isc dhcp server weird hostname option value

Bill Shirley bill at c3po.polymerindustries.biz
Fri Jun 23 14:23:04 UTC 2017


I have a some devices that don't provide a hostname.  Dynamic DNS
doesn't get updated without a hostname.  I use:
# ------------------------------------------------------------------------------
class "NoName" {
         match if not (
                 exists server.ddns-hostname
                 or exists fqdn.hostname
                 or exists host-name
         );

         ddns-hostname = concat("NoName-", binary-to-ascii(16, 8, "", substring(hardware, 4, 3)));
         option dhcp.host-name = config-option server.ddns-hostname;
#       option fqdn.hostname = config-option server.ddns-hostname;
}
# ------------------------------------------------------------------------------
to create a hostname for them.  It uses the last 3 bytes of the MAC address
as part of the host name.  Note: I'm using 'option dhcp.host-name' instead
of 'option host-name'.  I really don't know if that matters.  You might also
try 'option fqdn.hostname'.

You can even confine them to a pool:
         pool {
                 deny dynamic bootp clients;
                 allow members of "NoName";
.
.
         }

HTH,
Bill

On 6/23/2017 7:28 AM, Radoslav Pešek wrote:
>
> Hi all,
>
> I sent this email few weeks ago (without being subscribed here), and didn't receive any reply, nor do I see it in lists 
> archive. So I write once again after subscription, hope it will work now.
>
> ~~~~
>
> My isc dhcp server behaves strangely - it sends some clients dhcp hostname option value which I don't know where it is getting 
> it from.
>
> It's on Debian 8.5, version 4.3.1. It's in failover mode.
>
> I'm using dynamic hostname generation for some clients based on last byte of their mac address:
>
> |set mac_6 = suffix(concat("0", binary-to-ascii(16, 8, "", substring(hardware, 6, 1))), 2); ddns-hostname = concat("vm1", 
> mac_6); option host-name = config-option server.ddns-hostname; |
>
> So for example client with mac address ending 02 should get hostname vm102 - and it seems to work correctly - I added logging 
> to my dhcp config and I see in log file both option host-name and config-option server.ddns-hostname set to vm102.
>
> I also see in dhcpd.leases
>
> |set mac_6 = "02"; client-hostname "vm102"; |
>
> But when I run tcpdump for this client, I see
>
> |Hostname Option 12, length 5: "vm121" |
>
> and at least once it changed to vm103.
>
> So where is it getting it from? What are the sources for this option when dhcp server sends DHCPOFFER or DHCPACK?
>
> It works after dhcp server restart, but then it starts again after a whlie.
>
> I tried to read isc-dhcp-server's source code but got bit lost as it was hard to follow.
>
> Thanks for any help.
>
> Rado.
>
>
>
>
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> dhcp-users at lists.isc.org
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