How to configure DHCP Option 6

08/05/2020 3 1715

DESCRIPTION:

DHCP Option 6 provides a framework for passing DNS server IP addresses to hosts on a network. Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The data items themselves are called options. Here we show how to implement DHCP Option 6 for DNS servers within a DHCP scope. 

RESOLUTION:

• When set to ‘Inherit’, the DHCP Option 6 DNS Servers will be sent in order after those listed in the SonicWall’s DNS Settings.
• When set to ‘Manual’, the DHCP Option 6 DNS Servers will be sent in order after those listed in the DHCP Scope DNS Servers.
• Therefore, if you need your client target to use the specified custom DNS server the default scope must be set to 0.0.0.0. Otherwise the client will receive the default scope.

The default DNS settings for this DHCP scope are 8.8.8.8 and 4.2.2.2. If you add an IP address to the custom dns option 6  settings in the advanced tab the custom dns server will come after the two default servers listed here. 

Therefore you set the default DNS servers to 0.0.0.0

With the default scope set to 0.0.0.0 the client will now receive the custom DNS in the following example 4.2.2.2. 

The advanced options are created in the DHCP scope configuration advanced tab. 

The capture shows the SonicWall only sends the custom option. Had the DNS settings within the DHCP the scope had any IP configuration other than 0.0.0.0 this custom server IP would come after the default IPs and the client therefore may not use your intended DNS configuration.