Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

 
 

DHCP Snooping

DHCP snooping on Junos OS device validates DHCP messages and drops invalid traffic. You can configure how DHCP relay agent handles DHCP snooped packets. Depending on the configuration, DHCP relay agent either forwards or drops the snooped packets it receives. For more information, read this topic.

DHCP Snooping Support

DHCP snooping provides additional security by identifying the incoming DHCP packets and rejecting DHCP traffic determined to be unacceptable from untrusted devices in the network.

What is DHCP Snooping

DHCP allocates IP addresses dynamically, leasing addresses to devices so that the addresses can be reused when they are no longer needed by the devices to which they were assigned. Hosts and end devices that require IP addresses obtained through DHCP must communicate with a DHCP server across the LAN.

DHCP snooping looks into incoming DHCP packets and examines DHCP messages. It extracts their IP addresses and lease information allocated to clients and builds up a database. Using this database, it can determine if the packets arriving are from the valid clients—that is—the IP addresses of the clients was assigned by the DHCP server. In this way, DHCP snooping acts as a guardian of network security by keeping track of valid IP addresses assigned to downstream network devices by a trusted DHCP server (the server is connected to a trusted network port).

Benefits of DHCP Snooping

  • DHCP snooping provides an extra layer of security via dynamic IP source filtering.

  • DHCP snooping can prevent rogue DHCP activity in the network by filtering out DHCP packets that are arriving on the wrong ports, or with incorrect contents.

Configuring DHCP Snooping

In the default DHCP snooping configuration, all traffic is snooped.

On Junos OS device, DHCP snooping is enabled in a routing instance when you configure the following options in that routing instance:

  • dhcp-relay statement at the [edit forwarding-options] hierarchy level

  • dhcp-local-server statement at the [edit system services] hierarchy level

  • You can optionally use the forward-snooped-clients statement to evaluate the snooped traffic and to determine if the traffic is forwarded or dropped, based on whether or not the interface is configured as part of a group.

The router discards snooped packets by default if there is no subscriber associated with the packet. To enable normal processing of snooped packets, you must explicitly configure the allow-snooped-clients statement at the [edit forwarding-options dhcp-relay] hierarchy level.

You can configure DHCP snooping support for a specific routing instance for the following:

  • DHCPv4 relay agent—Override the router’s (or switch’s) default snooping configuration and specify that DHCP snooping is enabled or disabled globally, for a named group of interfaces, or for a specific interface within a named group.

    In a separate procedure, you can set a global configuration to specify whether the DHCPv4 relay agent forwards or drops snooped packets for all interfaces, only configured interfaces, or only nonconfigured interfaces. The router also uses the global DHCP relay agent snooping configuration to determine whether to forward or drop snooped BOOTREPLY packets. A renew request may be unicast directly to the DHCP server. This is a BOOTPREQUEST packet and is snooped.

  • DHCPv6 relay agent—As you can with snooping support for the DHCPv4 relay agent, you can override the default DHCPv6 relay agent snooping configuration on the router to explicitly enable or disable snooping support globally, for a named group of interfaces, or for a specific interface with a named group of interfaces.

    In multi-relay topologies where more than one DHCPv6 relay agent is between the DHCPv6 client and the DHCPv6 server, snooping enables intervening DHCPv6 relay agents between the client and the server to correctly receive and process the unicast traffic from the client and forward it to the server. The DHCPv6 relay agent snoops incoming unicast DHCPv6 packets by setting up a filter with UDP port 547 (the DHCPv6 UDP server port) on a per-forwarding table basis. The DHCPv6 relay agent then processes the packets intercepted by the filter and forwards the packets to the DHCPv6 server.

    Unlike the DHCPv4 relay agent, the DHCPv6 relay agent does not support global configuration of forwarding support for DHCPv6 snooped packets.

  • DHCP local server—Configure whether DHCP local server forwards or drops snooped packets for all interfaces, only configured interfaces, or only nonconfigured interfaces.

  • You can also disable snooping filters. In the preceding configurations, all DHCP traffic is forwarded to the slower routing plane of the routing instance before it is either forwarded or dropped. Disabling snooping filters causes DHCP traffic that can be forwarded directly from the faster hardware control plane to bypass the routing control plane.

Example: Configuring DHCP Snooping Support for DHCP Relay Agent

This example shows how to configure DHCP snooping support for DHCP relay agent.

Requirements

Overview

In this example, you configure DHCP snooping support for DHCP relay agent by completing the following operations:

  • Override the default DHCP snooping configuration and enable DHCP snooping support for the interfaces in group frankfurt.

  • Configure DHCP relay agent to forward snooped packets to only configured interfaces.

Note:

By default, DHCP snooping is disabled globally.

Configuration

Procedure

Step-by-Step Procedure

To configure DHCP relay support for DHCP snooping:

  1. Specify that you want to configure DHCP relay agent.

  2. Specify the named group of interfaces on which DHCP snooping is supported.

  3. Specify the interfaces that you want to include in the group. DHCP relay agent considers these as the configured interfaces when determining whether to forward or drop traffic.

  4. Specify that you want to override the default configuration for the group.

  5. Enable DHCP snooping support for the group.

  6. Return to the [edit forwarding-options dhcp-relay] hierarchy level to configure the forwarding action and specify that DHCP relay agent forward snooped packets on only configured interfaces:

  7. Enable DHCP snooped packet forwarding for DHCP relay agent.

  8. Specify that snooped packets are forwarded on only configured interfaces (the interfaces in group frankfurt).

Results

From configuration mode, confirm your configuration by entering the show forwarding-options command. If the output does not display the intended configuration, repeat the instructions in this example to correct it. The following output also shows a range of configured interfaces in group frankfurt.

If you are done configuring the device, enter commit from configuration mode.

Configuring DHCP Snooped Packets Forwarding Support for DHCP Relay Agent

You can configure how DHCP relay agent handles DHCP snooped packets. Depending on the configuration, DHCP relay agent either forwards or drops the snooped packets it receives.

DHCP relay uses a two-part configuration to determine how to handle DHCP snooped packets. This topic describes how you use the forward-snooped-clients statement to manage whether DHCP relay agent forwards or drops snooped packets, depending on the type of interface on which the packets are snooped. In the other part of the DHCP relay agent snooping configuration, you enable or disable the DHCP relay snooping feature.

Table 1 shows the action the router or switch takes on snooped packets when DHCP snooping is enabled by the allow-snooped-clients statement.

The router or switch also uses the configuration of the DHCP relay agent forwarding support to determine how to handle snooped BOOTREPLY packets.

Table 1: Actions for DHCP Relay Agent Snooped Packets When DHCP Snooping Is Enabled

forward-snooped-clients Configuration

Action on Configured Interfaces

Action on Non-Configured Interfaces

forward-snooped-clients not configured

snooped packets result in subscriber (DHCP client) creation

dropped

all-interfaces

forwarded

forwarded

configured-interfaces

forwarded

dropped

non-configured-interfaces

snooped packets result in subscriber (DHCP client) creation

forwarded

Table 2 shows the action the router (or switch) takes on snooped packets when DHCP snooping is disabled by the no-allow-snooped-clients statement.

Table 2: Actions for DHCP Relay Agent Snooped Packets When DHCP Snooping Is Disabled

forward-snooped-clients Configuration

Action on Configured Interfaces

Action on Non-Configured Interfaces

forward-snooped-clients not configured

dropped

dropped

all-interfaces

dropped

forwarded

configured-interfaces

dropped

dropped

non-configured-interfaces

dropped

forwarded

Table 3 shows the action the router (or switch) takes for the snooped BOOTREPLY packets.

Table 3: Actions for Snooped BOOTREPLY Packets

forward-snooped-clients Configuration

Action

forward-snooped-clients not configured

snooped BOOTREPLY packets dropped if client is not found

forward-snooped-clients all configurations

snooped BOOTREPLY packets forwarded if client is not found

Configured interfaces have been configured with the group statement in the [edit forwarding-options dhcp-relay] hierarchy. Non-configured interfaces are in the logical system/routing instance but have not been configured by the group statement.

To configure DHCP snooped packet forwarding and BOOTREPLY snooped packet forwarding for DHCP relay agent:

  1. Specify that you want to configure DHCP relay agent.
  2. Enable DHCP snooped packet forwarding.
  3. Specify the interfaces that are supported for snooped packet forwarding.

For example, to configure DHCP relay agent to forward DHCP snooped packets on only configured interfaces: