By default, Vigor Routers are DHCP servers, automatically assigning IP addresses to LAN clients. In the DHCP packet, the router can include information other than the IP address, which can be customised from the LAN General Setup page in the router’s web user interface. This article demonstrates how to configure the basic DHCP optionschange lease timeclear DCHP lease from inactive clientsadvanced options, and also use the router as a DHCP relay agent. (For DHCP reservation, see How to configure Bind IP to MAC for wired and wireless PC”).

The Basic Options

You can change the basic configuration in the LAN >> General Setup >> Details Page in the DHCP Server Configuration section, including The IP address pool, Subnet Mask, Router IP, Lease time and Domain Name Server.

Change Default Lease Time

The default DHCP lease time is 86400 seconds (1 day), which you can change in this section as well if necessary. Alternatively it can also be done by entering command “srv dhcp leasetime xxx” in the CLI, where xxx is the desired lease time in seconds, then enter sys reboot to restart the router.

Clear DHCP Lease from Inactive Clients

To prevent the router from running out of IP addresses, DrayOS also provides the option to reclaim IP addresses from inactive clients. It is especially useful for network environments where some clients are only temporary. With this option is enabled, the router will take the following actions when the IP pool has less than 30 IP addresses available:

  1. Check the activity of DHCP clients using ARP requests every minute
  2. Clear the DHCP lease when the client is not replying to the ARP requests

Advanced DHCP Options

More advanced DHCP options can be found by going to LAN >> General Setup and clicking on “DHCP Server Option” button. (Note: for older firmware versions, it may show “Advanced” instead).

To add a custom DHCP option:

a. Check Enable
b. Select the LAN Interface you want to provide the DHCP option
c. Enter the Option Number
d. Select a Data Type according to the option requirements. (Check Appendix below for some examples).
e. Enter the Data for the DHCP option (Note: ASCII characters ” and \ are not allowed due to security concerns. Use HEX 22 and 5C instead.)
f. Click Add to add the option into the Customised List.

DHCP Relay Agent

If you already have a DHCP server on your network, you can configure the router to act as a DHCP Relay Agent. This lets the router forward DHCP request to the IP address specified. (DHCP Relay Agent also works over VPN.)

Appendix – Examples of DHCP Options

 No.
 Option Name
 Datatype
 Data
 18
 Extensions Path
 ASCII
The path name, file name, and extension of the specified file, for example, “/path/2925001.cfg”
 33
 Static Route
 Address List
A pair of IP address separated by “,”, where the first one is the destination IP, and the second is the router’s IP, for example, “172.16.2.196, 172.16.2.1”
 66
 TFTP Server Name
 ASCII
The name of the TFTP server.
 67
 Bootfile Name
 Hexadecimal
The file name presented in hexadecimal format.
 119
 Domain Search
 Hexadecimal
The domain suffixes presented in hexadecimal with a number indicate the length in front of it. For example, “draytek.com” should be presented as “076472617974656b03636f6d” where 07 means there are 7 characters followed, 6472617974656b is “draytek”, 03 means there are 3 characters followed, and 636f6d is “com”
 121
 Classless Static Route  Option
 Hexadecimal
The width of subnet mask, the significant portion of the subnet number, and the IP address of the router presented in hexadecimal value. For example, a route “172.16.196.0/24 via 172.16.2.1” should be presented as “18ac10c4ac100201”, where 18 is the “24” in hex, ac10c4 is “172.16.196”, and ac100201 is 172.16.2.1
 150
 TFTP Server Address
 Address List
The IP address of TFTP server.