Implementing F5 DNS and Creating Custom CNAME Redirects

Your approach to use an iRule on F5 DNS to manipulate DNS requests and responses is correct. However, the issue you're facing might be due to how DNS resolution works and the context in which iRules operate. The iRule you provided is designed to modify the DNS request for "example.com" to "example.2.com", and then create a CNAME record in the DNS response. When you perform a DNS lookup for "example.com", the lookup is redirected to "example.2.com". However, this does not necessarily mean that "example.2.com" will resolve correctly.

when DNS_REQUEST priority 500 {
    set original_name [DNS::question name]
    if { [string tolower $original_name] ends_with "example.com" } {
        set modified_name [string map {"example.com" "example.2.com"} [string tolower $original_name]]
        # DNS::question name $modified_name
        set cname_record "${original_name}. 300 IN CNAME ${modified_name}."
        log local0. "cname_record is ${cname_record}"
        set new_rr [DNS::rr ${cname_record}]
        # log local0. "$new_rr"
        DNS::answer clear
        DNS::answer insert $new_rr
        DNS::header aa 1
        DNS::header rd 0
        DNS::header ra 0
        DNS::header ad 0
        DNS::return
    }
}

[root@NF42GTT1-A3:Active:Standalone] config # dig @172.16.53.53 www22.example.com

; <<>> DiG 9.11.31 <<>> @172.16.53.53 www22.example.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60738
;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www22.example.com. IN A

;; ANSWER SECTION:
www22.example.com. 300 IN CNAME www22.example.2.com.

;; Query time: 4 msec
;; SERVER: 172.16.53.53#53(172.16.53.53)
;; WHEN: Fri Sep 22 15:44:52 CST 2023
;; MSG SIZE rcvd: 76

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

I believe that what you want to do is use a single DNAME record that will effect the multi-CNAME behavior you want to implement.

Specifically, in your DNS zone EXAMPLE.COM, you can create a DNAME record:

example.com. <ttl> IN DNAME example.2.com

Then, for instance, using your "test" hostname, a DNS query for resolution of the IP address of FQDN test.example.com should result in the dig utility returning:

;; ANSWER SECTION:
example.com. IN DNAME example.2.com.
test.example.com. IN CNAME test.example.2.com.
test.example.2.com. IN A 1.2.3.4

assuming that that the hostname "test" is defined to resolve to IP address 1.2.3.4 in the EXAMPLE.2.COM domain.