INFORM getting different values for the same option than REQUEST

Bob Harold rharolde at
Mon Feb 15 21:19:33 UTC 2016

I am seeing  a case where my DHCP server is sending different DHCP
option values (for DNS Server) in response to an INFORM than it sends
in response to a REQUEST.
This particular DHCP range is set to use a different DNS server than
the default.
But an INFORM is getting the default DNS server instead of the
specific value for the client's Dynamic DHCP range.  Is that correct?
It was certainly not what I expected.

The packet flow during a renew is:
REQUEST from client
ACK from server contains "Domain Name Server:"
INFORM from client
ACK from server contains "Domain Name Server:"  Why?

Internet Systems Consortium DHCP Server 4.3.1
Windows, gets DHCP IP

dhcpd.conf is very long, the relevant bits (client's Client ID, MAC
Address, and IP Address are not in the file):
option domain-name-servers;
subnet netmask
    option subnet-mask;
    option broadcast-address;
    ddns-rev-domainname "";
    ddns-updates false;
    option domain-search ...;
    option routers;
        deny members of "DENY";
        option domain-name-servers;
        option time-offset 0;


I obfuscated as little as possible.  And I normally list two DNS
servers.  I did not ask why this user is using their own DNS for this
subnet.  For now, my concern is whether this is expected or a bug.

Bob Harold
University of Michigan

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