Overview

Kerio Control has a built-in DHCP server feature. From time to time, the DHCP server can stop assigning IP addresses for DHCP clients. The error log may show the following:

  DHCP error: Unable to allocate address, scope 192.168.10.0 is exhausted.

Warning logs output:

  DHCP server [Transaction ID 0x37E83A90]: Client 3c-15-c2-f2-a1-22 declined IP address 192.168.10.185.
  This address is probably in use and it will not be offered for the next 86400 seconds

Solution

DHCP server is allocating DHCP scope automatically or manually for the active interfaces listed in Trusted/Local interfaces group. If the device is not able to release the lease, the remaining DHCP address is still being assigned to the client.

The automatic scope configuration allows allocating up to 244 devices (hosts) within one interface.

If a free, newly released, or expired DHCP address cannot be assigned (or found) and DHCP allocation limit is reached, the "Unable to allocate address, scope x.x.x.x is exhausted" error is thrown.

For the DHCP "probably in use" warning, it might occur due to the device (in this example, workstation reported as a client with Mac address = 3c-15-c2-f2-a1-22) not releasing the address automatically due to local DHCP lease policy mismatch. This often happens in Apple macOS devices. For more information about DHCP in Mac, please refer to Adjust the DHCP lease time or Renew your IP address from the DHCP server on Mac.

Allocation and DHCP lease time settings can be adjusted in Kerio Control as well.

1. In Kerio Control administration navigate to DHCP server -> click Configure scopes manually -> Yes.
2. Select the necessary interface and click Edit.
3. Allocate more DHCP IP addresses by changing the first/last address or mask.
   
   

   Important: make sure you're not overlapping other interfaces' subnets.

4. Change the lease time in DHCP options table to a preferrable value (default is 4 days). Click OK.
5. Click Apply to save the settings. The allocated column will show a wider DHCP scope.
   

Testing

Ask client devices to renew DHCP lease (disable and re-enable network connection).