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Understanding and Using Trusted DHCP Servers

Understanding Trusted and Untrusted Ports and DHCP Servers

DHCP servers provide IP addresses and other configuration information to the network’s DHCP clients. Using trusted ports for the DHCP server protects against rogue DHCP servers sending leases.

Untrusted ports drop traffic from DHCP servers to prevent unauthorized servers from providing any configuration information to clients.

By default, all trunk ports are trusted for DHCP and all access ports are untrusted.

You can configure an override of the default behavior to set a trunk port as untrusted, which blocks all ingress DHCP server messages from that interface. This is useful for preventing a rogue DHCP server attack, in which an attacker has introduced an unauthorized server into the network. The information provided to DHCP clients by this server has the potential to disrupt their network access. The unauthorized server might also assign itself as the default gateway device for the network. An attacker can then sniff the network traffic and perpetrate a man-in-the-middle attack—that is, it misdirects traffic intended for a legitimate network device to a device of its choice.

You can also configure an access port as trusted. If you attach a DHCP server to an access port, you must configure the port as trusted. Before you do so, ensure that the server is physically secure—that is, that access to the server is monitored and controlled.

Enabling a Trusted DHCP Server (ELS)

Note:

This example uses Junos OS for EX Series switches with support for the Enhanced Layer 2 Software (ELS) configuration style.

You can configure any interface on a switch that connects to a DHCP server as a trusted interface (port). Configuring a DHCP server on a trusted interface protects against rogue DHCP servers sending leases.

By default, all access interfaces are untrusted, and all trunk interfaces are trusted. However, you can override the default setting for access interfaces by configuring a group of access interfaces within a VLAN, specifying an interface to belong to that group, and then configuring the group as trusted.

Before you can configure a trusted DHCP server, you must configure a VLAN. See Configuring VLANs for EX Series Switches with ELS Support (CLI Procedure).

To configure an untrusted access interface as a trusted interface for a DHCP server by using the CLI :

  1. Configure a group within a VLAN with a specific access interface:
  2. Configure that group as trusted to make the specified interface contained within the group a trusted interface:

Enabling a Trusted DHCP Server (non-ELS)

You can protect against rogue DHCP servers sending rogue leases on your network by using trusted DHCP servers and ports. By default, for DHCP, all trunk ports are trusted, and all access ports are untrusted. And you can only set up DHCP server on an interface; that is, using a VLAN is not supported.

Trusted ports allow DHCP servers to provide IP addresses and other information to requesting devices. Untrusted ports drop traffic from DHCP servers to prevent unauthorized servers from providing any configuration information to clients.

To configure a port to host a DHCP server, enter the following command from the Junos CLI:

where, the interface, ge-0/0/8 is any trusted and physically secure interface that is valid for your network.

Enabling a Trusted DHCP Server (MX Series Routers)

You can configure any interface on a switching device that connects to a DHCP server as a trusted interface (port). Configuring a DHCP server on a trusted interface protects against rogue DHCP servers sending leases.

By default, all access interfaces are untrusted, and all trunk interfaces are trusted. However, you can override the default setting for access interfaces by configuring a group of access interfaces within a bridge domain, specifying an interface to belong to that group, and then configuring the group as trusted.

Before you can configure a trusted DHCP server, you must configure a bridge domain.

To configure an untrusted access interface as a trusted interface for a DHCP server by using the CLI :

  1. Configure a group within a bridge domain with a specific access interface:

  2. Configure that group as trusted to make the specified interface contained within the group a trusted interface:

Verifying That a Trusted DHCP Server Is Working Correctly

Purpose

Verify that a DHCP trusted server is working on the switch. See what happens when the DHCP server is trusted and then untrusted.

Action

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

Display the DHCP snooping information when the interface on which the DHCP server connects to the switch is trusted. The following output results when requests are sent from the MAC addresses and the server has provided the IP addresses and leases:

Meaning

When the interface on which the DHCP server connects to the switch has been set to trusted, the output (see preceding sample) shows, for each MAC address, the assigned IP address and lease time—that is, the time, in seconds, remaining before the lease expires.

If the DHCP server had been configured as untrusted, no entries would be added to the DHCP snooping database and nothing would be shown in the output of the show dhcp snooping binding command.

Configuring a Trunk Interface as Untrusted for DHCP Security (CLI Procedure)

Before you can configure a group of interfaces, you must configure a VLAN. See Configuring VLANs for EX Series Switches with ELS Support (CLI Procedure).

Untrusted trunk interfaces support the following DHCP security features when they are enabled on the VLAN:

  • DHCP and DHCPv6 snooping

  • Dynamic ARP inspection

  • IPv6 neighbor discovery inspection

To configure a trunk interface as untrusted, you must configure a group of interfaces within a VLAN, add the trunk interface to the group, and then configure the group as trusted. A group must have at least one interface.

To configure a trunk interface as untrusted for DHCP security:

  1. Configure a group within a VLAN with the trunk interface as a member:
  2. Configure the group as untrusted to make the specified interface contained within the group an untrusted interface: