ddns-fwd-name using the wrong name

Rob Moser Rob.Moser at nau.edu
Wed Feb 7 22:30:24 UTC 2018

Hi All,

Short Version:

In most cases our DDNS setup works fine, but an issue has come up where for some of our hosts the DHCP server seems to pick the wrong ddns-fwd-name to send in the updates to DNS. How does dhcpd determine what name to use?

More Details:

I can see the DHCP server making updates in it's logs, and see the updates taking place successfully in the named logs on the other end. It's updating, it's just using the wrong name. From our dhcpd.leases file on one of the DHCP servers:

lease <ip address> {  
  starts 5 2018/02/02 18:54:43;
  ends 6 2018/02/03 02:54:43;
  tstp 6 2018/02/03 06:54:43;
  tsfp 6 2018/02/03 06:54:43;
  atsfp 6 2018/02/03 06:54:43;
  cltt 5 2018/02/02 18:54:43;
  binding state active;
  next binding state expired;
  hardware ethernet 18:03:73:41:33:5b;
  uid "\001\030\003sA3[";
  set ddns-fwd-name = "MATH222-20.math.our.domain";
  set ddns-txt = "REDACTED";
  set ddns-rev-name = "<reverse ip>.in-addr.arpa.";
  client-hostname "math133-01";

You can see that the client hostname is correctly identified as math133-01, but ddns-fwd-name gets set to "MATH222-20.math.our.domain". Apparently this machine did have that hostname at some point in the past, before being re-imaged and re-named, so I got them to do a release and renew, just in case it was somehow still carrying an old lease, and I observed in the logs that it went through the full RELEASE/DISCOVER cycle. No change.

The MATH222-20 host does not have a host entry in our dhcpd configs. The math133-01 has the following entry:

host math133-01.math.our.domain-dynamic {
     hardware ethernet 18:03:73:41:33:5b;
     ddns-hostname "math133-01";
     ddns-domainname "math.our.domain";
     option host-name "math133-01";
     option domain-name "math.our.domain";

The zone config for math.our.domain has nothing but the (correct) ip of the DNS server, and the key. I've grepped our entire DHCP config tree for any specific mention of MATH220-20, and found nothing.

Any ideas where dhcpd might be getting this old out-of-date value for ddns-fwd-name? Thanks for any help,

 - rob.

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