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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