DHCP Option 81 on SRX Devices

This article discuss the support of DHCP Option 81 on SRX devices.

  • SRXis acting as a DHCP Server.
  • DHCP option 81 “Client FQDN option” is being implemented on the SRX and Clients.
  • Intention is to make SRX update the A and PTRrecords on the DNS server, whneevr a Client gets a new IP or IP gets renewed.
  • SRX leases out the IP’s as expected.
  • The A and PTR records are not being updated on the DNS server by the SRX.

The DHCP option 81 is not supported on the SRX and J series Platforms.
But we have got a method to implement the feature if required.

Understanding DHCP option 81 :

  • This option also called the “Client FQDN Option” is used to update DNS servers with A and PTR DNS records when ever end hots either get a new Ip or Ip gets renewed.
  • This option needs to be requested by the client in DHCP discover and Request (the flags in the request determine the nature of the request. For details refer :RFC 4702)
  • The DHCP Server conveys its capapbility of wether it will perform the DNS update. or not to the clients.
  • To perform the DNS update the DHCP server should have the authority on the DNS server to update.
  • The DHCP Server if capable always performs the PTR update, the A update can be done either by CLIENT or the SERVER.

The below steps can be impleneted on the on the Active Directory, to get the updates of DNS A and PTR entries.

  1. On the Domain Controller open: Start Menu -> Administrative Tools -> Group Policy Management
  2. From within the Group Policy Management Console, right click on the configured domain and choose Create new GPO
  3. Apply in the new GPO the settings showing the in diagram below.
  4. Update the Group Policy on all the clients in the domain. (this can be done in several ways, either from cmd or PowerShell

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James Palmer

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