== Configure common reply options

FreeRADIUS includes a powerful xref:unlang/index.adoc[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
`DHCP-Client-Hardware-Address`, optional parameters such as
`DHCP-Requested-IP-Address`, and parameters synthesised by FreeRADIUS such as
`DHCP-Message-Type` and `DHCP-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.

[source,unlang]
----
dhcp_common {
    update reply {
        &DHCP-Domain-Name-Server := 8.8.8.8
        &DHCP-Domain-Name-Server += 8.8.4.4
        &DHCP-Subnet-Mask := 255.255.255.0
        &DHCP-Router-Address := 192.0.2.1
        ...
    }
}
----

Note, FreeRADIUS has four main operators for assigning values to attributes:

`=`:: Add the attribute to the list, if and only if an attribute of the same
  name is not already present in that list.
`:=`:: Add the attribute to the list.  If any attribute of the same name is
  already present in that list it is replaced with the new one.
`+=`:: Add the attribute to the tail of the list, even if attributes of the
  same name are already present in the list.
`^=`:: Add the attribute to the head of the list, even if attributes of the
  same name are already present in the list.

These operators allow for attributes to be set to default values and then
overwritten, e.g. setting a default lease time, but then 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:

[source,unlang]
----
update control {
    &Pool-Name := "local"
}
dhcp_sqlippool
----

The `Pool-Name` control attribute is used in looking up addresses in the
database.  The line containing `dhcp_sqlippool` is a call to invoke an
instance of a module with that name. This module is responsible for assigning a
free IP address into the `DHCP-Your-IP-Address` reply attribute from the pool
identified by `Pool-Name`.

Here `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
`update` block with logic to map clients to the correct pool, as described below.
