By design, the JDHCP config style.a default configuration for DHCP Relay caches the binding of an IP address to the MAC address and interface in its bind table. This creates an ARP entry in the ARP table set with flag “Permanent”. This is configuration is design to guard against ARP spoofing.

This article explains the concept and how a customer who does not require this default behavior can use knobs to alter it.

A typical configuration for DHCP relay:

set forwarding-options dhcp-relay overrides bootp-support
set forwarding-options dhcp-relay server-group relay-server z.z.z.z
set forwarding-options dhcp-relay group prod-server active-server-group relay-server
set forwarding-options dhcp-relay group prod-server interface rethX.Y

ARP entry for the client receiving IP address from relay is:

admin@fw1> show arp | matchz.z.z.z.z
MAC Address       Address         Interface     Flags
<...Output Snipped...>
ab:bc:ab:bc:ab:bc  z.z.z.z            rethX.Y permanent  <<<<<<

where z.z.z.z is the IP address of DHCP Server.
          X being the reth interface
          Y being the sub interface

Note : Customer does not want the SRX to hold the IP permanently.

By design the SRX caches the IP address received by client from DHCP Relay-Agent in JDHCP daemon.  The SRX creates a permanent entry into the ARP table denying any refresh.

There could be an issue for customers who have servers booting from a PXE and have a BOND interface with virtual-IP (received from DHCP Server), but the MAC address is  chosen dynamically of tone of the interface bonded to the BOND interface.

If the SRX default behavior is not desired and customers do not want the SRX to cache the IP to HW address and interface mapping, the following option can be used to alter the behavior.

This would relay all and any new DHCP requests received to the server without reviewing its local cache, when the following configuration is used:

set forwarding-options dhcp-relay overrides bootp-support
set forwarding-options dhcp-relay forward-only
set forwarding-options dhcp-relay server-group relay-server z.z.z.z
set forwarding-options dhcp-relay group prod-server active-server-group relay-server
set forwarding-options dhcp-relay group prod-server interface rethX.Y

Arp Table:

admin@fw1> show arp | match z.z.z.z
MAC Address       Address         Interface     Flags
<...Output Snipped...>
ab:bc:ab:bc:ab:bc  z.z.z.z              rethX.Y              none  <<<<<

• DHCP vs JDHCP changes documented TSB