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Understanding relay agents

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Understanding relay agents

A relay agent is a small program that relays DHCP/BOOTP messages between clients and servers on different subnets. DHCP/BOOTP relay agents are part of the DHCP and BOOTP standards and function according to the Request for Comments (RFCs), standard documents that describe protocol design and related behavior.

DHCP/BOOTP router support

In TCP/IP networking, routers are used to interconnect hardware and software used on different physical network segments called subnets and forward IP packets between each of the subnets. To support and use DHCP service across multiple subnets, routers connecting each subnet should comply with DHCP/BOOTP relay agent capabilities described in RFC 1542.

To be compliant with RFC 1542 and provide relay agent support, each router must be able to recognize BOOTP and DHCP protocol messages and process (relay) them appropriately. Because routers interpret DHCP messages as BOOTP messages (such as a UDP message sent through the same UDP port number and containing shared message structure), a router with BOOTP-relay agent capability typically relays DHCP packets and any BOOTP packets sent on the network.

If a router cannot function as a DHCP/BOOTP relay agent, each subnet must have either its own DHCP server or another computer that can function as a relay agent on that subnet. In cases where it is impractical or impossible to configure routers to support DHCP/BOOTP relay, you can configure a computer running Windows NT Server 4.0 or later to act as a relay agent by installing the DHCP Relay Agent service.

In most cases, routers support DHCP/BOOTP relay. If your routers do not, contact your router manufacturer or supplier to find out if a software or firmware upgrade is available to support this feature.

How relay agents work

A relay agent relays DHCP/BOOTP messages that are broadcast on one of its connected physical interfaces, such as a network adapter, to other remote subnets to which it is connected by other physical interfaces. The following illustration shows how client C on Subnet 2 obtains a DHCP address lease from DHCP server 1 on Subnet 1.

DHCP/BOOTP relay agent

  1. DHCP client C broadcasts a DHCP/BOOTP discover message (DHCPDISCOVER) on Subnet 2, as a User Datagram Protocol (UDP) datagram using the well-known UDP server port of 67 (the port number reserved and shared for BOOTP and DHCP server communication).

  2. The relay agent, in this case a DHCP/BOOTP relay-enabled router, examines the gateway IP address field in the DHCP/BOOTP message header. If the field has an IP address of 0.0.0.0, the agent fills it with the relay agent or router's IP address and forwards the message to the remote Subnet 1 where the DHCP server is located.

  3. When DHCP server 1 on remote Subnet 1 receives the message, it examines the gateway IP address field for a DHCP scope that can be used by the DHCP server to supply an IP address lease.

  4. If DHCP server 1 has multiple DHCP scopes, the address in the gateway IP address field (GIADDR) identifies the DHCP scope from which to offer an IP address lease.

    For example, if the gateway IP address (GIADDR) field has an IP address of 10.0.0.2, the DHCP server checks its available set of address scopes for a scope range of addresses that matches the class A IP network that includes the gateway address as a host. In this case, the DHCP server would make a check for a scope of addresses between 10.0.0.1 and 10.0.0.254. If a matching scope exists, the DHCP server selects an available address from the matched scope to use in an IP address lease offer response to the client.

  5. When DHCP server 1 receives the DHCPDISCOVER message, it processes and sends an IP address lease offer (DHCPOFFER) directly to the relay agent identified in the gateway IP address (GIADDR) field.

  6. The router then relays the address lease offer (DHCPOFFER) to the DHCP client.

    The client IP address is still unknown, so it has to be broadcast on the local subnet. Similarly, a DHCPREQUEST message is relayed from client to server, and a DHCPACK message is relayed from server to client, according to RFC 1542.

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