The helper subsystem is not running because the legacy DHCP relay process (fud) is not listening on UDP port 67.

JunOS returns the error below when issuing the following command:

There are two processes in JUNOS that provide DHCP relay service. One is the legacy DHCPD process - fud (UDP forwarding) process. Another is the JDHCPD process. They do not co-exist as they all listen on UDP port 67.

1. If DHCP relay configuration is under  [edit forwarding-options dhcp-relay] level , JDHCPD will be listening on UDP port 67.

   To check statistics, issue the command 'show dhcp relay statistics'

2. If DHCP relay configuration is under [edit forwarding-options helpers bootp] level, fud will be listening on UDP port 67.

   To check statistics, issue the command 'show helper statistics'

When JunOS returns the message, ' error: the helper subsystem is not running ', it means the legacy DHCP relay process (fud) is not listening on UDP port 67.

First, check which process is active by checking the output below:

1. If JDHCPD is active, DHCP relay configuration should be applied under  [edit forwarding-options dhcp-relay] . Issue the command below to check DHCPD relay statistics:

   show dhcp relay statistics

2. If JDHCPD is active but DHCP relay configuration is under helper hierarchy, check if any of the configuration below exist and remove them:

   # show interfaces fxp0
unit 0 {
    family inet {
        dhcp;  <-- remove
    }
}

# show system processes 
dhcp -service {  <-- remove
    traceoptions {
        file dhcp-debug size 10m files 10;
    }
}
    
   # show forwarding-options
   dhcp-relay {     <-- remove
    

3. If neither fud or JDHCPD is active, refer to the example below and make sure the configuration is correct.
   Learn more from Juniper documentation on Configuring Routers, Switches, and Interfaces as DHCP and BOOTP Relay Agents

   Example:

   forwarding-options {
    helpers {
        bootp {
            relay-agent-option;
            interface {
                irb.99 {
                    server 1.1.1.1 routing-instance test;
                    maximum-hop-count 16;
                    client-response-ttl 20;
                }
            }
        }
    }
}