On Sun, Dec 3, 2023 at 9:57 PM jeremy ardley <jeremy.ard...@gmail.com> wrote: > > > On 4/12/23 05:18, Geert Stappers wrote: > > That triggered me to ask "Has the DHCP server been restarted?" > > > The default behaviour of most dhcp clients when they can't connect to a > dhcp server is to maintain the settings from any previous lease. > > A second default behaviour is for clients to not request new dhcp until > their existing dhcp lease has expired. > > To make sure a new dhcp setting is taken by all clients you need to > > - reconfigure your dhcp server > > - restart your dhcp server > > - check your dhcp server logs for any problems > > - restart your dhcp clients > > - check your client dhcp logs to see if the new lease info has been taken. > > There can be problems where a client with a specific MAC address > requests a specific ip address based on a previous lease but the server > is unwilling or unable to allocate that address to that MAC. This can be > checked in the server and client logs. It may also require flushing the > server lease tables and client configs so they can both start out > completely fresh.
The "restart your dhcp clients" may have a sharp edge. Sometimes the clients have a touch of resiliency or hardening added so they contact their original dhcp server, and not a [possibly] rogue server setup by an unknowing developer or attacker. In the past, you used to have to reboot Microsoft clients to get them to use the new dhcp server. I don't know Linux behavior. But if the problem is, dhcp clients are using the old dhcp server or old dhcp lease info, then I would try to reboot a client if the service restart did not help. (I saw a similar attack happen as an undergrad at the University of Maryland. Someone in the CS department setup a Linux server with Kerberos running. Workstations got answers from the rogue server and got [useless] tickets granted, and the [useless] tickets could not be used for authenticating to other services on the UMBC network. It was an intermittent problem, and it took a couple of weeks to straighten it out). Jeff