Forum Discussion

VFB's avatar
Icon for Cirrus rankCirrus
Oct 16, 2022

Adding a dynamic generic host to GTM

I am attempting to load balance GTM traffic to some servers that dynamically update their IP addresses. Is there a way I can configure the GTM to update the IP address of these servers when I associate them with a WIP?

1 Reply

  • Hi

    I don't think you can do this in GTM as you can in LTM with ephemeral nodes.  A couple of altenate methods spring to mind... 

    Firstly you could do something with an iRule to intercept the DNS request as it goes to the listener, do a DNS lookup and then respond to the request based on this logic.  This would look something like

    when DNS_REQUEST {
        if {[DNS::question type] eq "A"}{
            if {[DNS::question name] eq "www.test.local"}{
                set result [RESOLVER::name_lookup "/Common/dns2" a]
                set answer [DNSMSG::section $result answer]
                set first_rr [lindex $answer 1]
                set rdata [DNSMSG::record $first_rr rdata]
            # Generate an answer
            DNS::answer insert "[DNS::question name]. 111 [DNS::question class] [DNS::question type] $rdata"
            # Stop further processing of the query after this iRule and send the answer to the client

    Or you could create some third party script to do a comparison between the DNS resolution or the server and the GTM Virtual Server address and then, if there are different, you can make some  calls to update the GTM VS and Pool config.  Below is a bash shell that I've cobbled together that shows the idea - this is very basic with no error handling so would need polishing.  But you could use something like this either on the BIGIP and use tmsh calls or externally and make API calls - if the script ran to an acceptable schedule then GTM should be able to keep up with 3rd party DNS changes.

    #Setup some variables
    #GTM Server Object Name
    #GTM Server-Virtual Server
    #DNS Name to lookup
    #DNS lookup
    dns_rec=$( dig +short $dns | grep -E '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}')
    #GTM Virtual Server 
    gtm_rec=$( tmsh list gtm server $gtm_srv virtual-servers { $gtm_vs { destination }} | grep -E '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' | sed 's/destination//g' | cut -f1 -d":" | sed 's/^ *//g')
    echo $dns_rec
    echo $gtm_rec
    #See if the values are different
    if [ "$dns_rec" == "$gtm_rec" ]; then
        echo "DNS and GTM are equal, no change"
    tmsh modify gtm server $gtm_srv virtual-servers delete { $gtm_vs }
    tmsh modify gtm server $gtm_srv virtual-servers add { $gtm_vs { destination $dns_rec:80}}
    tmsh modify gtm pool a hello members add { $gtm_srv:$gtm_vs }
        echo "GTM updated to reflect DNS change"