Automatic IP allocation


DHCP (Dynamic Host Configuration Protocol) is a system that allows IP addresses to be allocated on an network automatically.

The FireBrick® can issue addresses on a network as a DHCP Server, and can receive its IP address automatically as a DHCP client.

DHCP server

The FireBrick® will allocate up to 256 addresses on LAN or WAN, and the diagnostics page will show the addresses that have been allocated along with the machine name if known. Whilst each DHCP lease is only 2 hours long, the FireBrick® will keep track of old leases to ensure machines always get the same address even if it has been a long time since they were last on your network. This allows a semi-permanent allocation of IP addresses. Only if addresses run out will old ones be re-used.

To be a DHCP server simply requires that the subnet configuration has a range of IP addresses to allocate (and is not stealth). The addresses allocated are in the range specified where they are also valid on the subnet you have configured and obviously avoiding the FireBrick®s own address.

You can set up several identical subnets with different ranges of addresses to allocate for DHCP if required - this will allow several different ranges to be given out - perhaps avoiding addresses used for other purposes. If the requesting machine has a name that matches a restricted subnet, then it will only be allocated an IP from one of the restricted subnets for which it's name matches. If the requesting machine does not match a restricted subnet, then it can only be allocated from unrestricted subnets.

The FireBrick® automatically avoids giving addresses where there appears to be another machine already using the address, but careful planning should avoid this anyway.

You can also set Backup DHCP which means that the server will not answer the first request from a machine - allowing another DHCP the chance to answer first.

Normally the DHCP server provides DNS, Gateway, Domain, time server, and syslog server - however these can individually be excluded if required in the subnet configuration.

Note that if the time is not set, the the server will not track the lease times, but still allocates two hour leases.

DHCP client

Being a DHCP client means that the IP address, subnet mask, gateway address and various other settings for a computer are set automatically. On the FireBrick® this allows the IP, subnet mask, gateway, DNS server, Time server, Syslog server, and Domain to be set. Some of these can however be excluded allowing them to be set manually.

To be a DHCP client you should configure a subnet with and mark it as DHCP client. Going back to the subnet later, or looking at DNS server, etc, will show the current values. The diagnostic page also shows the DHCP server details.

Options

Backup DHCP As a DHCP server, the FireBrick® does not answer the first query from a host, allowing another DHCP server to answer first.
Don't check As a DHCP client, the FireBrick® does not check the address it is given is valid - needed on some cable modems
DHCP Mirror As a DHCP server, this subnet is configured based on the other interface as a DHCP client.
This makes the FireBrick® have the far side router address, and allocating a DHCP address if was given on the far side
DHCP Restrict As a DHCP server, this subnet is only used when the client machine name starts with the subnet name
Broadcast renewal As a DHCP client, when renewing addresses the request is broadcast rather than sent to the previous server