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Problem this snippet solves:

Currently there is no support for FQDN nodes in non-default route domains as per Article. With recent increase in cloud deployment, most of the time there is a requirement from F5 to load balance to servers or ELB in the cloud which has FQDN names as those are having dyanamic IP addresses. If you are using route domains in your BIGIP environment then this becomes a challenge.

Below iRule script can be used in those scenerios to use F5 to send traffic to FQDN nodes in non-default route domains.

How to use this snippet:

  • If FQDN needs to be resolved by your internal DNS, create performance layer 4 VIP (dns_53) load balancing your DNS servers.
  • DNS server can be used directly in the iRule itself, but it would be better to use a VIP to have redundancy.
  • Create iRule and apply to the VIP

Code :

set host [RESOLV::lookup @dns_53 ""]
set ip [getfield $host " " 1]
node [lindex $ip 0]% 

Tested this on version:


Thanks for this. i am not an irule expert, but i am trying to understand the code. 

set host [RESOLV::lookup @dns_53 ""] -> So you are resolving the FQDN
set ip [getfield $host " " 1] -> Extracting the hostname
node [lindex $ip 0]%  -> I couldn get this one though!!, what it does. 


Version history
Last update:
‎16-Apr-2021 01:03
Updated by: