security reasons for having to have a system renew leases on a frequent basis. One solution is to pick a time at night when there is little network traffic and turn off your connected server for a couple of hours. You can also write a script that does a disable/enable of your firewall's NIC.

For a nice short refresher on DHCP go to: http://www.comptechdoc.org/independent/networking/guide/netdhcp.html.

Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield, MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.

This was first published in November 2004

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