It’s been a while since I wrote up a new post, so I thought I would come back with a nice post about the changes in configuring DHCP Servers on the SRX’s. Since Junos 12.x a new DHCP process came out to help fix some long standing issues with the existing feature. In this post I will discuss the old configuration, some of the problems I would regularly encounter, and the configuration of the new DHCP process.

The SRX platform has had DHCP available since the platform originally was deployed. It is pretty simple to configure under System > Services > DHCP:

[edit system services]
    dhcp {
        pool 192.168.1.0/24 {
            address-range low 192.168.1.5 high 192.168.1.100;
            default-lease-time 3600;
            domain-name alostrealist.com;
            name-server {
                192.168.1.1;
            }
            router {
                192.168.1.1;
            }
        }
    }

[edit security zones security-zone trust]

interfaces { ge-0/0/1.0 { host-inbound-traffic { system-services { dhcp; } } } }

So this is pretty straightforward. In the case the SRX is passing out addresses for the 192.168.1.0/24 network, providing a name-server, domain name, and a default gateway.

This solution had its problems, and the big issue was related to setting up DHCP in routing instances. The SRX would drop the DHCP packets if the interface was bound to a routing an instance. In order to get around the limitation the interface would have to remain in the default routing instance, and a firewall filter would redirect DHCP packets from the routing instance into the default routing instance. This KB Article explains it in detail. In simple deployments this usually was not an issue, but for larger deployments with overlapping subnets this posed a huge problem without routing instance support.

The new DHCP daemon, JDHCP, was introduced in 12.1 for the SRX’s. This new daemon resolved the issue with DHCP in routing instances. Moreover the configuration was moved to the routing instance instead of globally.

[edit]
  routing-instances {
      JDHCP-VR {
          system {
              services {
                  dhcp-local-server {
                      group JDHCP-GROUP {
                          interface ge-0/0/1.0;
                      }
                  }
              }
          }
          access {
              address-assignment {
                  pool JDHCPVR-POOL {
                      family inet {
                          network 192.168.1.0/24;
                          range JDHCPVR-RANGE {
                              low 192.168.1.5;
                              high 192.168.1.100;
                          }
                          dhcp-attributes {
                              name-server {
                                  192.168.1.1;
                              }
                              router {
                                  192.168.1.1;
                              }
                          }
                      }
                  }
              }
          }
      }                                
  }

Now while setting up a DHCP Server in a cluster is still not officially supported I have found that JDHCP seems to be more stable on clustered SRX’s as well.

UPDATE: As of 12.1×47, DHCP on a SRX Cluster is now fully supported!