INFORM getting different values for the same option than REQUEST

[Bob Harold]

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: 192.168.146.49"
INFORM from client
ACK from server contains "Domain Name Server: 10.1.2.3"  Why?

Server:
Internet Systems Consortium DHCP Server 4.3.1
Client:
Windows, gets DHCP IP 10.244.132.37

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 10.1.2.3;
...
subnet 10.244.132.0 netmask 255.255.255.0
{
    option subnet-mask 255.255.255.0;
    option broadcast-address 10.244.132.255;
    ddns-rev-domainname "in-addr.arpa.";
    ddns-updates false;
    option domain-search ...;
    option routers 10.244.132.1;
    pool
    {
        deny members of "DENY";
        option domain-name-servers 192.168.146.49;
        option time-offset 0;
        range 10.244.132.11 10.244.132.253;
    }

}

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
DNS and DHCP
University of Michigan
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[glenn.satchell]

Hi Bob

I'm pretty sure this is expected behaviour. The INFORM request doesn't
have enough information for the server to work out pool memberships.

Try moving the dns server definition from the pool to the subnet.

I can't tell from your sample conf, but if you've got more than one pool
in that subnet then this won't help.

regards,
-glenn

On Tue, February 16, 2016 8:19 am, Bob Harold wrote:

> 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: 192.168.146.49"
> INFORM from client
> ACK from server contains "Domain Name Server: 10.1.2.3"  Why?
>
> Server:
> Internet Systems Consortium DHCP Server 4.3.1
> Client:
> Windows, gets DHCP IP 10.244.132.37
>
> 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 10.1.2.3;
> ...
> subnet 10.244.132.0 netmask 255.255.255.0
> {
>     option subnet-mask 255.255.255.0;
>     option broadcast-address 10.244.132.255;
>     ddns-rev-domainname "in-addr.arpa.";
>     ddns-updates false;
>     option domain-search ...;
>     option routers 10.244.132.1;
>     pool
>     {
>         deny members of "DENY";
>         option domain-name-servers 192.168.146.49;
>         option time-offset 0;
>         range 10.244.132.11 10.244.132.253;
>     }
>
> }
>
> 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
> DNS and DHCP
> University of Michigan
> _______________________________________________
> dhcp-users mailing list
> [hidden email]
> https://lists.isc.org/mailman/listinfo/dhcp-users
>

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[Simon Hobson]

Glenn Satchell <[hidden email]> wrote:

> I'm pretty sure this is expected behaviour. The INFORM request doesn't
> have enough information for the server to work out pool memberships.

I vaguely recall from past discussions that this is considered wrong but mandated by the RFCs. Specifically, the server is not allowed to look at any lease the client has been given when responding to an Inform message - and given that the client may not send all the same information, this means the server may respond with different answers.

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[Friesen, Don MTIC:EX]

Yes, that's what we determined.  That it was the Windows client's fault for updating network information when requesting additional browser related data.

Don Friesen


-----Original Message-----
Glenn Satchell <[hidden email]> wrote:

> I'm pretty sure this is expected behaviour. The INFORM request doesn't
> have enough information for the server to work out pool memberships.

I vaguely recall from past discussions that this is considered wrong but mandated by the RFCs. Specifically, the server is not allowed to look at any lease the client has been given when responding to an Inform message - and given that the client may not send all the same information, this means the server may respond with different answers.

_______________________________________________
dhcp-users mailing list
[hidden email]
https://lists.isc.org/mailman/listinfo/dhcp-users
_______________________________________________
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