Packet Capture records when a DHCP Client switches between different DHCP sever

Source: Internet
Author: User

When the author's DHCP Client switches between different DHCP sever (from 10.0.2. × to 10.0.9.x), the packet capture is as follows:


The data content of the fourth package Nak is as follows:

0110 00 00 00 00 00 00 63 82 53 63 35 01 06 36 04 0a ...... C. sc5 ..
0120 00 09 01 38 1f 72 65 71 75 65 73 74 65 64 20 61... 8.req uested
0130 64 64 72 65 73 20 6e 6f 74 20 61 76 61 69 6C ddress n ot avail
0140 61 62 6C 65 ff 00 00 00 00 00 00 00 00 00 00 able ............
0150 00 00 00 00 00 ......

It means that the new DHCP server finds that the DHCP request sent by the client itself cannot be processed, so he immediately sends a Nak so that he does not need to send it again.

Then the client sends a broadcast (discover) to get a new IP address. The server returns the result using the offer package.

Then the client sends a broadcast (request), and the server returns it using the ACK package. IP address allocated successfully.

The client uses the obtained IP address to broadcast (inform) to the group. This means that others will not use this IP address.


Comments: DHCP Nack shoudl be sent to any PC
Requesting DHCP connectivity from the firewall interface, if a dhcprequest for
Non-configured network is already ed. This terminates the dhcprequest sequence,
And starts the dhcpdiscover process by the computer.


In addition, whether the DHCP server can send a Nak depends on whether the server configuration file DHCP. conf contains authoritative. If no, the server will not send the Nak, and the client will repeatedly send 3 (Linux) or 7 (Windows) requests before sending diiscover to obtain the IP address again, which is very slow. If the client is deployed on Windows, it usually takes about one minute.


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