John, If you can, swap your server address with your old server address. Here is what I suspect is happening: In a normal dhcp request, the packet is a broadcast. All systems see the request. In most advanced (enterprise, university, corporate setups), the switches isolate broadcast traffic and in fact you might not even be serving dhcp for the same subnet you are on. So, enter the "dhcp helper" as Cisco calls it. A router sees the broadcast traffic and forwards it to where ever the admin has told it to go even if the dhcp server is not on the same broadcast domain. I say that only to let you know that this might not work. Here is what I use all the time: tcpdump -i enp1s0 port 67 or port 68 -e -n -vv change the enp1s0 to what ever your ethernet interface is. This will show both sides of a dhcp conversation. The reason I suggested you switch your addresses is that if your switch/router admins have told your dhcp where to go, it will work after you switch the addresses. Hopefully one or the other works for you. -- Kelly Prescott On Tue, 29 Dec 2020, John G Heim wrote:
I am trying to set up a new failover DHCP server using the Linux isc-dhcp-server package. I had a working failover server that needed to be replaced because it's old. I copied the dhcpd.conf file from the old server to the new one so I'm fairly sure it is correct. But if I shut down the primary server, it does not fail over. I suspect a config problem at the switch -- which I do not own or control. So what I want to do is to sniff packets to see if the DHCPREQUEST packets are even getting to the failover server. Anybody know how I can do that on Linux?
My preferred packet sniffer is ngrep but I'll use something else if someone can give me the command syntax.
I probably should mention that I need to do this remotely. I am working from home because of the pandemic, of course.
-- John G. Heim; jheim@math.wisc.edu; 608-263-4189 _______________________________________________ Blind-sysadmins mailing list -- blind-sysadmins@lists.hodgsonfamily.org To unsubscribe send an email to blind-sysadmins-leave@lists.hodgsonfamily.org