isc dhcp server weird hostname option value

Radoslav Pešek radoslav.pesek at
Fri Jun 23 11:28:40 UTC 2017

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.


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