DHCP Client Relay Server HowTo

Introduction

This document shows how to set up a DHCP server to provide leases to an IP subnet behind a DHCP relay agent.

The DHCP server is be configured with a default pool of addresses and a number of Static Leases. The DHCP relay is configured to append DHCP option 82 to each DHCP message on vlan1 and forward it to the DHCP sever.

                    |
                .--.-.
               ( (    )__
              (_,  \ ) ,_)  Internet/Intranet
                '-'--`--'
                 |
            .----+----.
            |         |
            | Server  |
            |         |
            '----+----'
                 | .1
  192.168.2.0/24 |
                 | .2
            .----+----.
            |         | iface: vlan2
            |  Relay  |
            |         | iface: vlan1
            '----+----'
                 | .1
  192.168.1.0/24 |
                 |
-----+-------+---+---+-------+-----
     |       |       |       |
  .--+--. .--+--. .--+--. .--+--.
  |  H1 | |  H2 | |  H3 | |  H4 |
  '-----' '-----' '-----' '-----'

Figure 1: DHCP server and relay setup with multiple clients.

Relay Configuration

Configure the Relay to forward any DHCP traffic towards the server.

Forward all DHCP messages to the server at 192.168.1.1 port 6767
Append option82, to all DHCP messages that pass through the relay

Relay:/#> configure
Relay:/config/#> dhcp-relay
Relay:/config/dhcp-relay/#> server 192.168.1.1:6767
Relay:/config/dhcp-relay/#> iface vlan1
Relay:/config/dhcp-relay/iface-vlan1/#> end
Relay:/config/dhcp-relay/#>
Relay:/config/dhcp-relay/#> option82
Activating Option 82.
Relay:/config/dhcp-relay/option82/#> end
Relay:/config/dhcp-relay/#> leave
Relay:/#>

Server Configuration

The DHCP server is set up to provide a dynamic pool of addresses and a number of static leases for the 192.168.1.0 subnet. With each lease the server will also provide some additional information like: gateway and DNS server. Worth noting is the use of the non-standard port 6767, this means clients cannot accidentally connect to the server for a pool address, but must always go through a relay agent.

Server:/config/#> dhcp-server
Server:/config/dhcp-server/#> server-port 6767
Server:/config/dhcp-server/#> name-server auto
Server:/config/dhcp-server/#> gateway auto

Next up is the subnet to serve:

Server:/config/dhcp-server/#> subnet 192.168.1.0/24
Created new DHCP server for subnet 192.168.1.0.
Server:/config/dhcp-server/subnet-192.168.1.0/#> pool 192.168.1.100 100
Server:/config/dhcp-server/subnet-192.168.1.0/#> end
Server:/config/dhcp-server/#>

We conclude the configuration by setting up a couple of static leases:

The first lease use match mac, meaning only the client with that particular MAC address will be offered the host 1 configuration, address 192.168.1.10
The second lease use match option82, meaning any host connected to port eth1 on the Relay (identified by its MAC address) will be offered the host 2 configuration, address 192.168.1.20

Server:/config/dhcp-server/#> host 1
Server:/config/dhcp-server/host-1/#> match mac 0c:50:e5:07:24:00
Server:/config/dhcp-server/host-1/#> address 192.168.1.10
Server:/config/dhcp-server/host-1/#> end
Server:/config/dhcp-server/#> host 2
Server:/config/dhcp-server/host-2/#> match option82 circuit-id eth1
Server:/config/dhcp-server/host-2/#> match option82 remoteid-id 0c:50:e5:65:97:00
Server:/config/dhcp-server/host-2/#> address 192.168.1.20
Server:/config/dhcp-server/host-2/#> end
Server:/config/dhcp-server/#> leave
Server:/#>

Status

To verify that addresses are distributed as intended any provided lease can be viewed on the server:

Server:/#> show dhcp
LEASE TIME  MAC ADDRESS       CLIENT ID            IP ADDRESS       HOSTNAME
864000      0c:50:e5:07:24:00 01:0c:50:e5:07:24:00 192.168.1.10     H1
864000      0c:50:e5:a0:b3:00 01:0c:50:e5:a0:b3:00 192.168.1.20     H2
864000      0c:50:e5:b2:55:00 01:0c:50:e5:b2:55:00 192.168.1.114    H3
864000      0c:50:e5:c6:27:00 01:0c:50:e5:c6:27:00 192.168.1.120    H4