DHCP Snooping and Neighbor Discovery inspection

DHCP Snooping

DHCP Snooping is a Layer 2 security feature, that provides network security by filtering untrusted DHCP messages received from the external network causing traffic attacks within the network. DHCP Snooping is based on the concept of trusted versus untrusted switch ports. Switch ports configured as trusted can forward DHCP Replies, and the untrusted switch ports cannot. DHCP Snooping acts like a firewall between untrusted hosts and DHCP servers.

Security is critically important in an access network because various devices can connect to an access network that may not be administratively controlled by a single administrator. Stateless Address Autoconfiguration (SLAAC) and Duplicate Address Detection (DAD) mechanisms used by IPv6 are more vulnerable to attacks from a malicious user. If any person, intentionally or unintentionally, configures an IP address on the device interface wrongly and advertises that IP address as one‘s own address during DAD mechanism initiated by other device, DAD initiated devices cannot assign this address. If a malicious user replies to all the DAD IP addresses as own address, none of the devices in the access network can assign any IP addresses to their interfaces. Thus, DoS attacks can be easily carried out by the malicious user making the entire network unfunctional. In another kind of attack, a malicious user can try to poison the neighbor cache of a host by sending ND packets with bogus MAC address which is learnt by other hosts into their neighbor table. Due to the infiltration of the bogus MAC address in the host‘s neighbor table, the packets destined to its neighbor is sent to the bogus MAC address and is eventually dropped or received by an unintended host.

In general, these kinds of attacks are carried out by sending different Neighbor Discovery (ND) packets – either through solicited ND packet exchanges or as a result of unsolicited ND packet exchanges triggered due to an event like the expiry of ND timers. These packets carry interface IP address information and link-layer address information. Other devices use this information to build their neighbor table for forwarding traffic to or through the malicious device. As part of ND inspection mechanism, ND (specifically, NS, NA, and redirect) packets from only trusted hosts are allowed to pass through and the packets from untrusted hosts are dropped in the switch itself. Other network devices can safely use ND mechanisms for correctly assigning IP address to their interfaces resulting in a smooth traffic flow.

For validating the ND packets, the switch must first learn the trusted information by various mechanisms and store the information in a DHCP binding table. If the switch receives ND packets on an untrusted port, the packets are validated against entries in the DHCP binding table. If the ND packets pass the validation, the packets are forwarded. If the packets fail the validation, they are dropped in the switch itself. This process avoids invalid NA packets from propagating beyond the access switch.

DHCP Binding Table

DHCP Snooping builds and maintains a binding table, this binding table contains the MAC address, IP address, lease time, binding type, VLAN number, and port information that correspond to the local untrusted ports of the switch. When the switch receives a DHCPRELEASE or DHCPDECLINE broadcast message, DHCP Snooping performs a lookup of the MAC address in the binding table to determine if the port information in the binding table matches the port on which the message was received. If the port information matches, the DHCP packet is forwarded, otherwise it is dropped.

Trust Bindings

Restrictions

In addition to the FHS restrictions, DHCP Snooping and ND inspection have the following restrictions:

• Link-local address validation is not supported under ND inspection. Thus, an First Hop Security (FHS) enabled switch is vulnerable to attackers who try to attack with link-local addresses.

• As a 5-second timer is used to cleanup expired DHCP binding table entries, the expired DHCP binding table entries can remain in the DHCP binding table for up to 5 seconds after they expire.

• If an FHS-enabled switch gets rebooted, all the dynamically-learned binding entries get flushed and those entries need to be re-learned for ND inspection to pass. However, when the switch is rebooted, DHCP clients connected to it do not re-initiate DHCP learning, due to which, the switch cannot learn these assigned IP addresses. As a result, ND inspection fails for these addresses. To overcome this problem either DHCP client must learn the IP address again through DHCP mechanisms or the administrator must add static entries for these addresses.

• For IPv6, DHCP binding table entries learned through DHCP are not removed from the DHCP table on DHCP clients that release these addresses. The administrator must manually remove these entries after the addresses are released.

• A dynamic DHCP binding table entry is learned only using the DHCP mechanism. For other modes of address configuration on the host, a relevant DHCP binding table entry must be configured on the FHS switch so that ND packets from such host are not blocked due to ND inspection processing.

• DHCP Snooping is not supported on:

  • DHCP Relay switch

  • Etree

  • Private VLANs

  • Split Multi-Link Trunking (SMLT)

• As a best practice, do not configure DHCP Snooping on core switches.

• Configure ip dhcp-snooping trusted only on the port that connects to the DHCP Server.

• You can configure DHCP Snooping on separate switches in the following scenarios:

  • on the switch where the DHCP server connects on the DHCP server VLAN.

  • on the switch where the DHCP client connects on the DHCP client VLAN(s).

• You can configure DHCP Snooping only on the switch where the DHCP server connects and only on a DHCP server VLAN.

• If DHCP clients connect to the same switch as the DHCP server, you can configure DHCP Snooping on the client VLANs.

• A best practice is to connect the DHCP server and DHCP clients to different switches.