FS S5500 DHCP-Snooping Configuration User Guide

: June 5, 2024
: FS

Table of Contents

FS S5500 DHCP-Snooping Configuration
DHCP-Snooping Configuration
References
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FS S5500 DHCP-Snooping Configuration

DHCP-Snooping Configuration

DHCP-Snooping Configuration Tasks

DHCP-Snooping is to prevent the fake DHCP server from providing the DHCP service by judging the DHCP packets, maintaining the binding relationship between MAC address and IP address. The L2 switch can conduct the DAI function and the IP source guard function according to the binding relationship between MAC address and IP address. The DHCP-snooping is mainly to monitor the DHCP packets and dynamically maintain the MAC-IP binding list. The L2 switch filters the packets, which do not meet the MAC-IP binding relationship, to prevent the network attack from illegal users.

Enabling/Disabling DHCP-snooping
Enabling DHCP-snooping in a VLAN
Setting an interface to a DHCP-trusting interface
Enabling DAI in a VLAN
Setting an interface to an ARP-trusting interface
Enabling source IP address monitoring in a VLAN
Setting an interface to the one which is trusted by IP source address monitoring
Configuring the TFTP server for backing up DHCP-snooping binding
Configuring a file name for DHCP-snooping binding backup
Configuring an interval for DHCP-snooping binding backup
Configuring or adding the binding relationship manually
Monitoring and maintaining DHCP-snooping
Examples for DHCP-snooping configuration

Enabling/Disabling DHCP-Snooping

Run the following commands in global configuration mode.

Command

|

Purpose

---|---
ip dhcp-relay snooping| Enables DHCP snooping.
no ip dhcp-relay snooping| Resumes the default settings.

This command is used to enable DHCP snooping in global configuration mode. After this command is run, the switch is to monitor all DHCP packets and form the corresponding binding relationship.

Note : If the client obtains the address of a switch before this command is run, the switch cannot add the corresponding binding relationship.

Enabling DHCP-Snooping in a VLAN

If DHCP snooping is enabled in a VLAN, the DHCP packets which are received from all distrusted physical ports in a VLAN will be legally checked. The DHCP response packets which are received from distrusted physical ports in a VLAN will then be dropped, preventing the faked or mis-configured DHCP server from providing address distribution services. For the DHCP request packet from distrusted ports, if the hardware address field in the DHCP request packet does not match the MAC address of this packet, the DHCP request packet is then thought as a fake packet which is used as the attack packet for DHCP DOS and then the switch will drop it. Run the following commands in global configuration mode.

Command

|

Purpose

---|---
Ip dhcp-relay snooping vlan _vlanid| Enables DHCP-snooping in a VLAN.
no ip dhcp-snooping vlan _vlanid| Disables DHCP-snooping in a VLAN.

Setting an Interface to a DHCP-Trusting Interface

If an interface is set to be a DHCP-trusting interface, the DHCP packets received from this interface will not be checked.
Run the following commands in physical interface configuration mode.

Command

|

Purpose

---|---
dhcp snooping trust| Sets an interface to a DHCP-trusting interface.
no dhcp snooping trust| Resumes an interface to a DHCP-distrusted interface.

The interface is a distrusted interface by default.

Enabling DAI in a VLAN

When dynamic ARP monitoring is conducted in all physical ports of a VLAN, a received ARP packet will be rejected if the source MAC address and the source IP address of this packet do not match up with the configured MAC-IP binding relationship. The binding relationship on an interface can be dynamically bound by DHCP or configured manually. If no MAC addresses are bound to IP addresses on a physical interface, the switch rejects forwarding all ARP packets.

Command

|

Purpose

---|---
ip arp inspection vlan vlanid| Enables dynamic ARP monitoring on all distrusted ports in a VLAN.
no ip arp inspection vlan vlanid| Disables dynamic ARP monitoring on all distrusted ports in a VLAN.

Setting an Interface to an ARP-Trusting Interface

ARP monitoring is not enabled on those trusted interfaces. The interfaces are distrusted ones by default. Run the following commands in interface configuration mode.

Command

|

Purpose

---|---
arp inspection trust| Sets an interface to an ARP-trusting interface.
no arp inspection trust| Resumes an interface to an ARP-distrusting interface.

Enabling Source IP Address Monitoring in a VLAN

After source IP address monitoring is enabled in a VLAN, IP packets received from all physical ports in the VLAN will be rejected if their source MAC addresses and source IP addresses do not match up with the configured MAC-to- IP binding relationship. The binding relationship on an interface can be dynamically bound by DHCP or configured manually. If no MAC addresses are bound to IP addresses on a physical interface, the switch rejects forwarding all IP packets received from the physical interface. Run the following commands in global configuration mode.

Command

|

Purpose

---|---
ip verify source vlan vlanid| Enables source IP address checkup on all distrusted interfaces in a VLAN.
no ip verify source vlan vlanid| Disables source IP address checkup on all interfaces in a VLAN.

**Setting an Interface to the One Which is Trusted by IP Source Address

Monitoring**

Source address checkup is not enabled on an interface if the interface has a trusted source IP address. Run the following commands in interface configuration mode.

Command

|

Purpose

---|---
ip-source trust| Sets an interface to the one with a trusted source IP address.
no Ip-source trust| Resumes an interface to the one with a distrusted source IP address.

Configuring the TFTP Server for Backing up Interface Binding

After the switch configuration is rebooted, the previously-configured interface binding will be lost. In this case, there is no binding relationship on this interface. After source IP address monitoring is enabled, the switch rejected forwarding all IP packets. After the TFTP server is configured for interface binding backup, the binding relationship will be backed up to the server through the TFTP protocol. After the switch is restarted, the switch automatically downloads the binding list from the TFTP server, securing the normal running of the network. Run the following commands in global configuration mode.

Command

|

Purpose

---|---
ip dhcp-relay snooping database-agent ip-address| Configures the IP address of the TFTP server which is to back up interface binding.
no ip dhcp-relay snooping database-agent| Cancels the TFTP Server for backing up interface binding.

Configuring a File Name for Interface Binding Backup

When backing up the interface binding relationship, the corresponding filename will be saved on the TFTP server. In this way, different switches can back up their own interface binding relationships to the same TFTP server.
Run the following commands in global configuration mode

Command

|

Purpose

---|---
ip dhcp-relay snooping db-file name| Configures a file name for interface binding backup.
no ip dhcp-relay snooping db-file| Cancels a file name for interface binding backup.

Configuring the Interval for Checking Interface Binding Backup

The MAC-to-IP binding relationship on an interface changes dynamically. Hence, you need check whether the binding relationship updates after a certain interval. If the binding relationship updates, it need be backed up again. The default interval is 30 minutes

Command

|

Purpose

---|---
ip dhcp-relay snooping write num| Configures the interval for checking interface binding backup.
no ip dhcp-relay snooping write| Resumes the interval of checking interface binding backup to the default settings.

Configuring Interface Binding Manually

If a host does not obtain the address through DHCP, you can add the binding item on an interface of a switch to enable the host to access the network. You can run no ip source binding MAC IP to delete items from the corresponding binding list. Note that the manually-configured binding items have higher priority than the dynamically-configured binding items. If the manually- configured binding item and the dynamically-configured binding item have the same MAC address, the manually-configured one updates the dynamically- configured one. The interface binding item takes the MAC address as the unique index. Run the following commands in global configuration mode.

Command

|

Purpose

---|---
ip source binding MAC IP interface name| Configures interface binding manually.
no ip source binding MAC IP| Cancels an interface binding item.

L2 Switch Forwarding DHCP Packets

The following command can be used to forward the DHCP packets to the designated DHCP server to realize DHCP relay. The negative form of this command can be used to shut down DHCP relay.

Note : This command can only be used to enable DHCP relay on L2 switches, while on L3 switches, DHCP relay is realized by the DHCP server. Run the following commands in global configuration mode

Command

|

Purpose

---|---
ip dhcp-relay agent| Enables DHCP relay.
ip dhcp-relay helper-address address vlan vlan-id| Configures the destination address and VLAN of the relay.

Monitoring and Maintaining DHCP-Snooping

Run the following commands in EXEC mode

Command

|

Purpose

---|---
show ip dhcp-relay snooping| Displays the information about DHCP- snooping configuration.
show ip dhcp-relay snooping binding| Displays the effective address binding items on an interface.
show ip dhcp-relay snooping binding all| Displays all binding items which are generated by DHCP snooping.
[ no ] debug ip dhcp-relay [ snooping | binding |

event ]

| Enables or disables the switch of DHCP relay snooping.

The following shows the information about the DHCP snooping configuration:

switch#show ip dhcp-relay snooping ip dhcp-relay snooping vlan 3 ip arp inspection vlan 3 DHCP Snooping trust interface: FastEthernet0/1 ARP Inspect interface: FastEthernet0/11 switch#show ip dhcp-relay snooping binding Hardware Address IP Address remainder time Type VLAN interface 00-e0-0f-26-23-89 192.2.2.101 86400 DHCP_SN 3 FastEthernet0/3 The following shows all binding information about dhcp-relay snooping switch#show ip dhcp- relay snooping binding all Hardware Address IP Address remainder time Type VLAN interface 00-e0-0f-32-1c-59 192.2.2.1 infinite MANUAL 1 FastEthernet0/2 00-e0-0f-26-23-89 192.2.2.101 86400 DHCP_SN 3 FastEthernet0/3 The following shows the information about dhcp-relay snooping. switch#debug ip DHCP-snooping packet DHCPR: receive l2 packet from vlan 3, diID: 3 DHCPR: DHCP packet len 277 DHCPR: add binding on interface FastEthernet0/3 DHCPR: send packet continue DHCPR: receive l2 packet from vlan 3, diID: 1 DHCPR: DHCP packet len 300 DHCPR: send packet continue DHCPR: receive l2 packet from vlan 3, diID: 3 DHCPR: DHCP packet len 289 DHCPR: send packet continue DHCPR: receive l2 packet from vlan 3, diID: 1 DHCPR: DHCP packet len 300 DHCPR: update binding on interface FastEthernet0/3 DHCPR: IP address: 192.2.2.101, lease time 86400 seconds DHCPR: send packet continue

Example of DHCP-Snooping Configuration

The network topology is shown in figure 1.

Enable DHCP snooping in VLAN 1 which connects private network A. Switch_config# ip dhcp-relay snooping Switch_config#ip dhcp-relay snooping vlan 1
Enable DHCP snooping in VLAN 2 which connects private network B. Switch_config# ip dhcp-relay snooping Switch_config# ip dhcp-relay snooping vlan 2
Sets the interface which connects the DHCP server to a DHCP-trusting interface. Switch_config_f0/1# dhcp snooping trust

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