dhcpd.conf fails to push options to fixed-address clients

[Simon]

Kenneth Porter <shiva at sewingwitch.com> wrote:

> I use host declarations in dhcpd.conf to assigned fixed addresses to wireless access points (WAP), printers, and other IoT gear. I'm trying to assign a fixed address to a TP-Link Archer AX 20 being used as a WAP. The router sends a DHCP request for a renewed address about every 30 seconds, filling my dhcpd.log. I ran tcpdump to see what's going on and it looks like dhcpd isn't sending the requested parameter list which includes the netmask, lease time, and other useful items that are definitely configured in dhcpd.conf and which work for all my dynamic assignments. I probably never noticed it before with other devices because they don't default to a very short lease time when they don't get one in the DHCPACK.
> 
> My dhcpd.conf (after expanding file includes) can be viewed here. I tried it with hosts at top-level and within a group with the desired options.
> 
> <https://pastebin.com/yAjNziqm>
> 
> Here's what tcpdump shows. Note how the client requests lease length, subnet-mask, etc. but the server doesn't send them.
> 
> <https://pastebin.com/EbvphXh2>
> 
> I'm using dhcp-4.2.5-83.el7.centos.1.x86_64 on CentOS 7.9.2009.

What is in the leases file for 10.96.0.250 ?

Without digging out the book to check, I’m guessing that the server is sending a BOOTP response rather than a DHCP response. You have BOOTP enabled, so if that device previously had a BOOTP lease, then it’s possible the server is using the and ignoring what the client is asking for. You may have to remove any exiting leases for that device/address.

I alway deny BOOTP, it’s a long time since I came across a device needing it, and as BOOTP has no concept of lease length, all BOOTP leases are effectively infinite which means network churn can leave you with no free leases. If you do have a device that needs it, you can allow it on a per-device basis.

Simon