Configure Common Reply Options

FreeRADIUS includes a powerful policy language called "unlang".

Statements in unlang may be used to call further policies, update attribute lists and invoke modules. There are also control flow statements (if, switch, etc.) typical of most imperative languages.

FreeRADIUS has a number attribute lists that it maintains as it processes packets within the virtual server sections. Most relevant to DHCP are request, control and reply.

The DHCP options from the current request packet are provided in the request list. This includes fixed DHCP parameters such as Client-Hardware-Address, optional parameters such as Requested-IP-Address, and parameters synthesised by FreeRADIUS such as Message-Type and Network-Subnet.

DHCP options can be set by updating their value in the reply list. This forms the basis of the packet returned to the client.

In the default DHCP server configuration, a "policy" (akin to a subroutine) is used to set common options for reply packets. The policy is found in <raddb>/policy.d/dhcp.

Look at the contents of the dhcp_common section and set any global options applicable to all clients in this policy.

dhcp_common {
    reply = {
        Domain-Name-Server = { 8.8.8.8, 8.8.4.4 }
        Subnet-Mask = 255.255.255.0
        Router-Address = 192.0.2.1
        ...
    }
}

See the edit documentation for instructions on creating and editing attributes.

These edit operation allow for attributes to be set to default values and then overwritten, e.g. setting a default lease time, but then later overwriting it for a particular group of clients.

Attributes in the control list are not returned in the DHCP reply packets but instead govern aspects of server’s behaviour.

To use an SQL backend for either static or dynamic IP allocation, un-comment the block:

control.IP-Pool.Name := "local"
sqlippool

The IP-Pool.Name control attribute is used in looking up addresses in the database. The line containing sqlippool is a call to invoke the sqlippool module. This module is responsible for assigning a free IP address into the Your-IP-Address reply attribute from the pool identified by IP-Pool.Name.

Here IP-Pool.Name is being set to a constant value (local) indicating that a single pool is to be used. If you have multiple pools, then replace this assignment with logic to map clients to the correct pool, as described below.