DHCP Snooping

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.

Activating DHCP Snooping

When you are using the DHCP snooping feature, it is important that you understand about enabling the DHCP snooping feature.

On Junos OS device, you cannot configure DHCP snooping feature as an independent feature. Whenever you configure DHCP security or DHCP relay or DHCP server for a specific VLAN or interface or routing instance of the device, the DHCP snooping is automatically enabled on that VLAN/interface/routing instance to perform it’s task.

For example:

• When you enable DHCP Relay on a given list of interfaces of a specific routing instance
• DHCP snooping gets automatically enabled on those interfaces for that routing instance.
• When you enable DHCP security on a specific VLAN, DHCP snooping gets automatically enabled on that VLAN.

Junos OS enables DHCP snooping on a switch/router/firewall 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.

• A switch when you configure the following option for any port security features:

  • dhcp-security statement at the [edit vlans vlan-name forwarding-options] hierarchy level.

Tip:

If you need to configure DHCP relay, use the forward-only statement unless you need subscriber management or class-of-service (CoS).

We recommend you read the DHCP documentation and use a lab with DHCP traceoptions enabled to check and understand the configuration.

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.