F5 is upgrading its customer support chat feature on My.F5.com. Chat support will be unavailable from 6am-10am PST on 1/20/26. Refer to K000159584 for details.

Forum Discussion

rameshr_132303's avatar
rameshr_132303
Icon for Nimbostratus rankNimbostratus
Aug 12, 2015

DNS Server forwarding requests to GTMs at two sites

Hi All,

 

First off, apologies if this sounds more like a DNS related query.

 

I'm currently putting in place a solution for a customer. We have two Datacenters - London and Bristol and there's on GTM at each DC.

 

Users from the customer's WAN need to be pointed to either DCs for the GTM to load balance URLs across the two DCs.

 

So for example: WAN (company) User --> send URL query to local DNS server --> Local DNS server needs to point to either London's GTM or Bristol's GTM.

 

Now i know that the local DNS server needs to have delegation records/forwarding records for the URL to point to the GTM as it would be the name server for the delegated zone.

 

The question here is, if the London DC fails, how would the DNS route the request to the Bristol GTM? Would the user keep waiting for a DNS time out and send the request to the local DNS server again and again till the local DNS server forwards the traffic to Bristol?

 

Thanks!!

 

Ramesh

 

3 Replies

  • Cyril's avatar
    Cyril
    Icon for Nimbostratus rankNimbostratus
    Aug 12, 2015

    Hi Ramesh,

     

    In GTM, Wide IP should be mapped with pool and this pool will have the configuration of Local Private IP pointing to the desired Application (Consider Virtual IP if you have LTM) and Public IP of the domain configured at the other DC.

     

    So if one of pool member (Local Private IP) is not responding, the traffic will be diverted to the other IP (Public IP of other DC). You can also have fallback IP configured for the same.

     

  • rameshr_132303's avatar
    rameshr_132303
    Icon for Nimbostratus rankNimbostratus
    Aug 12, 2015

    Hi Cyril,

     

    Thanks i understand that. But my question is, the Local DNS server in WAN, how would it forward the user traffic to either the London or Bristol GTM? Once the traffic reaches the GTM, the GTM can then return the IP address of either London or Bristol's LTM for the local server.

     

    But if the London DC fails, how would the local DNS route the request to the Bristol GTM? Would the user keep waiting for a DNS time out and send the request to the local DNS server again and again till the local DNS server forwards the traffic to Bristol? Or would the local DNS server after forwarding the traffic to London, wait for a response from London's GTM and when it doesnt receive a reply forward the traffic to the Bristol GTM after a timeout?

     

    Thanks

     

  • jonnyquest_2564's avatar
    jonnyquest_2564
    Icon for Nimbostratus rankNimbostratus
    Mar 30, 2016

    LDNS here acts as a resolver, so it doesn't forward the user traffic to anywhere, rather it helps user to resolver the FQDN.

     

    Both GTMs' records will be responded to the LDNS by the upper level domain, like this:

     

    lb1.redirect.group.com. 212.162.7.99

     

    lb2.redirect.group.com. 69.74.209.171

     

    And now (I'm not sure about this, sorry), the LDNS will try to contact lb1 and lb2 simultaneously (or the LDNS will just make contact with lb1 first, not sure), and both GTMs will reply back, say the LDNS got the reply from lb2 first, and it will choose to communicate with lb2 hereafter.

     

    lb2 will communicate with lb1 to decide which IP to send back to LDNS depends on the configurations. E.g., the availability of the site, GEO location, etc.

     

    Hope this helps.