Chapter 16: DHCP Snooping
228
Overview
The DHCP Snooping feature provides security by inspecting ingress
packets for the correct IP and MAC address information. The DHCP
Snooping feature defines the AT-GS950/24 ports as either trusted or
untrusted. With DHCP Snooping enabled, two network security issues are
addressed:
All ingress DHCP packets are examined on the untrusted ports
and only authorized packets are passed through the switch.
Unwanted ingress DHCP packets are discarded. See
"Unauthorized DHCP Servers".
DHCP ingress packets on an untrusted port are inspected to insure
that the source IP Address and MAC Address combination in each
packet is valid when compared to the DHCP Snooping Binding
Table. If match is not found, the packet is discarded.
Trusted Ports
By definition, trusted ports inherently trust all ingress Ethernet traffic.
There is no checking or testing on ingress packets for this type of port. A
trusted port connects to a DHCP server in one of the following ways:
Directly to the legitimate trusted DHCP Server
A network device relaying DHCP messages to and from a trusted
server
Another trusted source such as a switch with DHCP Snooping
enabled.
Untrusted Ports
The Ethernet traffic on an untrusted port is inherently not trusted. The
ingress packets are consequently tested against specific criteria to
determine if they can be forwarded through the switch or should be
immediately discarded. Untrusted ports are connected to DHCP clients
and to traffic that originates outside of the LAN.
Unauthorized
DHCP Servers
Normally in a network, a single DHCP server exists in a local area network
(LAN). The DHCP server supplies network configuration information to
individual devices on the network including the assigned IP address for
each host. A trusted DHCP server is connected to a trusted port on the
switch.
It is possible that another unauthorized and unwanted DHCP server could
be connected to the network. This situation can occur if a client on the
network happens to enable a DHCP server application on his workstation
of if someone outside the network attempts to send DHCP packets to your
network. These situations pose a security risk.
A network device initially sends out a DHCPDISCOVER packet so that a
DHCP server will respond. It waits for and then accepts the
Summary of Contents for AT-GS950/24
Page 12: ...Figures 12 ...
Page 14: ...List of Tables 14 ...
Page 18: ...Preface 18 ...
Page 60: ...Chapter 2 Basic Switch Configuration 60 ...
Page 68: ...Chapter 3 Port Configuration 68 ...
Page 74: ...Chapter 4 Port Mirroring 74 ...
Page 94: ...Chapter 5 Virtual LANs 94 ...
Page 102: ...Chapter 6 GVRP 102 ...
Page 132: ...Chapter 8 STP and RSTP 132 ...
Page 146: ...Chapter 9 Multiple Spanning Tree Protocol 146 ...
Page 156: ...Chapter 10 Static Port Trunking 156 ...
Page 178: ...Chapter 12 Quality of Service CoS 178 ...
Page 206: ...Chapter 13 Access Control Configuration 206 ...
Page 226: ...Chapter 15 MAC Address Table 226 ...
Page 238: ...Chapter 16 DHCP Snooping 238 ...
Page 244: ...Chapter 17 IGMP Snooping 244 ...
Page 288: ...Chapter 21 Simple Network Management Protocol SNMPv3 288 ...
Page 300: ...Chapter 22 RMON 300 ...
Page 322: ...Chapter 24 Management Software Updates 322 ...
Page 348: ...Appendix A MSTP Overview 348 ...
Page 366: ...Appendix A AT GS950 24 Default Parameters 366 ...