Implementing Client Subnet DNS Requests

Hi,

is there a way to tamper with the DNS response based on this information? We are running GTM. When the listener is receiving a DNS query, I would like to decide which virtual server out of the selected pool is used and posted back in the dns response depending on both location (based on the EDNS information) AND availability of the respective resource.

So, ideally, in analogy to the GSLB "Topology Record Builder" settings, I would like to set the datacenter information, but not based on the Client-IP in the Layer3 Header, but based on the EDNS client information, as shown in the code snippet.

Is this possible? Thanks!

Regards, Matt

Matt, if you have a second LTM/GTM device it is possible to SNAT the connection to match the ecs_address. You end up with something like:

when DNS_REQUEST {
...
  snat $ecs_address
...
}

This works by using a feature of LTM/GTM of auto last hop to route the snat'd traffic back to the original device that received the request. It assumes that both LTM/GTM devices are L2 adjacent.

The config is

Device 1: LTM listener with DNS profile/LTM iRule performing SNAT. Pool is GTM listener on Device 2. Device 2: GTM listener with DNS profile (vanilla GTM)

Traffic flow:

Internet (LDNS IP) -> Device 1 (SNAT to ECS IP) -> Device 2 -> Device 1 -> Internet

Be warned that you are trusting the ECS records to intentionally spoof traffic to Device 2. I have only tested this in a lab environment and have noted that it does some weird things to the EDNS records that get passed along to Device 2.

As an FYI for anyone testing, using a recent DIG version (eg 9.10), the "+subnet" is used to send a ENDS0 request , eg:

$ dig @10.1.10.53 +subnet=205.168.2.3/32

The response should show

;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096

; CLIENT-SUBNET: 205.168.2.3/32/0

;; QUESTION SECTION:

hello, is it possible to configure persistency based on the source IP captured via the eDNS header ?

@alex. Not sure if you saw that this is now a feature in BIG-IP DNS 14.0 (I just updated this post to mention that as well). Using Client Subnet in DNS Requests.

 

If you are not running BIG-IP DNS 14.0; then you could potentially use a custom persist method with an iRule: K7392 talks about using HTTP URI, but you could use the string value from eDNS in theory. I haven't thought it through fully, so not sure that would work.

 

hi Eric, i have the feeling version 14.0 + 14.1 are not stable and recommended versions yet. i am right to saythe irule in the K7392 only applies to LTM and not GTM ?

thanks in advance.