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DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
441
For the reason that basic ACL can only filter ARP packets based on packet type, VLAN ID, Source, and Destination
MAC information, there is a need for further inspections of ARP packets. To prevent ARP spoofing attack, we will
demonstrate here via using Packet Content ACL on the Switch to block the invalid ARP packets which contain faked
gateway’s MAC and IP binding.
Configuration
The configuration logic is as follows:
1. Only if the ARP matches Source MAC address in Ethernet, Sender MAC address and Sender IP address in
ARP protocol can pass through the switch. (In this example, it is the gateway’s ARP.)
2. The switch will deny all other ARP packets which claim they are from the gateway’s IP.
The design of Packet Content ACL on the Switch enables users to inspect any offset chunk. An offset chunk is a 4-
byte block in a HEX format, which is utilized to match the individual field in an Ethernet frame. Each profile is allowed
to contain up to a maximum of four offset chunks. Furthermore, only one single profile of Packet Content ACL can be
supported per switch. In other words, up to 16 bytes of total offset chunks can be applied to each profile and a switch.
Therefore, a careful consideration is needed for planning and configuration of the valuable offset chunks.
In Table 1, you will notice that the Offset_Chunk0 starts from the 127th byte and ends at the 128th byte. It also can be
found that the offset chunk is scratched from 1 but not zero.
Offset
Chunk
Offset
Chunk0
Offset
Chunk1
Offset
Chunk2
Offset
Chunk3
Offset
Chunk4
Offset
Chunk5
Offset
Chunk6
Offset
Chunk7
Offset
Chunk8
Offset
Chunk9
Offset
Chunk10
Offset
Chunk11
Offset
Chunk12
Offset
Chunk13
Offset
Chunk14
Offset
Chunk15
Byte
127
3
7
11
15
19
23
27
31
35
39
43
47
51
55
59
Byte
128
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
Byte
1
5
9
13
17
21
25
29
33
37
41
45
49
53
57
61
Byte
2
6
10
14
18
22
26
30
34
38
42
46
50
54
58
62
Offset
Chunk
Offset
Chunk16
Offset
Chunk17
Offset
Chunk18
Offset
Chunk19
Offset
Chunk20
Offset
Chunk21
Offset
Chunk22
Offset
Chunk23
Offset
Chunk24
Offset
Chunk25
Offset
Chunk26
Offset
Chunk27
Offset
Chunk28
Offset
Chunk29
Offset
Chunk30
Offset
Chunk31
Byte
63
67
71
75
79
83
87
91
95
99
103 107 111 115 119 123
Byte
64
68
72
76
80
84
88
92
96
100 104 108 112 116 120 124
Byte
65
69
73
77
81
85
89
93
97
101 105 109 113 117 121 125
Byte
66
70
74
78
82
86
90
94
98
102 106 110 114 118 122 126
Table 1 - Chunk and Packet Offset
The following figure indicates a completed ARP packet contained in Ethernet frame which is the pattern for the
calculation of packet offset.
Figure 14 - A Completed ARP Packet Contained in an Ethernet Frame
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