101 Labs - Wireshark WCNA by Browning Paul

Author:Browning, Paul [Browning, Paul]
Language: eng
Format: epub
Published: 2020-06-24T16:00:00+00:00

In this scenario, one or more hosts on the network have statically-assigned addresses and the DHCP server is unaware of this. In such a situation, the DHCP server may offer an address that is already in use in the network. This can cause a problem because two hosts with the same IP cannot exist on the same network. The figure below shows this problematic situation.

In the network, “Client 2” exists with a static IP1, while all other clients have dynamic IP. When “Client n+1” enters the network, the DHCP server offers the exact IP1 to the new client. The DHCP client can perform the duplicate address test. If the DHCP client locates another host with the same address, it must decline the IP address provided in the DHCP offer. It, however, remains with no IP address assigned.

Task 2:

A better alternative is that the DHCP server directly performs a duplicate address test (typically using ICMP Echo Requests) so that it offers only those IP addresses that are not currently not being used in the network.

The situation described above is not easy to understand, we should apply a DHCP approach based on “Capture Filter” instead of analyzing all the traffic.

Let’s suppose you capture traffic by using the capture filter port 67 or port 68 to capture only DHCP messages. In this situation, you cannot observe the duplicate address test (ICMP) made by the client or the duplicate ICMP test made by the DHCP server, thus, totally missing the problem.

The only thing you see is that the client declined the DHCP offer but you do not know why. The solution is to capture all the traffic and analyze it with display filters. The figure below shows some of the display filters available for DHCP. In the filter toolbar, type dhcp. The auto-complete feature displays a drop-down menu with all available DHCP filters, as shown in the figure below.

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