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Example: Protecting against Rogue DHCP Server Attacks

In a rogue DHCP server attack, an attacker has introduced a rogue server into the network, allowing it to give IP address leases to the network's DHCP clients and to assign itself as the gateway device.

This example describes how to configure a DHCP server interface as untrusted to protect the switch from a rogue DHCP server:


This example uses the following hardware and software components:

  • One EX Series switch or one QFX3500 switch

  • Junos OS Release 9.0 or later for EX Series switches or Junos OS Release 12.1 or later for the QFX Series

  • A DHCP server to provide IP addresses to network devices on the switch

Before you configure an untrusted DHCP server interface to mitigate rogue DHCP server attacks, be sure you have:

Overview and Topology

Ethernet LANs are vulnerable to address spoofing and DoS attacks on network devices. This example describes how to protect the switch from rogue DHCP server attacks.

This example shows how to explicitly configure an untrusted interface on an EX3200-24P switch and a QFX3500 switch. Figure 1 illustrates the topology for this example.


Figure 1: Network Topology for Basic Port SecurityNetwork Topology for Basic Port Security

The components of the topology for this example are shown in Table 1.

Table 1: Components of the Port Security Topology
Properties Settings

Switch hardware

One EX3200-24P, 24 ports (8 PoE ports) or one QFX3500 switch

VLAN name and ID

employee-vlan, tag 20

VLAN subnets through is the subnet's broadcast address

Interfaces in employee-vlan

ge-0/0/1, ge-0/0/2, ge-0/0/3, ge-0/0/8

Interface for DHCP server


In this example, the switch has already been configured as follows:

  • Secure port access is activated on the switch.

  • DHCP snooping is enabled on the VLAN employee-vlan.

  • The interface (port) where the rogue DHCP server has connected to the switch is currently trusted.


To configure the DHCP server interface as untrusted because the interface is being used by a rogue DHCP server:


CLI Quick Configuration

To quickly set the rogue DHCP server interface as untrusted, copy the following command and paste it into the switch terminal window:

Step-by-Step Procedure

To set the DHCP server interface as untrusted:

  • Specify the interface (port) from which DHCP responses are not allowed:


Check the results of the configuration:


Confirm that the configuration is working properly.

Verifying That the DHCP Server Interface Is Untrusted


Verify that the DHCP server is untrusted.


  1. Send some DHCP requests from network devices (here they are DHCP clients) connected to the switch.

  2. Display the DHCP snooping information when the port on which the DHCP server connects to the switch is not trusted.


There is no output from the command because no entries are added to the DHCP snooping database.