One DHCPACK too many in failover ISC DHCP

5 pts.
Tags:
DHCP
Hi Im running ISC DHCP 3.0.1 (yes old I know) and have recently experianced a strange thing when some clients are requesting to extend the lease. I have a failover solution and in some cases both servers answers the DHCPREQUEST with an DHCPACK on the same IP adress even thou, naturally, only one of them are specified in the option 54 field (dhcp-server-identifier). I cannot explain this and I am curious if there is anyone that can explain this or is it a bug?

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DHCP is a “dumb” protocol. If there is a DHCP request on a subnet, any DHCP server on the same subnet will send a response. What happens is the machine will take the first IP it was given and ignore all others. There is no priority in DHCP servers.

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Another way of saying this is that DHCP is a broadcast protocol. It is not unicast or multicast. The DHCP packets go to the broadcast (ff:ff:ff:ff:ff:ff or 255.255.255.255) address. So, any device running DHCP server services can and will respond. It is up to the client to pick the first server. Check this out for <a href=”http://www.tcpipguide.com/free/t_HostConfigurationandTCPIPHostConfigurationProtocol.htm”>everything you ever wanted to know about bootp and dhcp</a>.

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If you have multiple DHCP servers for backup purposes, have you made sure that the addresses issued don’t overlap between the two?

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  • Labnuke99
    Yes, you don't want overlapping IP address ranges although "intelligent" servers should ping the address to see if it is in use before assignment.
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