Hej, desperate here!

I'm running isc-dhcp-server 4.1 here (with webmin but that's another topic) and want to give some options (tftp-server and bootfile) to network-components that I can identify by MAC address and vendorstring.

However, I got to the point where the config matches the MAC address but still says it's an unknown client and won't proceed after DHCPDISCOVER. I do use deny unknown-clients but for a different shared-network and subnet.

What am I missing here and why do I only get this in the log?

dhcpd: found a board
dhcpd: DHCPDISCOVER from b8:27:eb:ab:cd:ef via eth0: unknown client

This is the current config:

shared-network COMPUTERS {
    subnet netmask {
        option subnet-mask;
        default-lease-time 3600;
        ignore client-updates;
        deny unknown-clients;
        ddns-updates off;
        pool {
        pool {

class "board" {
    match if substring (hardware, 1, 3) = b8:27:eb;
    log(info, "found a board");

shared-network hardware {
    # network for TFTP stuff
    subnet netmask {
        pool {
                allow unknown-clients;
                allow dynamic bootp clients;
                allow members of "board";
                filename "uboot.scr";
                log(info , "allocated to a board" );

Since it's a CentOS 6 I am using eth0 and eth0:1 config files and will post ifconfig as well as ip add output:

> ifconfig:
eth0      Link encap:Ethernet  HWaddr 00:26:AB:12:34:56
          inet addr:  Bcast:  Mask:
          RX packets:356698 errors:0 dropped:0 overruns:0 frame:0
          TX packets:224426 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:59600895 (56.8 MiB)  TX bytes:32866187 (31.3 MiB)

eth0:1    Link encap:Ethernet  HWaddr 00:26:AB:12:34:56
          inet addr:  Bcast:  Mask:

> ip add:
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
    link/ether 00:26:ab:12:34:56 brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
    inet brd scope global eth0:1
        valid_lft forever preferred_lft forever

Any help would be greatly appreciated.


  • Please show us ifconfig eth0
    – Tomek
    Jul 23, 2018 at 19:19
  • added eth0 information in the post above
    – Jeremiah
    Jul 23, 2018 at 20:49

1 Answer 1


Given your interface setup you have to put BOTH subnets ( & in THE SAME shared-network declaration.

From dhcpd.conf man page:

The shared-network statement is used to inform the DHCP server that some IP subnets actually share the same physical network. Any subnets in a shared network should be declared within a shared-network statement. Parameters specified in the shared-network statement will be used when booting clients on those subnets unless parameters provided at the subnet or host level override them. If any subnet in a shared network has addresses available for dynamic allocation, those addresses are collected into a common pool for that shared network and assigned to clients as needed. There is no way to distinguish on which subnet of a shared network a client should boot.

You probably can get around the restriction in the last sentence by properly using allow, deny and host declarations and you seem to be going in right direction.

You may also want to have a look at allow/deny members of "class". I am not sure however if it is available on your version of the DHCP server. It is documented in man page for ISC dhcpd 4.2.5 on CentOS 7.

  • That explained most things and brought me further a couple of steps but it seems that the class match isn't working as intended or, again, I am misunderstanding how to apply it properly. If I leave the deny unknown-clients; in the subnet/pool directive of the hardware section I still get unknown-client on the DHCPDISCOVER although I also get both loginfos for found and allocated to board. Totally confusing me. Once I comment/remove the deny unknown-clients I also get other MAC addresses added to the pool..
    – Jeremiah
    Jul 24, 2018 at 6:18
  • On a side note: dhcpd: Multiple interfaces match the same shared network: eth0 eth0:1
    – Jeremiah
    Jul 24, 2018 at 6:36
  • If you are using deny unknown-clients then you must have host entries for the clients you want to allocate addresses (just entry name and corresponding hardware ethernet address). As for the class match - I don't remember all of the details (it's been some time since I played with it) but you may need to prepend OUI part of MAC address with 1 to indicate ethernet address.
    – Tomek
    Jul 24, 2018 at 7:53

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