DHCPD gets confused about what IPs should be handed out on what interfaces

[Daniel C]

I had a device request an IP on one VLAN (one interface associated with the server -- ens256), and then I moved the device to a different VLAN (interface ens192 on the server), and was surprised when the DHCPD process continued to hand out the same IP on the new VLAN.

Here are the associated syslog entries, and the IP interface settings:

Jan 28 12:06:35 backup dhcpd[1020678]: DHCPREQUEST for 10.2.195.100 from ec:02:73:b8:4d:01 via ens256
Jan 28 12:06:35 backup dhcpd[1020678]: DHCPACK on 10.2.195.100 to ec:02:73:b8:4d:01 via ens256
Jan 28 12:08:43 backup dhcpd[1020678]: DHCPREQUEST for 10.2.195.100 from ec:02:73:b8:4d:01 via ens256
Jan 28 12:08:43 backup dhcpd[1020678]: DHCPACK on 10.2.195.100 to ec:02:73:b8:4d:01 via ens256
Jan 28 12:11:08 backup dhcpd[1024746]: DHCPREQUEST for 10.2.195.100 from ec:02:73:b8:4d:01 via ens192
Jan 28 12:11:08 backup dhcpd[1024746]: DHCPACK on 10.2.195.100 to ec:02:73:b8:4d:01 via ens192
Jan 28 12:11:23 backup dhcpd[1024746]: DHCPREQUEST for 10.2.195.100 from ec:02:73:b8:4d:01 via ens192
Jan 28 12:11:23 backup dhcpd[1024746]: DHCPACK on 10.2.195.100 to ec:02:73:b8:4d:01 via ens192

(I've trimmed out the IPv6 information):
backup$ ip addr list
...
3: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:9e:8b:d8 brd ff:ff:ff:ff:ff:ff
    inet 10.7.32.2/21 brd 10.7.39.255 scope global ens192
       valid_lft forever preferred_lft forever
...
5: ens256: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:9e:f2:e6 brd ff:ff:ff:ff:ff:ff
    inet 10.2.195.2/24 brd 10.2.195.255 scope global ens256
       valid_lft forever preferred_lft forever
...

When I rebooted the client, it went back to DHCPDISCOVER mode and then the DHCPD daemon correctly recognized that this client was on a new interface/VLAN and handed it a valid IP address for that interface/VLAN.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: isc-dhcp-server 4.4.1-2.1ubuntu5.20.04.2 [modified: usr/bin/omshell]
ProcVersionSignature: Ubuntu 5.4.0-94.106-generic 5.4.157
Uname: Linux 5.4.0-94-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.21
Architecture: amd64
CasperMD5CheckResult: pass
Date: Fri Jan 28 12:24:05 2022
InstallationDate: Installed on 2021-06-29 (212 days ago)
InstallationMedia: Ubuntu-Server 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
ProcEnviron:
 SHELL=/bin/bash
 TERM=xterm-color
 XDG_RUNTIME_DIR=<set>
 PATH=(custom, user)
SourcePackage: isc-dhcp
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.dhcp.dhcpd6.conf: [modified]
mtime.conffile..etc.dhcp.dhcpd.conf: 2022-01-28T12:09:41.747865
mtime.conffile..etc.dhcp.dhcpd6.conf: 2019-01-03T15:05:49