IP Register DHCP service

We run a DHCP service with a configuration that is partly generated from the IP Register database.

If you would like to make use of this service for a new subnet, please contact ip-register@uis.cam.ac.uk with the following details:

  • The subnet(s) that want DHCP support
  • The size of any dynamic pools you want in each subnet, or alternatively, how much space should be set aside for static registrations
  • Any special DHCP options you might require, such as different DNS resolver addresses, or PXE boot support
  • The domain name to use for the place-holder pool address registrations
  • The physical location of the subnet

Dynamic pools should normally be on CUDN-private subnets, unless there is a clear reason for using public addresses (such as external ACLs).

If you would like to convert an existing subnet to DHCP then we need the same information listed above, but there is a caveat: it will only be possible to provide a dynamic pool if there is a large enough block of addresses left clear of registrations.

Subnet ranges

When we set up a subnet for DHCP, we divide it into separate ranges:

  • A dynamic pool, exclusively for unregistered devices;

  • Space for static registrations of devices with fixed addresses.

Devices with their own entries in the IP Register database must use addresss from the static range. Devices can be set up in two ways:

  • Static address configuration without DHCP, where the device is manually configured to match the IP Register database;

  • Fixed address reservations for devices that use DHCP and configure themselves to match the IP Register database.

The DHCP servers only hand out addresses in the static range to devices with fixed address reservations.

Dynamic pool sizes

The capacity of a dynamic pool is half the number of addresses in the dynamic range.

We have two DHCP servers which have independent lease databases. Dynamic DHCP pools are divided in two, with half managed by each DHCP server. This allows us to cope with DHCP server failure without any complicated machinery.

Dynamic pool address registrations

When we set up a pool, we register all the dynamic addresses with names like dhcp-NN-NN-NN-NN.botolph.private.cam.ac.uk.

These pool names avoid accidental registration of a dynamic address for other purposes. So far we have not used restrictions in the IP Register database to prevent re-registering pool addresses, but this is likely to change.

Fixed reservations with IP Register

In subnets that have been set up with DHCP support, fixed DHCP reservations are automatically configured using a feed from the IP Register database.

These fixed address reservations are configured using the mac and dhcp_group fields on the single_ops form, which correspond to columns in the v4_address table.

The mac field accepts MAC addresses in any of the usual formats. They must be unique within any particular mzone/lan combination.

The dhcp_group allows you to choose different sets of DHCP options for different hosts, for example, different DNS servers or PXE boot options. You can ask us to set up groups in the DHCP server configuration with the options you require. The groups available in each subnet are listed in the dhcp_group table in the database.

Client devices in the default or blank group gets the default DHCP options for the subnet.

Lease times

The default lease time is 24 hours (max 48 hours, min 16 hours). This can be changed for special-case subnets if very short leases are required to cope with a high turnover of devices.

DNS resolver addresses

The IP Register database has a lan_ns4 table listing the default resolver IPv4 addresses for each lan; this table is not connected to the DHCP server configuration, and only provides informal advice about manual configuration settings. However it is wise to ensure it matches the DHCP server configuration.

Update times

DHCP updates happen hourly at 53 minutes past the hour, and they take a couple of minutes to complete