Listening on interface without IP address

Simon Hobson dhcp1 at
Thu Jun 13 17:36:57 UTC 2019

Miloslav Hůla <miloslav.hula at> wrote:

> we maintain about 200 VLANs with ISC DHCP server, relay running on central box.
> But the relay is somehow broken and I'm thinking how to bypass that. Unfornutaly, we have no free IPv4 addresses for 2 DHCP servers.
> Is it possible to configure ISC DHCP server to listen on given interface but without IPv4 address on it?
> No subnet declaration for eth0.376 (no IPv4 addresses).
> ** Ignoring requests on eth0.376.  If this is not what
>   you want, please write a subnet declaration
>   in your dhcpd.conf file for the network segment
>   to which interface eth0.376 is attached. **
> Somehow specify in dhcpd.conf where eth0.376 belongs to?
> shared-network VLAN-376 {
> 	... for interface eth0.376
> }

What could work would be something like this :

shared-network VLAN-376 {
  subnet a.b.c.d ... {
    range a.b.c.e ; };
  subnet ... {};

And give the DHCP server an IP address in the subnet.
The server will operate using the subnet, and the shared-network statement will make it associate the a.c.b.d subnet with the same interface. You will need to setup routing so that unicast packets from the client at a.b.c.e can reach the server at 10.1.120.nn. This could be either by adding a 10.1.120.xx address to the router on VLAN 376, or by adding a route via some other route - for example routing to to some other interface on the DHCP server. For the latter, the DHCP server will need to be listening on that interface.

Ideally - just fix the relay agent !

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