Implementing F5 DNS and Creating Custom CNAME Redirects

https://techdocs.f5.com/en-us/bigip-16-0-0/big-ip-dns-implementations/redirecting-dns-queries-using-a-cname-pool.html

where are you using or applying this irule in f5 , I guess a WIDEIP must be required to get the desired results.

DNS_REQUEST

Dns name to IP address is handled by system. Http request is handled by browser!

Browser is not aware if there is a DNS CNAME.

To make it simple, when you enter "a.example.com" in your browser, the DNS resolution is done by the system. but in no case a redirection is done by dns or browser.So CNAME can't redirect URL which we input to another URL...

If your service is hosted by F5, you can set a rule to allow you to redirect your request to another host.

but if this is not the case you have to find you have to find other solutions. I do not think that's the best solution, but you can for example hosted a service in F5 for host "a.example.com" and juste implement a redirection on host "b.login.aws.com". the other solutions is to see the side of your host so that he can make the configuration necessary to accpter the host "a.example.com".

But I'm not sure that in this case, GTM can help you because all it will do is return an IP and never redirect you...

Without involving a WIDEIP I dont think it will work.

for me on GTM-only iRules, but I think this will work for you:

when DNS_REQUEST {
  if { [wideip name] eq "www.easy.com" } {
     cname "www.verylongname.com"
 }
}

Configuration:

1). Configure WIP www.easy.com with A record and CNAME

2). Irules

1. when DNS_REQUEST {
2. if { [wideip name] eq "www.easy.com" } {
3. cname "www.verylongname.com"
4. }
5. }

https://techdocs.f5.com/en-us/bigip-16-0-0/big-ip-dns-implementations/redirecting-dns-queries-using-a-cname-pool.html