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Accepted
M. Schmidt
Asked: 2020-04-29 22:49:27 +0800 CST

ISC-DHCPd how to assign different DHCP settings even if request originated from different subnet?

  • 772

Network setup

  • 10.0.1.0/28 (VLAN 1)
  • 10.0.1.32/28 (VLAN 1)

ISC-DHCPd setup

subnet 10.0.1.0 netmask 255.255.255.240 {
  option routers 10.0.1.1;
}

subnet 10.0.1.32 netmask 255.255.255.240 {
  option routers 10.0.1.33;
}

host {
  fixed-address 10.0.1.34;
  hardware ethernet 00:11:22:33:44:55:66;
}

The networks are using a DHCP relay, that can only be configured on a per-VLAN basis.

The source of the relayed/proxied DHCPDISCOVER will, in both cases, be the router of the first subnet. Therefore ISC-DHCPd sends no DHCPOFFER and logs "no free leases" for any host that has a statically configured IP of the second subnet (10.0.1.32/28).

dhcpd.conf manual states

When dhcpd tries to find a host declaration for a client, it first looks for a host declaration which has a fixed-address declaration that lists an IP address that is valid for the subnet or shared network on which the client is booting.

Question

How can I configure the ISC-DHCPd in a way that it will accept relayed requests for all subnets, and assign a lease that is for a different subnet than where the request originated from?

dhcp isc-dhcp

1 Answers

  1. Best Answer

    Changing the ISC-DHCPd setup as follows

    shared-network twonetworks {
  subnet 10.0.1.0 netmask 255.255.255.240 {
    option routers 10.0.1.1;
  }

  subnet 10.0.1.32 netmask 255.255.255.240 {
    option routers 10.0.1.33;
  }
}

host {
  fixed-address 10.0.1.34;
  hardware ethernet 00:11:22:33:44:55:66;
}

    Makes the ISC-DHCPd answer to the DHCPDISCOVER as expected:

    dhcpd[20491]: DHCPDISCOVER from 00:11:22:33:44:55:66 via 10.0.1.1
dhcpd[20491]: DHCPOFFER on 10.0.1.34 to 00:11:22:33:44:55:66 via 10.0.1.1
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