dhcpd.leases overrides configuration in dhcpd.conf

Ronnie Flink ronnie.flink at gmail.com
Fri Oct 10 09:28:13 UTC 2014

Hi All.

I'm working at a small ISP where we among other access technologies use
DOCSIS and every modem receives a ip and download its configuration from
tftp based on information the modem gets from dhcp.

I have been having a problem since upgrading to dhcpd 4.2.4 (Ubuntu 14.04
stock) from dhcpd 3.0.5 (on FreeBSD).
The problem that occurs is when we make changes to the configuration of
dhcpd and restart it the client that should have been affected isn't (other
clients that shouldn't be affected remain unaffected) and other normal dhcp
operation works as expected.

Steps taken we take to notice the problem:

We take a modems mac address (as listed below) and change the class it is
associated with (to get the modem to load a different configuration file)
and then restart the dhcp server to activate this change.
Next we restart the modem to force it to make a new dhcp-request and in the
dhcp-reply we send the name of the configuration file the modem should
download but the modem doesn't download this file, looking at the reply we
still see the old filename.
The only way to force the modem to download the new file is to manually
remove all objects associated with the modems mac-address from the
dhcpd.leases file (with dhcp-server stoped and using a text editor), then
when we have saved the edited dhcpd.leases file,  started dhcpd again and
finally restarted the modem again it downloads the correct file.
If we look inside dhcpd.leases before and after this manual update the
modem is associated with one class before and another after.

Each class has a option set to so that each modem assigned to that class
should use a for the class specific configuration file.
Does anyone know if there a way we can get this working as before again so
that what is in dhcpd.conf doesn't get overridden by what's in dhcpd.leases
or have any input on the issue?

In the config-extract below is the tftp-server that serves the
modem configuration files and 172.16.x.x is used to represent public
There is a section from dhcpd.leases for a modem at the end for reference.

class "bf83941a-5e71-7628-e049-13413865ccbf" {
    match option dhcp-client-identifier;
    filename "8000-800-2.cm";
    lease limit 4000;

class "772d4661-9dd6-5228-3342-6547a59a980b" {
    match option dhcp-client-identifier;
    filename "8000-4000-2.cm";
    lease limit 4000;

class "none-docsis" {
        match if not (substring (option vendor-class-identifier, 0, 6) =

subnet netmask {
option subnet-mask;
option routers;
pool {
# Class Pool (START)
allow members of "bf83941a-5e71-7628-e049-13413865ccbf";
allow members of "772d4661-9dd6-5228-3342-6547a59a980b";
allow members of "77e0e1ee-96b0-f6c4-40f8-6abcb23db39d";
allow members of "e9fee5ad-0a10-de2e-82d2-98ec10e7f920";
allow members of "1d66adaf-d85d-fbfd-93b5-7d842d398208";
allow members of "20615bd4-23a5-d873-3f23-65df6a131944";
# Class Pool (END)

subnet netmask {
option subnet-mask;
option domain-name "domain.tld";
option routers;
pool {
deny members of "docsis";
allow members of "none-docsis";
log (info, concat("cmts-2-xyz (EuroDOCSIS): CM Mac = ",binary-to-ascii(16,
8, ":", option agent.remote-id), "; CPE IP = ",binary-to-ascii (10, 8, ".",


# Modems with DOCSIS 8/4
subclass "772d4661-9dd6-5228-3342-6547a59a980b" 1:e4:48:c7:b3:c7:86;
subclass "772d4661-9dd6-5228-3342-6547a59a980b" 1:00:23:ed:21:dc:01;
subclass "772d4661-9dd6-5228-3342-6547a59a980b" 1:e4:48:c7:b3:b1:46;

# Modems with DOCSIS 8
subclass "bf83941a-5e71-7628-e049-13413865ccbf" 1:00:23:ed:21:d5:9a;
subclass "bf83941a-5e71-7628-e049-13413865ccbf" 1:00:23:ed:21:dc:9f;
subclass "bf83941a-5e71-7628-e049-13413865ccbf" 1:00:23:ed:9b:0a:d9;
subclass "bf83941a-5e71-7628-e049-13413865ccbf" 1:00:23:ed:21:d7:99;

lease {
  starts 4 2014/10/09 08:55:19;
  ends 4 2014/10/09 14:55:19;
  tstp 4 2014/10/09 14:55:19;
  cltt 4 2014/10/09 08:55:19;
  binding state active;
  next binding state free;
  billing subclass "772d4661-9dd6-5228-3342-6547a59a980b"
  hardware ethernet e4:48:c7:b3:c7:86;
  uid "\001\344H\307\263\307\206";
  option agent.circuit-id 80:2:4:12;
  option agent.remote-id e4:48:c7:b3:c7:86;
  option agent.unknown-9 0:0:11:8b:6:1:4:1:2:3:0;

Ronnie F
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