Table of Contents

Chapter 1 ARP Configuration. 1-1

1.1 Introduction to ARP. 1-1

1.1.1 ARP Function. 1-1

1.1.2 ARP Message Format 1-1

1.1.3 ARP Table. 1-3

1.1.4 ARP Process. 1-4

1.1.5 Introduction to ARP Attack Detection. 1-4

1.1.6 Introduction to ARP Packet Rate Limit 1-6

1.1.7 Introduction to Gratuitous ARP. 1-6

1.2 Configuring ARP. 1-7

1.2.1 Configuring ARP Basic Functions. 1-7

1.2.2 Configuring ARP Attack Detection. 1-7

1.2.3 Configuring the ARP Packet Rate Limit Function. 1-9

1.3 Configuring Gratuitous ARP. 1-10

1.4 Displaying and Debugging ARP. 1-11

1.5 ARP Configuration Examples. 1-11

1.5.1 ARP Basic Configuration Example. 1-11

1.5.2 ARP Attack Detection and Packet Rate Limit Configuration Example. 1-12

 

Chapter 1  ARP Configuration

When configuring ARP, go to these sections for information you are interested in:

l           Introduction to ARP

l           Configuring ARP

l           Configuring Gratuitous ARP

l           Displaying and Debugging ARP

l           ARP Configuration Examples

 

 

1.1  Introduction to ARP

1.1.1  ARP Function

Address Resolution Protocol (ARP) is used to resolve an IP address into a data link layer address.

An IP address is the address of a host at the network layer. To send a network layer packet to a destination host, the device must know the data link layer address (MAC address, for example) of the destination host or the next hop. To this end, the IP address must be resolved into the corresponding data link layer address.

 

 

1.1.2  ARP Message Format

ARP messages are classified as ARP request messages and ARP reply messages. Figure 1-1 illustrates the format of these two types of ARP messages.

l           As for an ARP request, all the fields except the hardware address of the receiver field are set. The hardware address of the receiver is what the sender requests for.

l           As for an ARP reply, all the fields are set.

Figure 1-1 ARP message format

Table 1-1 describes the fields of an ARP packet.

Table 1-1 Description on the fields of an ARP packet

Field	Description
Hardware Type	Type of the hardware interface. Refer to Table 1-2 for the information about the field values.
Protocol type	Type of protocol address to be mapped. 0x0800 indicates an IP address.
Length of hardware address	Hardware address length (in bytes)
Length of protocol address	Protocol address length (in bytes)
Operator	Indicates the type of a data packets, which can be: l      1: ARP request packets l      2: ARP reply packets l      3: RARP request packets l      4: RARP reply packets
Hardware address of the sender	Hardware address of the sender
IP address of the sender	IP address of the sender
Hardware address of the receiver	l      For an ARP request packet, this field is null. l      For an ARP reply packet, this field carries the hardware address of the receiver.
IP address of the receiver	IP address of the receiver

 

Table 1-2 Description on the values of the hardware type field

Value	Description
1	Ethernet
2	Experimental Ethernet
3	X.25
4	Proteon ProNET (Token Ring)
5	Chaos
6	IEEE802.X
7	ARC network

 

1.1.3  ARP Table

In an Ethernet, the MAC addresses of two hosts must be available for the two hosts to communicate with each other. Each host in an Ethernet maintains an ARP table, where the latest used IP address-to-MAC address mapping entries are stored. An S3100-52P Ethernet switch provides the display arp command to display the information about ARP mapping entries.

ARP entries in an S3100-52P Ethernet switch can either be static entries or dynamic entries, as described in Table 1-3.

Table 1-3 ARP entries

ARP entry	Generation Method	Maintenance Mode
Static ARP entry	Manually configured	Manual maintenance
Dynamic ARP entry	Dynamically generated	ARP entries of this type age with time. The aging period is set by the ARP aging timer.

 

1.1.4  ARP Process

Figure 1-2 ARP process

Suppose that Host A and Host B are on the same subnet and that Host A sends a message to Host B. The resolution process is as follows:

1)         Host A looks in its ARP mapping table to see whether there is an ARP entry for Host B. If Host A finds it, Host A uses the MAC address in the entry to encapsulate the IP packet into a data link layer frame and sends the frame to Host B.

2)         If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request, in which the source IP address and source MAC address are respectively the IP address and MAC address of Host A and the destination IP address and MAC address are respectively the IP address of Host B and an all-zero MAC address. Because the ARP request is sent in broadcast mode, all hosts on this subnet can receive the request, but only the requested host (namely, Host B) will process the request.

3)         Host B compares its own IP address with the destination IP address in the ARP request. If they are the same, Host B saves the source IP address and source MAC address into its ARP mapping table, encapsulates its MAC address into an ARP reply, and unicasts the reply to Host A.

4)         After receiving the ARP reply, Host A adds the MAC address of Host B into its ARP mapping table for subsequent packet forwarding. Meanwhile, Host A encapsulates the IP packet and sends it out.

Usually ARP dynamically implements and automatically seeks mappings from IP addresses to MAC addresses, without manual intervention.

1.1.5  Introduction to ARP Attack Detection

I. Man-in-the-middle attack

According to the ARP design, after receiving an ARP response, a host adds the IP-to-MAC mapping of the sender into its ARP mapping table even if the MAC address is not the real one. This can reduce the ARP traffic in the network, but it also makes ARP spoofing possible.

In Figure 1-3, Host A communicates with Host C through a switch. To intercept the traffic between Host A and Host C, the hacker (Host B) forwards invalid ARP reply messages to Host A and Host C respectively, causing the two hosts to update the MAC address corresponding to the peer IP address in their ARP tables with the MAC address of Host B. Then, the traffic between Host A and C will pass through Host B which acts like a “man-in-the-middle” that may intercept and modify the communication information. Such an attack is called man-in-the-middle attack.

Figure 1-3 Network diagram for ARP man-in-the-middle attack

II. ARP attack detection

To guard against the man-in-the-middle attacks launched by hackers or attackers, an S3100-52P Ethernet switch supports the ARP attack detection function. All ARP (both request and response) packets passing through the switch are redirected to the CPU, which checks the validity of all the ARP packets by using the DHCP snooping table or the manually configured IP binding table. For description of DHCP snooping table and the manually configured IP binding table, refer to the DHCP snooping section in the part discussing DHCP in this manual.

After you enable the ARP attack detection function, the switch will check the following items of an ARP packet: the source MAC address, source IP address, port number of the port receiving the ARP packet, and the ID of the VLAN the port resides. If these items match the entries of the DHCP snooping table or the manual configured IP binding table, the switch will forward the ARP packet; if not, the switch discards the ARP packet.

l           With trusted ports configured, ARP packets coming from the trusted ports will not be checked, while those from other ports will be checked through the DHCP snooping table or the manually configured IP binding table.

l           With the ARP restricted forwarding function enabled, ARP request packets are forwarded through trusted ports only; ARP response packets are forwarded according to the MAC addresses in the packets, or through trusted ports if the MAC address table contains no such destination MAC addresses.

1.1.6  Introduction to ARP Packet Rate Limit

To prevent the man-in-the-middle attack, a switch enabled with the ARP attack detection function delivers ARP packets to the CPU to check the validity of the packets. However, this causes a new problem: If an attacker sends a large number of ARP packets to a port of a switch, the CPU will get overloaded, causing other functions to fail, and even the whole device to break down. To guard against such attacks, an S3100-52P Ethernet switch supports the ARP packets rate limit function, which will shut down the attacked port, thus preventing serious impact on the CPU.

With this function enabled on a port, the switch will count the ARP packets received on the port within each second. If the number of ARP packets received on the port per second exceeds the preconfigured value, the switch considers that the port is attacked by ARP packets. In this case, the switch will shut down the port. As the port does not receive any packet, the switch is protected from the ARP packet attack.

At the same time, the switch supports automatic recovery of port state. If a port is shut down by the switch due to high packet rate, the port will revert to the Up state after a configured period of time.

1.1.7  Introduction to Gratuitous ARP

The following are the characteristics of gratuitous ARP packets:

l           Both source and destination IP addresses carried in a gratuitous ARP packet are the local addresses, and the source MAC address carried in it is the local MAC addresses.

l           If a device finds that the IP addresses carried in a received gratuitous packet conflict with those of its own, it returns an ARP response to the sending device to notify of the IP address conflict.

By sending gratuitous ARP packets, a network device can:

l           Determine whether or not IP address conflicts exist between it and other network devices.

l           Trigger other network devices to update its hardware address stored in their caches.

With the gratuitous ARP packet learning function enabled:

A device receiving a gratuitous ARP packet adds the information carried in the packet to its own dynamic ARP table if it finds no corresponding ARP entry for the ARP packet exists in the cache.

1.2  Configuring ARP

1.2.1  Configuring ARP Basic Functions

Follow these steps to configure ARP basic functions:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Add a static ARP entry	arp static ip-address mac-address [ vlan-id interface-type interface-number ]	Optional By default, the ARP mapping table is empty, and entries are created dynamically by ARP.
Configure the ARP aging timer	arp timer aging aging-time	Optional 20 minutes by default.
Enable the ARP entry checking function (that is, disable the switch from  learning ARP entries with multicast MAC addresses)	arp check enable	Optional Enabled by default.

 

 

1.2.2  Configuring ARP Attack Detection

Follow these steps to configure the ARP attack detection function:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable DHCP snooping	dhcp-snooping	Required Disabled by default.
Enter Ethernet port view	interface interface-type interface-number	—
Specify the current port as a trusted port	dhcp-snooping trust	Required By default, after DHCP snooping is enabled, all ports of a switch are untrusted ports.
Quit to system view	quit	—
Enter VLAN view	vlan vlan-id	—
Enable the ARP attack detection function	arp detection enable	Required By default, ARP attack detection is disabled on all ports.
Quit to system view	quit	—
Enter Ethernet port view	interface interface-type interface-number	—
Configure the port as an ARP trusted port	arp detection trust	Optional By default, a port is an untrusted port.
Quit to system view	quit	—
Enter VLAN view	vlan vlan-id	—
Enable ARP restricted forwarding	arp restricted-forwarding enable	Optional Disabled by default. The device forwards legal ARP packets through all its ports.

 

 

1.2.3  Configuring the ARP Packet Rate Limit Function

Follow these steps to configure the ARP packet rate limit function:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enter Ethernet port view	interface interface-type interface-number	—
Enable the ARP packet rate limit function	arp rate-limit enable	Required By default, the ARP packet rate limit function is disabled on a port.
Configure the maximum ARP packet rate allowed on the port	arp rate-limit rate	Optional By default, the maximum ARP packet rate allowed on a port is 15 pps.
Quit to system view	quit	—
Enable the port state auto-recovery function	arp protective-down recover enable	Optional Disabled by default.
Configure the port state auto-recovery interval	arp protective-down recover interval interval	Optional By default, when the port state auto-recovery function is enabled, the port state auto-recovery interval is 300 seconds.

 

 

1.3  Configuring Gratuitous ARP

Follow these steps to configure gratuitous ARP:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable the gratuitous ARP packet learning function	gratuitous-arp-learning enable	Required Disabled by default.

 

 

1.4  Displaying and Debugging ARP

To do…	Use the command…	Remarks
Display specific ARP mapping table entries	display arp [ static | dynamic | ip-address ]	Available in any view
Display the ARP mapping entries related to a specified string in a specified way	display arp [ dynamic | static ] | { begin | include | exclude } regular-expression
Display the number of the ARP entries of a specified type	display arp count [ [ dynamic | static ] [ | { begin | include | exclude } regular-expression ] | ip-address ]
Display the statistics about the untrusted ARP packets dropped by the specified port	display arp detection statistics interface interface-type interface-number
Display the setting of the ARP aging timer	display arp timer aging
Clear specific ARP entries	reset arp [ dynamic | static | interface interface-type interface-number ]	Available in user view

 

1.5  ARP Configuration Examples

1.5.1  ARP Basic Configuration Example

I. Network requirements

l           Disable ARP entry check on the switch.

l           Set the aging time for dynamic ARP entries to 10 minutes.

l           Add a static ARP entry, with the IP address being 192.168.1.1, the MAC address being 000f-e201-0000, and the outbound port being Ethernet 1/0/10 of VLAN 1.

II. Configuration procedure

<Sysname> system-view

[Sysname] undo arp check enable

[Sysname] arp timer aging 10

[Sysname] arp static 192.168.1.1 000f-e201-0000 1 Ethernet1/0/10

1.5.2  ARP Attack Detection and Packet Rate Limit Configuration Example

I. Network requirements

As shown in Figure 1-4, Ethernet 1/0/1 of Switch A connects to DHCP Server; Ethernet 1/0/2 connects to Client A, Ethernet 1/0/3 connects to Client B. Ethernet 1/0/1, Ethernet 1/0/2 and Ethernet 1/0/3 belong to VLAN 1.

l           Enable DHCP snooping on Switch A and specify Ethernet 1/0/1 as the DHCP snooping trusted port.

l           Enable ARP attack detection in VLAN 1 to prevent ARP man-in-the-middle attacks, and specify Ethernet 1/0/1 as the ARP trusted port.

l           Enable the ARP packet rate limit function on Ethernet 1/0/2 and Ethernet 1/0/3 of Switch A, so as to prevent Client A and Client B from attacking Switch A through ARP traffic.

l           Enable the port state auto recovery function on the ports of Switch A, and set the recovery interval to 200 seconds.

II. Network diagram

Figure 1-4 ARP attack detection and packet rate limit configuration

III. Configuration procedure

# Enable DHCP snooping on Switch A.

<SwitchA> system-view

[SwitchA] dhcp-snooping

# Specify Ethernet 1/0/1 as the DHCP snooping trusted port and the ARP trusted port.

[SwitchA] interface Ethernet1/0/1

[SwitchA-Ethernet1/0/1] dhcp-snooping trust

[SwitchA-Ethernet1/0/1] arp detection trust

[SwitchA-Ethernet1/0/1] quit

# Enable ARP attack detection on all ports in VLAN 1.

[SwitchA] vlan 1

[SwitchA-vlan1] arp detection enable

# Enable the ARP packet rate limit function on Ethernet 1/0/2, and set the maximum ARP packet rate allowed on the port to 20 pps.

[SwitchA] interface Ethernet1/0/2

[SwitchA-Ethernet1/0/2] arp rate-limit enable

[SwitchA-Ethernet1/0/2] arp rate-limit 20

[SwitchA-Ethernet1/0/2] quit

# Enable the ARP packet rate limit function on Ethernet 1/0/3, and set the maximum ARP packet rate allowed on the port to 50 pps.

[SwitchA] interface Ethernet1/0/3

[SwitchA-Ethernet1/0/3] arp rate-limit enable

[SwitchA-Ethernet1/0/3] arp rate-limit 50

[SwitchA-Ethernet1/0/3] quit

# Configure the port state auto recovery function, and set the recovery interval to 200 seconds.

[SwitchA] arp protective-down recover enable

[SwitchA] arp protective-down recover interval 200